Delphine Poquet

Emilie Ginon

Baptiste Goubel

Claire Chabanet

Stephan S Marette

Sylvie Issanchou

Sandrine Monnery Patris

Sandrine Monnery-Patris
email: sandrine.monnery-patris@inra.fr

Impact of a front-of-pack nutritional traffic-light label on the nutritional quality and the hedonic value of mid-afternoon snacks chosen by mother-child dyads

Keywords: Nutri-Score label, mid-afternoon snack, food choices, hedonic cost, healthiness, intervention

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The French National Nutrition and Health Program (PNNS) recommends a mid-afternoon snack for children to help them have energy throughout the day and diversify their diet [START_REF] Francou | Le goûter en perte de vitesse et loin des recommandations[END_REF]. In France, the mid-afternoon snack is a common practice among children [START_REF] Francou | Le goûter en perte de vitesse et loin des recommandations[END_REF] but also among their mothers. Mothers with a child in the household have been found to consume snacks with a higher energy density and a lower nutrient density than women without a child in household (Si Hassen et al., 2018). This eating occasion, which represents 14% of total daily energy intake in French children up to 10 years of age, is usually characterized by the consumption of energydense and fatty, sweet foods (Anses, 2017). These survey results on the nutritional composition of children's mid-afternoon snacks are in line with those obtained by sociologists. In social representations, this eating occasion remains resolutely associated with the universe of sweetness, pleasure of eating and gluttony [START_REF] Comoretto | Le goûter de 16h30 comme symbole du patrimoine alimentaire enfantin ? Analyse des transactions non marchandes dans deux cours de récréation (France)[END_REF][START_REF] Tibère | Le goûter résiste à la nutritionnalisation[END_REF]. In this context, it appears relevant to identify ways to orient mid-afternoon snack choices towards beverages and food items with good nutritional quality for mother-child dyads.

One way to increase healthy food choices is to deliver information about the nutritional quality of a food product. Providing nutrition information via front-of-pack labels improves consumers' awareness of the healthiness of food products [START_REF] Campos | Nutrition labels on pre-packaged foods: a systematic review[END_REF][START_REF] Cowburn | Consumer understanding and use of nutrition labelling: a systematic review[END_REF][START_REF] Grunert | A review of European research on consumer response to nutrition information on food labels[END_REF][START_REF] Hawley | The science on front-of-package food labels[END_REF][START_REF] Hersey | Effects of front-of-package and shelf nutrition labeling systems on consumers[END_REF].

There are two main types of front-of-pack labelling systems: nutrient-specific systems, in which the product is characterized in terms of specific nutrients (sugar, fat, saturated fats, salt and energy), and summary systems, which provide a global evaluation of the nutritional quality of the food [START_REF] Hersey | Effects of front-of-package and shelf nutrition labeling systems on consumers[END_REF].

In France, the principle of front-of-pack nutritional labelling was planned in the 2011-2015 National Programme for Nutrition and Health (PNNS) in line with European Regulation 1169/2011 and with the WHO recommendation of promoting consumer-friendly labelling [START_REF] Who | European Food and Nutrition Action Plan 2015-2020[END_REF]. Two experiments were launched in 2016 to compare the efficiency of five different formats of front-of-pack nutritional labels, one in 60 supermarkets (Allais, Albuquerque, Bonnet, & Dubois, 2017; Ministère des Affaires Sociales et de la Santé, 2017) and one in an experimental food store [START_REF] Crosetto | Modification des achats alimentaires en réponse à cinq logos nutritionnels[END_REF].

Among the five different formats, we decided to test the impact of the Nutri-Score, a 5-colour synthetic labelling system, on the nutritional quality of mid-afternoon snacks based on the results of previous experiments. Research found that a five-colour nutrition label was perceived as the easiest label to identify and as the label requiring the lowest amount of effort and time to understand [START_REF] Ducrot | Effectiveness of front-of-pack nutrition labels in French adults: results from the NutriNet-Santé cohort study[END_REF]. Moreover, a study on the impact of different front-of-pack nutrition labels on consumer purchasing intentions showed that in a virtual web-based supermarket, the five-colour nutrition label was associated with a significantly higher nutritional quality of shopping cart items compared with a control condition without any front-of-pack label [START_REF] Ducrot | Impact of different front-of-pack nutrition labels on consumer purchasing intentions: a randomized controlled trial[END_REF]. Finally, in an experimental supermarket, the five-colour nutrition label, which was associated with communication about the logo, led to a significant improvement in the nutritional quality of purchased items for the sweet biscuits category, one of the three food categories that were tested, compared to the control condition without a label or communication [START_REF] Julia | Impact of the front-of-pack 5-colour nutrition label (5-CNL) on the nutritional quality of purchases: an experimental study[END_REF]. To the best of our knowledge, no study has investigated the impact of the Nutri-Score logo on the nutritional quality of mid-afternoon snack choices in motherchild dyads.

