Human punishment is motivated by inequity aversion, not a desire for reciprocity

Humans involved in cooperative interactions willingly pay a cost to punish cheats. However, the proximate motives underpinning punitive behaviour are currently debated. Individuals who interact with cheats experience losses, but they also experience lower payoffs than the cheating partner. Thus, the negative emotions that trigger punishment may stem from a desire to reciprocate losses or from inequity aversion. Previous studies have not disentangled these possibilities. Here, we use an experimental approach to ask whether punishment is motivated by inequity aversion or by a desire for reciprocity. We show that humans punish cheats only when cheating produces disadvantageous inequity, while there is no evidence for reciprocity. This finding challenges the notion that punishment is motivated by a simple desire to reciprocally harm cheats and shows that victims compare their own payoffs with those of partners when making punishment decisions.     

Attending to the outcome of others: disadvantageous inequity aversion in male capuchin monkeys (Cebus apella)

Brosnan and de Waal [Nature 425:297-299, 2003] reported that capuchin monkeys responded negatively to unequal reward distributions between themselves and another individual when comparing their own rewards with that of their partner. It was suggested that social emotions provided the underlying motivation for such behavior and that this inequity aversion is specific to the social domain. However, alternative hypotheses such as the "frustration effect" or the "food expectation hypothesis" may provide more parsimonious explanations for Brosnan and de Waal's [Nature 425:297-299] results, while others have argued that these findings are not congruent with the Fehr-Schmidt inequity aversion model cited by the authors. The claim that inequity aversion behavior is specific to the social domain has also been questioned, as primates also develop expectations about rewards in the absence of partners, and react negatively when those expectations are violated. In this study, a modified Dictator game was used to investigate whether capuchins would exhibit either disadvantageous inequity aversion behavior or reference-dependent expectancy violation in social and nonsocial conditions, respectively. When given the choice between an equitable and an inequitable outcome, the subjects showed disadvantageous inequity aversion behavior, choosing the equitable outcome significantly more in the social condition. In the nonsocial condition, however, subjects did not show negative expectancy violation resulting from the formation of reference-dependent expectations, choosing the equitable outcome at chance levels. These results suggest that capuchins attend to differential payoffs and that they are averse to inequity, which is disadvantageous to themselves.     

An Evolutionary Model of Cooperation,Fairness and Altruistic Punishment in Public Good Games

We identify and explain the mechanisms that account for the emergence of fairness preferences and altruistic punishment in voluntary contribution mechanisms by combining an evolutionary perspective together with an expected utility model. We aim at filling a gap between the literature on the theory of evolution applied to cooperation and punishment, and the empirical findings from experimental economics. The approach is motivated by previous findings on other-regarding behavior, the co-evolution of culture, genes and social norms, as well as bounded rationality. Our first result reveals the emergence of two distinct evolutionary regimes that force agents to converge either to a defection state or to a state of coordination, depending on the predominant set of self- or other-regarding preferences. Our second result indicates that subjects in laboratory experiments of public goods games with punishment coordinate and punish defectors as a result of an aversion against disadvantageous inequitable outcomes. Our third finding identifies disadvantageous inequity aversion as evolutionary dominant and stable in a heterogeneous population of agents endowed initially only with purely self-regarding preferences. We validate our model using previously obtained results from three independently conducted experiments of public goods games with punishment.     

Social Influences on Inequity Aversion in Children

Adults and children are willing to sacrifice personal gain to avoid both disadvantageous and advantageous inequity. These two forms of inequity aversion follow different developmental trajectories, with disadvantageous inequity aversion emerging around 4 years and advantageous inequity aversion emerging around 8 years. Although inequity aversion is assumed to be specific to situations where resources are distributed among individuals, the role of social context has not been tested in children. Here, we investigated the influence of two aspects of social context on inequity aversion in 4- to 9-year-old children: (1) the role of the experimenter distributing rewards and (2) the presence of a peer with whom rewards could be shared. Experiment 1 showed that children rejected inequity at the same rate, regardless of whether the experimenter had control over reward allocations. This indicates that children’s decisions are based upon reward allocations between themselves and a peer and are not attempts to elicit more favorable distributions from the experimenter. Experiment 2 compared rejections of unequal reward allocations in children interacting with or without a peer partner. When faced with a disadvantageous distribution, children frequently rejected a smaller reward when a larger reward was visible, even if no partner would obtain the larger reward. This suggests that nonsocial factors partly explain disadvantageous inequity rejections. However, rejections of disadvantageous distributions were higher when the larger amount would go to a peer, indicating that social context enhances disadvantageous inequity aversion. By contrast, children rejected advantageous distributions almost exclusively in the social context. Therefore, advantageous inequity aversion appears to be genuinely social, highlighting its potential relevance for the development of fairness concerns. By comparing social and nonsocial factors, this study provides a detailed picture of the expression of inequity aversion in human ontogeny and raises questions about the function and evolution of inequity aversion in humans.     

