Dishonesty is more affected by BMI status than by short-term changes in glucose.

There is evidence that human decision-making is affected by current body energy levels and physiological states. There is less clear evidence linking decision-making to long-term changes in energy, as those associated with obesity. We explore the link between energy, obesity and dishonesty by comparing the behaviour of obese and lean subjects when hungry or sated while playing an anonymous die-under-cup task. Participants performed the task either before or after breakfast. We find that short-term switches in energy have only a mild effect on dishonesty, as only lean females lie less when sated. By contrast, obese subjects lie more than lean subjects in both conditions, and they lie more to avoid the lowest payoff than to get the highest payoff. Our findings suggest that the observed patterns are more likely mediated by factors associated with obesity than by short term energy dynamics, and call for a better integration of the psychological, economic and biological drivers of moral behaviour.

Introduction

Food intake is fundamental in living organisms to secure survival. The main source of energy is glucose, whose metabolization from the bloodstream allows each brain region to carry out its function[1]. Energy levels, though, fluctuate considerably over the course of a day and across individuals. Decreasing energy levels through acute fasting has been shown to affect a wide range of cognitive processes, including reduction of self-control[2], increase in risk taking[3,4], and reduction in patience[5]. On the other hand, increased levels of peripheral blood glucose have been found to enhance cognitive performance in terms of memory, attention, reaction times, reasoning, and inhibitory control (for reviews, see[6]). Such results provide insights into the potential role that physiological processes and energy dynamics can play also in domains of social decision making, such as morality and ethical choices, which has been much less investigated and delivered contradictory findings. For example, hunger has been shown to reduce moral disapproval ratings for ethical violations[7], while a high-carbohydrate/protein ratio meal increased the severity of punishment in response to norm violations[8]. On the opposite, ego-depletion has been found to make fasted judges more likely to accept status quo, leading to less favourable ruling[9].

Beyond the effects of short-term changes in energy levels, there is increased evidence linking obesity, a condition characterized by excessive energy intake, with alterations of executive functions and inhibitory mechanisms[10,11]. Obese individuals tend to be more risk prone, more impulsive and discount future rewards more than lean subjects[12-15]. However, a clear understanding of the causal link between cognitive disfunctions, obesity and energy homeostasis is still lacking, making it difficult to pinpoint the exact role of energy dynamics in affecting cognition. For example, research has shown that differences in self-control are predictive of weight gain in children[16], supporting the view that behavioural differences are responsible for the emergence of obesity. Furthermore, several physiological processes typically associated with obesity (e.g., insulin resistance) have direct effects on body energy regulation (e.g., glucose homeostasis, fat deposition) but also on behaviour and cognition (e.g., memory)[17,18]. At the same time, an emerging literature suggests that obesity is not causal to insulin resistance and type-2 diabetes, but the behavioural factors that cause obesity and those that cause insulin resistance are largely overlapping[19]. Taken together, the above findings suggest that although human decision making might be sensible to transient fluctuations in body energy levels, behavioural variations associated with long-term energy imbalances (e.g., obesity) may not necessarily be explained only through an energy dynamic perspective.

This study aims to better understand if, and to what extent, energy dynamics can explain individuals’ ability to refrain from the unethical temptation to lie in order to earn more money. While small-scale dishonesty is pervasive in human societies, most individuals either do not lie, or lie but not to the maximum extent even when there is no risk of being detected, while a minority is unconditionally dishonest[20-22]. Variations in the levels of honesty depend on the way individuals respond to economic incentives, moral norms, psychological factors and on neurobiological predispositions (for reviews on lying behavior see[20,21,23]. Of particular interest is the role of self-control. Self-control is defined as a process that allows people to alter or override their impulses, thoughts, emotions, and behaviors to achieve overarching goals[24]. From a self-control perspective, engaging in unethical behavior often involves a trade-off between short-term (e.g., monetary rewards) and long-term benefits (e.g., moral self-image, reputation, and social acceptance). Several studies have shown that resisting the temptation to cheat requests self-regulatory resources[25], and that unethical behaviour is more likely under conditions of reduced self-control such as sleep deprivation[26,27] and time pressure[28], and when self-control is experimentally reduced[29,30]. Such results have often been interpreted in light of ego depletion and of the limited resource model, that views self-control as a finite resource that can become temporarily depleted by previous self-regulatory efforts (e.g.,[30-32]). An intriguing but controversial hypothesis is that glucose could be the limited resource behind self-control, as self-control acts seemingly reduce glucose levels while consuming glucose would restore the capacity for self-control[33]. Self-control has been therefore considered as a “moral muscle” (with glucose potentially acting as its biological modulator) which helps people override selfish impulses and behave morally[24]. Depletion of (self-regulatory) resources appears not only to impact active deception ( e.g.,[25,29,34]) but also to impair lie detection[35], possibly by reducing the differences in the response time between truthful and untruthful answers[36]. Despite the potential link between self-control, glucose and deception, the effects of current body energy levels on lying behaviour remain largely unexplored. Also, no study has compared the behavioural patterns of subjects differing in BMI status. By designing an experiment in which both elements interact, we try to better identify the extent to which energy dynamics can explain changes in dishonest behaviour and compare that with alternative explanations.