The present research was conducted within mother-child dyads because food purchases are still mostly made by mothers in French households [START_REF] Mathé | Comment consomment les hommes et les femmes ?[END_REF] and because it appears that mothers take their children's desires into account when offering them foods for their mid-afternoon snack [START_REF] Tibère | Le goûter résiste à la nutritionnalisation[END_REF]. Some studies have reported the effect of nutritional information [START_REF] Bannon | Impact of nutrition messages on children's food choice: Pilot study[END_REF][START_REF] Gonçalves | The impact of exposure to cartoons promoting healthy eating on children's food preferences and choices[END_REF][START_REF] Miller | Children's use of on-package nutritional claim information[END_REF]) on children's food choices, but only a few studies have investigated the impact of front-of-pack labels on school-age children's food choices [START_REF] Graham | Impact of explained v. unexplained front-of-package nutrition labels on parent and child food choices: a randomized trial[END_REF][START_REF] Privitera | Emolabeling increases healthy food choices among grade school children in a structured grocery aisle setting[END_REF]. These studies produced divergent results, which could be partly due to the different label formats and different settings they used. However, in all these studies, the choices were only declarative; in other words, the chosen food items were not consumed [START_REF] Graham | Impact of explained v. unexplained front-of-package nutrition labels on parent and child food choices: a randomized trial[END_REF][START_REF] Privitera | Emolabeling increases healthy food choices among grade school children in a structured grocery aisle setting[END_REF]. Declarative methods have been described to bias participants toward the choice of the more socially desirable option [START_REF] Camerer | The effects of financial incentives in experiments: a review and capital-labor-production framework[END_REF]. In this context, the aim of the present study was to assess the impact of the nutritional label Nutri-Score on the nutritional quality of mid-afternoon snack choices in mother-child dyads while limiting the effect of social desirability by using a protocol that strengthened participant involvement. We hypothesized that the nutritional quality of mothers' and children's choices would be higher after labelling. We also studied the effect of socio-demographic characteristics on the potential change in the nutritional quality of chosen snacks since it has been shown that such characteristics, particularly level of education and income, have an impact on the understanding and use of food labels [START_REF] Campos | Nutrition labels on pre-packaged foods: a systematic review[END_REF][START_REF] Cowburn | Consumer understanding and use of nutrition labelling: a systematic review[END_REF]. In the literature, it is well established that healthier diets cost more than unhealthy diets. Lower-quality diets, with a higher content of added sugars and fats, are generally less expensive on a per calorie basis [START_REF] Darmon | Contribution of food prices and diet cost to socioeconomic disparities in diet quality and health : a systematic review and analysis[END_REF][START_REF] Darmon | In foods, energy is cheap where it is abundant and expensive where it is scarce: this is a fact, not an artifact[END_REF]. Since price could hinder the adoption of healthy food choices among a population with a low socio-economic level, we studied the budget for snacks chosen before and after labelling.

Research has shown that delivering information about the health benefits of a food product could lead to a counterproductive effect, particularly in children. A study showed that presenting food as instrumental in achieving a goal, for example, outlining the health benefits of consuming a food product, reduced perceived tastiness and decreased consumption in pre-school children [START_REF] Maimaran | If it's useful and you know it, do you eat? Preschoolers refrain from instrumental food[END_REF]. Another study showed that children rated a "healthy labelled" drink as less pleasant than the same drink presented without such a label [START_REF] Wardle | An experimental investigation of the influence of health information on children's taste preferences[END_REF]. Since focusing attention towards healthiness of a food can decrease its liking and consumption, the second main goal of our study was to assess the potential hedonic cost associated with a change in favour of healthier snack choices.

Material and method

General design

The experiment was run in Dijon, Burgundy, France, from June-July 2017, before the market introduction of food products labelled with the Nutri-Score. (The French government signed a decree announcing the voluntary adoption of the Nutri-Score front-of-pack nutritional labelling in October 2017.) The general design is presented in Fig. 1 and summarized afterwards. During the sessions conducted in the laboratory, participants were asked to choose one beverage and two food items for a mid-afternoon snack, first for themselves and then for the other dyad member (step 1). Then, participants completed a questionnaire in which they rated their liking and perceived healthiness of all items, first for themselves then for the other dyad member (step 2). An explanation of the Nutri-Score label was then provided via a video (step 3). Mothers and children were again asked to choose one beverage and two food items first for themselves and then for the other dyad member among the same set of products, which were now labelled with the Nutri-Score label (step 4). Finally, participants completed a questionnaire in which they rated their perceived healthiness for all items, first for themselves then for the other dyad member (step 5). At the beginning of the experiment, participants were informed that one of the four chosen snacks -the two selected by the participants for themselves and the two selected by the other member of the dyad for this participant -would be randomly selected for their consumption on site at the end of the session.

Products

Preliminary tests (qualitative survey and pre-tests) were conducted with children who did not participate in the main experiment in order to establish a possible list of products for our study. Then, a Nutri-Score was calculated for all these products (see 2.2.3). A final list was then established.

