Margot Déage

'Don't you trust me?': teenagers challenging friendship on Snapchat

Keywords: Trust, Snapchat, Friendship, Reputation, Youth

This study explores the use of Snapchat among teenagers in France, examining how its unique features shape social dynamics and trust among young users. The research was conducted between 2016 and 2018 in four middle schools located in Parisian suburbs, and included 150 interviews with pupils aged 10 to 17, a questionnaire with 236 responses, and nethnography of 100 Snapchat profiles. Additionally, education professionals and the French Snapchat CEO were interviewed. The findings reveal that Snapchat is used by teenagers to test the trust boundaries of their peers, resulting in significant changes to friendship dynamics. This is illustrated through the use of snap streaks, where teenagers ask friends for their passwords to maintain the streaks. Refusal to share passwords can lead to suspicion and mistrust, creating an environment of secrecy and betrayal. Despite the ephemeral nature of snaps, teenagers find ways to save them, leading to the perpetuation of rumors and misrepresentation of facts. The study highlights how abusive trust on Snapchat can result in a loss of reputation and negative consequences for users. Through an analysis of cases of betrayal and reputation shifts, this paper illustrates how private sharing on Snapchat creates an ambivalent trust relationship within teenage friendships.

Introduction

The internet has become an integral part of our lives, creating a virtual extension of the real world where people interact and experience real-life consequences. However, online communication is fundamentally different from face-to-face interactions, as digital interfaces can modify the nature of our relationships. John Suler, a renowned cyber psychologist, demonstrated that online chatting can produce a disinhibition effect, leading to both confidences and malevolence (2004). Trust is essential to any friendship, as it serves as the main tie in this unconstrained relationship [START_REF] Bidart | L'Amitié, un lien social[END_REF]. With the emergence of social media, friendship has evolved to grow and be maintained online, particularly among the Z generation, the new digital native generation (Palfrey & Grasser, 2008). In contrast to the Y generation's online experience in the early 2000s, where teenagers required a computer to access the internet and online interactions were limited by time and space, the Z generation, born after 2000, is introduced to a world of blurred spatiotemporal borders through smartphones. Messages can arrive at any time, from anyone, and they can chat with people they met offline, online, or simultaneously on and offline, reconfiguring relationships and interactions. In France, Snapchat has become the preferred mode of communication for teenagers as it aims to create an interaction similar to faceto-face conversations by making the device and social network disappear. Despite this minimalistic approach, the question remains: does Snapchat influence the French Z generation's conception and experience of friendship? On Snapchat, friendship is defined by the regularity, reciprocity, and intensity of exchanges. Although friendship is a privileged relationship, in an unlimited co-presence, teenagers distinguish their true friends through evidence of trustworthiness, sharing intimacies and secrets. However, sending such signs of reliability through a device renders them vulnerable to virality, prompting teenagers to take precautions to protect their reputation in the event of betrayal.

A (n)ethnographical method

Between 2016 and 2018, I conducted a two-year research study that took place in four secondary schools located in Paris, which catered to students from various socioeconomic backgrounds. The schools were categorized based on their geographical locations and the demographic makeup of their student body. School A was situated in the city center and enrolled mostly students from higher social classes, while school B was located in a gentrified suburb where the majority of pupils were from lower classes. School C was a suburban area enrolling students from both higher and lower social classes, with some coming from nearby ghettos. Lastly, school D was situated in an inner-city ghetto, with pupils only from lower social classes and recent migration.

During the study, I observed the behavior of students in various settings, including the schoolyard, hallways, classrooms, and interactions with their peers, supervisors, teachers, nurses, and administrators. The students affectionately referred to me as 'Miss Interview', and I conducted around 150 interviews with students aged 10 to 17. The interviews focused on the students' experiences with friendship, reputation both online and offline, rumors, and conflicts they encountered. To gain a comprehensive understanding of the students' online behavior, I followed around 100 of them on both Instagram and Snapchat.

In addition to conducting interviews and following students on social media, I attended bullying and cyberbullying prevention sessions organized by teachers, nurses, and police officers. I also interviewed school administrators, teachers, nurses, police officers, and the CEO of Snap Inc.'s French subsidiary to gather their perspectives on the topic. To augment the research data further, I shared a Google Form on each of the four schools' websites, which received voluntary responses from approximately 15% of the student body.

Being friends on Snapchat

In the context of French teenage social media usage, it is evident that multiple platforms serve specific purposes for this demographic. Facebook, for instance, is primarily utilized as a means to remain connected with family members. Conversely, Instagram is predominantly viewed as a virtual business card to showcase meticulously selected and edited photographs. Amongst surveyed French teenagers, Snapchat emerges as the dominant messaging platform, with a significant proportion of respondents indicating its use. Indeed, an overwhelming majority (69.2%) of those surveyed reported having a Snapchat account, highlighting its immense popularity. Furthermore, over half (58.6%) of the respondents identified Snapchat as their favored social media platform, followed by Instagram at 20.2%. [START_REF] Labourdette | Des vies en images[END_REF] describes as 'locutions'. In this sense, Snapchat recreates face-to-face orality conditions for its users beyond the physical distance separating them. Furthermore, the app can be described as a 'ghost network' [START_REF] Déage | S'exposer sur un réseau fantôme: Snapchat et la réputation des collégiens en milieu populaire[END_REF] due to the ephemeral sharing of messages that disappear upon viewing and the invisible communities formed, where contact information such as community, nicknames, numbers, relations, and reactions remain unknown.

The ephemeral and fleeting nature of Snapchat may belie a more nuanced conception of friendship that is configured by the app's unique features. Indeed, the app's emphasis on instantaneity and ephemerality suggests a type of friendship that prioritizes frequent, reciprocal, and intense exchanges between two individuals. These interactions are time-limited, occurring within the present moment and disappearing quickly thereafter. Despite this, Snapchat remains a crucial social media tool among French teenagers, with a substantial majority of 69.2% reporting an account and 58.6% citing it as their preferred platform. This attests to the app's significant role in shaping the social lives and relationships of this demographic.

Intimate connections

Snapchat's unique design distinguishes it from most other social media platforms, as it functions as a close friends' network, requiring users to provide specific information, such as a phone number or a snapcode screen, to add contacts. While concerns about cyberbullying have received considerable attention in public debate and educational programs, French teenagers are more preoccupied with privacy issues on social media (Génération Numérique, 2018). They tend to avoid leaving any traces of their online activities that could be discovered by others, and Snapchat's features enable them to share with peers while avoiding the possibility of unwanted attention from predators. They also trust their peers and believe that the app's features limit the potential dissemination of their information, as they can restrict their audience and receive notifications when their content is received, opened, replayed, or captured.

