Brain stimulation reveals distinct motives underlying reciprocal punishment and reward.

Reciprocal fairness, in the form of punishment and reward, is at the core of human societal order. Its underlying neural mechanisms are, however, not fully understood. We systemize suggestive evidence regarding the involvement of the right dorsolateral prefrontal cortex (rDLPFC) and medial prefrontal cortex (mPFC) in reciprocal fairness in three cognitive mechanisms (cognitive control, domain-general and self-reference). We test them and provide novel insights in a comprehensive behavioural experiment with non-invasive brain stimulation where participants can punish greedy actions and reward generous actions. Brain stimulation of either brain area decreases reward and punishment when reciprocation is costly but unexpectedly increases reward when it is non-costly. None of the hypothesized mechanisms fully accounts for the observed behaviour, and the asymmetric involvement of the investigated brain areas in punishment and reward suggests that different psychological mechanisms are underlying punishing selfishness and rewarding generosity. We propose that, for reciprocal punishment, the rDLPFC and the mPFC process self-relevant information, in terms of both personal cost and personal involvement; for reciprocal reward, these brain regions are involved in controlling selfish and pure reciprocity motives, while simultaneously promoting the enforcement of fairness norms. These insights bear importance for endeavours to build biologically plausible models of human behaviour.

Interplay between different forms of power and meritocratic considerations shapes fairness perceptions.

Power imbalance often leads to unequal allocations. However, it remains largely unknown how different forms of power and meritocratic considerations interact to shape fairness perceptions. Using modified Ultimatum Games, we examined how two power forms-decision power and availability of attractive outside option-affect bargaining behavior and fairness perceptions, and how meritocratic considerations are incorporated into the fairness perceptions of powerful and powerless individuals. We identified an asymmetric power effect: having increased decision power or attractive outside options independently increased self-advantageous allocations and self-serving fairness perceptions, whereas the combined lack of both power forms led to self-disadvantageous allocations but had no influence on fairness perceptions. The power effect on fairness perceptions became symmetric when power was obtained through a meritocratic process (procedural justice). In contrast, relative contributions to resource production (distributive justice) did not moderate power effects. We provide causal evidence that the powerful, but not the powerless, strive to minimize cognitive dissonance between behavior and fairness perceptions by interpreting fairness in self-serving ways. This study contributes novel insights into the interplay between different power forms, the asymmetry of power effects, the moderating role of procedural justice, and the mediating role of behavior in the power-driven adjustment of fairness perceptions.

Free neighborhood choice boosts socially optimal outcomes in stag-hunt coordination problem.

Situations where independent agents need to align their activities to achieve individually and socially beneficial outcomes are abundant, reaching from everyday situations like fixing a time for a meeting to global problems like climate change agreements. Often such situations can be described as stag-hunt games, where coordinating on the socially efficient outcome is individually optimal but also entails a risk of losing out. Previous work has shown that in fixed interaction neighborhoods agents' behavior mostly converges to the collectively inefficient outcome. However, in the field, interaction neighborhoods often can be self-determined. Theoretical work investigating such circumstances is ambiguous in whether the efficient or inefficient outcome will prevail. We performed an experiment with human subjects exploring how free neighborhood choice affects coordination. In a fixed interaction treatment, a vast majority of subjects quickly coordinates on the inefficient outcome. In a treatment with neighborhood choice, the outcome is dramatically different: behavior quickly converges to the socially desirable outcome leading to welfare gains 2.5 times higher than in the environment without neighborhood choice. Participants playing efficiently exclude those playing inefficiently who in response change their behavior and are subsequently included again. Importantly, this mechanism is effective despite that only few exclusions actually occur.

Building the Leviathan--Voluntary centralisation of punishment power sustains cooperation in humans.

The prevalence of cooperation among humans is puzzling because cooperators can be exploited by free riders. Peer punishment has been suggested as a solution to this puzzle, but cumulating evidence questions its robustness in sustaining cooperation. Amongst others, punishment fails when it is not powerful enough, or when it elicits counter-punishment. Existing research, however, has ignored that the distribution of punishment power can be the result of social interactions. We introduce a novel experiment in which individuals can transfer punishment power to others. We find that while decentralised peer punishment fails to overcome free riding, the voluntary transfer of punishment power enables groups to sustain cooperation. This is achieved by non-punishing cooperators empowering those who are willing to punish in the interest of the group. Our results show how voluntary power centralisation can efficiently sustain cooperation, which could explain why hierarchical power structures are widespread among animals and humans.

Be nice if you have to--the neurobiological roots of strategic fairness.