Qualitative survey

First, from January to March 2017, twenty-two semi-structured interviews were carried out to identify the mid-afternoon snack habits of mothers and children aged 9 to 11 years. These interviews were conducted at home with twenty-two mothers residing in Dijon and its suburbs. Second, observations were performed during extracurricular time in four schools located in Dijon and its suburbs to determine the kinds of beverages and food items that were distributed by leisure centres to children aged 9 to 11 years for their mid-afternoon snack. Following this qualitative study, a list of eight beverages and eighteen food items (not shown), representative of mid-afternoon snack habits, was established to formulate pre-tests.

Pre-tests

In April 2017, pre-tests were conducted to evaluate children's liking and perceived healthiness of the eight beverages and eighteen food items. These pre-tests were conducted in three leisure centres located in Dijon and its suburbs with twenty-nine children aged 8 to 11 years. During the face-to face interviews, children were presented the eight beverages and eighteen food items one at time and asked to rate their liking by answering the question "How much do you like this beverage/food?" using smiley face items and to rate their perceived healthiness by answering the question "How healthy is this beverage/food? using a thumb scale [START_REF] Marty | When do healthiness and liking drive children's food choices? The influence of social context and weight status[END_REF]. This pre-test phase allowed us to compare the beverage and food items in terms of perceived healthiness and liking by children.

Nutri-Score calculation

The nutritional score based on the 5-C system was calculated using the data available on the products' nutrition labels regarding their contents in terms of energy density, five nutrients and fruits and vegetables (Anses, 2016). According to the final score, a letter from A to E was assigned to each product. Each letter was written on a colour background, with green for A and red for E. The five colours appear on the label with a magnifying glass placed over the colour and letter assigned to the product (Fig. 2).

Final list of products

The final choices of products to include in the experiment were made based on the pre-test results. The products were contrasted in terms of their nutritional quality. All products were available in packaging with individual portions. Table 1 shows the list of 6 beverages and 9 food items used in the present study along with their Nutri-Score. During the laboratory experiment, the banana was labelled even though the Nutri-Score is intended to label only manufactured items.

Table 1. Beverages and food items and their

Participants

Ninety-five mother-child dyads participated in this study. They were recruited through our internal database (Chemosens Platform's PanelSens, CNIL no. 1148039), through leaflets distributed at the exits of schools located in Dijon, and by a recruitment agency. The inclusion criterion for mothers was to have an afternoon snack at least once to twice a month. The inclusion criterion for children was grade level (3 rd , 4 th or 5 th grade). Participants who indicated that they had a food allergy or a chronic health disease were excluded. At the beginning of the experiment, mothers and children signed a consent form to indicate their agreement to participate in the study. A consent form was also obtained from mothers for the participation of their child. The research was approved by the Ethics Evaluation Committee of Inserm (IRB00003888). At the end of the experiment, mothers received a €20 voucher to thank them for their participation. The mean age of the participating children was 9.38 years (range=7-11 years). Table 2 shows the socio-demographic characteristics of the participants.

Experimental procedure

The procedure was based on a protocol previously used with mothers and children in the same age range [START_REF] Marette | Are children more paternalistic than their mothers when choosing snacks[END_REF]. During the experiment, mothers and children were placed in front of a wall. Moreover, to avoid oral and visual interactions between mothers and their children they sat back to back with one another and screens were installed between them at the centre of the room. Thus, participants' choices were not made in front of the experimenter.

To ensure anonymity, each participant was identified by a code. Once participants were seated, the experimenter provided instructions. At the beginning of the experiment, participants were advised that one of the four chosen snacks (the two selected by the participant and the two selected by the other member of the dyad for this participant) would be randomly selected for their consumption.

Participants' snack choices for themselves and for the other dyad member among products

without the Nutri-Score label (step 1, Fig. 1)

Two boxes, one containing six beverages and another containing nine food items, were distributed to each participant (Fig. 3). Mothers and children were asked to choose one beverage and two food items for their own mid-afternoon snack. The three selected products were placed in the first bag, and once filled, the bags were taken away by the experimenters. Boxes containing beverages and those containing food items were replenished with the missing products. Next, mothers and children were asked to choose one beverage (among six) and two food items (among nine) for a mid-afternoon snack for the other dyad member. After this second choice, full bags and boxes were taken away. The experimenters distributed a questionnaire to each participant. First, mothers and children were asked to guess the three products (one beverage and two food items) that the other dyad member had chosen for them (results not shown). Then, they rated their liking of all items, first for themselves ("How much do you like this food?") and then for the other dyad member ("How much do you think your mom/child likes this food?"). Responses were given on a 5-point scale labelled at the left anchor with "I don't like it at all" and at the right anchor with "I like it very much" (Fig. 4. A) and coded from 1 to 5 for the analyses. Lastly, they rated their perceived healthiness of all items, first for themselves ("How healthy do you think this food is for you?") and then for the other dyad member ("How healthy do you think this food is for your mom/child?"). Responses were given on a 5-point scale labelled at the left anchor with "It is not healthy at all" and at the right anchor with "It is very healthy" (Fig. 4. B)

and coded from 1 to 5 for the analyses. The scales were based on the scales used by [START_REF] Marty | When do healthiness and liking drive children's food choices? The influence of social context and weight status[END_REF]. While participants completed the questionnaires, the experimenters recorded the contents of the bags. 2.4.3. Nutri-Score information presentation (step 3, Fig. 1)

