Zakaria Babutsidze

Nobuyuki Hanaki

Adam Zylbersztejn

Carla Lapointe

Ismaël Rafaï

Charlotte Saucet

Ann Thampi

Vincent Théroude

Mira Toumi

Dorian Vincileoni

Giuseppe Attanasi

Brice Corgnet

John Duy

Martin Dufwenberg

Astrid Hopfensitz

Nicolas Jacquemet

Annett John

Michal Krawczyk

Charles Noussair

Nonverbal content and swift trust: An experiment on digital communication

Keywords: Digital communication, Trust, Hidden action, Nonverbal content, Principal-agent relationship, Promises C72, D83 Digital communication, Trust, Hidden action, Nonverbal content, Principal-agent relationship, Promises. JEL Code: C72, D83

We experimentally study the effect of the mode of digital communication on the emergence of swift trust in a principal-agent relationship. We consider three modes of communication that differ in the capacity to transmit nonverbal content: plain text, audio, and video. Communication is pre-play, one-way, and unrestricted, but its verbal content is homogenized across treatments. Overall, both audio and video messages have a positive (and similar) effect on trust as compared to plain text; however, the magnitude of these effects depends on the verbal content of agent's message (promise to act trustworthily vs. no such promise). In all conditions, we observe a positive e ect of the agent's promise on the principal's trust. We also report that trust in female principals is sensitive to the availability of nonverbal cues about interaction partners.

Introduction

Trust is of utmost value for economic activities. It can be dened as the belief that others act in the interest of some measure of fairness or social welfare rather than their own self-interest [START_REF] Bracht | Whatever you say, your reputation precedes you: Observation and cheap talk in the trust game[END_REF]Feltovich, 2009, p. 1036).

1 Various institutions, spanning from centralized justice and law enforcement systems to self-governance within a community, foster and secure trust in economic interactions by means of incentives and punishment [START_REF] Farrell | Trust, institutions, and institutional change: Industrial districts and the social capital hypothesis[END_REF]. But trust may also emerge through tacit social agreements and informal norms of behavior [START_REF] Greif | Contract enforceability and economic institutions in early trade: The maghribi traders' coalition[END_REF]. Such informal trust often hinges on reputation which, in turn, requires repeated interactions and ways of providing and receiving feedback [START_REF] Bolton | How eective are electronic reputation mechanisms? an experimental investigation[END_REF][START_REF] Bracht | Whatever you say, your reputation precedes you: Observation and cheap talk in the trust game[END_REF][START_REF] Simpson | The enforcement of moral boundaries promotes cooperation and prosocial behavior in groups[END_REF]. Importantly, however, even when reputation lacks relevance, is hard to build, or when repeated interactions are unlikely, people may still exhibit a specic form of trust towards others known as swift trust [START_REF] Meyerson | Swift trust and temporary groups[END_REF]. Swift trust arises during a short process of interaction between previously unacquainted parties and (temporarily) aects their behavior.

2

Recent experimental evidence suggests that face-to-face communication can be highly conducive in establishing trust in social interaction by facilitating the detection of other's trustworthiness [START_REF] He | The sources of the communication gap[END_REF]. Albeit most primal, this form of communication is multilayer and encompasses not only the verbal components (e.g., verbal content and style of the message), but also the nonverbal one (e.g., a speaker's facial displays, voice, body movements).

3 However, recent 1 As they note, trust and trustworthiness (i.e., the extent to which trust in a person is warranted) are commonly considered as primary components of social capital (see also [START_REF] Knack | Does social capital have an economic payo ? a cross-country investigation[END_REF].

2 Swift trust dieres from the general willingness to trust other people an other-regarding preference that can be measured either in behavioral or attitudinal manner (or both). A commonplace methodology for measuring behavioral trust in laboratory settings is the incentivized, one-shot and anonymous trust game based on [START_REF] Berg | Trust, reciprocity, and social history[END_REF]. In this context, trust is dened as a willingness to bet that another person will reciprocate a risky move (at a cost to themselves) (Camerer, 2003, p. 85) and captured by the amount transfered from the trustor to the trustee; accordingly, trustworthiness is the amount subsequently retransfered in the opposite direction. See [START_REF] Johnson | Trust games: A meta-analysis[END_REF] for a meta-analytic study of trust game experiments. Attitudinal trust, in turn, is often measured in large-scale surveys using items such as Generally speaking, would you say that most people can be trusted, or that you can't be too careful in dealing with people? (World Values Survey; see, e.g., [START_REF] Knack | Does social capital have an economic payo ? a cross-country investigation[END_REF],

In general, one can trust other people (German Socio-Economic Panel, also used herein), or I assume that people have only the best intentions (Global Preference Survey being part of the Gallup World Poll; see [START_REF] Falk | Global evidence on economic preferences[END_REF].

In addition, [START_REF] Falk | The preference survey module: A validated instrument for measuring risk, time, and social preferences[END_REF] provide evidence that the two types of measures are correlated.

3 As noted by [START_REF] Chovil | Why emotionality cannot equal sociality: Reply to buck[END_REF], [f ]acial displays are a means by which we communicate with others.

Like words and utterances, they are more likely to be emitted when there is a potential recipient, when they are useful in conveying the particular information, and when that information is pertinent or appropriate to the social interaction (p. 163). This echoes a broad denition of communication used in animal studies which we also IT revolution has been increasingly replacing direct face-to-face interactions with those mediated by new digital technologies. Many strangers are now transacting in open online marketplaces without ever meeting in person. People are donating to charity online (e.g. Qgiv ) or contribute to online crowd-funding campaigns (e.g. Kickstarter ). Teenagers increasingly prefer communicating with their friends indirectly via texting, social media, and video-chatting rather than interacting face-to-face [START_REF] Rideout | Social media, social life: Teens reveal their experiences[END_REF]. Furthermore, the proliferation of IT technologies has led to an upswing in gig economy and abundance of distributed enterprises. In these new forms of organizations, communications among team members are less often face-to-face than in the more traditional ones (see [START_REF] Marlow | Communication in virtual teams: a conceptual framework and research agenda[END_REF], for a recent review).

Herein, we focus on the role digital communication in building swift trust in human interactions.

The main question we are asking in this paper is: does the performance of a digital communication tool in inducing swift trust depend on its capacity to transmit nonverbal content? 4 To answer this question, we conduct an incentivized laboratory experiment that implements a classic principalagent paradigm (known as the hidden action game; see [START_REF] Charness | Promises and partnership[END_REF] with one-way, pre-play communication. Before making decisions, the principal receives a message from the agent which is transmitted in one of the following ways: plain text message, auditory message, or video message. We believe that the three communication protocols studied in this paper are rather standard in the light of both the existing literature (see [START_REF] Bicchieri | Computer-mediated communication and cooperation in social dilemmas: an experimental analysis[END_REF] and everyday experience. As we discuss in the next section, the novelty of our design allows us to homogenize the verbal content of communication across treatments, and thus isolate the causal link between the richness of nonverbal content transmitted through a communication channel and the subsequent emergence of swift trust. We achieve this by inserting the recordings made in prior face-to-face interactions into our three communication treatments.

adopt for the purpose of the present study: behavior in [...] the sender [...] which evokes a response in [...] the receiver (Ekman, 2006, p. 21). Clearly, this denition includes both verbal and nonverbal forms or components of communication.

4 Numerous experiments point to the importance of nonverbal content of communication. A survey by [START_REF] Bonnefon | Can we detect cooperators by looking at their face?[END_REF] suggests that observing physical cues (such as facial displays or, more broady, bodily movements) may help detect cooperation in various economic games. See, for instance, [START_REF] Fetchenhauer | Not only states but traitshumans can identify permanent altruistic dispositions in 20 s[END_REF]; [START_REF] Bonnefon | The modular nature of trustworthiness detection[END_REF]; [START_REF] Vogt | Can we see inside? predicting strategic behavior given limited information[END_REF]; [START_REF] Centorrino | Honest signaling in trust interactions: Smiles rated as genuine induce trust and signal higher earning opportunities[END_REF]; De [START_REF] De Neys | Adolescents gradually improve at detecting trustworthiness from the facial features of unknown adults[END_REF]; van [START_REF] Van Leeuwen | Predictably angry facial cues provide a credible signal of destructive behavior[END_REF]. [START_REF] Tognetti | Does he sound cooperative? acoustic correlates of cooperativeness[END_REF] provide related evidence for auditory cues. Furthermore, [START_REF] Eckel | Face value[END_REF] and [START_REF] Zylbersztejn | Preferences for observable information in a strategic setting: An experiment[END_REF] document that people tend to exhibit a preference for receiving nonverbal cues in social interactions.

Our study aims to contribute to the experimental literature that highlights the importance of communication in fostering trust between humans and building sound economic partnerships.

With some exceptions (that are discussed in the following section), it is commonplace in this research to employ a minimalistic communication protocol in which written messages are transmitted between anonymous parties. By design, these studies are well suited for investigating the role of verbal content, but not the nonverbal one. In the context of the hidden action game that we use, several experiments highlight a particular feature of verbal messages a voluntary, non-binding promise to cooperate as a credible signal of trustworthiness and an enhancer of trust. 5 We conrm this nding for a large set of communication protocols. Furthermore, we extend this literature by sheding light on the role on nonverbal content of communication, as well as its interplay with the verbal content. Our main ndings are as follows. In aggregate, plain text messages entail less trust than either audio or video messages (which, in turn, yield similar trust rates). Providing auditory cues about the speaker seems to play a key role in inducing trust, as compared to providing a plain text content of the message. Adding visual cues in the video-based communication increases the observed trust rates only slightly (and not signicantly). Furthermore, we also nd evidence that these aggregate eects are moderated by the verbal content of communication. Including auditory cues generates a statistically signicantly increases trust levels when the message conveys a promise to cooperate. However, in the the absence of such a promise, only the combination of auditive and visual cues signicantly increases trust levels compared to the baseline textual communication. Finally, in the additional analyses we also relate our investigation of the role of nonverbal content of communication to the experimental literature on social distance. Following some of the previous experiments on strategic communication, we focus on players' gender. We observe systematic gender dierences in the environments with a restricted access to nonverbal cues. Females are generally less trusting than males when solely the vebal content of a message is transmitted (which usually does suce to identify the other person's gender). Adding nonverbal content based on auditory cues helps close this gap. However, it also leaves females more trusting towards other females than towards males (which is a bias, since it stands at odds with the degree of trustworthiness observed in both genders). These dierences disappear once we further include visual cues about the sender.

Altogether, our experimental evidence points to the positive eect of the transmission of nonverbal content of communication on human trust, but also to the heterogeneity of this eect. It may be sensitive not only to the factors that are endogeneous to the process of communication (such as the verbal content that is transmitted by the sender alongside the nonverbal content), but also to the exogeneous ones (such as the players' gender).

Empirical strategy

The main methodological objective of our study is to provide an experimental design allowing to isolate the eect of the capacity to transmit nonverbal content on inducing trusting behavior in an experimental hidden action game with one-way, pre-play communication between players.

The main virtues of our experimental design lie in the (i) ecological validity of the communication protocol (since we allow our subjects to address others in an unrestricted manner) while maintaining (ii) full control over the verbal content of communication across dierent communication modes, so as to (iii) draw causal inference about the impact of the dierent layers of nonverbal communication on swift trust in economic interactions.