Adolescence is a critical period of transition from parental to self-identity, where young people become progressively more independent [START_REF] Galland | Sociologie de la jeunesse. L'entrée dans la vie[END_REF]. Social media provides them with an opportunity to connect with their peers and express their identity, away from the surveillance of parents and teachers [START_REF] Metton | Les usages de l'Internet par les collégiens[END_REF]. Similar to the Y generation's use of blogs (Fluckiger, 2006, p.113), teenagers use the internet to experiment and absorb youth culture norms, which may sometimes conflict with school or parental rules. They tend to avoid Facebook and its adult-dominated presence, fearing predators, parental and teacher surveillance, and potential sanctions.

Snapchat, on the other hand, is particularly popular among teenagers, with 71% of people under 25 reportedly using the platform in 2017 (Coëffe, 2018). The app creates a strong generational homogamy, as few parents or teachers have a Snapchat account due to its non-intuitive and childish design. The CEO of Snap Inc.'s French subsidiary suggests that the company deliberately developed a hidden design to create an initiated user network. Connecting to Snapchat is thus considered a rite of passage into adolescence, providing teenagers with the means to integrate into their peers' groups.

It is true that Snapchat is designed to be unreadable. Originally, this non-readability was intended to initiate real-life conversations. For example, one person might say, 'Oh, you're not on Snapchat? Well, let me show you.' This would start a discussion, initiating offline virality. In fact, there are no online features such as views, shares, comments or likes on Snapchat. The app's cryptic style was initially seen by teenagers as something that was against adults. Secondly, it was present in people's top-ofmind, and the spread of the application was always organized between insiders and outsiders. 'Let me show you how the app works, otherwise, you won't be able to do it yourself', creating a status for the one who knows and then for the one who did not know and who becomes the one who knows. (CEO of Snap Inc.'s French subsidiary, April 2018)

Rewarded shares

The flashy and entertaining interface of Snapchat reminds users of video games. User interactions are rewarded with scores and trophies, which are public indicators of user activity. However, the score does not reflect the amount of snaps sent and received. Instead, the score promotes interaction and encourages the use of various features. For example, the score rewards reactivity, as more points are given when users respond to messages immediately. Despite this, the calculation of the score remains unclear to most users.

Behavioral psychology suggests that random rewards may reinforce behavior [START_REF] Ferster | Schedules of reinforcement[END_REF]. Thus, the random rewards in Snapchat may stimulate users' desire to share on the app.

Snapchat is well-known for its stories, pictures, and short videos that are publicly accessible for 24 hours. However, private chats are where most users' activities are concentrated. Among teenagers using Snapchat, 30.4% never publish stories, and only 17% publish every day (n=171). The prioritization of interpersonal communication over public self-display allows users to assume different social roles. Facebook and Instagram pages are front-stage interactions, where individuals present an artificially unified self [START_REF] Goffman | La Mise en Scène de la Vie Quotidienne[END_REF](Goffman, [1959]]). However, people known from different spheres may have different or even contradictory versions of the self.

In this sense, a wider network requires stronger self-censorship. In contrast, Snapchat's private and restricted network may disinhibit teenagers for sharing content. They can experiment with an online self closer to their offline social selves, without assuming a multi-normative social control. Communication on Snapchat is presented as a fun and light-hearted activity. The French CEO of Snapchat has stated that 'Snapchat is designed to be ridiculous because we know it does not matter,' as users share with close friends. Lenses, which are augmented reality masks, are used to reduce users' embarrassment in showing themselves.

According to the CEO, these lenses are meant to cultivate a 'sane self-esteem' by encouraging users to laugh instead of falsely embellishing their lives. Thus, the platform stands for an internet derision culture (Dagnaud, 2013, p. 91-119).

Intensified co-presences

The aim of Snapchat is to facilitate face-to-face connections despite physical distance, with the ultimate goal of promoting the common good. (The CEO of Snap Inc.'s French subsidiary, April 2018) Snapchat's purported goal is to replicate the intimacy of face-to-face relationships, even over long distances. This mission appears to be a noble one, driven by a desire to serve the common good. Nevertheless, the platform's evaluative mechanisms may promote a narrow conception of relationships. As previously noted, the interface rewards prompt responses with scores, causing messages to be treated as urgent matters. This requirement not only enables social control but also prioritizes online communication over offline activity. As most snaps vanish quickly, users must pay full attention to them, effectively isolating themselves from the surrounding world. Although the platform's temporal scope extends several years back, with features such as saving messages and content, ephemeral communication remains the norm.

In this evanescent configuration, relationships must be continuously maintained. The snap streak feature encourages daily sharing, with a flame emoji appearing next to conversations after several days of uninterrupted communication. The app counts the number of days that communication has continued and displays it next to the symbol. When nearly 24 hours have elapsed since the last message, a sandglass icon appears, warning that time is running out before the flame goes out. This feature defines the value of a relationship based on interaction frequency and reciprocity.

In Parisian middle schools, 60.1% of teenagers who use Snapchat have a flame with their friends, and 36.5% try to achieve the highest possible snap streak (n=170). Sometimes, they ask in their story if someone wants to have a flame with them. In this rush to share, they may be less cautious about the content they share, opting to share any mundane event just to keep in touch. Phatic conversations [START_REF] Licoppe | Sociabilite et technologies de communication. Deux modalités d'entretien des liens interpersonnels dans le contexte du déploiement des dispositifs de communication mobiles[END_REF]Velten, Arif, & Moehring, 2017, p. 238) consisting of spontaneous, low-quality, and hastily edited 'fast food' images (Verstraete, 2016, p. 113) create a continuous co-presence. These informal, suggestive, and elliptical videos (ibid, p. 109) provide a realistic chronicle of friends' experiences, creating a sense of immersion. Through voices, settings, facial expressions, and imagery, classmates can see and be seen in each other's lives outside of school, even if they are not yet permitted to go out together.

However, teenagers often subvert this sharing imperative by sending black screens or flame emojis without any other content. When they have flames with many people, they usually create a list of recipients for these contentless snaps. In this case, daily sharing becomes a means to earn flames rather than maintaining friendships.