Since the experiment took place before the Nutri-Score label was actually used in the market, it was necessary to provide some information about this labelling system. To limit a potential desirability bias, information about the Nutri-Score label was not read by an experimenter but was provided via a short video. The message, delivered by a voice-over, was "The Nutri-Score logo -what is it? This label guides us to choose healthy foods: low in fat, low in sugar, low in salt and high in fibres and vitamins. Food products with the letter 'A' or 'B' are healthy. Food products with the letter 'C' are neither healthy nor unhealthy. Food products with the letter 'D' or 'E' are unhealthy. In summary, the greener the letters are, the healthier the foods are, and the redder the letters are, the unhealthier the foods are".

Participants' snack choices for themselves and for the other dyad member among products with

the Nutri-Score label (step 4, Fig. 1)

After this information was provided, new boxes with products that were the same as those used in step 1 but were now labelled with the Nutri-Score were distributed to each participant. Fig. 5 represents an example of one of the nine food items without and with the Nutri-Score label. The procedure was the same as in step 1. A second questionnaire was distributed to each participant. Mothers and children were asked to guess the three products (one beverage and two food items) chosen for them by the other dyad member (results not shown). Then, they rated their perceived healthiness of all items (results not shown), first for themselves ("How healthy do you think this food is for you?") and then for the other dyad member ("How healthy do you think this food is for your mom/child?"). Then, mothers answered questions on their socio-demographic characteristics, and children answered questions on their mother's feeding practices regarding restrictions and pressure to eat [START_REF] Monnery-Patris | Parental practices perceived by children using a French version of the Kids' Child Feeding Questionnaire[END_REF]. As participants completed the questionnaires, the experimenters recorded the contents of the bags.

2.4.6. Drawing a snack and end of the experiment (steps 6 and 7, Fig. 1)

The four bags (the two chosen by participants for themselves and the two chosen for participants by the other dyad member) were shown to each participant. Mothers and children were asked to indicate which bag they wanted to have (results not shown). Then, one of the four snacks was randomly selected, and participants were asked to indicate their satisfaction with the snack (results not shown) before consuming it in another room that was especially designed for a social eating occasion. Before leaving, they were asked to note their impression in a guestbook.

Statistical analyses

Statistical analyses were performed with R software for Windows version 3.4.2.

Liking and perceived healthiness ratings

Friedman tests were carried out to compare the medians of the liking and perceived healthiness ratings issued by participants for themselves and for the other dyad member for beverages and for food items.

To complete these analyses, multiple comparison tests were performed based on pairwise comparisons using the Wilcoxon signed rank test with Bonferroni adjustment.

The slopes of the individual regressions of the perceived healthiness ratings on the Nutri-Score values,

given by the children and the mothers for themselves, were calculated separately for the beverages and the food items. Then, we tested whether the mean of the individual slopes was different from 0. The same process was performed for the liking ratings.

Nutritional quality of choices

For each chosen snack, the nutritional quality was evaluated by calculating a score, called the "nutritional score", according to the Nutri-Score of the chosen products. Five points were assigned to a product with a Nutri-Score of "A", four points to a product with a Nutri-Score of "B", three points to a product with a Nutri-Score of "C", two points to a product with a Nutri-Score of "D" and one point to a product with a Nutri-Score of "E". Thus, the nutritional score could range from 3 to 15. A bilateral Wilcoxon test for paired samples was used to compare the medians of the nutritional scores of the snacks chosen by children and mothers for themselves and for the other dyad member before and after receiving information about the Nutri-Score system and product labelling.

Hedonic scores of choices

For each chosen snack, a hedonic score was calculated according to the ratings, coded from 1 to 5, given in the first questionnaire for each chosen product (step 2, Fig. 1, i.e., before nutritional labelling). Thus, these scores could range from 3 to 15 for the snacks. A bilateral Wilcoxon test for paired samples was used to compare the medians of the hedonic scores of the chosen snacks before and after information was provided on the Nutri-Score system and product labelling. These comparisons were conducted on the snacks chosen by the children and the mothers for themselves.

Two Kendall tau correlation tests were carried out to evaluate the link between the change in nutritional scores and the change in hedonic scores given by mothers and children.

Effect of socio-demographic characteristics on the change in the nutritional quality of product choices

Two multiple linear regression models were carried out to explain the difference in the nutritional quality of the chosen snacks (by children for themselves and by mothers for themselves) before and after the presence of nutritional labelling by the socio-demographic characteristics of children and mothers.