Our experiment relates to the early articles by [START_REF] Brosig | The eect of communication media on cooperation[END_REF] and [START_REF] Bicchieri | Computer-mediated communication and cooperation in social dilemmas: an experimental analysis[END_REF] who compare the performance of dierent modes of pre-play communication in inducing cooperation in public goods games. Like them, we employ a rich set of communication protocols: plain text message transmitted through a computer interface, audio message, and video message.

The main ndings from this literature are: (i) communication generally improves cooperation as compared to a no-communication control condition, (ii) communication via plain text messages and audio messages has a similar eect on cooperation, and (iii) communication via video messages and face-to-face communication perform equally well and further enhance cooperation. 6 However, such outcomes are far from being stable and the literature disagrees on the relative performance of various communication channels in inducing cooperative behavior. In a related experiment with public goods games, [START_REF] Bochet | Communication and punishment in voluntary contribution experiments[END_REF] nd no dierence between a communication condition with computerized plain text messages and another condition with face-to-face communication.

The same null result is observed for trust and trustworthiness in a trust game experiment by [START_REF] Bicchieri | The medium or the message? communication relevance and richness in trust games[END_REF].

Importantly, the determinants of behavior in this body of experiments are twofold: rst, how people communicate (which is our variable of interest); second, what is the verbal content of their communication (which may impede the measurement of our variable of interest). Thus, these previous studies impose dierent communication channels as exogenous experimental treatments, but they leave the verbal content of messages to be endogenous and to vary freely across treatments.

7 Thus, behaviors observed in these studies may stem not only from an exogenous variation in the modes of communication allowing for dierent transmission of nonverbal content, but also from an endogenous and uncontrolled variation in the verbal content of communication. We conjecture that this may be the cause underlying the diverging results in this literature. For the sake of illustration of the interplay between the treatment variable, the content of communication, and the eventual behavior, consider two examples drawn from the experimental economics literature on promises.

Example 1. Using a set of communication conditions that is closely related to ours (spanning from computer chat to audio communication, then to face-to-face communication), [START_REF] Conrads | The eect of communication channels on promisemaking and promise-keeping: experimental evidence[END_REF] report that the outcome of communication (i.e., the rate of promise-making) varies of communication and reports that cooperation is higher under face-to-face communication than under communication via written messages. Other experiments provide similar evidence in the contexts of conict resolution [START_REF] Rockmann | To be or not to be trusted: The inuence of media richness on defection and deception[END_REF] and bargaining [START_REF] Valley | A matter of trust: Eects of communication on the eciency and distribution of outcomes[END_REF]; see also [START_REF] Charness | What's in a name? anonymity and social distance in dictator and ultimatum games[END_REF] for a related discussion of earlier experimental evidence.

7 Recent studies that investigate the relationship between the mode of communication and lying behavior (see, for example, [START_REF] Abeler | Representative evidence on lying costs[END_REF][START_REF] Conrads | The eect of communication channels on dishonest behavior[END_REF][START_REF] Cohn | Honesty in the digital age[END_REF] do better in controlling the content of the communication across dierent channels. In these studies, subjects simply need to report the outcomes of coin tosses in all the communication channels (voice, text, online-form, or face-to-face). [START_REF] Abeler | Representative evidence on lying costs[END_REF] nd no dierence in the reported outcome between phone call and on-line form. [START_REF] Cohn | Honesty in the digital age[END_REF] report that the presence of a human counter part (who receives the reported outcome) is a more important determinant of lying than the mode of communication (text vs voice). Finally, [START_REF] Naquin | The ner points of lying online: E-mail versus pen and paper[END_REF] nd that people are more likely to lie when sending e-mails than in hand-written messages.

in a systematic way: the richer and more direct the communication protocol, the more likely a subject is to make a promise to the other person. 8 At the same time, they observe little variation in the subsequent rates of promise-keeping.

Example 2. In the context of the hidden action game, the data from [START_REF] Charness | Self-serving cheap talk: A test of aumann's conjecture[END_REF]Dufwenberg (2006, 2010) point to an opposite phenomenon: in a richer communication environment, subjects are less likely to make promises. 9

However, the eects of communication (overall, as well as conditional on a promise to be trustworthy) on trust and trustworthiness are stronger in the richer communication environment.

We also note that the existing literature proposes some methods for controlling specic features of communication. One way to control the variability of its content is to restrict the allowed topics of conversation (see [START_REF] Bouas | Group discussion and cooperation in social dilemmas[END_REF], for an early experiment applying this method).

For instance, on top of varying modes of communication (either computer chat or face-to-face) in the trust game experiment by [START_REF] Bicchieri | The medium or the message? communication relevance and richness in trust games[END_REF], players are either allowed or not allowed to make a promise. In a similar setting, Ismayilov and Potters (2016) allow pre-play written message which can or cannot be related to the experimental game. However, while controlling the variation in the characteristic of interest, this method fails to control for the variation in other characteristics. For example, [START_REF] He | The sources of the communication gap[END_REF] observe that when people are not allowed to talk about the game they are about to play, some of them may resort to using signs and body language to non-verbally transmit strategic content.

In this paper, we provide a simple yet novel experimental method for comparing the performance of dierent modes of digital communication, which solves the problem of the endogeneity of the verbal content of communication. We focus on one-way communication from the agent to the principal in order to capture the causal eect of the transmission of nonverbal content of 8 Their experiment is based on an individual task rather than an experimental game. Communication takes place between a subject and the experimenter (a research assistant). In all communication conditions, the latter uses a standardized script presenting the task (an online survey to be lled in within 24h without any additional gratication) and asking a subject whether he or she promises to perform it.

9 In those studies, authors consider two kinds of one-way, written messages: a xed-form message (41/47 messages contain a promise to cooperate) and a free-form, hand-written statement (24/42 messages contain a promise to cooperate). We use Fisher's exact test to compare these two rates and obtain p = 0.004.

communication on trust. 10 In order to homogenize the communication content across treatments, we rst video-record messages transmitted in an experiment with direct (face-to-face, henceforth, F2F) interactions. Then, we use these recordings in the our three main treatments. Therefore, all treatments are based on resampling from the same set of messages, and thus the verbal content is homogenous across treatments, while the richness of nonverbal cues transmitted in the communication process varies across communication modes.

3 Experimental design and procedures

Our three main experimental treatments are based on an earlier set of video recordings from the pre-play face-to-face communication from agents to principals in a classic hidden action game by [START_REF] Charness | Promises and partnership[END_REF]. Participants acting as agents in those earlier sessions with faceto-face communication were clearly informed that they would be video-recorded and that these recordings could be used in future experiments. Participants to our three main treatments with digital communication are clearly informed that the recorded messages they are about to receive (either as video recordings, audio recordings, or bare content transcripts) come from the previous experimental sessions. Furthermore, the experimental instructions handed out to all participants specify that their own payos may depend on the earlier decision of the agent who sent the message, but that their own decisions have no further impact on that agent's payos. 11 Below, we rst outline the experimental game, then the face-to-face experiment used to gather the recordings, and then our three subsequent experimental treatments that exploit those aforementioned recordings.

10 We also note that when communication is real-time and multi-directional like in [START_REF] Bochet | Communication and punishment in voluntary contribution experiments[END_REF][START_REF] Bicchieri | The medium or the message? communication relevance and richness in trust games[END_REF], several treatments in [START_REF] Brosig | The eect of communication media on cooperation[END_REF], as well as other experimental data included in [START_REF] Bicchieri | Computer-mediated communication and cooperation in social dilemmas: an experimental analysis[END_REF] and [START_REF] Balliet | Communication and cooperation in social dilemmas: A meta-analytic review[END_REF], each interacting group becomes an endogenously formed cell with no counterpart in other conditions. In this case, it becomes even harder (if not impossible) to establish a causal link from an individual message to behavior, since all the messages are correlated, and each player acts as both the sender and the receiver of messages.

11 By the standard denition of trust adopted in the opening section the belief that others act in the interest of some measure of fairness or social welfare rather than their own self-interest, the way we link player A's payo to player B's earlier decision allows us to measure player A's trust in player B's cooperative intentions even though both parties do not directly interact. Don't roll

In

(5, 5) Out

Experimental hidden action game

Figure 1 presents the classic hidden action game by [START_REF] Charness | Promises and partnership[END_REF]. All payos are in Euros. The game is played between two parties: the principal (henceforth, player A) and the agent (henceforth, player B). Player A may either choose an outside option Out which yields 5 to both players and ends the interaction, or go In. Then, player B may either choose to Roll a die (which yields 12 to A and 10 to B with the probability of 5/6, and 0 to A and 10 to B with the probability of 1/6), or not to Roll (yielding 0 to A and 14 to B with certainty). This game provides a simple setting for studying principal-agent relationships with moral hazard: incentives are not aligned between the two parties, and earning 0 is not perfectly informative for player A about player B's action.

Following [START_REF] Charness | Promises and partnership[END_REF], in face-to-face implementation we simultaneously elicit both players' decisions. Namely, the player B makes a decision without knowing player A's decision. Player B's decision is only implemented had player A gone In. In our three main treatments the player B's decision is pre-recorded, and again player A takes the decision without observing it. In addition, in all experimental conditions the game is preceded by a pre-play communication stage in which player B delivers a message to player A.

Procedure

First, we have collected the dataset for the direct face-to-face (F2F) communication condition.

We have video-recorded all the pre-play statements made by player Bs, and kept records of their subsequent choices (to Roll or not), as well as of the outcomes of die rolls. Based on that information, we have then constructed our three treatments of interest VIDEO, AUDIO, and TEXT in which we vary the channel through which player Bs' messages are transmitted to player As.

Experiment begins with preliminary measurements (distributional preferences, risk preferences, cognitive skills, emotional intelligence, general trust towards others) and a basic socio-demographic questionnaire. This is followed by treatment-specic experimental instructions. In VIDEO, AU-DIO, and TEXT, those instructions also include a one-page summary of the F2F instructions.

Furthermore, after making the decision in the game (to go In or to stay Out ), player As ll in a short questionnaire about their perceptions of player Bs.

12 Below, we overview the implementation of this hidden action game.

F2F. Each experimental sessions involves 6 player As and 6 player Bs. All player As remain in one room during the whole experiment. They are seated in a single row, isolated one from another by separators, and not allowed to talk. The space in front of them is left open and used by a player B to make a brief statement. A video camera recording player Bs' statement is discretely positioned in the middle of the player A's raw. Player Bs enter the room one by one, so that player As play six rounds of the game (which is common knowledge). Each time, player B faces the center of player As' row, and all player As have a clear view on the speaker. Player B also has a clear, unobstructed view on all six player As. After making a statement, player B is invited to a separate room where s/he privately decides whether to Roll a die or not. Then, s/he is asked to leave the laboratory and wait outside until the end of the experiment. At the same time, each player A makes a decision whether to go In or stay Out. All decisions are made on a sheet of paper, which is then put in an envelope, sealed, and collected by the laboratory sta after each round. In addition, once player B has made a decision and left the separate room, a laboratory sta member rolls a die in private and marks the outcome on player B's sealed envelope.

At the end of the experiment, player As and Bs are randomly and anonymously matched in pairs. The outcome of the game for each pair is based on the payo structure described in Figure 1 and dened by the decision made by player A after player B's statement, as well as the decision made by player B in a private room had A chosen to go In. For B's decision to Roll, the outcome

of the die roll is also taken into account.

