Nichola J. Raihani

Punishment and cooperation in nature

Humans use punishment to promote cooperation in laboratory experiments but evidence that punishment plays a similar role in non-human animals is comparatively rare. In this article, we examine why this may be the case by reviewing evidence from both laboratory experiments on humans and ecologically relevant studies on non-human animals. Generally, punishment appears to be most probable if players differ in strength or strategic options. Although these conditions are common in nature, punishment (unlike other forms of aggression) involves immediate payoff reductions to both punisher and target, with net benefits to punishers contingent on cheats behaving more cooperatively in future interactions. In many cases, aggression yielding immediate benefits may suffice to deter cheats and might explain the relative scarcity of punishment in nature.

Punishers Benefit From Third-Party Punishment in Fish

In cleaner fish, punishment of foraging partners who cheat a client benefits punishers by increasing future cooperation. In cases where uninvolved bystanders pay to punish defectors, this behavior has typically been interpreted in terms of group-level rather than individual-level benefits. Male cleaner fish, Labroides dimidiatus, punish their female partner if she cheats while inspecting model clients. Punishment promotes female cooperation and thereby yields direct foraging benefits to the male. Thus, third-party punishment can evolve via self-serving tendencies in a nonhuman species, and this finding may shed light on the evolutionary dynamics of more complex behavior in other animal species, including humans.

Resolving the iterated prisoner’s dilemma: theory and reality

Pairs of unrelated individuals face a prisoner’s dilemma if cooperation is the best mutual outcome, but each player does best to defect regardless of his partner’s behaviour. Although mutual defection is the only evolutionarily stable strategy in one‐shot games, cooperative solutions based on reciprocity can emerge in iterated games. Among the most prominent theoretical solutions are the so‐called bookkeeping strategies, such as tit‐for‐tat, where individuals copy their partner’s behaviour in the previous round. However, the lack of empirical data conforming to predicted strategies has prompted the suggestion that the iterated prisoner’s dilemma (IPD) is neither a useful nor realistic basis for investigating cooperation. Here, we discuss several recent studies where authors have used the IPD framework to interpret their data. We evaluate the validity of their approach and highlight the diversity of proposed solutions. Strategies based on precise accounting are relatively uncommon, perhaps because the full set of assumptions of the IPD model are rarely satisfied. Instead, animals use a diverse array of strategies that apparently promote cooperation, despite the temptation to cheat. These include both positive and negative reciprocity, as well as long‐term mutual investments based on ‘friendships’. Although there are various gaps in these studies that remain to be filled, we argue that in most cases, individuals could theoretically benefit from cheating and that cooperation cannot therefore be explained with the concept of positive pseudo‐reciprocity. We suggest that by incorporating empirical data into the theoretical framework, we may gain fundamental new insights into the evolution of mutual reciprocal investment in nature.

Identifying teaching in wild animals

After a long period of neglect, the study of teaching in nonhuman animals is beginning to take a more prominent role in research on social learning. Unlike other forms of social learning, teaching requires knowledgeable individuals to play an active role in facilitating learning by the naive. Casting aside anthropocentric requirements for cognitive mechanisms assumed to underpin teaching in our own species, researchers are now beginning to discover evidence for teaching across a wide range of taxa. Nevertheless, unequivocal evidence for teaching remains scarce, with convincing experimental data limited to meerkats, pied babblers, and tandem-running ants. In this review, our aim is to stimulate further research in different species and contexts by providing conceptual and methodological guidelines for identifying teaching, with a focus on natural populations. We begin by highlighting the fact that teaching is a form of cooperative behavior that functions to promote learning in others and show that consideration of these key characteristics is critical in helping to identify suitable targets for future research. We then go on to discuss potential observational, experimental, and statistical techniques that may assist researchers in providing evidence that the criteria that make up the accepted operational definition of teaching have been met. Supplemental materials for this article may be downloaded from http://lb.psychonomic-journals.org/content/supplemental.