Friendship, trust and reputation

Discrepancies between snapchat and teenagers' friendship conception

Although Snapchat refers to contacts as 'friends' and sharing with them adds value to conversation, teenagers are able to distinguish between permanent co-presence and friendship. As explained by Clair Bidart (2002[START_REF] Bidart | Les âges de l'amitié Cours de la vie et formes de la socialisation[END_REF], adolescence is a period in life when informal co-presence is highly developed. Joining middle school, which is typically larger than elementary school, and connecting to social media (81.1% of teenagers receive a smartphone before or at 12 years old) can drastically increase relational options.

Paradoxically, while Snapchat is designed for people who already know each other in real life, it often serves as the first digital meeting place for young people. They frequently use their Snapchat stories to share their friends' Snapcodes or nicknames, encouraging their community to add them. Therefore, Snapchat is not necessarily considered a close friend social media platform by teenagers. Teenagers often interact with new people both online and offline, but it does not necessarily imply that these individuals are considered as friends. Their socialization is constrained by the educational institution, which defines class groups, timetables, obligations, and sanctions, among other factors. According to young people, trust is the main criterion for distinguishing between friends and other types of relationships. Even for them, friendship is considered an exceptional bond. On average, teenagers report that they only have 4.8 schoolmates whom they trust (see Table 4). The concept of trust is multifaceted and can be further elucidated. Trust is not an automatic, optimal, or complete gift in practice. Individuals may consider someone reliable without trusting them, accept to interact with someone they deem untrustworthy, or trust someone for one aspect but not another. However, it is possible to articulate values and expectations that should be met by individuals we trust.

In the case of teenagers, sincerity (27.5%), a sense of humor (22.5%), and respect (17.8%) are the key qualities that facilitate the formation of friendships. Expectations for friendship vary by gender, with boys valuing friends who support them during difficult times (29.3%), those who are fun (29.3%), or those who share similar interests (12.2%). Girls also prioritize support (21.3%) but also value friends who are always there for them (20%) and with whom they can be their authentic selves (18%). Masculine friendship is characterized by entertainment, while feminine friendship is primarily focused on intimacy (see Table 5). As such, the definition of friendship proposed by Snapchat is not prevalent among teenagers: only 15.1% of teenagers consider permanence to be the most important criterion in distinguishing friends from acquaintances, while only 2.6% believe that duration is the most significant factor.

Numerical trust as friendship bound

During the transitional phase from childhood to adolescence, behavioral norms undergo rapid changes, but students integrate them at varying rates. It is common for former friends to stop communicating due to perceived changes in one another, resulting in 39.4% of students feeling abandoned by a friend. Middle school marks a period where ties between adults and children become increasingly undone. As parents grant their children more independence, and with the multiplicity of teachers, pupils begin to distance themselves from adult authority. In this process of automation, youth culture and language, which tend to confront adult norms, gain prominence. Adolescents assert themselves within their peer groups through their music, movies, clothing, language, and sense of humor, influenced by popular culture and transgressive behaviors. However, exaggerating one's belonging to this youth culture can sometimes threaten friendships. Those who abandon their initial group to stand out in supposed popular circles may be denigrated. Leila and Lina, two 13-year-old girls from School, recounted how their friend had changed.

Leila: Actually, the way she changed is not necessarily for the worse. Some people change for the worse, but that's not the case here. Before, she used to speak in a gentle manner. But now, she speaks in a way that surprises us. She's acting like a diva [Elle fait sa meuf]. It's hard to explain, but we use the phrase 'elle fait sa meuf' to describe this behavior.

Interviewer: What do you mean exactly? Leila: She acts as if she's superior to others. Lina: Yes, that's right. Often, when she becomes friends with someone new, she changes her behavior, her thoughts, her clothing style, and even her way of speaking. For instance, someone told her about something, and she responded by saying 'yes, I like it'. But when another girl told her that the same thing sucks, she changed her opinion and agreed with her.

In situations of relational instability, there are two ways to reduce uncertainty: 1) through trust, where an individual suspends their search for information and believes in the reliability of the other person to avoid betrayal, and 2) through mistrust, where an individual seeks guarantees of reliability to protect themselves from betrayal. Isabelle Clair (2008, p. 88-89) observed that in youth communities, the constant threat of rumors and reputational damage creates an atmosphere of mistrust. Adolescents often believe that adults are incapable of securing their peer relationships, resulting in them placing their trust in very few people, and when they do, it tends to be unconditional.

As a manifestation of this trust, teenagers often give their best friends their Snapchat passwords so that they can keep an eye on their flame' and receive their friend's password in exchange. This exchange of passwords serves to reinforce the exceptional nature of their friendship and their confidence that it will endure.

Reputation as a guarantee

In the context of Snapchat, conversations may be considered intimate, yet not entirely confidential. Adolescents are aware that the content they share could potentially be exposed. Despite their knowledge of Snapchat's features, including views and captures, they possess alternative techniques (such as the use of an external camera or specific apps, and Wi-Fi disconnection) to save content surreptitiously. They are also cognizant of the fact that password sharing among peers is commonplace, and that multiple users may access the same account, potentially leading to identity theft. Unlike Google, Snapchat does not immediately notify users when someone logs in from another device; this feature must be enabled by the user, which is a rarity among teenagers.

Friendship holds paramount importance as a source of social and emotional support for teenagers. On the one hand, shared activities and co-presence facilitate information exchange among peers. On the other hand, students are constrained in the schoolyard as they feel the control and scrutiny of their peers whom they do not fully trust. As a result, adolescents seek refuge in Snapchat to share confidential information with their most trusted friends. Despite its significance, friendships at this stage of life are vulnerable and delicate. To safeguard against potential deception, teenagers tend to collect a 'dossier' of compromising information that they could use as leverage to disseminate rumors if a former friend were to threaten them. Fear and revenge can create negative reputations that result in exclusion from the peer group. In the digital age, apps make it effortless to save and disseminate content, while possessing someone's password enables teenagers to access and share compromising information with their entire community. According to Imen, a 14-year-old student at School C, streaks on Snapchat tend to bolster the spread of rumors.

Imen: There was a girl, it's been a long time, maybe two years ago, her boyfriend sent her a message on Snapchat saying: Aida, I want to have sex with you. They were both in this school and dating at that time. She didn't say yes or no. I didn't say anything either. But this year, the rumor surfaced again! She's now in 3e. Her ex-boyfriend told a group of guys, who then spread it to everyone. I think he got mad when they broke up.

Interviewer: Students often mention using a dossier to protect themselves.