Budget for choices

The budget for the chosen snacks was estimated according to the purchase prices of the products at the time of experiment (Table 1). The median budgets for the snacks chosen before and after the provision of information about the Nutri-Score system and product labelling were compared with a bilateral Wilcoxon test for paired samples. These comparisons were conducted on the snacks chosen by the children and the mothers for themselves and for the other dyad member.

Results

First, in this section, we describe the liking and perceived healthiness ratings for the different beverages and food items provided to children and mothers to make their snack choices. Second, we present the results related to our main objectives, i.e., the nutritional and hedonic scores of choices before and after the provision of nutritional information. Finally, we present the effect of sociodemographic characteristics on the change in the nutritional quality of choices and then the estimated budget for the different choices. Following the mislabelling of the vanilla cream (as noted in table 1), statistical analyses were also conducted without considering the data for children and mothers who have chosen this item. Since the conclusions obtained with these analyses were identical to the conclusions obtained with all data, the results without considering the data for children and mothers who have chosen the vanilla cream are presented only in Supplementary Fig. 1 and Fig. 2.

Liking and perceived healthiness ratings for beverages and food items

Our aim was to propose beverages and food items that were relatively well liked but contrasted in terms of perceived healthiness. Thus, the distributions of liking and perceived healthiness ratings given to beverages and food items by participants for themselves and for the other dyad member were examined and are presented in Supplementary Fig. 3. Friedman tests indicated significant differences between products (beverages and food items) in terms of the liking and perceived healthiness ratings given by children and mothers for themselves and for the other dyad member (all P < 0.001).

The results showed that water was the beverage most liked by children and mothers, with a median equal to 5. The beverage least liked by children was sparkling water, with a median equal to 2, and the beverage least liked by mothers was the chocolate dairy drink Candy'Up, with a median equal to 2.

Among food items, the chocolate bar Kinder Bueno was product the most liked by children, with a median equal to 5. The chocolate bar Kinder Bueno was also one of the products most liked by mothers. The least liked food item was vanilla cream, with a median equal to 3 for children and 2 for mothers. Despite significant differences, all medians were equal to or higher than 3 for five out of six beverages for children and mothers, for all food items for children, and for eight out of nine food items for mothers.

Water was significantly perceived by children and mothers as the best for their health, with a median equal to 5, while tea peach was considered by mothers as the worst for their health, with a median equal to 2. Surprisingly, the health value of sparkling water was underestimated by children and mothers compared with the Nutri-Score value. This misperception may be attributed to the fact that sparkling water is a fizzy drink. Among food items, the banana was significantly perceived to the best for their health, with a median equal to 5, whereas the chocolate bar Kinder Bueno had a median equal to 2.

The results indicate that the means of the individual slopes of the regressions of the liking ratings on the Nutri-Score values for beverages were significantly negative for children but significantly positive for mothers (Table 3). The same result was found for food items for children and mothers. The means of the individual slopes of the regressions of the perceived healthiness ratings on the Nutri-Score values for beverages and food items were significantly positive for children and mothers. Thus, the perceived healthiness ratings were higher for higher Nutri-Scores. This shows that children and mothers had a good perception of the nutritional value of the beverages and food items. The lower mean slope observed for beverages in children was due to the underestimation of the healthiness of sparkling water.

Nutritional scores of children's and mothers' chosen snacks

The percentages of each beverage and food item chosen by children and mothers before and after labelling are presented in Supplementary Tables 1 and2.

As shown in Fig. 6, children's and mothers' choices for themselves and for the other dyad member were significantly more oriented towards products with good nutritional quality after labelling than before labelling (P < 0.0001), which is in accordance with our hypothesis. Moreover, the increase in nutritional quality of snacks that participants chose for themselves was higher (P < 0.05) among children than among mothers (Fig. 7). More precisely, the median nutritional quality of choices participants made for themselves increased by 4 points among children and 2 points among mothers.

This result could be partly attributed to the fact that the nutritional quality of choices made by children for themselves was initially lower than the nutritional quality of choices made by mothers for themselves (P < 0.0001). The median nutritional quality of choices made by children for their mother and by mothers for their child increased by 4 points. Moreover, as shown in Fig. 7, the nutritional score of participants' choices for themselves increased for more than 75% of participants. * P < 0.05, bilateral Wilcoxon test for paired samples.

Hedonic scores of children's and mothers' choices

As shown in Fig. 8, the hedonic scores were very high before and after labelling, but the snacks children chose for themselves were significantly (P < 0.001) less liked after labelling than before labelling. The same result was observed for mothers (P = 0.004), and as shown in Fig. 9, no difference was found between the change in hedonic score of snacks chosen by children compared with mothers (P = 0.35). Moreover, a decrease in the hedonic score was observed for 75% of participants. Negative associations between the change in the nutritional score and the change in the hedonic score of chosen snacks were found. These associations were significant for mothers' choices for themselves (τ = -0.19, P = 0.02) but did not reach significance for children's choices for themselves (τ = -0.15, P = 0.052).