Neural Correlates of Advantageous and Disadvantageous Inequity in Sharing Decisions

Humans have a strong preference for fair distributions of resources. Neuroimaging studies have shown that being treated unfairly coincides with activation in brain regions involved in signaling conflict and negative affect. Less is known about neural responses involved in violating a fairness norm ourselves. Here, we investigated the neural patterns associated with inequity, where participants were asked to choose between an equal split of money and an unequal split that could either maximize their own (advantageous inequity) or another person’s (disadvantageous inequity) earnings. Choosing to divide money unequally, irrespective who benefited from the unequal distribution, was associated with activity in the dorsal anterior cingulate cortex, anterior insula and the dorsolateral prefrontal cortex. Inequity choices that maximized another person’s profits were further associated with activity in the ventral striatum and ventromedial prefrontal cortex. Taken together, our findings show evidence of a common neural pattern associated with both advantageous and disadvantageous inequity in sharing decisions and additional recruitment of neural circuitry previously linked to the computation of subjective value and reward when violating a fairness norm at the benefit of someone else.     

The Effect of Power Asymmetries on Cooperation and Punishment in a Prisoner’s Dilemma Game

Recent work has suggested that punishment is detrimental because punishment provokes retaliation, not cooperation, resulting in lower overall payoffs. These findings may stem from the unrealistic assumption that all players are equal: in reality individuals are expected to vary in the power with which they can punish defectors. Here, we allowed strong players to interact with weak players in an iterated prisoner''s dilemma game with punishment. Defecting players were most likely to switch to cooperation if the partner cooperated: adding punishment yielded no additional benefit and, under some circumstances, increased the chance that the partner would both defect and retaliate against the punisher. Our findings show that, in a two-player game, cooperation begets cooperation and that punishment does not seem to yield any additional benefits. Further work should explore whether strong punishers might prevail in multi-player games.     

Individual heterogeneity and costly punishment: a volunteer's dilemma

Social control and the enforcement of social norms glue a society together. It has been shown theoretically and empirically that informal punishment of wrongdoers fosters cooperation in human groups. Most of this research has focused on voluntary and uncoordinated punishment carried out by individual group members. However, as punishment is costly, it is an open question as to why humans engage in the punishment of wrongdoers even in one-time-only encounters. While evolved punitive preferences have been advocated as proximate explanations for such behaviour, the strategic nature of the punishment situation has remained underexplored. It has been suggested to conceive of the punishment situation as a volunteer''s dilemma (VOD), where only one individual''s action is necessary and sufficient to punish the wrongdoer. Here, we show experimentally that implementing the punishment situation as a VOD sustains cooperation in an environment where punishers and non-punishers coexist. Moreover, we show that punishment-cost heterogeneity allows individuals to tacitly agree on only the strongest group member carrying out the punishment, thereby increasing the effectiveness and efficiency of social norm enforcement. Our results corroborate that costly peer punishment can be explained without assuming punitive preferences and show that centralized sanctioning institutions can emerge from arbitrary individual differences.     

Theft in an ultimatum game: chimpanzees and bonobos are insensitive to unfairness

Humans, but not chimpanzees, punish unfair offers in ultimatum games, suggesting that fairness concerns evolved sometime after the split between the lineages that gave rise to Homo and Pan. However, nothing is known about fairness concerns in the other Pan species, bonobos. Furthermore, apes do not typically offer food to others, but they do react against theft. We presented a novel game, the ultimatum theft game, to both of our closest living relatives. Bonobos and chimpanzee ‘proposers’ consistently stole food from the responders'' portions, but the responders did not reject any non-zero offer. These results support the interpretation that the human sense of fairness is a derived trait.     