Imen: Yes, that's how it works here. If I'm mad at you, I threaten to expose your dossier in front of everyone. In Aida's case, the boy didn't like her and said, 'Shut up, stop talking, or else I'll reveal your dossier.' It's like an atomic bomb, and everyone will know about it. They'll be shocked and won't talk to you. That's how we do it. If you're angry at someone, you can use their dossier against them.

In the realm of social media, teenagers exhibit a sense of apprehension towards unfamiliar adults. This cautiousness is further exemplified by the actions of the French CEO of Snapchat, who prohibits his 14-year-old son from creating a profile on the application due to the potential threat posed by predators. However, studies have demonstrated that a significant proportion of cyberbullying incidents are perpetrated by individuals known to the victim within the school environment (Blaya, 2016, p. 59). Such aggression may be instigated by factors such as love, or in the context of betrayal among friends.

Conclusion

Snapchat, while purporting to recreate face-to-face interaction online, is not a neutral interface. Among its features, the snap streak stands out as having a strong influence on teenagers' interactions. Through symbolic rewards, it promotes an imperative of permanent and intensive co-presence between 'friends'. Teenagers are keen on this kind of reward and many of them strive to obtain it as frequently as possible. However, they also find ways to circumvent the imperative of sharing by sending contentless snaps. Although Snapchat was designed for close friends, it has become the primary meeting place for Parisian teenagers, who, at this stage of their lives, experience relational autonomy that is both new and unstable. Consequently, they often live in a mistrusting atmosphere, with many acquaintances but few trusted friends (an average of 4.8). Snap streaks are a pretext for them to test the reliability of their friends and exchange passwords to avoid losing their symbolic flames. They are aware that such actions increase the risk of deception and, therefore, protect themselves by amassing information they can use to threaten their friends if they feel their own reputation is endangered.

Table 1 Characteristic of questionnaire respondents (n=236)

 1

		Number	Frequency
		School grade
	6 e (11-12-year-old)	36	15,3%
	5 e (12-13-year-old)	75	31,8%
	4 e (13-14-year-old)	60	25,4%
	3 e (14-15-year-old)	65	27,5%
		School
	A	35	14,8%
	B	111	47%
	C	52	22%
	D	38	16,1%
		Sex
	Girl	150	64%
	Boy	86	36%

Table 2 Teenagers' favorite social media (n=203)

	informal conversational temporality, or what
		Number	Frequency
	Facebook	8	3.9%
	Instagram	41	20.2%
	Snapchat	119	58.6%
	Twitter	7	3.4%
	WhatsApp	28	13.8%
	Snap Inc., the company behind Snapchat, does not classify its product as a traditional social network, as it
	differs from popular platforms such as Facebook, Instagram, and Twitter in several key ways. Most notably,
	Snapchat is unique in the temporary nature of its messages, which are primarily ephemeral, and the
	prevalence of pictures and videos over text. Upon opening the app, users are immediately directed to the
	camera, and the quick capture of visual content is encouraged. The app also features playful filters that mock
	the discomfort of imperfect appearances and disinhibit users from taking selfies.
	The use of Snapchat as a communication tool disrupts traditional conceptions of relatively polished images
	used to supply collective memory, as the ephemeral nature of the images and messages create a friendly and

Table 3

 3 Social media where teenagers encounter most new people (n=226)

		Number	Frequency
	None	114	50.4%
	Facebook	7	3.1%
	Instagram	35	15.5%
	Snapchat	62	27.4%
	Twitter	4	1.8%
	WhatsApp	4	1.8%

Table 4

 4

	Schoolmates teenagers are trusting (n=219)
		Number	Frequency
	Aucun	17	7,2 %
	1 schoolmate	21	8,9 %
	2 schoolmates	35	14,8 %
	3 schoolmates	29	12,3 %
	4 schoolmates	44	18,6 %
	From 5 to 9 schoolmates	36	15,3 %
	10 schoolmates and more	37	15,7 %

Table 5 .

 5 True friend main characteristic according to gender (n=232)

		True love	difficult times	Support in	Confidant	for you	Always there	acquaintance	Long-standing	Fun	Authenticity	Shared values	interests	Shared	Total
	Girl	8,0%	21,3%	7,3%	20,0%	0,0%	14,0%	18,0%	1,2%	6,0%	100%
	Boy	3,7%	29,3 % 4,9%	6,1%	7,3%	29,3%	6,1%	5,3%	12,2%	100%
	total	6,5 %	23,7 % 6,5%	15,1 % 2,6 %	19,4%	13,8 % 3,9 %	8,1%	100%
	χ2 : 35,242, ddl = 18, p-value = 0,008812


Thomas Castelain

Vittorio Girotto

Frank Jamet

Hugo Mercier
email: hugo.mercier@unine.ch.

Evidence for benefits of argumentation in a Mayan indigenous population

Keywords: Reasoning, argumentation, myside bias, traditional populations

Group discussion improves on individual reasoning performance for a wide variety of tasks. This improvement, however, could be largely specific to members of modern, schooled, affluent Western cultures. In two studies, we observed the same improvement in the members of a traditional population-indigenous Maya from Guatemala. Two features of reasoning can account for this improvement: the myside bias, which precludes individuals from improving their performance on their own, and the ability to soundly evaluate others' arguments, which allows individuals to benefit from group discussions. These two features were observed in the traditional population studied: solitary reasoning performance was marked by the myside bias; individuals were more likely to be convinced by arguments for the correct answer rather than for a wrong answer. Together with previous evidence, the present results strengthen the conclusion that these features are adaptive features of reasoning.

At least since Descartes, reasoning has generally been understood as a tool of individual cognition: by carefully evaluating and weighing one's reasons, one should arrive at sounder beliefs and better decisions [START_REF] Descartes | Discours de la méthode[END_REF][START_REF] Kahneman | A perspective on judgment and choice: Mapping bounded rationality[END_REF][START_REF] Stanovich | The Robot's Rebellion[END_REF].

Opposed to this individualistic view, an alternative is that reasoning's main function is social: to find reasons in order to convince others, and to evaluate others' reasons in order to adopt better supported beliefs. The argumentative theory of reasoning [START_REF] Mercier | Why do humans reason? Arguments for an argumentative theory[END_REF]) is a recent instantiation of this social view of reasoning (for other social views of reasoning, see [START_REF] Baumeister | Conscious thought is for facilitating social and cultural interactions: How mental simulations serve the animal-culture interface[END_REF][START_REF] Billig | Arguing and Thinking: A Rhetorical Approach to Social Psychology[END_REF][START_REF] Gibbard | Wise Choices, Apt Feelings[END_REF][START_REF] Piaget | Judgment and Reasoning in the Child[END_REF].