VIDEO, AUDIO, and TEXT. In these three treatments, each participant acts as a player A and is exposed to 10 items randomly drawn from the set of 41 recordings of player Bs' statements made in F2F. 13 Subjects are clearly informed that these recordings have been gathered in experiments conducted in the past, and that although their own decisions do not inuence the payos of the player Bs behind those messages, their own payos may depend on those player Bs' antecedent decisions. In each round, each participant receives a message (a statement made by player B). In the VIDEO treatment, participants watch the video recording of the message. In AUDIO, they listen to an audio recording. They can watch/listen to a given recording only once. In TEXT, precise transcripts of the statements (including inarticulate sounds) are displayed on player As' computer screens for the amount of time equal to the duration of the corresponding recording. The experiment took place at a large university in a medium-sized city in France, between April 2017 and February 2018. All treatments were implemented through a between-subject design. In addition to 83 subjects who participated in F2F experiment, a total of 217 subjects participated in VIDEO (62), AUDIO (70), and TEXT (85) treatments. 14 Our subjects are predominantly students (296/300), their average age is 21.28 (SD 3.69), 59.3% of them are females. See Appendix D for a summary of other characteristics of our sample.

Results

4.1 Summary of player Bs' behavior.

Before investigating player As' behavior, let us rst briey summarize the behavior observed among their counterparts. 61% of player Bs in the F2F experiment decide to Roll, while a promise to Roll is made by 51.2% of them. Following [START_REF] Charness | Promises and partnership[END_REF], a promise is dened as statement of intent to Roll. 15 There is no signicant dierence between the duration of the messages with promise as compared to the ones without (average duration: 28.90 seconds vs. 23.75 seconds; p = 0.108 using ranksum Mann-Whitney test).

There is a large dierence between the frequencies of Roll conditional on a promise to Roll (71%) and without such promise (50%). The result of a logit regression of the Roll dummy on the Promise to Roll dummy (N = 41) which also controls for session xed eects 16 suggests that a promise to Roll is highly predictive of a subsequent decision to Roll : the average marginal change of the likelihood of Roll due to a Promise to Roll is estimated at 0.394 (SE 0.122, p = 0.001). In the additional analyses reported in Appendix F, we also oer a screening exercise exploiting the 14 Number of participants in VIDEO, AUDIO, and TEXT varies due to the natural variation in show-up rates across sessions. 15 As raised by [START_REF] Houser | Classication of natural language messages using a coordination game[END_REF], the ex post interpretation of free-form messages is a major methodological challenge for the experimenter. The literature still lacks a common consensus on whether this should involve content analysis carried out by the experimenter [START_REF] Charness | Promises and partnership[END_REF], by independent coders [START_REF] He | The sources of the communication gap[END_REF], through an incentivized coordination game [START_REF] Houser | Classication of natural language messages using a coordination game[END_REF], or by asking the subjects for their own interpretation [START_REF] Servátka | Words speak louder than money[END_REF]. Echoing a recent study by [START_REF] Schwartz | Why do people keep their promises? a further investigation[END_REF], herein we implement a method combining the rst two approaches. Player Bs' statements were classied as promises or non-promises by two independent coders (research assistants). The rst coder classied the content of messages (promise or not) while preparing the transcripts of player Bs' statements. Then, another coder received the full list of 41 transcripts and independently classied each of them as a promise or a non-promise. Ties were broken by one of the authors.

All the transcripts (translated from French to English) and their classication may be found in the Appendix E.

16 Session dummies (7 sessions, rst session dummy dropped) allow us to account for the fact that player Bs face the same audience of player As within a session, but the audiences vary between sessions. These coecients are jointly insignicant (p = 0.287).

information about player B's individual characteristics to investigate who makes a promise. We nd that pro-social preferences (measured by the SVO angle) stand out as a signicant predictor of promise-making.

The Roll (promise-making) rates in our three treatments are as follow: 66.6% (53.9%) in VIDEO, 64.7% (52.9%) in AUDIO, and 60.0% (49.3%) in TEXT. These small variations are due to random sampling, and both rates are equally distributed across treatments. 17

Trust and promises with digital communication

Figure 2 summarizes aggregate trust rates across our experimental conditions. The statistical comparisons of these rates are based on linear probability models (LPM) reported in Table 1. In those models, we regress the trust indicator variable (= 1 if player A goes In, = 0 otherwise) on the set of condition indicator variables corresponding to various conditions (= 1 for a given condition, = 0 otherwise). The models are constructed such that each coecient corresponds to the trust rate observed in a given condition. We estimate cluster-robust standard errors (individual-level clustering) to account for the potential unobserved correlation between individual observations and the heteroscedasticity of residuals in the LPM. The rst model (Table 1a) uses treatment indicators as a set of explanatory variables. In the second model (Table 1c), these variables are further interacted with promise/non-promise indicators. For each model, the equality of trust rates between two (multiple) experimental conditions is parametrically tested by comparing dierent coecients through t-test (F -test). Finally, in Tables 1b andTable 1d we compare aggregate trust rates across the experimental conditions.

18 Panel (a) of Figure 2 provides the average trust rates, i.e. the likelihood that a player A goes In, in VIDEO, AUDIO, and TEXT. 19 Overall, we observe similar trust rates in VIDEO and 17 To test whether each of these variables is equally distributed across the three treatments, for every player A we calculate the frequency of decision Roll and the frequency of promise-making among the ten player Bs that person faced during the experiment. Comparing the distributions of those frequencies across the three treatments using Fisher's exact test, we nd p = 0.258 for Roll and p = 0.123 for the promise to Roll. Given that the size of each contingency table is relatively large (9 × 3), both p-values are computed using Monte Carlo simulations.

18 As a robustness check, Table 8 in the Appendix G re-assesses the aggregate comparisons from Table 1b after including player B xed eects. This is a general parametric way to account for the fact that dierent player As are exposed to a message coming from the same player B. All the results remain robust to this extension.

19 See Appendix H for details (as well as caveats) of the comparisons of these three treatments with the F2F experiment. In a nutshell, the trust rate in F2F is 40.2% descriptively higher than in TEXT, and lower than in AUDIO and VIDEO; yet, none of those dierences is found to be statistically signicant.

20 Another way to look at those data is to take each player B as an individual observation and compare the trust rates (i.e., the likelihood that a given player B is trusted by a player A) across the three experimental conditions.

The main patterns in the data remain unchanged: trust rates are lower in TEXT than in either AUDIO (p = 0.012) or VIDEO (p = 0.001), and do not dier signicantly between the last two conditions (p = 0.241).

and statistically signicant in all the treatments (p < 0.001). Furthermore, these eects do not vary signicantly across the three treatments (p = 0.402, F -test).

To further investigate the robustness of the eect of player Bs' promises on player As' trust, Table 2 provides average marginal eects from a logistic regression of the In dummy on the 1[P romise] dummy, coupled with a set of individual-level control variables. These controls are related to two questions: who is the subject in the role of player A that is more likely to trust, and how does that person perceive the participant in the role of player B? To empirically address the rst question, we exploit the set of individual characteristics we have gathered. For the second question, we exploit the information about player A's perceptions of player B's characteristics:

beauty (collected everywhere but in TEXT), intelligence, trustworthiness, and self-condence, all measured on a 1-8 scale. 22 Since this information has been elicited from every player A in each round only after the decision-making stage of the game, we stress the correlational nature of its relationship with the dependent variable.

The main result of this additional analysis is that the eect of a promise on trust remains positive and signicant even after controlling for a wide range of individual-level variables. Not surprisingly, we also nd that player As are more likely to trust player Bs the more they see them as trustworthy. We also observe a positive association between one's trust and the perceived intelligence of the other person in most conditions (p = 0.070 in VIDEO; p < 0.001 in TEXT;

insignicant in AUDIO). The perceived beauty or self-condence of a player B are not signicantly predictive of a player A's trust. Finally, in the absence of visual cues about player Bs, player As' behavior seems to be guided by their general sense of trust towards strangers: the marginal eect of SOEP trust 2 (i.e., the answer to the question How much do you trust people you just met?

on a 1-4 scale; see Appendix B.1) is positive and signicant at the 1% level in AUDIO (p = 0.006)

and at the 10% level in TEXT (p = 0.072) models, but not in the VIDEO model (p = 0.953).

22 Although promise-makers receive slightly higher average scores from player As for each of the four character- istics (see Appendix I), we fail to detect any signicant dierences between promise and perceived trustworthiness across treatments. For each condition, ranksum test using player Bs' average score on trustworthiness as independent observation fails to identify signicant dierences between promise-makers (N = 21) and others (N = 20).

Regarding player Bs' intelligence, the same test indicates signicant dierences in VIDEO and AUDIO.

Additional analyses: social distance and gender eects in trust

Following [START_REF] Bohnet | The sound of silence in prisoner's dilemma and dictator games[END_REF], [START_REF] Homan | Social distance and other-regarding behavior in dictator games[END_REF], and [START_REF] He | The sources of the communication gap[END_REF], we dene social distance as the extent to which the other person can be identied by the decision-maker. 23 Our experimental conditions vary social distance in a meaningful way: TEXT provides the lowest degree of identication of a player B (restricted to the content of the statement), AUDIO improves it thanks to the additional auditive cues, and VIDEO further enhances it by providing both visual and auditive cues about a player B. In particular, one crucial dierence between TEXT and the remaining treatments is that in the former, a player B's gender remains mostly unknown to player As. 24 In this part of our analysis, we consider the availability of information about the counterpart's gender as a factor reducing social distance in the interaction, and explore how it aects trust in dierent communication environments.

We note that the existing experimental literature on communication lacks a consensus on the relationship between (the availability of cues about the other person's) gender and trust. [START_REF] Belot | Promises and cooperation: Evidence from a tv game show[END_REF][START_REF] Belot | Can observers predict trustworthiness?[END_REF] and Van den Assem et al. ( 2012) analyze a dataset from a high-stake prisoners' dilemma game played in a TV show that females are more likely to share the prize than males, and that external observers can accurately predict this gender dierence. [START_REF] Bicchieri | The medium or the message? communication relevance and richness in trust games[END_REF], in turn, nd no statistical evidence for gender dierences in trust and trustworthiness in a trust game played with dierent modes of communication (either computer-mediated or face-to-face), and disregard this aspect in their analyses; similar conclusions can be found in [START_REF] Chen | Promises and lies: can observers detect deception in written messages[END_REF].

In our data, gender does not appear to be an important predictor of player Bs' behavior. The promise-making rates are fairly similar for both genders (females: 9/20; males: 12/21; Fisher's exact test yields p = 0.538). The average marginal eect of being a female on the likelihood of choosing to Roll is negative (-0.113), yet insignicant (p = 0.479). 25 Thus, we interpret any dierence in player As' behavior that stems from player Bs' gender as a behavioral bias.

We further investigate if there is a gender dierence in player As' trust, and whether the trust in our experiment is gender-biased. To answer these questions, let us suppose that people may have dierent trust attitudes towards males and females, depending on their own gender (which is known to the experimenter) and some other (unobserved) preferences. To capture these individual trust attitudes, for each player A, we construct two variables: Ratio trust M and Ratio trust F.

They capture the fraction of decisions In in all the interactions with a player B of either gender (M stands for male, F stands for female). 26 The sample means of those individual ratios are summarized in Table 3.