Reputational benefits for altruistic punishment

Many studies show that people act cooperatively and are willing to punish free riders (i.e., people who are less cooperative than others). However, nonpunishers benefit when free riders are punished, making punishment a group-beneficial act. Presented here are four studies investigating whether punishers gain social benefits from punishing. Undergraduate participants played public goods games (PGGs) (cooperative group games involving money) in which there were free riders, and in which they were given the opportunity to impose monetary penalties on free riders. Participants rated punishers as being more trustworthy, group focused, and worthy of respect than nonpunishers. In dyadic trust games following PGGs, punishers did not receive monetary benefits from punishing free riders in a single-round PGG, but did benefit monetarily from punishing free riders in iterated PGGs. Punishment that was not directed at free riders brought no monetary benefits, suggesting that people distinguish between justified and unjustified punishment and only respond to punishment with enhanced trust when the punishment is justified.     

Behavioral Responses to Inequity in Reward Distribution and Working Effort in Crows and Ravens

Sensitivity to inequity is considered to be a crucial cognitive tool in the evolution of human cooperation. The ability has recently been shown also in primates and dogs, raising the question of an evolutionary basis of inequity aversion. We present first evidence that two bird species are sensitive to other individuals'' efforts and payoffs. In a token exchange task we tested both behavioral responses to inequity in the quality of reward (preferred versus non-preferred food) and to the absence of reward in the presence of a rewarded partner, in 5 pairs of corvids (6 crows, 4 ravens). Birds decreased their exchange performance when the experimental partner received the reward as a gift, which indicates that they are sensitive to other individuals'' working effort. They also decreased their exchange performance in the inequity compared with the equity condition. Notably, corvids refused to take the reward after a successful exchange more often in the inequity compared with the other conditions. Our findings indicate that awareness to other individuals'' efforts and payoffs may evolve independently of phylogeny in systems with a given degree of social complexity.     

Do descriptive social norms drive peer punishment? Conditional punishment strategies and their impact on cooperation

Peer punishment is widely considered a key mechanism supporting cooperation in human groups. Although much research shows that human behavior is shaped by the prevailing social norms, little is known about how punishment decisions are impacted by the social context. We present a set of large-scale incentivized experiments in which participants (999 American participants recruited via Amazon Mechanical Turk) could punish their partner conditional on either the level of cooperation or the level of punishment displayed by others who previously interacted in the same setting. While many participants punish independently of levels of cooperation or punishment, a substantial portion punishes free riding more severely when cooperation is more common (‘norm enforcement’), or when free riding is more severely punished by others (‘conformist punishment’). With a dynamic model we demonstrate that conditional punishment strategies can substantially promote cooperation. In particular, conformist punishment helps cooperation to gain a foothold in a population, and norm enforcement helps to maintain cooperation at high levels. Our results provide solid empirical evidence of conditional punishment strategies and illustrate their possible implications for the dynamics of human cooperation.     

Competing demands of prosociality and equity in monkeys

Prosocial decisions may lead to unequal payoffs among group members. Although an aversion to inequity has been found in empirical studies of both human and nonhuman primates, the contexts previously studied typically do not involve a trade-off between prosociality and inequity. Here we investigate the apparent coexistence of these two factors, specifically the competing demands of prosociality and equity. We directly compare the responses of brown capuchin monkeys (Cebus apella) among situations where prosocial preferences conflict with equality, using a paradigm comparable to other studies of cooperation and inequity in this species. By choosing to pull a tray towards themselves, subjects rewarded themselves and/or another in conditions in which the partner either received the same or different rewards, or the subject received no reward. In unequal payoff conditions, subjects could obtain equality by choosing not to pull in the tray, so that neither individual was rewarded. The monkeys showed prosocial preferences even in situations of moderate disadvantageous inequity, preferring to pull in the tray more often when a partner was present than absent. However, when the discrepancy between rewards increased, prosocial behavior ceased.     

Third‐party punishers are rewarded,but third‐party helpers even more so

Punishers can benefit from a tough reputation, where future partners cooperate because they fear repercussions. Alternatively, punishers might receive help from bystanders if their act is perceived as just and other‐regarding. Third‐party punishment of selfish individuals arguably fits these conditions, but it is not known whether third‐party punishers are rewarded for their investments. Here, we show that third‐party punishers are indeed rewarded by uninvolved bystanders. Third parties were presented with the outcome of a dictator game in which the dictator was either selfish or fair and were allocated to one of three treatments in which they could choose to do nothing or (1) punish the dictator, (2) help the receiver, or (3) choose between punishment and helping, respectively. A fourth player (bystander) then sees the third‐party's decision and could choose to reward the third party or not. Third parties that punished selfish dictators were more likely to be rewarded by bystanders than third parties that took no action in response to a selfish dictator. However, helpful third parties were rewarded even more than third‐party punishers. These results suggest that punishment could in principle evolve via indirect reciprocity, but also provide insights into why individuals typically prefer to invest in positive actions.     