An important result supporting the argumentative theory of reasoning is that, provided minimal conditions are met (e.g. the expression of disagreement, see [START_REF] Janis | Groupthink (2nd Rev)[END_REF], argumentation in the course of group discussion produces sizeable improvements in reasoning performance on a variety of tasks, such as logical, mathematical, and inductive problems [START_REF] Laughlin | Group Problem Solving[END_REF][START_REF] Moshman | Collaborative reasoning: Evidence for collective rationality[END_REF][START_REF] Trouche | Arguments, more than confidence, explain the good performance of reasoning groups[END_REF], work related tasks (Mercier, 2011c), forecasting [START_REF] Mellers | Psychological strategies for winning a geopolitical forecasting tournament[END_REF][START_REF] Rowe | The Delphi technique as a forecasting tool: issues and analysis[END_REF], and school tasks [START_REF] Mercier | Reasoning serves argumentation in children[END_REF][START_REF] Slavin | Cooperative Learning: Theory, Research, and Practice[END_REF][START_REF] Smith | Why peer discussion improves student performance on in-class concept questions[END_REF]. The argumentative theory suggests that the gap in performance between individual reasoning and reasoning in discussion stems from a combination of two features of reasoning. The first is the myside bias (or confirmation bias): individuals overwhelming produce reasons for their preferred opinions (Mercier, in pressa;[START_REF] Nickerson | Confirmation bias: A ubiquitous phenomena in many guises[END_REF]. As a result, reasoning rarely allows the lone reasoner to correct mistaken intuitions. Instead, the myside bias can lead to overconfidence [START_REF] Koriat | Reasons for confidence[END_REF] and polarization [START_REF] Tesser | Some effects of time and thought on attitude polarization[END_REF]. The second feature is reasoning's ability to soundly evaluate others' arguments, rejecting weak arguments and accepting strong enough ones [START_REF] Hahn | The rationality of informal argumentation: A bayesian approach to reasoning fallacies[END_REF][START_REF] Petty | Attitude change: Multiple roles for persuasion variables[END_REF]. In a discussion group members only provide arguments for their side, but they also evaluate each other's arguments. They change their minds when the arguments are good enough, which usually means changing their mind for the best. These features of reasoning, along with the gap in reasoning performance they seek to explain, could be a peculiarity of WEIRD (Western Educated Industrialized Rich Democratic) cultures. These cultures, in which the research mentioned above has been conducted, differ from other cultures on a number of traits [START_REF] Henrich | The weirdest people in the world[END_REF]. In particular, participants from WEIRD cultures are sometimes at the far end of the distribution-for instance in terms of individualism [START_REF] Henrich | The weirdest people in the world[END_REF]. Schooling, which is comprehensive in WEIRD cultures but absent from many other cultures, can exert a profound influence on cognition, for instance through the acquisition of literacy [START_REF] Dehaene | Reading in the brain: The new science of how we read[END_REF] and numeracy [START_REF] Dehaene | The Number Sense: How the Mind Creates Mathematics[END_REF][START_REF] Gordon | Numerical cognition without words: Evidence from Amazonia[END_REF].

Regarding reasoning and argumentation, WEIRD cultures have a series of traits that, while not necessarily specific to these cultures, might conspire to create the features mentioned above. Compared to many other cultures, members of Western cultures: 1) have long put a higher value on argumentation in their institutions, from science to law or politics [START_REF] Lloyd | Adversaries and Authorities[END_REF][START_REF] Peng | Culture, dialectics and reasoning about contradiction[END_REF]; especially compared to Eastern cultures, see [START_REF] Becker | Reasons for the lack of argumentation and debate in the Far East[END_REF][START_REF] Nakamura | Ways of Thinking of Eastern Peoples: India[END_REF]; 2) put relatively less stress on face-saving and social harmony [START_REF] Kim | Deviance or uniqueness, harmony or conformity? A cultural analysis[END_REF][START_REF] Oetzel | Face and facework in conflict: A cross-cultural comparison of China, Germany, Japan, and the United States[END_REF], which might allow for more confrontational and open debates [START_REF] Mercier | The benefits of argumentation are cross-culturally robust: The case of Japan. Thinking & Reasoning[END_REF]; 3) adopt a different parenting style in which children tend to argue more with adults: they question more freely their decisions, and receive more explanations-although this difference seems to be restricted to the middle and upper classes [START_REF] Chouinard | Children's questions: A mechanism for cognitive development[END_REF]; indeed, they talk more with adults generally, [START_REF] Hart | Meaningful differences in the everyday experience of young American children[END_REF][START_REF] Heath | Ways with words: Language, life and work in communities and classrooms[END_REF]. All of these factors might make argumentation a cognitive skill particularly valued in participants from WEIRD cultures.

The myside bias might be a cognitive response to a specific cultural environment in which argumentation is highly valued and it is particularly important to be able to defend one's point of view. Two other traits might help explain the existence of a myside bias in WEIRD cultures. First, in WEIRD cultures, individuals are confronted with a variety of choices, values, and worldviews. In such cultures, it makes sense to anticipate having to defend one's choices, since it is likely that one will encounter people who have made different choices [START_REF] Schwartz | The Paradox of Choice: Why More is Less[END_REF]. Second, many of these choices mostly have a symbolic value, so that it arguably matters more to make a decision that is socially acceptable than an intrinsically good decision. For instance, reasoning has been shown to lead customers towards products they enjoy less, but which allow them to be perceived more positively by others [START_REF] Thompson | The social utility of feature creep[END_REF] for review, see [START_REF] Mercier | Why do humans reason? Arguments for an argumentative theory[END_REF]. Even socially consequential choices, such as voting, might mostly have a symbolic value [START_REF] Sears | Self-interest vs. symbolic politics in policy attitudes and presidential voting[END_REF]. The symbolic value of voting, and of political opinions more generally, might help explain why voters in WEIRD cultures have a consistent myside bias when reasoning about politics: it matters more that they have arguments to justify their opinions than that they make otherwise sound opinions [START_REF] Kahan | The polarizing impact of science literacy and numeracy on perceived climate change risks[END_REF][START_REF] Taber | Motivated skepticism in the evaluation of political beliefs[END_REF].