We then conduct the following estimation exercise. For ith player A, the overall trust ratio towards player Bs of gender g ∈ {F, M } depends on i's gender in the following way:

Ratio_trust ig = γ 0 + γ 1 × 1[F emale_A i ] + u ig , (1)
where 1[F emale_A i ] = 1 if an ith player A is a female, and = 0 otherwise. Since both Ratio_trust iF and Ratio_trust iM may also stem from i's unobserved preferences (so that u iF and u iM may be correlated), these models are estimated as seemingly unrelated regressions (see, Cameron and Trivedi, 2005, pp. 209-210) and presented in Table 4.

The main results are as follows. In TEXT (where player B's gender is predominantly unobonce we additionally control for the presence of a promise to Roll (the marginal eect of which is once again positive, 0.390, and highly signicant, p = 0.001). servable for player As), female player As are found to be generally less trusting than male player

As (H 0 : γ 1 = 0 for Ratio trust F : p = 0.041; for Ratio trust M : p = 0.055). For both male and female player As, we do not nd a signicant dierence between the two rates: coecients γ 0 as well as γ 0 + γ 1 are not dierent in equations Ratio trust F and Ratio trust M (p = 0.871 for γ 0 and p = 0.629 for γ 0 + γ 1 ). In AUDIO, male player As' trust does not vary as a function of player B's gender (coecients γ 0 do not vary between the two equations, p = 0.283). However, female player As now exhibit stronger trust towards player Bs of their own gender: the model suggests that γ 0 + γ 1 is statistically signicantly higher in the equation Ratio trust F than in Ratio trust M (p = 0.007). On the other hand, we also note that the eect of player A's gender on trust in AUDIO is less pronounced than in TEXT: for neither ratio, we detect a statistically signicant dierence between male and female player As (H 0 : γ 1 = 0 in Ratio trust F : p = 0.701; Ratio trust M : p = 0.208). Finally, all dierences between trust ratios fade away in the VIDEO treatment.

To conclude, we nd evidence for gender dierences in trust that can be overriden by means of providing nonverbal content in the process of communication. First, when no nonverbal content is transmitted and player B's gender remains unknown (TEXT treatment), we observe that female player As are generally less trustful than males. The AUDIO treatment (in which the auditory cues provide straightforward information about the speaker's gender) helps close this gap. However, it also gives rise to a gender bias in trust: female player As tend to favor players Bs of their own gender. Finally, any gender dierences fade away in VIDEO that enriches the nonverbal content by visual cues about the sender.

Discussion

A large body of experimental research in economics and other social sciences explores the role of communication in various types of economic interactions, such as cooperation, coordination, trust and reciprocity (see, among others, [START_REF] Ostrom | Covenants with and without a sword: Selfgovernance is possible[END_REF][START_REF] Cooper | Communication in coordination games[END_REF][START_REF] Charness | Self-serving cheap talk: A test of aumann's conjecture[END_REF][START_REF] Duy | Do actions speak louder than words? an experimental comparison of observation and cheap talk[END_REF][START_REF] Ellingsen | Promises, threats and fairness[END_REF][START_REF] Duy | Words, deeds, and lies: Strategic behaviour in games with multiple signals[END_REF][START_REF] Bochet | Not just babble: Opening the black box of communication in a voluntary contribution experiment[END_REF][START_REF] Bracht | Whatever you say, your reputation precedes you: Observation and cheap talk in the trust game[END_REF][START_REF] Gangadharan | Normative conict and the limits of self-governance in heterogeneous populations[END_REF]. This body of empirical evidence generally points to the conclusion that communication increases the eciency of interactions [START_REF] Sally | Conversation and cooperation in social dilemmas: A meta-analysis of experiments from 1958 to 1992[END_REF][START_REF] Crawford | A survey of experiments on communication via cheap talk[END_REF][START_REF] Balliet | Communication and cooperation in social dilemmas: A meta-analytic review[END_REF]. Here, we complement this literature by asking a dierent question: how important is the nonverbal content of communication in inducing swift trust? Overall, our experimental data highlight an interplay between verbal and nonverbal content of communication in the process of inducing swift trust in human interactions.

To answer our research question, we use a controlled laboratory experiment based on a principalagent relationship with dierent modes of one-way communication. Our design allows us to homogenize the verbal content of communication across the experimental conditions. We document that the degree to which communication channels enable the transmission of nonverbal content is important for the emergence of swift trust in human interactions. In aggregate terms, providing auditory cues about the speaker seems to play a key role in inducing swift trust, as compared to providing a plain text content of the message. Adding visual cues in video-based communication does not further lead to a statistically signicant increase in the observed rates of trust, notwithstanding the general ndings in [START_REF] Bicchieri | Computer-mediated communication and cooperation in social dilemmas: an experimental analysis[END_REF]. However, we nd evidence that these eects are moderated by the verbal content of communication. They hold for messages containing a promise to cooperate (which constitutes a signicant and well recognized predictor of trustworthiness); in the absence of such a promise, a combination of auditive and visual cues is necessary to signicantly enhance trust as compared to a plain text message.

In addition, we discuss whether and how digital communication protocols may give rise to gender dierences in trust, depending on the amount of transmitted nonverbal cues. We relate these ndings to the concept of social distance [START_REF] Bohnet | The sound of silence in prisoner's dilemma and dictator games[END_REF][START_REF] Homan | Social distance and other-regarding behavior in dictator games[END_REF] which has been previously studied in the context of (face-to-face) communication by [START_REF] He | The sources of the communication gap[END_REF] and which can be dened as the extent to which the other person can be identied by the decisionmaker. The most restricted communication environment in which players send plain text messages and that makes it virtually impossible to identify the other person (not even his or her gender),

gives rise to a gender dierence in trust: females tend to be generally less trustful than males.

27

Providing auditory cues that reveal the sender's gender, but otherwise provide only a limited identication of that person, leads to a gender bias in trust: female principals tend to trust other females more than to trust other males, even though gender is not predictive of the agents' trustworthiness in our experiment (if anything, females happen to be slightly, yet insignicantly, less trustworthy). All those dierences disappear once visual cues also become available, ensuring a high degree of identication of the other person.

In the experiment by [START_REF] He | The sources of the communication gap[END_REF], subjects keep a direct visual contact (except for the control condition), while the scope of communication varies across treatments: from no communication at all to restricted communication (no promise-making) to free communication (including promise-making). Their data suggest that social distance does not play an important role in explaining the eect of communication on cooperation in a social dilemma; rather, that eect is due commitment building and type recognition (which is particularly pronounced under face-to-face communication). Our experimental design and the conclusions thereof are somewhat dierent.

We study a free communication environment with a highly homogeneous message content. We observe that the extent to which the mode of digital communication allows the decision-maker to identify the other person plays a role in the emergence of trust towards that person. If the available cues are too scarce to fully identify the other person, female decision-makers may be either relatively wary of trusting others, or focus on (and seek commonalities in) a characteristic that may be correctly identied (i.e. the other person's gender).

Despite its methodological virtues, our study has one limitation that is worth emphasizing in the closing lines. The stimuli used herein is based on a specic sample of 41 recordings that has been resampled and employed throughout the whole experiment. Thus, an improved experimental control and the ability to draw causal inference about the eect of the digital communication mode on behavior comes with a price of reducing the natural heterogeneity of human communication (related to the variation in the content of messages, as well to the variety of senders' individual characteristics). Thus, there is certainly a need for further evidence, as well as out-of-the-sample replications.

No envelope will be opened before the end of the experiment.

At the end of the experiment, each player A is anonymously and randomly matched with one player B. The outcome of the game for each pair of players is determined by the decisions made by both playes (and also by the outcome of the die roll if the decisions in a pair are Right' and Roll') in the round in which the player B was in player As' room. Players are only informed about their personal payos, and not about the payos of or the decisions made by other players, or about the outcome of the die roll.

Additional information

Note that this set of instructions is provided to and read by each player A and each player B.

Furthermore, player Bs cannot communicate between themselves at any point of the experiment.

The same applies to the communication between player As.

You will play the role of player [A for player As, B for player Bs ]

A.3 Specic instructions for VIDEO, AUDIO, and TEXT Note: Below, the parts of instructions that are distinct for each treatment are marked with (treatment's name). Other parts are common to all three treatments.

(VIDEO) In this part of the experiment, you will watch a series of video recordings. In each recording, you will see a person speaking to other people. You will also answer a series of questions after each recording; some of them will determine your gain from this experience.

(AUDIO) In this part of the experiment, you will listen to a series of audio recordings. In each recording, you will hear a person speaking to other people. You will also answer a series of questions after each recording; some of them will determine your gain from this experience.

(TEXT) In this part of the experiment, you will read a series of transcripts. Each transcript comes from a person speaking to other people. You will also answer a series of questions after each recording; some of them will determine your gain from this experience.

B Additional measures B.1 Measures of individual characteristics

The preliminary stage of the experiment consists of basic socio-demographic questions (age, gender, education, major, current occupation, score at the baccalauréat exam at the end of high school), as well as a battery of incentivized and non-incentivized computerized tasks designed to measure specic individual characteristics. These tasks are administered in ve blocks presented to each individual in random order. In the incentivized tasks, all payos were expressed in Experimental Currency Units (ECU), with the exchange rate of 100 ECU = 2.50 EUR. No feedback on any decision or answer is provided at any stage. Below, we describe the content of those parts (labelled A-E).

28

Part A. Other-regarding preferences are measured using the Social Value Orientation (SVO) slider task by [START_REF] Murphy | Measuring social value orientation[END_REF]. In each of six distributional tasks, the decision-maker chooses an allocation of money for himself and for another person among nine possible allocations. These choices are then transformed into a score, labelled as SVO angle: the higher the angle, the stronger one's concerns about others' welfare. We use the original set of distributional tasks (all the amounts are expressed in ECU), and the strategy method to elicit responses in the role of the decision-maker from each participant. We also inform them that they will be randomly and anonymously matched in pairs at the end of the experiment, that in each pair one person will be randomly chosen as the decision-maker, and that both players' payos from this task will correspond to the decision-maker's choice in a randomly selected task. 28 Table 5 in Appendix D oers a summary of the elicited characteristics. We also provide evidence that the sample has been properly randomized with respect to all of these characteristics across treatments (VIDEO, AUDIO, or TEXT) and roles (player A or player B). Finally, we replicate several results that have been previously documents by the experimental studies employing certain measures used herein. In addition, Table 6 summarizes physical characteristics of the sample of player Bs.

1. A notebook and a pencil cost 1.10 Euros in total. The notebook costs 1 Euro more than the pencil. How much does the pencil cost?

2. If it takes 5 machines 5 minutes to make 5 widgets, how long would it take 100 machines to make 100 widgets?

3. In a lake, there is a patch of lily pads. Every day, the patch doubles in size. If it takes 48 days for the patch to cover the entire lake, how long would it take for the patch to cover half of the lake?

Subjects are informed that this set of three questions should be answered within 30 seconds (although we allow them to provide answers even after this time has elapsed). In this way, subjects can be classied according to their overall score (that is, the total number of correct answers) which can range from 0 to 3. No monetary incentive was provided for giving correct answers to these questions.

Part C. The Reading the Mind in the Eyes Test is an advanced test of theory of mind (ToM) developed by [START_REF] Baron-Cohen | The reading the mind in the eyes test revised version: a study with normal adults, and adults with asperger syndrome or high-functioning autism[END_REF]. As noted by [START_REF] Baron-Cohen | The reading the mind in the eyes test: Complete absence of typical sex dierence in ∼400 men and women with autism[END_REF], ToM entails attribution and recognition of mental states in oneself or others [. . .] and to use such information to make inferences and predict behaviour (p. 2). The test is non-incentivized and consists of 36 black-and-white photos of people. Each photo is cropped and rescaled so that only the area around the eyes can be seen. Photos are displayed one-by one and the participants are asked to choose the word that best describes what the person in the photo is thinking or feeling among four possible words shown underneath (only one of which is correct). Time is limited to 10 minutes.