A positive effect of flowers rather than eye images in a large-scale, cross-cultural dictator game

People often consider how their behaviour will be viewed by others, and may cooperate to avoid gaining a bad reputation. Sensitivity to reputation may be elicited by subtle social cues of being watched: previous studies have shown that people behave more cooperatively when they see images of eyes rather than control images. Here, we tested whether eye images enhance cooperation in a dictator game, using the online labour market Amazon Mechanical Turk (AMT). In contrast to our predictions and the results of most previous studies, dictators gave away more money when they saw images of flowers rather than eye images. Donations in response to eye images were not significantly different to donations under control treatments. Dictator donations varied significantly across cultures but there was no systematic variation in responses to different image types across cultures. Unlike most previous studies, players interacting via AMT may feel truly anonymous when making decisions and, as such, may not respond to subtle social cues of being watched. Nevertheless, dictators gave away similar amounts as in previous studies, so anonymity did not erase helpfulness. We suggest that eye images might only promote cooperative behaviour in relatively public settings and that people may ignore these cues when they know their behaviour is truly anonymous.

Male cleaner wrasses adjust punishment of female partners according to the stakes.

Punishment is an important deterrent against cheating in cooperative interactions. In humans, the severity of cheating affects the strength of punishment which, in turn, affects the punished individuals future behaviour. Here, we show such flexible adjustments for the first time in a non-human species, the cleaner wrasse (Labroides dimidiatus), where males are known to punish female partners. We exposed pairs of cleaners to a model client offering two types of food, preferred ‘prawn’ items and less-preferred ‘flake’ items. Analogous to interactions with real clients, eating a preferred prawn item (‘cheating’) led to model client removal. We varied the extent to which female cheating caused pay-off reduction to the male and measured the corresponding severity of male punishment. Males punished females more severely when females cheated during interactions with high value, rather than low value, model clients; and when females were similar in size to the male. This pattern may arise because, in this protogynous hermaphrodite, cheating by similar-sized females may reduce size differences to the extent that females change sex and become reproductive competitors. In response to more severe punishment from males, females behaved more cooperatively. Our results show that punishment can be adjusted to circumstances and that such subtleties can have an important bearing on the outcome of cooperative interactions.

The evolution of punishment in n-player public goods games: a volunteer's dilemma.

The evolution of punishment to stabilize cooperation in n‐player games has been treated as a second‐order social dilemma, where contributions to punishment of free‐riders are altruistic. Hence it may only evolve under highly restricted conditions. Here, we build on recent insights using the volunteers dilemma as an alternative payoff matrix for the evolution of cooperation. The key feature of a volunteers dilemma is that the benefits of cooperation are a nonlinear function of the number of contributors, meaning that cooperation is negatively frequency dependent. We propose that nonlinear returns are also an inherent feature of punishment and that this insight allows for a simple and novel explanation of how punishment evolves in groups.

The reputation of punishers

Punishment is a potential mechanism to stabilise cooperation between self-regarding agents. Theoretical and empirical studies on the importance of a punitive reputation have yielded conflicting results. Here, we propose that a variety of factors interact to explain why a punitive reputation is sometimes beneficial and sometimes harmful. We predict that benefits are most likely to occur in forced play scenarios and in situations where punishment is the only means to convey an individuals cooperative intent and willingness to uphold fairness norms. By contrast, if partner choice is possible and an individuals cooperative intent can be inferred directly, then individuals with a nonpunishing cooperative reputation should typically be preferred over punishing cooperators.

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.

Are cleaner fish, Labroides dimidiatus, inequity averse?

Inequity aversion (IA), a willingness to incur temporary costs to prevent unequal outcomes, is common in humans and thought to be beneficial in the context of cooperative relationships with nonkin, since it might allow individuals to regulate contributions to cooperative activities. Attempts to address whether nonhuman animals also show IA have produced mixed results: some studies found that cooperative species are more likely to show IA while others did not. This ambiguity may arise because animals are typically tested for an aversion to working for differential food rewards, even though most tested species do not regularly cooperate to access food. We used the interspecific mutualism between cleaner fish and their reef-fish ‘clients’ to investigate whether IA exists in a species that regularly cooperates with unrelated individuals in the food domain. Cleaners were tested in pairs of actors and recipients. Actors had to perform a task to provide a food reward to both actor and recipient. Cleaners show consistent food preferences in the wild and under laboratory conditions, allowing us to vary the value of the food reward offered to actor and recipient to test whether actors were less likely to work when recipients received higher value rewards. We performed two experiments: actors worked either for their opposite-sex partner or for a same-sex competitor. We found no evidence that cleaners were sensitive to inequity: actors were equally likely to perform the task in all experimental conditions. We discuss these results in light of theories of the evolution of IA.