Effect of socio-demographic characteristics on the change in the nutritional quality of chosen snacks

Table 4 presents the results of two regressions conducted to estimate the effect of socio-demographic characteristics on the change in the nutritional quality of the chosen snacks. The only significant effect was the mother's age, which had a negative effect on the change in nutritional quality. This result could be partly attributed to the fact that the nutritional quality of choices made by the youngest mothers for themselves was initially lower than the nutritional quality of choices made by the older mothers for themselves (result not shown). The value in bold is statistically significant with P < 0.05

Estimated budget for children's and mothers' chosen snacks

As shown in Fig. 10, there was no significant difference between the budget for snacks chosen by children for themselves (P = 0.50) and snacks chosen by mothers for their child (P = 0.11) after labelling compared with before labelling. In contrast, the budget for snacks chosen by mothers for themselves and by children for their mother were significantly lower after labelling than before labelling (P = 0.01)More precisely, the median budget for choices made by mothers for themselves decreased by € 0.11, and for choices made by children for their mothers from € 0.15..

Discussion

Our results showed that the nutritional quality of snacks chosen by participants for themselves and for the other dyad member was significantly better when products were labelled with the Nutri-Score, which was in accordance with our hypothesis. These findings are in line with those from a recent study showing an improvement in the nutritional quality of shopping cart items in a condition in which food products were labelled with the 5-colour nutrition label compared with a control condition without product labelling [START_REF] Ducrot | Impact of different front-of-pack nutrition labels on consumer purchasing intentions: a randomized controlled trial[END_REF]. Our findings are also consistent with those from a study that was conducted in an experimental food store and found an improvement in the nutritional quality of food products chosen by adults when products were labelled with the Nutri-Score than when three other nutrition labelling systems were used [START_REF] Crosetto | Modification des achats alimentaires en réponse à cinq logos nutritionnels[END_REF]. The efficacy of the Nutri-Score label in terms of improvement in the nutritional quality of chosen snacks could be due to this label's summary and prescriptive format [START_REF] Crosetto | Modification des achats alimentaires en réponse à cinq logos nutritionnels[END_REF][START_REF] Crosetto | Réponses des consommateurs à trois systèmes d'étiquetage nutritionnel face avant[END_REF][START_REF] Hersey | Effects of front-of-package and shelf nutrition labeling systems on consumers[END_REF]. A simple label that provides an overall evaluation of the nutritional value of a food product seems to be more effective [START_REF] Hersey | Effects of front-of-package and shelf nutrition labeling systems on consumers[END_REF] and easier to understand and to interpret [START_REF] Muller | Modification des achats en réponse à l'apposition de différents logos d'évaluation nutritionnelle sur la face avant des emballages[END_REF]) than a label with a nutrient-specific format, i.e., a label that provides information about the content of different specific nutrients. The efficacy of the Nutri-Score label could also be linked with its colours, which draw consumers' attention and resemble traffic-light signals. Red colour is typically associated with avoidance and danger [START_REF] Elliot | Color and psychological functioning[END_REF], and French teachers usually use red pencil to underline students' errors, both of which increase the alarm connotation of this colour. The representations associated with red colour are thus well identified by school-aged children, which could explain why the Nutri-Score label seems to be particularly effective in children. Indeed, the results of our study showed that the difference between the nutritional quality of chosen snacks before and after labelling was more significant for the snacks chosen by children for themselves than for the snacks chosen by mothers for themselves. These findings are in accordance with those from a study that showed that when choosing snacks for themselves, children reacted more to health-based information than their mother [START_REF] Marette | Are children more paternalistic than their mothers when choosing snacks[END_REF]. Interestingly, our results highlight that both children and mothers have accurate knowledge of the nutritional value of the targeted food products.

In fact, the mean of the individual slopes of the regressions of the perceived healthiness ratings, given before information was provided, on the Nutri-Scores values for beverages and food items was significantly positive for children and mothers. Children's ability to correctly classify foods in the categories of "good" or "bad" for health was previously demonstrated [START_REF] Nguyen | Children's evaluative categories and inductive inferences within the domain of food[END_REF]. Moreover, a cognitive processing model posits that consumers with prior nutrition knowledge pay attention to information on a food label, are able to understand this information, and store this information to apply it to a food related-decision [START_REF] Soederberg | The Effects of Nutrition Knowledge on Food Label Use: A Review of the Literature[END_REF]. Thus, one could argue that the significant effect of the Nutri-Score labels is due to the activation of a previous nutritional knowledge by the presence of the Nutri-Score which acts as a nudge leading to healthier food choices.