Alongside a set of precise instructions about the content and duration of the task, the participants go through one trial round with an immediate feedback on the accuracy of their answers; other than that, no feedback is provided.

Part D. Risk preferences are captured by the amount invested in the incentivized task based on [START_REF] Gneezy | An experiment on risk taking and evaluation periods[END_REF]. A decision-maker holds an initial endowed of 100 ECU, some (or all) of which he can invest in the following lottery: 50% chance of multiplying the investment by the factor of 2.5 times, and 50% chance of losing the invested amount. Any decision-maker who does not invest the whole amount is considered as being risk averse, and the invested amount is used as a measure of risk aversion.

Part E.

B.2 Post-decision questionnaire

After every decision (and before moving on to the next round), player As ll in a short questionnaire (based on [START_REF] Centorrino | Honest signaling in trust interactions: Smiles rated as genuine induce trust and signal higher earning opportunities[END_REF]. In all treatments but TEXT, player As are asked if they know the person with whom they have just interacted. Then, they evaluate that person's: beauty (inverse 1-8 scale: 1 beautiful, 8 not beautiful), trustworthiness (inverse 1-8 scale: 1 trustworthy, 8 untrustworthy), intelligence (1-8 scale: 1 not too intelligent, 8 very intelligent), and self-condence (1-8 scale: 1 shy, 8 self-condent). For the sake of the simplicity of exposition, the statistical analysis reverses those scales for beauty and trustworthiness.

In F2F, the questionnaire is pen-and-paper. Each completed questionnaire sheet is put in an envelope which is then sealed and collected by a laboratory sta member. In VIDEO, AUDIO, and TEXT the questionnaire is embedded in the computer interface used in those treatments. 29 Cronbach's α = 0.7 suggests that the individual answers are internally consistent. [START_REF] Tavakol | Making sense of cronbach's alpha[END_REF] describe internal consistency as the extent to which all the items in a test measure the same concept (here, trust towards others). It is connected to the inter-relatedness of the items within the test (p. 53). In the literature, α = 0.7 is usually considered as acceptable (p. 54).

C Implementation details

Recruitment of participants. We made sure that player Bs were aware of (and give their consent to) the fact that they would be recorded, as well as the future use of those recordings for scientic purposes. The invitation email sent to player Bs included the following information:

During the experiment, your photos and video recordings will be taken. All pictures and video recordings produced during this experiment will only serve strictly scientic purposes of this research project, but they will be used in other experimental sessions related to the research project.

Therefore, please register only if you agree that your photos and video recording will taken for this scientic purpose. 30 Similar information is repeated in the preliminary part of the experimental instructions (see Appendix A). For obvious reasons, the invitation email emphasizes that subjects who want to participate must be uent French speakers.

In order to minimize the chance of subjects in the opposite roles knowing each other, we tried to separate our subject pools and recruited participants from two dierent majors (player As: BA students in economics and some other majors with the exception of public administration; player Bs: BA students in public administration, MA students in economics, if necessary completed by students from other departments). Acquaintance between player As and player Bs occurs at a relatively low rate. In F2F, in 14.6% (36 in 246) of times a player A admits to know a player B.

In VIDEO and AUDIO, this rate is even lower (1.1% and 1.3%, respectively). We suspect that the dierence between F2F and the two other conditions stems from the fact that in F2F subjects in both roles physically participated in the same session. Thus, despite our attempts to limit the correlation between player As and player Bs, factors such as shared courses, overlapping time schedules, or simple word of mouth, may be still be at play. This problem happens to be marginal in VIDEO and AUDIO in which we only recruit player As, and becomes irrelevant in TEXT.

Gender composition of the experimental sample. In order to homogenize the audience faced by player Bs in F2F and enhance our control over the experimental environment, we attempted to have gender-balanced groups of player As in each session. We achieved this in 5 sessions out of 7.

In session 2, there are 2 males and 4 females, while the opposite holds in session 3. Despite these 30 The recruitment of the participants has been carried out using ORSEE [START_REF] Greiner | Subject pool recruitment procedures: organizing experiments with orsee[END_REF].

small variations, altogether the sample of player As in F2F is gender-balanced. In the remaining treatments, the gender-balance constraint is relaxed. Female player As account for 64.5% of the sample in VIDEO, 67.1% in AUDIO, and 57.1% in TEXT. Overall, this procedure does not produce a statistically signicant variations in the gender composition of player As' samples. Comparing the four samples using Fisher's exact test yields p = 0.273.

Logistics of the F2F condition. For the sake of logistics and ecient time management, player Bs arrive 30 minutes prior to player As. First, they are asked to take up several computerized tasks that measure their preferences and characteristics (see Section B.1). 31 Then, they are all led to a waiting room. Then, each subject is seated back in his cubicle with headphones on. He now listens to an audio le containing the experimental instructions (paper version is also provided). There is a brief comprehension quiz assisted by a laboratory sta member. Finally, he receives additional paper instructions about the upcoming statement in front of player As, as well as a pen and an empty sheet of paper, and is given approximately two minutes to prepare his message. 33 After that, a player B is invited to player As' room where he delivers a statement, leaves for another room, and the game proceeds as explain above in Section 3. The average duration of a message is 26.39 seconds (SD 2.09).

Upon their arrival to the laboratory, player As also take up the set of preliminary questionnaires. Then, they receive and read paper instructions for the experimental game, and nally they ll in a short comprehension quiz. A laboratory sta member then reads aloud all the questions from the quiz along with the corrects answers, and answers any remaining questions. Finally, 31 The computerized parts of the experiment are all programmed in z-tree [START_REF] Fischbacher | z-tree: Zurich toolbox for ready-made economic experiments[END_REF].

player As are asked to wait for the arrival of the rst player B.

Recordings. Player Bs' statements are recorded using a small, non-intrusive video camera set up in the middle of player As' row, right in front or player Bs' zone, so that the perspective in the video camera recording resembles the one of a player A. The camera is always adjusted to the height of player B (so as to capture head, shoulders, and thorax), and to the luminosity in the room. The that: promises do not cause trustworthiness, they are just more likely to be sent by trustworthy players than by untrustworthy ones. 35 Our experimental design allows us to screen the prospective promise-makers based on a rich set of individual characteristics elicited in the preliminary stage of the experiment and summarized in Table 5.

36

Below, we show that pro-social distributional preferences (measured through the Social Value

Orientation slider task by [START_REF] Murphy | Measuring social value orientation[END_REF] are highly predictive of promise-making. Importantly, such preferences have been previously shown to explain trustworthy behavior in the trust game [START_REF] Cox | How to identify trust and reciprocity[END_REF][START_REF] Ashraf | Decomposing trust and trustworthiness[END_REF]. Our result suggests that they also explain one's willingness to make a promise which corroborates the conjecture made by [START_REF] Ismayilov | Why do promises aect trustworthiness, or do they?[END_REF].

The econometric estimates of this screening exercise are reported in Table 7. For reasons stated in footnote 13, our econometric model accounts for session xed eects. In our dataset, this precludes tting a model using standard maximum likelihood method because of quasi-complete separation of observations. 37 Rather, we use two alternative specications. First, we report coecients of a linear probability model (LPM) with robust standard errors estimated through standard OLS procedure. As a robustness check, we report the outcomes of Firth's logit model estimated through penalized maximum likelihood (ML) procedure.

38

The main nding is that player Bs with a stronger pro-social orientation (as measured by their SVO angle) are more likely to make a promise to Roll. The positive eect of pro-sociality on promise-making is highly signicant and robust to the econometric specication (p LP M = 0.001;

p F irth = 0.010). 39 39 Firth's logit provides some evidence that promise-making may also depend on emotional intelligence (OR increasing in the RMET score) and risk preferences (OR decreasing in the investment in the Gneezy-Potters task).

However, these results are only (marginally) signicant at the 10% level (p F irth = 0.096 and 0.061, respectively), and they are not robust to the econometric specication of the model (respective coecients are insignicant in the LPM with pLP M = 0.182 and 0.185).

H Trust and direct communication

Here, we compare player As' trust under direct (F2F) and indirect communication (VIDEO, AU-DIO, and TEXT). Given that the experimental design of F2F diers from the remaining treatments in several ways, we would like to start by discussing the most important caveats for such comparison. Although we respond to each of them below, the results presented in the remaining part this section should be taken with a grain of salt.

One important concern is that the game is more interactive in F2F then elsewhere. In F2F, player A's decision aects both players' payos, while in the remaining treatments it does not aect player B's gains (only the vice versa holds). As a result, as the role distributional concerns (kindness) may be more pronounced in player As' decisions in F2F that elsewhere. Based on some the previous evidence on distributive preferences and trust (see, e.g., [START_REF] Cox | How to identify trust and reciprocity[END_REF], one may expect this design asymmetry per se to stimulate trust in F2F relative to other treatments. We respond to (and allay) this concern in two ways. First, we do not observed such a pattern in our data: F2F Note. Estimates of linear probability models (LPM). Dependent variable: indicator of player A's decision to go In. In the rst model, the dependent variable is regressed on the all-experiment set of treatment dummies (= 1 for a given treatment, = 0 otherwise). Each of the estimated coecients corresponds to the average eect of the underlying experimental condition on the probability of observing a given outcome, relative to the reference F2F treatment (the outcome of which is captured by the intercept). The second model also include a dummy variable 1[P romise] (= 1 if a player A received a promise to Roll from player B, = 0 otherwise), and its interactions with each of the treatment indicator variables. Models use cluster-robust standard errors (with session-level clustering for the F2F treatment and individual-level clustering in the remaining treatments) to account for the potential unobserved correlation between individual observations, as well as the heteroscedasticity of residuals in the LPM. There are 246 observations (7 clusters) in F2F, 620 (62) in VIDEO, 700 (70) in AUDIO, and 850 (85) in TEXT.

Figure

  Figure 1: Experimental hidden action game

  After receiving a message, each participant decides whether to go In or stay Out and lls in the post-decision questionnaire. Participants are not informed about the choice made by the player B whose message they have just received, or about the outcome of die roll. After completing 10 rounds, each player A is randomly matched with one of the ten player Bs. The payo structure corresponds to player A's payos in Figure1. Player A receives 5 Euros when staying Out, 0 Euros when going In with a player B who does not Roll, or either 12 Euros (with 5/6 chance) or 0 Euros (with 1/6 chance) if player B chooses to Roll.

  Trust rates and their SE from Fig. 2a (b) Comparing trust rates from Fig. 2a Rate p (SE) 1[V IDEO, N ON -P ROM ISE] DIO, N ON -P ROM ISE] EXT, N ON -P ROM ISE] Trust rates and their SE from Fig. 2b and 2c (d) Comparing trust rates from Fig. 2b and 2c Note. (a) and (c): OLS estimates from linear probability models (LPM) regressing the trust indicator variable (= 1 if player A goes In, = 0 otherwise) on the all-experiment set of condition indicator variables (= 1 for a given condition, = 0 otherwise). The models are intercept-free, and each coecient provides the aggregate rate (Rate) of trust in a given experimental condition. p-values correspond to standard t-test of a coecient's nullity. (b) and (d): coecient comparisons based on the models (a) and (c), respectively. p-values correspond to t-tests for the nullity of the dierences (Di.) between a given pair of coecients; the nal test in (d) is an F -test for the joint nullity of three dierences. In all models, observations are clustered and the individual level and estimated standard errors are cluster-robust. There 620 observations (62 clusters) in VIDEO, 700 (70) in AUDIO, and 850 (85) in TEXT.