In our experiment, subjects played a die-under-cup task (adapted from[37]). They received a sealed cup containing a three-color die and had to privately roll the die within the cup and observe the outcome through a hole in the lid. The colour reported (blue, yellow, or red) as the outcome of the first die roll determined the payoff (0, 3 or 5 Euros, respectively) for answering to a final questionnaire. Subjects could cheat by reporting a higher outcome than the observed one without any risk of detection since the experimenter could not observe the true outcome.

To measure the impact of energy and BMI status on individuals’ propensity to cheat, metabolic levels were varied in two ways. We invited 85 lean (BMI—Body Mass Index- ≤ 25) and 65 obese (BMI ≥ 30) fasted subjects to participate in our experiment (see characteristics in Table S1 in Supplementary Information). Baseline blood glucose level and subjective hunger were measured upon arrival (Time 1). After performing an unrelated task, subjects in the “Sated” condition were provided with a standardized breakfast and after a rest, were tested again (Time 2) for blood glucose and hunger before performing the same unrelated task and then, the die task. Subjects in the “Fasted” condition underwent the same procedure except for the timing of the meal, that was provided after performing the die task. All subjects were informed from the beginning that they would receive a breakfast.

In line with theories and models on the role of physiological states on cognition and behaviour (e.g.,[38,39]) we may expect to find major variations between fasted and sated subjects, regardless of BMI status (e.g., main effect of energy status). Given previous literature linking ethical behaviour to behavioural control, we should observe more honesty under satiation. According to the literature linking obesity to impairments in self-control abilities, we may also expect obese subjects to lie more (e.g., refrain less from increasing their payoffs) than lean subjects (e.g., main effect of BMI status), and/or to show reduced response to short-term energetic fluctuations given a reduced behavioural flexibility in reaction to satiation cues[40] (e.g., interaction effect between BMI and energy status). While finding major variation according to energy status (fasted vs. sated) would bring support to the hypothesis of energy dynamics driving changes in unethical behaviour (H1), in line with theories attributing to glucose an important role in counteracting ego-depletion in deceptive behaviour, observing major variation between lean and obese subjects would point against it (H2), in line with theories supporting a neuro-hormonal-behavioural origin of obesity.

Results

Blood glucose and hunger levels

Table 1 provides blood glucose levels and hunger index scores as measured before and after breakfast manipulation in the overall sample, and in the lean and obese subsamples separately. Baseline blood glucose levels and hunger index scores measured at the beginning of the experiment do not differ between fasted and sated groups. As expected, both measures differ significantly between groups after manipulation. Results are confirmed for both females and males separately (see SI). Overall, obese subjects have a slightly but significantly higher baseline blood glucose level and a lower initial hunger index than lean subjects. Higher glucose levels in obese compared to lean subjects are also found after breakfast consumption (see Table 1), and the same pattern is found in females but less so in males (see SI). Differences in hunger weaken after breakfast consumption for both the entire sample and for males and females separately (see SI).

Table 1

Glucose and hunger levels before and after treatment manipulation.