No Evidence for Moral Reward and Punishment in an Anonymous Context

Human social interactions are regulated by moral norms that define individual obligations and rights. These norms are enforced by punishment of transgressors and reward of followers. Yet, the generality and strength of this drive to punish or reward is unclear, especially when people are not personally involved in the situation and when the actual impact of their sanction is only indirect, i.e., when it diminishes or promotes the social status of the punished or rewarded individual. In a real-life study, we investigated if people are inclined to anonymously punish or reward a person for her past deeds in a different social context. Participants from three socio-professional categories voted anonymously for early career violinists in an important violin competition. We found that participants did not punish an immoral violin candidate, nor did they reward another hyper-moral candidate. On the contrary, one socio-professional category sanctioned hyper-morality. Hence, salient moral information about past behavior did not elicit punishment or reward in an impersonal situation where the impact of the sanction was indirect. We conclude that contextual features play an important role in human motivation to enforce moral norms.     

Tentative evidence for inequity aversion to unequal work-effort but not to unequal reward distribution in Goffin's cockatoos

One hallmark in the evolution of cooperation is the ability to evaluate one's own payoff for a task against that of another person. To trace its evolutionary history, there has recently been a surge in comparative studies across different species. In non-human animals, evidence of inequity aversion has so far been identified in several primate species, dogs, and rats. Research in birds revealed mixed findings so far: among corvids, crows and ravens did react sensitively to unequal payoffs and work-effort, while New Caledonian crows did not. Among psittacids, kea were studied so far: Yet, despite the fact that they live in large, hierarchically organized social groups that show complex interactions, they did not show a significant reaction to inequitable payoffs. Here we tested for the first time a Cacatua, the Goffin's cockatoo, using a standardized token exchange paradigm in which first the partner and then the subject could exchange a token for a food reward. Our results show that subjects did not react to unequal reward distributions. However, in comparison to the Equity Condition, the likelihood to exchange was lower in the condition in which the partner received the same reward as a gift (without having to work for it) whereas the subject had to perform a task involving substantial work-effort, suggesting that the Goffin's cockatoos do react aversively to work-effort inequity. In a follow-up experiment, subjects never received a reward but observed a conspecific receive a high-quality reward depending on condition. We found again no evidence for an aversion for the unequal reward distribution, but only that, independent of condition, subjects quickly lost their motivation to participate due to not receiving a reward. In summary, Goffins showed some sensitivity to increased unequal work-effort, but did not react to unequal reward distribution.     

喝含糖饮料对金钱表征和金钱捐献意向的影响

目的:探讨喝含糖饮料后是否影响大学生金钱表征和金钱捐献意向。方法:170名大学生,随机分成三组,喝雪碧组、喝纯净水组和什么都不喝组。在抄写完一段文字后,填写自编问卷一和PANAS量表。结果:(1)饮用含糖饮料后并非显著性地影响到对金钱表征;(2)喝含糖饮料在一定程度上能够增加金钱捐献意向,但喝雪碧组、喝纯净水组和什么都不喝组,三组被试金钱捐献意向未达显著性差异水平;(3)喝雪碧组、喝纯净水组和什么都不喝组三组被试在积极情绪和消极情绪维度上均未达到显著性水平。结论:喝含糖饮料并未显著地影响其金钱表征或促进其金钱捐献意向。     

Why punish cheaters? Those who withdraw cooperation enjoy better reputations than punishers,but both are viewed as difficult to exploit

Negatively sanctioning cheaters promotes cooperation. But do all negative sanctions have the same consequences? In dyadic cooperation, there are two ways that cooperators can sanction failures to reciprocate: by inflicting punishment or withdrawing cooperation. Although punishment can be costly, it has been proposed that this cost can be recouped if punishers acquire better reputations than non-punishers and, therefore, are favored as cooperation partners. But the evidence so far is mixed, and nothing is known about the reputations of those who sanction by withdrawing cooperation. Here, we test two novel hypotheses about how inflicting negative sanctions affects the reputation of the sanctioner: (i) Those who withdraw cooperation are evaluated more favorably than punishers, and (ii) both sanctioners are viewed as less exploitable than non-sanctioners. Observers (US online convenience sample, n = 246) evaluated withdrawers as more cooperative and less vengeful than punishers and preferred withdrawers as a partner. Sanctioners were also viewed as more difficult to exploit than non-sanctioners, with no difference between punishers and withdrawers. The results were the same when punishment was costly (US college sample, n = 203) with one exception: Costly punishers, who lost their payoffs by punishing, were viewed as more exploitable than withdrawers. Our results indicate that withdrawing cooperation has advantages over punishing: Withdrawers are favored as cooperative partners while gaining a reputation as difficult to exploit. The reputational consequences of the three responses to defectors—punishing, withdrawing cooperation, and not sanctioning at all—were opposite to those predicted by group selection models.     