Figure

  Figure 2: Average Trust Rate

21

  Panels (b) and (c) of Figure2separates the trust rates depending on whether player B's message contains a promise to Roll (Panel b) or not (Panel c). Trust rates following a promise do not signicantly vary between VIDEO and AUDIO (55% vs. 54%, p = 0.742), and they are both signicantly dierent to 43% observed in TEXT (p = 0.016 vs. VIDEO, p = 0.037 vs. AUDIO).Without a promise, trust rates vary from 42% in VIDEO to 34% in AUDIO to 30% in TEXT, and this time only the dierence between VIDEO and TEXT is signicant (p = 0.020; other pairwise comparisons yield p > 0.150). The eects of a player B's promise to Roll on the overall trust rate are positive (13 percentage points in VIDEO and TEXT, and 19 percentage points in AUDIO)

  Part B. Cognitive skills are measured through standard (3-item) Cognitive Reection Test (Frederick, 2005) which measures cognitive reectiveness or impulsiveness, respondents' automatic response versus more elaborate and deliberative thought (Brañas-Garza et al., 2012, p.255). The questions are as follows:

29 Finally,

 29 political views are elicited using another SOEP question: In politics, people often talk about left and right when describing dierent political views. When you think about your own political view, how would you rate them on the scale below? (11 point scale with 0 left, and 10 right).

  sake of the quality of the video recordings, the background in player Bs' zone is covered with light canvas. While making a statement, each player B also has a portable microphone attached below their face. The distance between player As and a player B is set to 2.50 meters. The information recorded in the F2F treatment (i.e., player Bs' messages and the content of their decisions sheets and die rolls) is re-used in the remaining experimental conditions: VIDEO, AUDIO, and TEXT.Duration and payos. For player As, the experiment takes roughly 1h. At the end of the experiment, subjects are paid according to their earnings in the incentivized pre-experimental tasks, earnings in the experimental game, in addition to the show-up fee of 5 euros. All payments are made privately and in cash. For player Bs, the experiments takes 30 minutes longer, and the show-up fee is increased to 10 euros. D Individual characteristics: randomization and key empirical patterns in the sample

  0 : λ 4 + λ 5 = 0) < 0.001 AUDIO (H 0 : λ 4 + λ 6 = 0) < 0.001 TEXT (H 0 : λ 4 + λ 7 = 0) < 0.001 Joint F -test (H 0 : λ 5 = λ 6 = λ 7 = 0) 0.514

Table 1 :

 1 Statistical support for Figure 2 and trust rate comparisons

		Rate	p	Di.
		(SE)
	1[V IDEO] (α1)	0.490	< 0.001
		(0.032)
	1[AU DIO] (α2)	0.444	< 0.001
		(0.029)
	1[T EXT ] (α3)	0.365	< 0.001
		(0.026)

  AUDIO (49.0% and 44.4%, respectively; p = 0.289) which are, in turn, signicantly higher than in TEXT (36.5%, with p = 0.003 vs. VIDEO, and p = 0.042 vs. AUDIO).

20

At the treatment level, these rates are insensitive to the conditioning on player B's actual trustworthiness: ex post, we hardly observe any variation between the aggregate rates of trust displayed towards player Bs who decided to Roll as compared to those who did not (VIDEO: 49.2% vs. 48.8%; AUDIO: 45.5% vs. 42.5%; TEXT: 37.3% vs. 35.3%). This suggests that, in aggregate terms, player As' trust is disconnected from player Bs' trustworthiness.

Table 2 :

 2 Promises and individual predictors of trust: marginal eects from logit regression models . Average marginal eects from logistic regression models. First/second/third model: data from VIDEO/AUDIO/TEXT treatment. Observations are clustered and the individual level and estimated standard errors are cluster-robust.

		VIDEO	AUDIO	TEXT
		M E, (SE)	p	M E, (SE)	p	M E, (SE)	p
	1[P romise]	0.102	0.001	0.171	< 0.001	0.099	0.002
		(0.030)		(0.037)		(0.031)
	Player A's perception of player B's characteristics (1-8 scale):
	beauty	-0.016	0.232	-0.010	0.449
		(0.014)		(0.013)
	intelligence	0.033	0.070	0.019	0.352	0.066	< 0.001
		(0.018)		(0.020)		(0.010)
	trustworthiness	0.088	< 0.001	0.058	< 0.001	0.033	< 0.001
		(0.013)		(0.015)		(0.009)
	self-condence	0.009	0.465	-0.001	0.920	-0.007	0.461
		(0.012)		(0.012)		(0.009)
	Player A's characteristics:
	SVO angle	-0.002	0.249	< 0.001	0.928	< 0.001	0.954
		(0.002)		(0.002)		(0.002)
	1[CRT > 0]	0.020	0.759	0.069	0.233	-0.052	0.437
		(0.066)		(0.058)		(0.067)
	RMET score	0.003	0.680	-0.004	0.469	0.002	0.782
		(0.007)		(0.005)		(0.006)
	GP investment	0.001	0.285	0.003	0.013	< 0.001	0.988
		(0.001)		(0.001)		(0.001)
	SOEP trust 1	-0.074	0.161	-0.072	0.172	-0.068	0.207
		(0.053)		(0.053)		(0.054)
	SOEP trust 2	-0.003	0.953	0.097	0.006	0.084	0.072
		(0.050)		(0.035)		(0.046)
	SOEP trust 3	0.081	0.260	-0.016	0.813	0.116	0.118
		(0.072)		(0.068)		(0.074)
	Political views	-0.001	0.943	0.003	0.840	0.003	0.767
		(0.017)		(0.013)		(0.010)
	Pseudo-R 2	0.142	0.100	0.115
	Nb of obs./clusters	620/62	700/70	850/85

Note

Table 3 :

 3 Trust and gender: aggregate results Ratio trust M (F ) provides the rate at with that player trusted male (female) player Bs. The average values in the sample are conditioned treatment and player A's gender.

	Ratio trust Male player A	VIDEO F M 54.1% 49.6% 53.5% 46.4% 43.0% 43.7% AUDIO TEXT F M F M
	Female player A 48.9% 46.2% 50.4% 37.7% 30.7% 32.4%
	Note. For each player A,

Table 4 :

 4 Trust and gender: seemingly unrelated regressions Note. For each treatment, we provide estimated coecients (and standard errors) of seemingly unrelated regression models. The dependent variables are Ratio trust F and Ratio trust M, the explanatory variable is an indicator variable 1[F emale_A]. a/b/c indicate statistical signicance at the 1/5/10% level.

	Ratio trust	F	VIDEO M	F	AUDIO M	F	TEXT	M
	Intercept (γ 0 )	0.541 a 0.496 a 0.535 a 0.464 a 0.430 a 0.437 a
		(0.065) (0.061) (0.067) (0.056) (0.046) (0.045)
	1[F emale_A] (γ 1 ) -0.052 -0.034 -0.031 -0.087 -0.123 b -0.113 c
		(0.080) (0.076) (0.081) (0.069) (0.060) (0.059)

  The non-incentivized measures of individual trust attitudes are based on the German Socio-Economic Panel Study (SOEP), an ongoing longitudinal survey of German households. In what follows, we present them in order of appearance. Variable SOEP trust 1 is the answer to In general, one can trust other people (4 point scale with 1 strong disagreement, and 4 strong agreement); variable SOEP trust 2 is the answer to How much do you trust people you just met? (4 point scale with 1 no trust at all, and 4 full trust); variable SOEP trust 3 is the average score on 7 items measuring general trust in socio-economic institutions (How much trust do you have in market economy/large companies/courts/government/parliament/other citizens/banks , same scale as for SOEP trust 2 ).

  To avoid any communication or subjects overhearing what others are saying or doing, each participant is seated in a separate cubicle, puts on a headphone and listens to a classical music until further notice. Then, they are taken one by one to a separate room for a mugshot picture and a short, standardized video recording.

	32

Table 5

 5

	provides a summary of the individual measures outlined in Section B.1. To check if our
	ve experimental subsamples (player As in F2F, VIDEO, AUDIO, and TEXT, as well as player
	Bs in VIDEO) have been properly randomized with respect to all of these characteristics, we
	adopt a parametric procedure that is parsimonious (it involves a single test) and provides a simple
	way to deal with the risk of false rejection due to multiple comparisons (see Veazie, 2006, for an
	insightful discussion). We regress each outcome of interest (i.e, SVO angle, CRT and RMET scores,
	investment in the Gneezy-Potters task, SOEP trust 1, 2, and 3, and political views) on the set of
	four experimental sample dummies (each set to 1 if an observation comes from a given sample,

and 0 otherwise; the fth sample is the reference). These eight regression models are estimated

Table 5 :

 5 Individual characteristics measured in the experimental sample: average outcomes Such nding is not new in the literature, and echoes a strongly left-skewed distribution of correct answers in the lower bound sample from the seminal study by[START_REF] Frederick | Cognitive reection and decision making[END_REF].

			SVO	CRT	RMET	GP	SOEP trust	Polit.
		N	angle	score	score	inves't	1	2	3	views
	Player As:
	F2F	42	21.613	0.405	24.000	48.643	2.310	1.905	2.255	4.452
	VIDEO	62	26.973	0.403	26.790	44.065	2.339	1.984	2.306	4.645
	AUDIO	70	28.078	0.529	26.871	43.271	2.443	2.129	2.422	4.143
	TEXT	85	26.487	0.353	26.435	48.624	2.412	2.047	2.380	4.094
	All player As	259	26.243	0.424	26.243	46.089	2.386	2.031	2.354	4.297
	Player Bs:	41	21.797	0.415	25.951	46.512	2.390	2.122	2.303	4.683
	All participants:	300	25.635	0.420	26.203	46.147	2.387	2.043	2.347	4.350

as seemingly unrelated regressions (SUR). This, in turn, enables us to perform F -test of joint insignicance of all between-sample dierences (i.e., joint insignicance of all sample dummies) simultaneously across all the SUR models. The resulting joint test of 32 null hypotheses yields p = 0.148.

We also replicate several results previously documents by the experimental studies employing CRT and RMET. First, we nd gender dierences in both measurements: males score signicantly higher than females in the CRT (0.543 vs. 0.316; two-sided t-test: p = 0.005), and signicantly lower in the RMET

(25.442 vs. 26.637

; two-sided t-test: p = 0.019). This stands in line with the ndings by

[START_REF] Brañas-Garza | Cognitive eort in the beauty contest game[END_REF]

,

[START_REF] Corgnet | The cognitive basis of social behavior: cognitive reection overrides antisocial but not always prosocial motives[END_REF]

, and

[START_REF] Hanaki | Fluid intelligence and cognitive reection in a strategic environment: Evidence from dominance-solvable games[END_REF]

for the CRT, and by

[START_REF] Baron-Cohen | The reading the mind in the eyes test: Complete absence of typical sex dierence in ∼400 men and women with autism[END_REF]

for RMET.