	Glucose (mg/dl)	Hunger index
Time 1 (baseline)
Fasted group	89.57 (8.94)	6.58 (1.73)
Sated group	89.75 (10.75)	6.75 (1.52)
Test sated vs. fasted	z = 0.295 p = 0.768	z = − 0.423 p = 0.672
Lean group	86.52 (7.90)	6.98 (1.37)
Obese group	93.77 (10.67)	6.25 (1.84)
Test lean vs. obese	z = − 4.38 p < 0.001	z = 2.416 p = 0.016
Time 2
Fasted group	87.55 (9.22)	7.29 (1.65)
Sated group	127.76 (23.27)	2.16 (2.01)
Test sated vs. fasted	z = − 9.73 p < 0.001	z = 9.774 p < 0.001
Lean group	103.14 (24.94)	5.09 (3.12)
Obese group	113.55 (28.15)	4.24 (3.17)
Test lean vs. obese	z = − 2.589 p = 0.010	z = 1.812 p = 0.070

Blood glucose levels are expressed in mg/dl. Hunger index range is between 1 and 10. Fasted and Sated refer to between-group treatments. Comparisons between Sated and Fasted, Obese and Lean are two-sided Mann–Whitney rank-sum tests with each individual taken as an independent observation. Standard deviations are in parentheses.

As expected, within-subject analyses show that blood glucose level increases after breakfast consumption in the sated group (Wilcoxon signed-rank tests, Time 1 vs Time 2: all: z = 7.512 p < 0.001, lean: z 5.013 p < 0.001 obese : z = 5.627 p < 0.001 ), while it decreases in the fasted group (Wilcoxon signed-rank tests, Time 1 vs Time 2: all: z = − 2.796 p = 0.005; lean: z = − 2.038 p = 0.041; obese: z = − 1.950 p = 0.051). The results hold when repeating the analyses by gender (see SI).

A regression analysis testing the influence of breakfast manipulation and BMI status on the change in blood glucose and hunger over time, controlling for gender, confirms the effect of our breakfast time manipulation on blood glucose levels and hunger in both lean and obese subjects (see Supplementary Table S2, S3 and Supplementary Fig. S1, S2).

Effect of metabolic alterations on cheating

If no individual lies, each die outcome should be reported in 33.34% of the observations. Figure 1 displays the distribution of the relative frequency of reported outcomes. The left panel is for lean subjects and the right panel for obese subjects. Table 2 shows the corresponding average payoff in the fasted and sated group for each subsample of subjects.

Figure 1

Proportions of subjects reporting red (€5), yellow (€3), and blue (€0) outcomes. Panel (a) is for lean subjects and panel (b) for obese subjects under the Sated and Fasted conditions. The dotted line represents outcomes chance level (0.334). ***, **, and * indicate significance at the 0.1%, 1%, 5%, in binomial and chi-square tests. NS for no significance.

Table 2

Average payoffs expressed in euros for sated and fasted subjects, by BMI status and gender.

	N	Average Payoff
		All	Females	Males
Lean
Sated	42	3.14 (1.98)	2.59 (1.99)	3.75 (1.83)
Fasted	43	3.72 (1.86)	3.66 (1.88)	3.78 (1.87)
Obese
Sated	33	3.85 (1.39)	3.87 (1.29)	3.77 (1.71)
Fasted	32	4.09 (1.23)	3.91 (1.31)	4.55 (0.88)

Standard deviation in parentheses.

We do not find evidence of lying in lean sated subjects. Indeed, the distribution of reported outcomes does not differ from a uniform distribution (χ2 test: p = 0.683). The frequency of each reported outcome does not differ from what is expected by chance: blue is reported in 23.81% of the observations (binomial test, p = 0.251), yellow in 33.33% (p = 1.000) and red in 42.86% (p = 0.194). In contrast, the distribution of outcomes reported by lean subjects when fasted differs significantly from a uniform distribution (χ2 test: p = 0.001). Subjects significantly under-report the lower outcome (blue: 16.28%, binomial test p = 0.015) and over-report the higher outcome (red: 60.47%, p < 0.001), while the medium outcome is reported less than expected but not significantly so (yellow: 23.26%, p = 0.196). This suggests that breakfast consumption by lean subjects is associated with more honest behaviour. The effect of breakfast manipulation in lean subjects appears to be mainly due to females: lean females report significantly more the higher outcome when fasted compared to when sated (Fisher tests p = 0.042).

Results differ for obese subjects, who lie regardless of breakfast manipulation. As found for lean subjects, the distribution of outcomes reported by fasted obese participants remains significantly different from a uniform distribution (χ2 test: p = 0.001). Indeed, these subjects significantly under-report the lower outcome (3.12%, binomial test p < 0.001) and over-report the higher outcome (59.38%, p = 0.004), while the frequency of the medium outcome does not differ from chance (37.50%, p = 0.708). However, in contrast to lean subjects, the distribution of outcomes reported by sated obese participants is significantly different from a uniform distribution (χ2 test: p = 0.008). Accordingly, the frequencies of the lower and higher reported outcomes differ significantly from chance: blue is reported in 6.06% of the observations (p < 0.001) and red in 51.52% (p = 0.040). Yellow is reported 42.42% of the time, which does not differ from chance level (p = 0.273).