Both mothers and children had a significantly higher liking for the snacks before they were labelled with the Nutri-Score than after they were labelled. Moreover, a significant negative association between the change in the nutritional score and the change in the hedonic score of chosen snacks was found in mothers and a tendency was observed in children. This means that when the Nutri-Score of the chosen snack increased, its liking scores decreased. This hedonic cost was observed despite the fact that several products with good nutritional quality were as well liked as products with poor nutritional quality (see Supplementary Fig. 3). Since liking is a strong driver of food choices, especially in children [START_REF] Nguyen | Predictors of children's food selection: The role of children's perceptions of the health and taste of foods[END_REF], the hedonic cost associated with a change in favour of healthier snack choices after labelling raises the question of the sustainability of such a behavioural change. A recent study [START_REF] Marty | Do hedonicversus nutrition-based attitudes toward food predict food choices? a cross-sectional study of 6-to 11-year-olds[END_REF] highlighted that children with more hedonically based implicit and/or explicit attitudes towards food were more likely to choose healthy food options from a buffet. Conversely, children with both implicit and explicit nutrition-based attitudes chose less healthy foods [START_REF] Marty | Do hedonicversus nutrition-based attitudes toward food predict food choices? a cross-sectional study of 6-to 11-year-olds[END_REF]. These results suggest that food preferences do not necessarily constitute a threat in the adoption of a healthy diet and underline that pleasure could constitute a lever in the implementation of interventions to encourage healthy snack choices by highlighting the attractiveness of healthy food products [START_REF] Marty | When do healthiness and liking drive children's food choices? The influence of social context and weight status[END_REF].

The result of the regressions showed that mothers' age had a negative effect on the change in the nutritional quality of snacks they chose for themselves. This result could be partly attributed to the fact that the nutritional quality of the choices made by the youngest mothers for themselves was initially lower than the nutritional quality of the choices made by older mothers for themselves. This finding is in accordance with those of a recent study showing that energy density and energy intake from daily snacks decreased with age in women (Si Hassen et al., 2018). However, this result is different from those of studies showing that people with lower levels of education are likely to have the most difficulty understanding food labels [START_REF] Campos | Nutrition labels on pre-packaged foods: a systematic review[END_REF][START_REF] Cowburn | Consumer understanding and use of nutrition labelling: a systematic review[END_REF]. This may be because more than half of the mothers of our sample had a high level of education.

In this study, we also examined the economic cost of the chosen snacks. Our results showed that the budget for snacks with a higher nutritional quality, i.e., those chosen after labelling with the Nutri-Score, was not higher. On the contrary, while the nutritional quality significantly increased, the price significantly decreased in two out of four cases, i.e., for the snacks chosen by the children for their mother and the snacks chosen by the mothers for themselves. Of course, these results were obtained with a given set of food products. Our results are consistent with those of a study showing that higher diet quality is not necessarily costlier [START_REF] Marty | Higher nutritional quality at no additional cost among low-income households: insights from food purchases of "positive deviants[END_REF]. According to the results of our study, the lower budget associated with healthy snack choices could constitute an argument to promote healthy snack choices among a population with a low socio-economic level. Thus, this budget argument could also help reduce social inequality in food accessibility.

The present study has several limitations. First, although the message was not read by an experimenter during the experiment, a social desirability bias cannot be totally excluded. However, we tried to reduce this effect since the voice of the person delivering the message that explained the nutritional labelling system was a neutral voice that did not belong to one of the experimenters. In addition, with the aim of involving mothers and children and to limit the social desirability bias, we informed participants at the beginning of the session that one of the four chosen snacks would be randomly selected for consumption on site. Nevertheless, the within-subject design used in the present experiment could have induced a purely cognitive demand effect. However, we have chosen a within-subject design because this design has greater statistical power than a between-subject design.

Moreover, since our experiment was conducted before the market introduction of food products labelled with the Nutri-Score, our purpose was to test what could happen when such a labelling would be introduced and thus a within-subject design makes sense. Additionally, our results showed that the Chocolate bar Kinder Bueno, which was the most liked product (see Supplementary Fig. 3), was still the most chosen item by children after labelling (see Supplementary Table 1). This result clearly

shows the limited impact of the social desirability bias or a relatively "weak wish" to satisfy the organizer. Our results also showed that while the vanilla cream and the drinkable strawberry yogurt were both labelled with a Nutri-Score B, the children's and mothers' choices for themselves of the vanilla cream did not increase as much as the choices for the drinkable strawberry yogurt (see Supplementary Tables 1 and2), and this is in line with the fact that the drinkable strawberry yogurt was more liked than the vanilla cream (see Supplementary Fig. 3). This indicates that the choices were mostly guided by the liking and not by the demand effect. Second, as mentioned by Crosetto et al.

(2017), the experimental approach directed participants' attention to the nutritional labelling system, which could lead to an amplification of the effects measured. Eventually, we know that a laboratory experiment is not a field experiment (directly in stores), which is a limitation of our study. Indeed, several studies showed that the proliferation of messages, the imperfect recall, the lack of attention to messages before purchasing, and the great number of purchased products affect many consumers in stores. Reaction in the laboratory experiments with focused consumers is an upper boundary regarding the possible consumers' reactions in the store. Thus, one can suppose that the effects that would be observed in a real shopping experience would be lower than the ones observed in our experiment.