[START_REF] Hello | [END_REF]

34

[START_REF] Frederick | Cognitive reection and decision making[END_REF]

, the respective frequencies are 64%, 21%, 10%, 5% (N=138). Similar ndings are also reported by Brañas-Garza et al. (2012) 67%, 23%, 9%, and 1% (N = 191), and by

[START_REF] Hanaki | Fluid intelligence and cognitive reection in a strategic environment: Evidence from dominance-solvable games[END_REF]

70%, 16%, 8%, and 6% (N = 200).

Table 6 :

 6 Player Bs' physical characteristics So, you can choose between Right and Left. So, Left is the "safe" choice, that is to say that when you earn 5e, I earn 5e. And Right is the risky choice, i.e. you may win 0e and I may win 14e. On the otherhand, it is also a choice that could make us all earn money, in the sense that you would earn 12e if I remember correctly, and me 10. So what does that mean? This means you have to have condence. Well, that's complicated, I understand that, especially since we don't know each other at all, and from what I've read, it's going to be intermingled over there. All I can tell you is that no matter what choice you make, I will choose to roll the die, because in any case this solution will best maximize the prot of all of us. Whether you choose the "safe" solution, that is Left, or the other a 80% chance. . . if we. . . and for the 20%, we have, uh, only 10e for me, I guess, and you, you get nothing. We have a greater chance to gain together, well, individually, to gain more. Full stop. [Classication: promise] 20. Uh, hello. Actually, there is. . . I'm player B, I'm going to show you that. . . uh, you. . . you choose the. . . uh. . . Actually. . . because I forgot the thing, I don't know. . . Well, uh, I suggest you choose, choose Don't roll. Because we, uh, it's not too benecial compared to the. . . I don't know. [Classication: non-promise] 21. So as you know, you have the opportunity to. . .. two possibilities, I meant. If you choose Right, you have the possibility. . . your expected earnings can reach up to 12e. So you have more chances to win than by choosing Left. If, on the other hand, you choose Left, your expectation of gain is set at 5e. So your decision depends on whether or not you want to earn more money. Think carefully before making a decision because if you choose Right, you can win up to 12e. If, on the other hand, you choose Left, it is set at 5e. So, actually, for this experiment, I think that it's in the common interest that player B makes a decision, and I think rolling the die is a good decision. And player As, by choosing Right, I think it's a good decision too, given that. . .. By choosing Left, there is a minimum gain, but by choosing Right, there is more chance of getting a maximum gain, and it is more interesting and wiser to. . . to choose. . . uh. . . the possibility of earning a prot higher than what you could get at a minimum. So I think. . . it's wiser for player As to. . . to decide to choose the action Right, and player B, well, to. . . to roll the die. [Classication: promise]and address the following questions: Who makes a promise? Which individual characteristics can predict a player B making a promise to Roll in front of player As? To address this question, we follow the lead of a recent experimental study byIsmayilov and Potters (2016) who report (p. 390)

	Mean age	22.51
	Male (%)	51.22
	Ethnicity (%):
	African	7.32
	Caucasian	63.41
	Middle-eastern	26.83
	South Asian	2.44
	Native speaker (%)	80.49
	Wearing glasses (%)	29.27
	Bearded male (%)	31.71
	Female wearing hijab (%)	7.32
	Female with clivage exposed (%)	34.15
	a.

E Translated transcripts and classication of player Bs' statements Note. For player As, the experimental instructions use a neutral frame: Right or Left, which is analogous to "In" or "Out" in the game from Figure

1

.

1. So, I don't know if you know about the experience that's coming, but you have two choices to make.

[Classication: non-promise] 22. Hello. In fact, in. . . we're in a game. And like in a game, we may or may not take risk. If you choose to take risk, you can earn even more, while if you do not take risk, the gains are certain, but. . . but they are low. OK. [Classication: non-promise] 23. Hello. Dear shareholders, I can reassure you that this action is a win-win situation. So don't think much and, uh, I suggest you go for it. [Classication: non-promise] 24.

Table 7 :

 7 Individual predictors of promise-making The rst two columns provide OLS estimates (β OLS ) of a linear probability model (heteroscedasticity-robust standard errors are given in parentheses) along with p-values from t-test for statistical signicance. R 2 = 0.586. The second part of the table provides the results from an alternative specication, Firth's logit: estimated coecients (β F irth ), odds ratios (OR) and p-value from LR test for statistical signicance. Joint signicance tests in the lower part of the table are: F -test (LR test) for the left-hand (right-hand) side model.

		LPM		Firth's logit
		β OLS , (SE)	p	β F irth	OR	p
	Intercept	0.214	0.764	0.178 1.195 0.976
		(0.705)
	SVO angle	0.014	0.001	0.120 1.128 0.010
		(0.004)
	1[CRT > 0]	0.152	0.384	1.162 3.197 0.377
		(0.172)
	RMET score	0.026	0.182	0.302 1.353 0.096
		(0.019)
	GP investment	-0.004	0.185	-0.041 0.960 0.061
		(0.003)
	SOEP trust 1	0.064	0.494	2.201 9.035 0.443
		(0.092)
	SOEP trust 2	-0.072	0.389	-2.625 0.072 0.318
		(0.082)
	SOEP trust 3	-0.154	0.294	-2.973 0.051 0.194
		(0.144)
	Political views	0.007	0.810	0.203 1.225 0.570
		(0.028)
	Session FE's	YES			YES
		Joint insignicance tests
	Session FE's		0.001			0.029
	β's		<0.001			0.057
	β's and FE's		<0.001			0.060

Note. N = 41.

Table 8 :

 8 Robustness check: treatment comparisons controlling for player Bs xed eects

		Di.	p
		(SE)
	VIDEO vs. AUDIO	0.036	0.388
	(H 0 : α 1 -α 2 = 0)	(0.042)
	VIDEO vs. TEXT	0.104	0.012
	(H 0 : α 1 -α 3 = 0)	(0.041)
	AUDIO vs. TEXT	0.067	0.087
	(H 0 : α 2 -α 3 = 0)	(0.039)
	G Robustness analysis for Figure 2

Table 8 re

 8 -assesses the comparisons reported in 1b after including player B xed eects(41 indicator variables). Note that such robustness check is not meaningful for 1d, i.e. after including the promise/non-promise indicators which (unlike the treatment indicator variables) do vary within a given recording.

Table 9 :

 9 Comparing F2F to other treatments: regression analysis

	Frequency of:	In		In
		λ, (SE)	p	λ, (SE)	p
	Intercept (λ 0 )	0.402	< 0.001	0.350	< 0.001
		(0.047)		(0.055)
	1[V IDEO] (λ 1 )	0.088	0.128	0.070	0.309
		(0.058)		(0.068)
	1[AU DIO] (λ 2 )	0.042	0.451	-0.008	0.908
		(0.055)		(0.065)
	1[T EXT ] (λ 3 )	-0.038	0.488	-0.049	0.434
		(0.054)		(0.063)
	1[P romise] (λ 4 )			0.102	0.099
				(0.062)

See, for instance,[START_REF] Charness | Promises and partnership[END_REF];[START_REF] Vanberg | Why do people keep their promises? an experimental test of two explanations[END_REF];[START_REF] Ismayilov | Why do promises aect trustworthiness, or do they?[END_REF][START_REF] Schwartz | Why do people keep their promises? a further investigation[END_REF]. For a systematic review of the experimental literature on promise-making, see[START_REF] Woike | Most people keep their word rather than their money[END_REF]. Other characteristics of messages may also matter. Using a modied two-person trust game with an additional external observer,[START_REF] Chen | Promises and lies: can observers detect deception in written messages[END_REF] report that other important features of a written message are its length (which increases trust) and the fact of mentioning money (which is associated with untrustworthiness).On the other hand, factors such as the use of encompassing words (like we or us) do not seem to matter.

This echoes the ndings from a meta-analytic study[START_REF] Balliet | Communication and cooperation in social dilemmas: A meta-analytic review[END_REF]. This study distinguishes between two forms

An English translations of the instructions, a summary of the measured characteristics, and further details on how the experiment was implemented in the lab (including extra care we have put in to minimize the likelihood that player As and player Bs know each other) are provided in Appendices A, B, and C, respectively.

We have conducted 7 F2F sessions. However, one player B in session 6 of F2F decided to quit after the preliminary measurements and before receiving the instruction of the hidden action game, and was replaced by a research assistant unknown to player As. To avoid any contamination of player As' behavior, that research assistant acted as player B in the nal round of the experimental game. The data from that round were dismissed and our dataset from that session only covers 5 player Bs, and thus 41 player Bs in total.

This observation echoes[START_REF] Belot | Promises and cooperation: Evidence from a tv game show[END_REF] and[START_REF] Van Den Assem | Split or steal? cooperative behavior when the stakes are large[END_REF] who use prisoner's dilemma data from a high-stake TV with a pre-play, free-form communication stage, and nd no evidence of a correlation in players' decisions.

behavior in economic games[START_REF] Dufwenberg | Gender composition in teams[END_REF][START_REF] Deck | An examination of the eect of messages on cooperation under double-blind and single-blind payo procedures[END_REF].24 Only 3 out of 41 transcripts reveal the sender's gender: twice as a female and once as a male. In French, the grammar rule of accord may impose gender-specic forms of verbs (depending on the tense) and adjectives in written which, however, are not always distinguishable in spoken (so that the masculine and the feminine forms may sound exactly the same). In our transcripts, we respect the French grammar and follow the rule of accord.[START_REF] Well | [END_REF] This result stems from a logit regression with session xed eects, and holds (marginal eect of -0.088, p = 0.534)

On average, a player A encounters a female player B in 4.71 rounds out of 10 in VIDEO, in 4.57 in AUDIO, and in 5.08 in TEXT. Replicating the procedure described in footnote 14 for these data gives p = 0.287.

This stands in line with[START_REF] Buchan | Trust and gender: An examination of behavior and beliefs in the investment game[END_REF] who report that men are more trusting than women. However, they also nd that women are more trustworthy which is not conrmed by our data.

Like in van[START_REF] Van Leeuwen | Predictably angry facial cues provide a credible signal of destructive behavior[END_REF], subjects are asked to read neutral content (a short extract from a printer instruction manual) and keep a neutral face expression. The recording takes about 30 seconds. This information is not part of the present investigation and is not reported in the paper.

Those additional instructions remind the subject about his role in the game; emphasize the fact that the message may aect player A's decisions and, consequently, the subject's gain from the experiment; instruct the subject to avoid making a visual or verbal contact with the experimenter, to aim at communicating with all player As, and not to introduce oneself or give any details about one's own identity.

This conjecture diers at face value from the perspective commonly adapted by modern behavioral theories, like the lying aversion theory[START_REF] Ellingsen | Promises, threats and fairness[END_REF], or the guilt aversion theory[START_REF] Charness | Promises and partnership[END_REF], that assume a causal link from promises to trustworthy behavior and explain its behavioral underpinnings.

The distribution of the CRT scores is strongly left-skewed (see Appendix D for details) and thus the number of observations with a positive score is relatively small and quickly decreasing in the score. For this reason, in the regression analysis we use a binary variable 1[CRT > 0] that splits the sample into two categories: those who correctly answered at least one question (1[C RT > 0] = 1), and those who did not (1[C RT > 0] = 0).

In session 6, all player Bs made a promise to Roll, so that the corresponding indicator variable is a perfect predictor of success. This, in turn, makes it impossible to t a binary response model through a standard ML exercise, since the value of the xed eect for session 6 that maximizes the likelihood function is unbounded and tends to innity.