Comparing the behaviour of obese and lean subjects when fasted or sated reveals that obese subjects tend to lie more on (by reporting less often) the lower outcome than their lean counterparts when sated (two-sided Fisher test: p = 0.056), while no significant difference is found for the medium and higher outcomes. This difference between obese and lean subjects is driven by the female sub-sample (p = 0.020), while no significant difference is found for males. No significant difference is found between lean and obese subjects when fasted, suggesting that hunger reduces the heterogeneity in behavioural responses (see Supplementary Table S4 for full results).

Lying to maximize payoff or to avoid lowest payoff?

To explore the effect of energy shifts and BMI status on lying beyond aggregated data, we estimate the mean percentage of subjects that lied using the method developed by Garbarino and colleagues[41]. We distinguish two categories of lies: those motivated by payoff maximization (e.g., reporting the maximum payoff regardless of the colour observed) and those motivated by the avoidance of the lowest payoff (e.g., reporting any higher payoff when observing the lowest one). Figure 2a,b display the estimated mean percentage of liars for each category of lie for lean and obese subjects when sated and when fasted, respectively, along with 95% confidence intervals (CI, hereafter).

Figure 2

Estimated mean percentage of lies motivated either by payoff maximization or by avoidance of the lower payoff, under the Sated and Fasted Conditions. Panel (a) is for lean subjects and panel (b) for obese subjects. Bars indicate 95% confidence intervals.

Breakfast consumption in lean subjects has a stronger effect on reducing lies driven by the maximization of payoff compared to those driven by the willingness to avoid the lowest outcome. As shown in Fig. 2a, the estimated mean percentage of liars motivated by payoff maximization switches from 40% in fasted subjects to 15% in sated ones. Such difference is significant at 90% CI (lower bound in Fasted: 29.19%; upper bound in Sated: 28.52%) but not at 95% CI. For females this percentage goes from 36.25% when fasted to 5.32% when sated (95% CI lower bound in Fasted: 20.71%; upper bound in Sated: 18.57%, see Supplementary Fig. S3). The estimated percentage of lean liars motivated by the avoidance of the lowest outcome decreases more moderately with satiation, from 48.85% to 34.07%. This difference is not significant (see Fig. 2a); the same applies to females (see Supplementary Fig. S3).

Since obese people lie in the same proportion both when sated and when fasted, there is no reason to expect a change in the nature of lies following our experimental manipulation. However, it is interesting to note that the estimated percentage of obese subjects that lie to avoid the lowest outcome (80.50% when sated and 89.92% when fasted) is disproportionately and significantly higher than that of those lying to maximize payoff, regardless of the condition (see Fig. 2b). It is also significantly higher than any other category of lies of lean subjects, regardless of the condition. This suggests that compared to lean, obese subjects have a harder time to refrain from lying when the underlying motivation is avoiding a bad outcome, regardless of their current energetic state. This cannot be explained by a difference in income, as obese and lean subjects report a similar economic status (unpaired t-tests: t148 = − 1.59 p = 0.113).

The role of individual factors on lying

Finally, we explore the likelihood of reporting each die outcome (using high reports as a proxy for lying propensity) by estimating ordered probit models on the full subjects sample, and on BMI (obese, lean) and gender (female, male) subsamples separately (see Method section). This analysis reveals several main findings. First, greater inter-individual variation is found when subjects are sated. Specifically, in this condition lean subjects are less likely to lie by reporting higher vs. lower die outcomes (e.g., behave more honestly) compared to their obese counterparts (sated lean vs. sated obese, β = 0.701 p = 0.023). Such difference in behaviour disappears under fasting (fasted lean vs. fasted obese, χ2 tests, p = 0.444). Further results suggest that breakfast manipulation influences lean subjects behaviour, as their likelihood of reporting a higher outcome is higher under fasting (lean sated vs. lean fasted β = 0.688 p = 0.049) while no difference is found for obese subjects (obese sated vs. obese fasted χ2 tests, p = 0.591, see model (1) in Supplementary Table S5 for full results). When only lean subjects are taken into account, a gender gap emerges under satiation: females are less likely to report a higher outcome than males when sated (sated females vs. sated males: β = 0.782 p = 0.034). Such gender gap disappears with fasting (fasted females vs. fasted males; χ2 tests: p = 0.881), and this effect is due to a significant decrease in females’ lying behaviour under satiation (sated females vs. fasted females: β = 0.979 p = 0.039, see model (2) in Supplementary Table S5 for full results). No such difference is found in the obese subsample (model (3), Supplementary Table S5). Repeating the analyses for female and male subsamples further confirm the role of gender in driving the effects of breakfast manipulation: the significance level of results is increased in only-female sample, while the main effects are lost in the only-male sample (see model (4) and (5) in Supplementary Table S5).