WEIRD cultures thus seem to have a number of traits that favor the development of argumentative skills, traits which in turn might favor the emergence of a myside bias. In order to test whether these cultural traits partly or entirely account for the relevant features of reasoning-the efficiency of argumentation and the deficiencies of solitary reasoning-it is necessary to study reasoning and argumentation in a population that does not share these cultural traits. We first argue that a broad type of cultures-which we will call, in an ad-hoc fashion, traditional cultures-differs in many relevant ways from WEIRD cultures. Then we will argue that the population to which the participants recruited here belong, the K'iche' Maya in Guatemala, share these traits.

Here we define traditional cultures as the human groups which are the broad opposite of WEIRD cultures: small-scale groups which are not Western, educated, industrialized, or rich (note that many human groups would thus fall between WEIRD and traditional cultures as so defined). The fact that these cultures are not Western means that they do not share the hypothetical Western values which cast argumentation in a positive light (although they might hold other values to the same effect, see, e.g. [START_REF] Gluckman | The judicial process among the Barotse of Northern Rhodesia (Zambia)[END_REF]. Regarding lack of education (in the sense of formal schooling), one of its most relevant consequences is a reluctance to engage in hypothetical thinking. Experiments conducted in several unschooled populations have revealed that most of their members fail to complete even very simple hypothetical reasoning tasks [START_REF] Cole | The cultural context of learning and thinking[END_REF][START_REF] Luria | Cognitive Development its Cultural and Social Foundations[END_REF][START_REF] Scribner | Modes of thinking and ways of speaking: Culture and logic reconsidered[END_REF]. This reluctance to engage in hypothetical thinking could hinder argumentation, since argumentation often relies on hypotheticals. Besides the lack of schooling, education also differs in traditional societies in other ways. Of particular relevance here is that parenting in traditional populations conforms to the pattern observed outside of middle and upper middle class WEIRD populations: relatively little talk addressed to children, and in particular a very small number of exchanges requiring reasons and explanations [START_REF] Gauvain | Children's questions in cross-cultural perspective a four-culture study[END_REF]. Because members of traditional cultures, compared to members of WEIRD cultures, might value argumentation less, be more reluctant to engage in hypothetical thinking, and are not trained to argue early on, they might be less likely to develop the argumentative skills evidenced in the members of WEIRD cultures.

We argued above that the members of WEIRD cultures-particularly in the middle and upper middle classes-are faced with a great variety of choices, and that many of these choices are in large part symbolic. The choices facing members of traditional cultures differ in both respects. As a rule, members of these cultures have far fewer choices: far fewer (if any) products to buy, far fewer (if any) choice of occupation, far fewer (if any) choice of places to live, far fewer (if any) choice of religion, far fewer (if any) choices of people to befriend, etc. (see, [START_REF] Lévi-Strauss | The scope of anthropology[END_REF]. This relative lack of choice suggests a lighter burden of justification. Members of traditional cultures might thus have less use of a myside bias which would help them defend their choices. Moreover, it has been argued that members of traditional cultures make more life and death decisions than members of WEIRD cultures (Diamond, 2012). Contrary to the members of WEIRD cultures, members of traditional cultures do not live only in a human created environment in which many natural dangers are eliminated (e.g. predation) or drastically reduced (e.g. many pathogens). This is reflected in the lower life expectancy in traditional cultures compared to WEIRD cultures (e.g. [START_REF] Gurven | Longevity among hunter-gatherers: a cross-cultural examination[END_REF]. It is thus plausible that for most decisions, intrinsic value matters more than symbolic value for the members of traditional cultures relative to members of WEIRD cultures. As a result, the myside bias could be disadvantageous for the former, as it leads to intrinsically worse decisions, and advantageous for the latter, as it leads to easier to defend symbolic decisions.

On the whole, it is thus plausible that members of traditional cultures, relative to members of WEIRD cultures, reason in a way that is more practical (see [START_REF] Luria | Cognitive Development its Cultural and Social Foundations[END_REF][START_REF] Medin | The native mind: Biological categorization and reasoning in development and across cultures[END_REF]) and more in line with the predictions of the individualistic view of reasoning.

Relative to members of WEIRD cultures, members of traditional cultures might suffer less from the myside bias, and thus be more efficient solitary reasoners, while benefitting less from argumentation.

In light of the evidence available, the relevant traits of traditional cultures mentioned above appear to be present in the K'iche' Maya who participated in the present experiments.

The K'iche' are a preliterate Maya Amerindian group living in rural areas of Guatemala. Men are mainly subsistence farmers and women do household maintenance work. They are obviously not Western, and there is no evidence that their culture puts a particular value on argumentation. The participants in our experiments had received no schooling as children and were in the process of receiving a very moderate amount of schooling as adults. The pattern of interactions with the children seems to follow the pattern typically found outside of middle and upper classes in WEIRD cultures. A study of the interactions between K'iche' adults and children revealed that most of the utterances adults address to children are imperative and very few are questions [START_REF] Pye | Quiché Mayan speech to children[END_REF]. There is thus little opportunity for argumentative exchanges between children and adults. Overall, the K'iche' do not seem to enjoy any of the cultural traits that might make WEIRD cultures particularly congenial to argumentation.

Like other small-scale societies relying on substance farming, the K'iche' face fewer choices than members of WEIRD cultures. Few products are available for purchase, and they have very little money to purchase them with (UNDP, 2010a(UNDP, , 2010b)). The choice of occupation and place to live is extremely restricted (UNDP, 2010a(UNDP, , 2010b(UNDP, , 2012(UNDP, , 2014)).

Moreover, the environment is harsher than that faced by most members of WEIRD cultures (for instance 94% of the population in the relevant district-Sololá-lacks food security, see INE, 2011, p. 29;UNDP, 2010a;UNFPA, 2014). The risk of disease and early death is much higher (INE, 2011;UNDP, 2010a;UNFPA, 2014). A myside bias might thus be less adaptive in such an environment than in that of WEIRD cultures.

The improvement in performance yielded by argumentation, the myside bias which explains poor solitary performance, and the argumentative skills which explain good performance in group discussion are the main pieces of evidence in favor of the argumentative theory of reasoning. This theory posits that reasoning is an evolved adaptation, and that these features, which make it adaptive, should be relatively universal [START_REF] Mercier | Why do humans reason? Arguments for an argumentative theory[END_REF]. However, these features have mostly been documented among WEIRD cultures. We have argued above that several traits of Western cultures might conspire to explain these features, and that reasoning might exhibit different features in traditional populations such as the K'iche' Maya recruited in the present study. Experiments carried out in such populations can thus provide crucial evidence to understand the function of human reasoning. Accordingly, the goal of the present experiments is twofold. First, to test whether, in this traditional population, discussion improves performance compared to solitary reasoning. Second, if such an improvement was observed, to determine whether it could be explained by the two features of reasoning that explain the gap in reasoning performance in WEIRD populations: myside bias and sound ability to evaluate others' arguments.