[START_REF] Firth | Bias reduction of maximum likelihood estimates[END_REF] method has been proposed as a remedy for data separation problems[START_REF] Heinze | A solution to the problem of separation in logistic regression[END_REF]. For Firth's logit, it is recommended to use likelihood ratio (LR) test rather than the Wald test for signicance testing (given that the estimated Hessian matrix does not allow for a reliable estimation of standard errors); for the same reason, odds ratios (OR) should be used instead of marginal eects.

* This ia a revised version of GATE WP 1909 entitled Digital communication and swift trust. The authors acknowledge the support from the following programs operated by the French National Research Agency (Agence Nationale de Recherche): DigiCom as a part of U CA JEDI (ANR-15-IDEX-01) and LABEX CORTEX (ANR-11-LABX-0042) as a part of Université de Lyon (ANR-11-IDEX-007). Ynès Bouamoud, Imen Bouhlel, Maxime David,

Appendix A Experimental instructions

A.1 Preliminary instructions given to all subjects in all the treatments You are about to take part in an experiment in which you can earn money. The amount of your gains will depend on your decisions, as well as on the decisions made by other participants. In addition, you will receive a xed fee of [5 for player As, 10 for player Bs ] EUR for completing the experiment. Your total earnings will be paid privately in cash at the end of the experiment.

The experiment consists of several parts. Each part will involve tasks the rules of which will be explained to you in due time. It is crucial that you understand and obey the rules of this experiment. Violation of these rules might result in an exclusion from the experiment and all payments. Please raise your hand whenever you have questions or need assistance.

All the information you provide, as well as the amount of your gains from this experiment, will remain strictly condential and anonymous.

We would now like to ask you to answer a series of preliminary questions. You will answer these questions using the interface on your computer screen. Some of these questions will generate monetary gains. These gains will be determined and added to your overall earnings at the end of the experiment.

Note: the following instructions were given only to player Bs in F2F for the preliminary recordings, not reported in the paper. Now, we would like to take a picture and video recoding of you.

First, you will be asked to stand by the wall and look into the camera. Please, try to keep a neutral facial expression.

Second, you will be asked to read aloud the content display on the screen in front of you. While reading, you will be video recorded.

All pictures and video recordings produced during this experiment will only serve strictly scientic purposes of this research project. They may be used in other experimental sessions related to this research project.

A.2 Specic instructions for F2F treatment

Rules of the game

You will now play a game with monetary stakes. The rules of the game are as follows.

The game is played by two players: player A and player B. Each player must choose between two possible actions. Player A chooses between actions Lef and Righ. Player B chooses whether she want a six-sided die to be rolled (action Roll) or not (action Don't roll).

You will play the role of player [A for player As, B for player Bs ]

Each players' payo depends on the actions chosen by herself as well as the other player:

• if player A chooses Lef , then regardless of player Bs' choice: player A's payo is 5 EUR and player B's payo is 5 EUR;

• if player A chooses Righ and player B chooses Don't roll: player A's payo is 0 EUR and player B's payo is 14 EUR;

• if player A chooses Right and player B chooses Roll: if the number of on the die is between 1 and 5, then player A's payo is 12 EUR and player B's payo is 10 EUR;

if if the number of on the die is 6 player A's payo is 0 EUR and player B's payo is 10 EUR;

How the game proceeds

The game will consist of six identical rounds.

At the beginning of a round, one player B is asked to enter the room in which there are six players As. Player As are separated one from another and are not allowed to talk.

Player B is then placed in front of player As and remains silent. Then, player B is allowed to talk for no longer than 20 seconds, and then asked to leave player As' room. While talking, player B is video recorded and should look straight into the camera.

Once player B leaves player As' room:

• player B makes a decision in a separate room. Player B privately and individually indicates her decision (either Roll or Don't roll) on a separate answer sheet, puts it in an envelope and seals the envelope. The experimenter collects the envelope and player B leaves the room.

Then, the experimenter privately rolls a six-sided die and marks the result on the envelope (without opening it). The outcome of the die roll will only be taken into account if player A's decision is Left and player B's decision is Roll.

• each player A privately and individually indicates her decision (either Left or Right) on a separate answer sheet, puts it in an envelope and seals the envelope. Then, all the envelopes are collected by the experimenter. Player As are either asked to remain silent and await the next player B, or informed that the experiment is over and given further instructions about their payment.

(VIDEO) Video recordings; (AUDIO) Audio recordings; (TEXT) Transcripts

(VIDEO) Here is some information about the recordings you will be watching. The recording have been made during several laboratory experiment sessions.

(AUDIO) Here is some information about the recordings you will be listening to. The recording have been made during several laboratory experiment sessions.

(TEXT) Here is some information about the transcripts you will read. The transcripts have been made during several laboratory experiment sessions.

In each session, two groups of participants (six players A and six players B) were installed in two dierent rooms. Participants in each room could not communicate with each other. Players were informed that their decisions and earnings would remain private and anonymous, and would never be disclosed to other participants.

Each session was organized as follows:

1. One by one, player Bs entered the room in which players A were sitting. Then, each player B made a short speech in front of player As.

(VIDEO) All the speeches have been recorded, and you will be watching some of them.

(AUDIO) All the speeches have been recorded, and you will be listening to some of them.

(TEXT) All the speeches have been recorded, and you will read some of their transcripts.

2. After his speech, player B left player As' room, and entered an empty room. The experiment ended when all the players had completed their task.

At the end of the experiment, each player A was anonymously and randomly matched with a player B. The outcome of the game for each pair of players was determined by the decisions made by both players following player B's speech:

• if player A chose Left, then regardless of player Bs' choice: player A's payo was 5 EUR and player B's payo was 5 EUR;

• if player A chose Right and player B chose Don't roll: player A's payo was 0 EUR and player B's payo was 14 EUR;

• if player A chose Right and player B chose Roll: if the number of on the die was between 1 and 5, then player A's payo was 12 EUR and player B's payo was 10 EUR; if the number of on the die was 6, then player A's payo was 0 EUR and player B's payo was 10 EUR; Your role (VIDEO) You will now watch ten recordings randomly selected from all the recordings described above. After each recording, you will make a decision: either Left or Right. This decision is important for your nal earnings. You will also answer some questions about the person seen in the recording. Finally, either you will proceed to the next recording, or you will be informed that the experiment is over.

At the end of the experiment, one of the recordings you have viewed will be drawn at random. Your earnings will depend on the decision (Left or Right) you have made after watching that recording. It will also depend on the decision previously made by the person from the recording following his or her speech in the laboratory session during which the recording was made:

(AUDIO) You will now hear ten recordings randomly selected from all the recordings described above. After each recording, you will make a decision: either Left or Right. This decision is important for your nal earnings. You will also answer some questions about the person heard in the recording. Finally, either you will proceed to the next recording, or you will be informed that the experiment is over.

At the end of the experiment, one of the recordings you have listened to will be drawn at random. Your earnings will depend on the decision (Left or Right) you have made after listening to that recording. It will also depend on the decision previously made by the person from the recording following his or her speech in the laboratory session during which the recording was made:

(TEXT) You will now see ten transcripts randomly selected from all the transcripts described above. After each transcript, you will make a decision: either Left or Right. This decision is important for your nal earnings. You will also answer some questions about the person that made the speech presented in the transcript. Finally, either you will proceed to the next transcript, or you will be informed that the experiment is over.

At the end of the experiment, one of the transcripts you have read will be drawn at random. Your earnings will depend on the decision (Left or Right) you have made after reading that transcript. It will also depend on the decision previously made by the person whose speech was presented in the transcript, during the laboratory session in which that speech was made:

• If you chose Left, then whatever the choice of the person from the recording: you earn 5 EUR ;

• [Classication: non-promise] 6. So. . . uh, I'm a competitor. I came here to play. Uh. . . so. . . uh, I'd like to have competitors in front of me. And. . . always choose an action, and. . . I will always roll my. . . my game.

That's it. [Classication: promise] 7. Hello everyone, uh, I think that in life sometimes you need to stick your neck out and. . . here, if you make that choice, you still have 5 chances out of 6 that it's a good choice. Otherwise, you can decide to stay in your comfort zone, and make sure you get the minimum. And then, if everything is. . .. if everything is a matter of luck, then maybe today you will make the right choice by trusting a stranger. That's it. [Classication: non-promise] does not generate more trust than other conditions. We also note that so far, the experimental evidence is mixed regarding the role of distributional concerns in explaining trust. [START_REF] Cox | How to identify trust and reciprocity[END_REF] estimates that distribututional concerns are as important as the expected reciprocity. However, in a subsequent study [START_REF] Ashraf | Decomposing trust and trustworthiness[END_REF] nd that kindness is not a primary driver of trust most of the variance in trust is explained by expectations of return (which is a common factor in all our treatments). Second, in a study closely related to ours (in which players in a laboratory trust game communicate through pre-recorded, standardized video messages), [START_REF] Centorrino | Honest signaling in trust interactions: Smiles rated as genuine induce trust and signal higher earning opportunities[END_REF] collect data in two waves of experimental sessions. In the rst wave (N = 84), both players' choices aected both players' payos. The second wave (N = 114) uses the same set of recordings, and this time player As' decisions have no impact on player Bs' gains. They do not nd evidence that this design aects trust, that is, player As' trust does not depend on the presence of monetary consequences for player Bs. [START_REF] Well | Well, I'm going to choose to roll[END_REF] Another design feature that distinguishes F2F from the other treatments is the number of decisions made by each player A (6 decisions in F2F and 10 decisions elsewhere). When parametrizing the VIDEO treatment, we also carried out a separate experimental session with 6 videos instead of 10. Clearly, the advantage of having 10 recordings per subject in an increase in the amount of experimental data. We did not nd any evidence (either descriptive or statistical) that this increase in the number of observations per subject comes at a cost of aecting player As' behavior. [START_REF] Hello | I'm not very good at speeches, and I didn't really prepare anything[END_REF] The overall trust rate in F2F is 40.2%. Descriptively, it is higher than in TEXT (36.5%), and lower than in AUDIO (44.4%) and VIDEO (49.0%); however, none of those dierences is statistically signicant (for statistical support, see Table 9). Like elsewhere, in F2F there is also a positive eect (equal to 10.2 percentage points, p = 0.099) of player B's promise to Roll on player A's trust. Pairwise comparisons suggest that this eect is not signicantly dierent from those observed in any of the remaining treatments; a joint F -test leads us to the same conclusion (p = 0.514).

Altogether, these results suggest that making communication direct i.e. moving from video 40 Importantly, their experimental sample comes from a subject pool (university students in a large French city) that is quite similar to ours (university students in another large French city). [START_REF] Hello | I'm not very good at speeches, and I didn't really prepare anything[END_REF] We observed similar trust rates in the 6-round game, and in rounds 1-6 and 7-10 of VIDEO. messages to face-to-face interaction does not improve trust. If anything, it may be counterproductive (although the dierences between those two conditions are not statistically signicant), and the outcomes from the face-to-face communication treatment are very close to those in the baseline treatment with plain text messages. This is in line with the previous results from public goods games and trust games reported in [START_REF] Bochet | Communication and punishment in voluntary contribution experiments[END_REF]; [START_REF] Bicchieri | The medium or the message? communication relevance and richness in trust games[END_REF], but goes against the ndings in Balliet (2010) who suggests that face-to-face communication is a far more powerful device for inducing cooperation than written messages.

I Player As' perceptions of player Bs' characteristics