Discussion

This is the first study that investigates the role of short-term energy dynamics, BMI status and their interactions on individuals’ ability to refrain from lying. We found that only a fraction of subjects (specifically, lean females) becomes more honest after consuming breakfast. Major differences in behaviour are instead found between lean and obese subjects, especially under satiation. Such results provide limited support for an effect of short term energetic shifts on moral decision making. Importantly, they reject H1 in favour of H2, as energy dynamics alone cannot explain the observed differences in unethical behaviour. These findings complement the analyses of the economic, psychological and cognitive determinants of small-scale dishonesty. A growing body of research has started to identify the psychological factors underlying unethical behaviour[25,42,43], often opposing alternative views about how the integrity of cognitive functions (in particular, self-control) affects the ability for refraining from lying. For example, it has been shown that people are more likely to lie under conditions of reduced self-control[29,30,42], while resisting the temptation both requires and depletes self-regulatory resources[25]. Similarly, sleep-deprivation and time pressure have been shown to increase the likelihood of engaging in unethical behaviour in both work-related[27] and lab settings[28]. Overall, our results fail to support the hypothesis that glucose acts as a general modulator of self-control resources underlying honest behaviour, as obese subjects cheat more despite higher blood glucose levels.

In our study, lean female subjects become more honest when sated, but males fail to do so. Increasing evidence of sex differences in the neural activity related to hunger and satiety[44,45] and in cortical areas processing food-related stimuli[46,47] supports the hypothesis that women are more sensitive to food-related cues than men and may have a greater sensitivity to humoural signals of hunger and satiation[48]. Similarly, a heightened malleability and sensitivity of women’s preferences to the context of an experiment has been suggested to explain gender differences in some economic games[49]. Given the overlap of brain areas (e.g., orbifrontal cortex) involved in processing food rewards and money rewards[50-52] and evidences showing the reciprocal association between the incentive value of food and of money[53], we suggest that a higher sensitivity of women to energetic shifts could facilitate the substitution of a primary reward (e.g., the calories provided by the food) to a secondary reward obtained at the moral cost of lying (e.g., the extra money earned by over-reporting the die outcome). Alterations of physiological state (e.g., hunger) have been suggested to modulate the emergence of gender gaps in economic behaviour[54]. In line with that, our finding can help interpret the contrasting results on gender and honesty in the past literature[55-59].

Major differences in lying behaviour emerge between obese and lean subjects especially after breakfast consumption. Such finding suggests that obesity may be associated to a reduced sensitivity to short-term energetic shifts. In support of this interpretation, it has been shown that the brain’s ability to respond to alterations in glucose metabolism becomes aberrant in both individuals predisposed to become obese (obesity prone) and those already obese and diabetic[60]. Moreover, while fluctuations in the motivational value of food are thought to contribute to the control of eating behaviour, there is evidence that such processes are impaired in individuals with obesity. For example, Castellanos and colleagues[40] show that while lean and obese have similar attentional bias to food-related cues when hungry, obese but not lean keep a high attentional bias even after eating, possibly due to a reward system dysregulation. In support of it, sensitivity to reward devaluation decreases with increasing BMI[61]. As dishonest behaviour has been linked to heightened responses in specific reward-related brain areas (e.g., nucleus accumbens[62]), obese subjects’ inability to correctly devaluate rewards in post-meal contexts may possibly contribute to explain the observed levels of dishonest behaviour.