Despite this limitation stemming from the artificial environment, an experiment conducted in the laboratory allows a precise observation of participants' choices. As mentioned by several authors [START_REF] Crosetto | Modification des achats alimentaires en réponse à cinq logos nutritionnels[END_REF][START_REF] Marette | Are children more paternalistic than their mothers when choosing snacks[END_REF], the experimental method constitutes a strength since an experiment conducted in the laboratory allows the isolation of participants' choices for themselves, which is particularly difficult to study in a natural field experiment.

Conclusion

The front-of-pack nutrition label "Nutri-Score" can help improve the healthiness of mid-afternoon snack choices in mother-child dyads. However, the experimental situation could have strengthened the effect of this label. Moreover, the hedonic cost associated with this change raises the question of the sustainability of such a behavioural change, since pleasure is a strong driver of food choices, particularly in children. Thus, it seems important to develop strategies to increase the pleasure of consuming healthy foods in order to avoid a potential hedonic cost. One strategy could be the promotion of healthy food products to increase their attractiveness. This study provides implications for public health policy, emphasizing the importance of reinforcing the association between healthiness and sensory pleasure.

Fig. 1 .

 1 Fig. 1. General design of the experimental procedure

Fig. 2 .

 2 Fig. 2. Nutri-Score labels ranging from A to E

  The Nutri-Score of this food item was C but was mislabelled during the experiment. The score shown during the experiment was taken into account in the calculations.

Fig. 3 .

 3 Fig. 3. Picture of two boxes with food products without labels

Fig. 4 .

 4 Fig. 4. Liking scale (A) and perceived healthiness scale (B).

Fig. 5 .

 5 Fig. 5. Picture of a food item without and with the Nutri-Score label

Fig. 6 .Fig. 7 .

 67 Fig. 6. Distributions of the nutritional scores of snacks chosen before and after labelling by participants for themselves (a, b) and for the other dyad member (c, d). For each boxplot, the bottom and top of the box indicate the 25 th and 75 th percentiles, respectively, and the line within the box indicates the median. The whiskers extend from the box as far as the data extend to a maximum distance of 1.5 × the interquartile range. Any values more extreme than this value are marked by a circle (°). *** P < 0.0001, bilateral Wilcoxon test for paired samples.

Fig. 8 .

 8 Fig. 8. Distributions of the hedonic score of snacks chosen by participants for themselves before and after labelling (a, b). ** P < 0.001, * P < 0.004, bilateral Wilcoxon test for paired samples.

Fig. 9 .

 9 Fig.9. Distribution of the variation in hedonic scores between "before labelling" and "after labelling" conditions for participants' choices for themselves.

Fig. 10 .

 10 Fig. 10. Distributions of the estimated budget for snacks chosen before and after labelling by participants for themselves (a, b) and for the other dyad member (c, d). * P < 0.01, bilateral Wilcoxon test for paired samples

Table 2 . Socio-demographic characteristics of participants (n=95 dyads)

 2

	Children's sex (%)
	Male

Table 3 . Mean slopes of the regressions of liking ratings or perceived healthiness on Nutri-Scores

 3

			Mean slopes of the regressions of
		liking ratings on Nutri-Scores	perceived healthiness ratings on Nutri-Scores
		Beverages	Food items	Beverages	Food items
	Children	-0.07 (P = 0.03)	-0.08 (P = 0.006)	0.19 (P < 0.001)	0.49 (P < 0.001)
	Mothers	0.31 (P < 0.001)	0.11 (P < 0.001)	0.42 (P < 0.001)	0.58 (P < 0.001)

Table 4 .

 4 Effect of the socio-demographic characteristics on the change of the nutritional quality of chosen snacks Dependent

variable: variation in the nutritional score

	Children's grade level (5 th grade) Mother's diploma (low)	-0.567 (0.802) Participants' choices for themselves 0.046 (0.727)	-0.485 (0.593) -0.261 (0.537)
	Mother's age	ns 0.006 (0.072)		-0.139 (0.053)
	Δ Hedonic score	-5 0 5
		-10
		Children's choices Mothers' choices
	Independent variables	Children for themselves	Mothers for themselves
	Children's sex (male)	-0.061 (0.642)		-0.423 (0.474)
	Children's grade level (3 rd grade)	1.411 (0.768)		-0.844 (0.568)

Numbers in brackets are standard errors.

Acknowledgments: This work was supported by grants from ANR PUNCH (ANR-15-CE21-0014), the Conseil Régional Bourgogne, Franche-Comté (PARI grant) and the FEDER (European Funding for Regional Economic Development. We acknowledge the contribution of F Bouillot for assisting with the recruitment of the mother-child dyads and V Feyen and E Szleper for assisting with data collection.

Declarations of interest: None.

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