Methods

Participants. One hundred and forty preliterate participants were involved in this research, 57 in study 1 (48 females, M age = 35.12 years, SD = 12.16, range 18-62 years) and 83 in study 2 (78 females, M age = 43.89 years, SD = 12.49, range 21-76 years). The participants were K'iche' from ten different villages around the city of Nahualá in the department of Sololá, Guatemala. The participants were recruited through an alphabetization program set up by the CONALFA (National Committee for Alphabetization) in which they take part. The program consisted of arithmetic and literacy classes dispensed in K'iche'. The participants were supposed to spend three hours per week to learn to read and write, first in their native language and later in Spanish, but few of them were able to attend the classes regularly.

Nearly all participants (N = 127, 91%) were in their first year of alphabetization, the rest in their second year. Verbal informed consent was obtained from the participants. Each participant received a reward-seeds or sewing threads-that they had requested during the recruitment phase.

Design. Two studies were conducted and both had the same design (see Supplemental Material for details). The tasks used in this research-conservation tasks-are designed to objects (left and right) were the same during the pre and post-test, and were inverted in the test. During the pre-and the post-test, 114 participants were asked to justify their answers, while 26 were asked instead to think aloud. For all participants, the justifications and think aloud protocols in the pre-and post-test, as well as the discussion during test were recorded, transcribed, translated from K'iche' to Spanish, and coded by two individuals, one of whom was blind to the hypotheses (inter-coder reliability: Kappa = .79, p < .001).

Procedure. Two native female experimenters were trained by one of the authors (T.C.), and they conducted all the experiments. All the experiments were conducted in 2014. The experimenters interacted with the participants in their native language. The translations from Spanish to K'iche' were done by one of the experimenters, the back-translation by the other experimenter, with discrepancies resolved through discussion with one of the authors (T.C.).

The experiments took place in the village of each community, in the rooms where the participants follow their alphabetization lessons. In order to ensure that the participants were not discussing the tasks outside of the experimental context, when they were not being tested they had to reintegrate their classroom and to continue to follow the lesson of the day. They were also instructed not to interact until the end of the experiment.

In the familiarization phase, the participants were presented with the material: two glasses and two pieces of clay (Play-Doh) (see Supplemental Material, Figure S1). We used the same materials in the following tasks, except for Tasks 2 and 4 in which we used one piece of clay and one piece of metal. The experimenter started the familiarization phase by pouring the two glasses with water and invited the participants to observe that their level was equal. Then she presented the first pair of objects in front of the two glasses. In Tasks 1 and 3, the transformations-cut the ball in two, turn it into a cylinder-were made in front of the participants, and the differences or similarities in weight or size were notified to them before each trial. The experimenter then asked the participants: "If I put the balls in the two glasses, their level of water will rise, do you think that it will rise more in this glass [The experimenter pointed to one of the glasses]? Do you think it will rise more in this glass [The experimenter pointed to the other glass]? Or it will rise the same in the two glasses?" All the tasks followed the same instructions. During the test, participants were invited to discuss the tasks in groups and to arrive at a consensual answer: "You can discuss now and agree on an answer." After the discussion, the experimenter asked: "Do you all agree? What is your answer?"

Results

At each stage of the studies, we computed the individual scores, assigning 1 to correct answers and 0 to incorrect answers, and the group scores (addition of individual scores), using real groups for the tests, and the equivalent nominal groups for the pre-and post-tests (Figure 1B To test whether the myside bias could explain wrong answers at the pre-test, we analyzed the reasons participants provided. The myside bias should limit the number of reasons that go against the participants' answers. Out of 141 explanations given by participants providing the wrong answer, 1 contained no reason, 134 contained only reasons supporting the participants' answer, and 6 contained reasons against it. This result could also reflect a task demand: participants were asked to explain their answers. Therefore, they might have failed to mention reasons that they had considered but dismissed. To test this possibility, a subset of participants had been asked not to explain their answers, but to think aloud while they were trying to solve the tasks. Out of the 32 explanations given by participants providing wrong answers in think aloud protocols, 3 mentioned reasons going against their final answer, 2 contained no reasons, while 27 contained only reasons supporting the participants' final answer (see Supplemental Material, Results for the detail of the coding).

To evaluate the soundness of argument evaluation, we contrasted the effects good and bad arguments had on the outcome of the discussion. An argument could convince other group members not because they think it is good, but because it is defended by the majority.

To examine such majority effects, we first analyzed the 43 discussions in which a participant who had given the correct answer and the correct justification in the pre-test faced group members who all agreed on the same wrong answer. In 33 (77%) of these discussions, the group agreed on the correct answer. Moreover, in 8 out of the 10 remaining discussions, no argument for the correct answer was offered. We also analyzed the 23 discussions in which a participant who had given a wrong answer in the pre-test faced group members who all agreed on the correct answer. In these discussions, the participant with a wrong answer never convinced the group. Thus, a participant who defended an answer on her own was significantly more likely to convince the group if she defended the correct answer than a wrong answer (p < .001). As an illustration, Figure 2 If the participants who adopted the correct answer did so because they had understood the good argument, they might be able to use this new understanding to solve a transfer task.

In Study 1, we used a novel conservation task (Task 4) after the post-test. In Study 2, Task 4 was used in the pre-test. In Study 1, participants solved this task correctly more often (63%) than in Study 2 (39%; p = .006).

Conclusion

The results of these studies show that preliterate individuals share some of the most important features of reasoning previously documented in WEIRD populations: reasoning performance significantly improves with discussion; argument production is marked by the myside bias; argument evaluation is efficient, enabling participants to discriminate between good and bad arguments and to be convinced by good arguments.

These results do not demonstrate that these features are universal. We might have considered the wrong traits of WEIRD cultures as being potentially relevant to the shaping of reasoning and argumentation. Other cultural traits, which are shared by WEIRD cultures and the Mayan indigenous group studied here but absent in other cultures, might in fact be determinant. Still, the K'iche' differ from WEIRD cultures in many of the traits that could have plausibly explained why the relevant features of reasoning are observed in WEIRD cultures: the value placed on argumentation, the fact that children and adults exchange many arguments, the fact that people feel they have to justify many of their choices, and the relatively low costs of poor decisions. We thus suggest that the present investigation helps make the case that these features are not culturally acquired.