Investigating the nature of lies can help us better characterize the motivations behind dishonest behaviour. We found that obese people’s misreporting behaviour is mainly motivated by the willingness to avoid the lower payoff in the die task. This could be related to loss aversion[63], echoing studies showing differential neural responses of obese subjects to monetary losses and to the anticipation of such losses compared to lean people[64]. If loss aversion is a permanent trait, then it might not be surprising that the estimated percentage of lies to avoid the lower payoff remains the same regardless of their metabolic state. In contrast, the willingness to maximize one’s payoff and the willingness to avoid the lowest payoff have a more similar weight in lean subjects.

Due to the correlational nature of our study, we are not able to infer causality between obesity and moral behaviour. Obesity stems from a complex interaction between behavioural, neuronal and metabolic processes and is associated (but not necessarily causally) to a dysregulation of the mechanisms governing energy homeostasis. In support of this view, recent genetic studies concluded that obesity is less metabolic and more driven by neuro-behavioural disorders[65]. From an evolutionary perspective, it has been suggested that insulin resistance, a metabolic condition often associated to obesity and type-2 diabetes, might have evolved as a socio-ecological adaptation allowing a shift from muscle-dependent to brain-dependent life strategies, and that the pathological consequences of obesity are likely to be caused by immune chronic inflammation rather than by changes in the homeostatic regulation system[66]. These studies challenge traditional views supporting the metabolic origins of obesity[67] and suggest a more intertwined role of social, hormonal and immunological factors in the emergence of obesity. Given the literature, we may postulate that the same behavioural patterns associated with obesity might be responsible for the observed variation in dishonest behaviour. Importantly, this suggests that although energy shifts might impact honesty, results cannot be explained by energy dynamics alone.

Finally, our study adds novel findings to the growing literature exploring the cognitive and economic determinants of unethical behaviour, and calls for a deeper understanding of the intertwined neurological, physiological and socio-economic factors that shape our ability to comply with moral norms.

Methods

Laboratory methods

The experiment has been conducted at GATE-Lab, CNRS, Lyon, France. A total of 182 subjects were recruited through ads in local newspapers, universities, and health care centres. Subjects were excluded from the experiment in case of severe illnesses (such as diabetes), psychological disorders, pregnancy and dieting. In total, we conducted 15 sessions. A priori power tests revealed that a reliable significant effect, with alpha level set at 0.5, power at 0.8 and effect size at 0.25, could be achieved with a minimum sample size of 136 participants. After setting exclusion and BMI criteria, the sample pool of interest consisted of a total of 150 subjects. Supplementary Table S1 reports the socio-demographic characteristics of our final subject sample pool.

All subjects were asked to arrive at the lab early in the morning (around 8.30 am) without having eaten since dinner on the day before. On arrival, they were randomly assigned to a cubicle that preserved anonymity where they filled out a consent form. Then, they moved one by one to a separate room where their baseline blood glucose level was measured (time 1), using a commercially available blood glucose self-monitoring tool-kit for diabetes control. Their height and weight were also measured to determine their BMI. Following the standard, we defined lean subjects as those having BMI ≤ 25 (Nlean = 85, 54% females) and obese those with BMI ≥ 30 (Nobese = 65, 72% females) (National Centre for Health Statistics 2000). Subjects falling between the two categories (N = 32) were disregarded as our hypotheses focus on effect of obesity on behaviour. After coming back to their seat, subjects proceeded by filling out a computerized multi-item 10-point scale questionnaire on hunger (adapted from[68]) and a socio-demographic questionnaire. In the data analysis, we computed a hunger index score by pooling subjects’ separate sub-scores.

The die task was administered at the end of a larger experimental session investigating subjects’ risk attitudes under different energy levels. Subjects were randomly assigned to two different conditions that were run in separate sessions. Participants in the “Sated” condition (Ntot = 75, 57% lean, 61% females) were provided with a standardized breakfast of about 600 cal, consisting of orange juice, detheinated tea, decaffeinated coffee and butter croissants. They also received a choice of magazines (leisure, sport, travelling, fashion, etc.) during waiting time. Blood glucose and hunger levels were measured again thirty minutes after meal provision (time 2). Then subjects returned to their cubicle, completed their other tasks; they played the die game approximately an hour after receiving their meal. Subjects in the “Fasted” condition (Ntot = 75, 56% lean, 62% females) underwent the same procedure with the exception that the same meal was provided at the end the experiment, after payment. To limit the risk that reporting behaviour of the subjects in the Sated condition could be affected by positive reciprocity toward the experimenter, subjects in the Fasted condition were also reminded at the beginning of the session that they would be served a breakfast later and they were also given a choice of magazines during the waiting time before the second measurement of glucose and hunger levels. When we collected this second set of measures, no subject in this treatment complained about not having received the meal yet.