Social norms, such as treating others fairly regardless of kin relations, are essential for the functioning of human societies. Their existence may explain why humans, among all species, show unique patterns of prosocial behaviour. The maintenance of social norms often depends on external enforcement, as in the absence of credible sanctioning mechanisms prosocial behaviour deteriorates quickly. This sanction-dependent prosocial behaviour suggests that humans strategically adapt their behaviour and act selfishly if possible but control selfish impulses if necessary. Recent studies point at the role of the dorsolateral prefrontal cortex (DLPFC) in controlling selfish impulses. We test whether the DLPFC is indeed involved in the control of selfish impulses as well as the strategic acquisition of this control mechanism. Using repetitive transcranial magnetic stimulation, we provide evidence for the causal role of the right DLPFC in strategic fairness. Because the DLPFC is phylogenetically one of the latest developed neocortical regions, this could explain why complex norm systems exist in humans but not in other social animals.

Value signals in the prefrontal cortex predict individual preferences across reward categories.

Humans can choose between fundamentally different options, such as watching a movie or going out for dinner. According to the utility concept, put forward by utilitarian philosophers and widely used in economics, this may be accomplished by mapping the value of different options onto a common scale, independent of specific option characteristics (Fehr and Rangel, 2011; Levy and Glimcher, 2012). If this is the case, value-related activity patterns in the brain should allow predictions of individual preferences across fundamentally different reward categories. We analyze fMRI data of the prefrontal cortex while subjects imagine the pleasure they would derive from items belonging to two distinct reward categories: engaging activities (like going out for drinks, daydreaming, or doing sports) and snack foods. Support vector machines trained on brain patterns related to one category reliably predict individual preferences of the other category and vice versa. Further, we predict preferences across participants. These findings demonstrate that prefrontal cortex value signals follow a common scale representation of value that is even comparable across individuals and could, in principle, be used to predict choice.

To trust or not to trust: the dynamics of social interaction in psychosis.

Psychotic illness is a disorder of social interaction unique to humans. However, up to now research has failed to pin down the exact determinants of the complex and interactive processes associated with the development of trust and reciprocity in psychosis. Utilizing a novel multi-round version of an interactive trust game experiment, we show that patients with psychosis and healthy relatives with a heightened risk for the illness exhibit lower baseline levels of trust compared with healthy controls. This effect partly overlapped with a reduced general intelligence. Furthermore, patients were unable to modify their trusting behaviour neither in response to information about the general trustworthiness of their interaction partner, nor in response to their partners' specific direct behavioural feedback. Relatives, in contrast, modified their trusting behaviour towards similar levels as healthy subjects in response to both. The results show that behavioural flexibility in response to socially relevant information is a critical determinant of success in the instantiation and maintenance of social relationships. A lack thereof may drive social dysfunction and the progression from subclinical symptoms to a full-blown psychosis. This offers a testable mechanistic hypothesis for progression from prodrome to psychotic illness, and may provide a therapeutic avenue to grapple the psychotic symptoms of social dysfunction.

Indirect punishment and generosity toward strangers.

Many people incur costs to reward strangers who have been kind to others. Theoretical and experimental evidence suggests that such "indirect rewarding" sustains cooperation between unrelated humans. Its emergence is surprising, because rewarders incur costs but receive no immediate benefits. It can prevail in the long run only if rewarders earn higher payoffs than "defectors" who ignore strangers' kindness. We provide experimental evidence regarding the payoffs received by individuals who employ these and other strategies, such as "indirect punishment," by imposing costs on unkind strangers. We find that if unkind strangers cannot be punished, defection earns most. If they can be punished, however, then indirect rewarding earns most. Indirect punishment plays this important role, even if it gives a low payoff and is rarely implemented.

The economics of altruistic punishment and the maintenance of cooperation.

Explaining the evolution and maintenance of cooperation among unrelated individuals is one of the fundamental problems in biology and the social sciences. Recent findings suggest that altruistic punishment is an important mechanism maintaining cooperation among humans. We experimentally explore the boundaries of altruistic punishment to maintain cooperation by varying both the cost and the impact of punishment, using an exceptionally extensive subject pool. Our results show that cooperation is only maintained if conditions for altruistic punishment are relatively favourable: low cost for the punisher and high impact on the punished. Our results indicate that punishment is strongly governed by its cost-to-impact ratio and that its effect on cooperation can be pinned down to one single variable: the threshold level of free-riding that goes unpunished. Additionally, actual pay-offs are the lowest when altruistic punishment maintains cooperation, because the pay-off destroyed through punishment exceeds the gains from increased cooperation. Our results are consistent with the interpretation that punishment decisions come from an amalgam of emotional response and cognitive cost-impact analysis and suggest that altruistic punishment alone can hardly maintain cooperation under multi-level natural selection. Uncovering the workings of altruistic punishment as has been done here is important because it helps predicting under which conditions altruistic punishment is expected to maintain cooperation.