Behavioral and physiological response to inequity in bonobos (Pan paniscus)

Inequity aversion (IA), the affective, cognitive, and behavioral response to inequitable outcomes, allows individuals to avoid exploitation and therefore stabilizes cooperation. The presence of IA varies across animal species, which has stimulated research to investigate factors that might explain this variation, and to investigate underlying affective responses. Among great apes, IA is most often studied in chimpanzees. Here, we investigate IA in bonobos, a reputedly tolerant and cooperative species for which few IA studies are available. We describe how bonobos respond to receiving less preferred rewards than a partner in a token exchange task. We show that bonobos respond to receiving less preferred rewards by refusing tokens and rewards, and by leaving the experimental area. Bonobos never refused a trial when receiving preferred rewards, and thus showed no advantageous IA. We also investigate the variability in the disadvantageous IA response on a dyadic level, because the level of IA is expected to vary, depending on characteristics of the dyad. Like in humans and chimpanzees, we show that the tolerance towards inequity was higher in bonobo dyads with more valuable relationships. To study the affective component of IA, we included behavioral and physiological measures of arousal: a displacement behavior (rough self-scratching) and changes in salivary cortisol levels. Both measures of arousal showed large variability, and while analyses on rough self-scratching showed no significant effects, salivary cortisol levels seemed to be lower in subjects that received less than their partner, but higher in subjects that received more than their partner, albeit that both were not significantly different from the equity condition. This suggests that although overcompensated bonobos showed no behavioral response, they might be more aroused. Our data support the cooperation hypothesis on an interspecific and intraspecific level. They show inequity aversion in bonobos, a reputedly cooperative species, and suggest that the variability in IA in bonobos can be explained by their socioecology. Most successful cooperative interactions happen between mothers and their sons and among closely bonded females. The limited need to monitor the partners' investment within these dyads can result in a higher tolerance towards inequity. We therefore suggest future studies to consider relevant socioecological characteristics of the species when designing and analyzing IA studies.     

Defectors,not norm violators,are punished by third-parties

Punishment of defectors and cooperators is prevalent when their behaviour deviates from the social norm. Why atypical behaviour is more likely to be punished than typical behaviour remains unclear. One possible proximate explanation is that individuals simply dislike norm violators. However, an alternative possibility exists: individuals may be more likely to punish atypical behaviour, because the cost of punishment generally increases with the number of individuals that are punished. We used a public goods game with third-party punishment to test whether punishment of defectors was reduced when defecting was typical, as predicted if punishment is responsive to norm violation. The cost of punishment was fixed, regardless of the number of players punished, meaning that it was not more costly to punish typical, relative to atypical, behaviour. Under these conditions, atypical behaviour was not punished more often than typical behaviour. In fact, most punishment was targeted at defectors, irrespective of whether defecting was typical or atypical. We suggest that the reduced punishment of defectors when they are common might often be explained in terms of the costs to the punisher, rather than responses to norm violators.     

Choosy moral punishers

The punishment of social misconduct is a powerful mechanism for stabilizing high levels of cooperation among unrelated individuals. It is regularly assumed that humans have a universal disposition to punish social norm violators, which is sometimes labelled "universal structure of human morality" or "pure aversion to social betrayal". Here we present evidence that, contrary to this hypothesis, the propensity to punish a moral norm violator varies among participants with different career trajectories. In anonymous real-life conditions, future teachers punished a talented but immoral young violinist: they voted against her in an important music competition when they had been informed of her previous blatant misconduct toward fellow violin students. In contrast, future police officers and high school students did not punish. This variation among socio-professional categories indicates that the punishment of norm violators is not entirely explained by an aversion to social betrayal. We suggest that context specificity plays an important role in normative behaviour; people seem inclined to enforce social norms only in situations that are familiar, relevant for their social category, and possibly strategically advantageous.