To elicit cheating we employed an anonymous die-under-cup task adapted from[37], using a three-color six-sided die as in[69]. This paradigm is particularly useful to reveal small scale cheating that well reflects day-by-day dishonesty also in real life[69,70]. Subjects were informed that they could earn different payoffs (0, 3 or 5 Euros) depending on the outcome of their first three-color die roll (blue, yellow and red, respectively), as compensation for having answered to a previous set of questions. The die was placed inside a sealed cup allowing only the subject to see the outcome of the roll, as in[43]. Subjects were instructed to privately roll the die in their cubicle as many times as they like, but they had to report the outcome of the first and the second rolls only, by clicking the corresponding colour on their monitor. As indicated in the instructions, only the first roll generated payoff. This method guarantees to the subjects that a lie has no risk of being detected by the experimenter.

All the instructions (see SI) were displayed on the monitor screen and all questions were answered in private. The experiment was programmed using the Java language. The whole session lasted approximately 150 min. On average, subjects earned €36.70 (SD = 5.32), including a €5 participation fee; for the sole die task, they earned on average €3.67 (SD = 1.71). Earnings in cash were placed in an envelope and brought to each subject’s cubicle.

The study was performed in accordance with relevant guidelines and regulations. The study design was approved by the CEEI-IRB (Comité d’Evaluation Ethique, Institutional Review Board) of INSERM (Institut National de la Santé et de la Recherche Médicale), France (IRB 00,003,888, approval no.16–316). Informed consents were obtained from all the participants.

Statistical methods

In the die task, no individual report can be identified as truthful or not since the experimenter does not know the true outcome observed by the subject. Thus, we use three main techniques to measure cheating. First, at the aggregate level, we compare the distribution of reported outcomes against their theoretical distribution, using χ2 goodness-of-fit tests, and we use binomial tests to compare each reported outcome against chance level (0.334). This indicates whether the proportion of participants reporting a given outcome differs from the expected proportion if all participants reported truthfully. We also report two-sided exact Fisher tests that compare the differences in reports between groups (obese vs. lean, sated vs. fasted subjects).

Second, to provide an estimate of the full distribution on the proportion of participants who lied we used the econometric method and the software developed by Garbarino and colleagues[41]. This method allows us to estimate precisely the mean and the lower and upper bounds of the proportion of people lying, by taking into account the information about the distribution of the possible outcomes to calculate the PDF and the CDF of lying. We distinguish two categories of lies: those motivated by payoff maximization and those motivated by the avoidance of the lowest payoff. In our experiment, the first category of lies is captured by the mean percentage of subjects who lie by reporting the highest outcome (red, which pays €5) when they observed the lower (blue) or the medium (yellow) outcome. The second category of lies is instead captured by the percentage of subjects that, having observed the lower outcome (blue, which pays €0), report any other outcome.

Third, to identify the individual determinants of the likelihood of reporting a higher outcome, we estimate ordered probit models in which the dependent variable is the value of the outcome reported by the participant (€0, €3 or €5). Model (1) pools all the observations together. Models (2) and (3) provide estimates for lean and for obese participants separately. Models (4) and (5) split the sample by gender and provides estimates for males and for females separately. The independent variables in models 1 (-all subjects-) 4 (-only females-) and 5 (-only males-) include dummies for obese and for lean subjects in each of the two conditions, with the lean participants in the Sated group taken as the reference category. In models 2 (-only lean-) and 3 (-only obese-), these variables are replaced with dummies for male and for female participants in each of the two conditions, with females in the Sated condition taken as the reference category. All models include controls for individual characteristics, such as weekly spending categories (excluding rents: 1 for 0–€149, 2 for €150–€299, 3 for €300–€449, 4 for €450–€599, 5 for €600–€749, 6 for €750 and more), age and the squared value of age (to account for a possible non-linear effect of aging), educational attainment (1 for primary education, 2 for secondary education, 3 for high school, 4 for vocational training, 5 for some University to Bachelor degree, and 6 for Master degree and above), status of student (1 if student and 0 otherwise) and hunger shift (absolute difference between time 2 and time 1 measurements) . In model (1) we added a control for gender (1 female, 0 male). The full results are provided in Supplementary Table S5. All analyses are two-tailed and the alpha value was set at 0.05.

Supplementary file1