The potential universality of these features of reasoning is relevant to the debates about the function of reasoning. Individualist theories of reasoning do not necessarily predict poor argumentative performance, but it is not clear how they can explain the improvement in performance following group discussion. As for the myside bias, it runs directly against their predictions. By contrast, these features of reasoning support the argumentative theory of reasoning.

The argumentative theory of reasoning suggests that these features are adaptive features of reasoning. They would be adaptive because they enable reasoning to fulfill its main function, argumentation, which in turns greatly facilitates communication (the evolutionary rationale is exposed in [START_REF] Mercier | Why do humans reason? Arguments for an argumentative theory[END_REF]. As expected if these features are adaptive, not only are they extremely robust in adults from WEIRD populations (Mercier, in pressb;[START_REF] Mercier | Why do humans reason? Arguments for an argumentative theory[END_REF]), but they have now been observed in a culture that differs in many relevant ways from WEIRD cultures, and there is no strong evidence that they are absent from other cultures (Mercier, 2011a;[START_REF] Mercier | The benefits of argumentation are cross-culturally robust: The case of Japan. Thinking & Reasoning[END_REF]. Moreover these features seem to develop early (Castelain, Bernard, Van der Henst, & Mercier, in press;[START_REF] Mercier | Reasoning serves argumentation in children[END_REF].

Although research into cultural variation can yield precious insights about cognition, it is possible that cross-cultural psychology is overly geared towards differences rather than similarities. Focusing on potential universals is also of tremendous theoretical importance, in particular for testing adaptive hypotheses. We hope the present research can be another step in this direction (for another recent example using the same population, see [START_REF] Fontanari | Probabilistic cognition in two indigenous Mayan groups[END_REF].

Fig. 1 .

 1 Fig. 1. (A) Illustration of the four tasks. (B) Box plots representing the normalized scores for the groups in Study 1 (Tasks 1, 2, 3) and Study 2 (Tasks 2, 4). The groups are the real discussion groups in the test and the nominal groups comprising the same participants in the pre-and post-tests. The boxes represent the middle half of each sample.

  , see Supplemental Material, Results for details). Comparing the individual scores, in both studies we observed a significant improvement in performance between the pre-test and the post-test (Study 1: Mdn Pre-test = 2, Mdn Post-test = 3, Z = 4.79, p < .001, r = .44; Study 2: Mdn Pre- test = 0, Mdn Post-test = 2, Z = 5.33, p < .001, r = .43). The pre-test / post-test improvement was also significant when we counted as correct only correct answers accompanied by a correct explanation, e.g. "because the objects have the same size" (Study 1: Mdn Pre-test = 2, Mdn Post-test = 3, Z = 5.04, p < .001, r = .45; Study 2: Mdn Pre-test = 0, Mdn Post-test = 2, Z = 4.94, p < .001, r = .36). Regarding group scores, in both studies we observed a significant improvement in performance between the pre-test and the test (Study 1: Mdn Pre-test = 6, Mdn Test = 9, Z = 3.64, p < .001, r = .67; Study 2: Mdn Pre-test = 2, Mdn Test = 6, Z = 3.80, p < .001, r = .53) but not between the test and the post-test (Study 1: Mdn Test = 9 Mdn Post-test = 8, Z = 1.47, p = .14; Study 2: Mdn Test = 6, Mdn Post-test = 5, Z = .064 p = .95). Thus, group discussion significantly improved the reasoning performance of preliterate individuals.

  represents the effects of correct and incorrect arguments on the outcome of group discussion for the Task 4 of Study 2.

Fig. 2 .

 2 Fig. 2. Details of the performance for Task 4 in Study 2. For the pre-test and the post-test, each disk represents a nominal group, and the proportion of red (resp. green) in the disks represents the proportion of incorrect (resp. correct) answers. For the test, each disk represents a group and the color represents the outcome of the discussion. A red (resp. green) surrounding circle indicates that an argument for the wrong answer (resp. the correct argument for the correct answer) was uttered at least once during the discussion.

Acknowledgments: This research was carried out thanks to an Ambizione grant (PZ00P1_142388 / 1) from the Swiss National Science Foundation to H.M. We thank all the participants of the study, the CONALFA association and their teachers from the Sololá department, Francisca Ixquiactap Lopez for the logistic, and María Guarchaj and Catalina Sac Tambriz, the two experimenters who conducted the studies.

Data availability: All the data are available at this url: https://sites.google.com/site/hugomercier/Data%20Evidence%20for%20core%20features%20 of%20reasoning.zip?attredirects=0

assess the understanding of the invariance of physical quantities across physical transformations [START_REF] Dasen | Cross-cultural Piagetian research: A summary[END_REF][START_REF] Piaget | The Child's Construction of Quantities[END_REF], and discussion has been shown to improve performance in WEIRD cultures [START_REF] Doise | The Social Development of the Intellect[END_REF][START_REF] Perret-Clermont | Social Interaction and Cognitive Development in Children[END_REF].

We used weight/volume conservation tasks, in which participants must indicate how the water level in two glasses changes when one object is introduced in each glass (Figure 1A). The objects differ in some irrelevant dimension(s), such as weight or shape, but not volume; hence the correct answer is that the water rises as much in the two glasses. All the participants first completed two familiarization trials (whose presentation order was counterbalanced): In both tasks, the experimenter provided participants with feedback (see Figure S1; no feedback was provided at any other point of the experiment). The participants then completed a series of three (Study 1) or two (Study 2) volume conservation tasks on their own (pre-test). Groups of three (Study 1, N = 19, Study 2, N = 25) or four (Study 2, N = 2) individuals were then formed randomly. The groups were given the instruction to arrive at a consensual answer for each task (test). Finally, each participant had to solve the task on her own again (post-test) (see Supplemental Material, Figure S1). In Study 1, a fourth, new task was presented to the participants in the post-test, in order to test for transfer effects (Task 4).

The tasks used in Study 1 (Tasks 1, 2, and 3 at all phases, Task 4 as a transfer task) were chosen based on results obtained by one of the authors (F.J.) with another traditional population (in Papua New Guinea), in which participants only had to complete the tasks on their own. We chose tasks for which there were no ceiling or floor effects in this other traditional population. The tasks used in Study 2 were based on the results of Study 1. We chose Task 2 because it elicited a lower rate of performance than Tasks 1 and 3 at the pre-test of Study 1. Task 4 provided a baseline to evaluate the performance in Study 1, in which it was used as a transfer task.

In each phase, the tasks were always presented in the same order. The positions of the