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Recent experimental research has revealed forms of human behavior involving interaction among unrelated individuals that have proven difficult to explain in terms of kin or reciprocal altruism. One such trait, strong reciprocity is a predisposition to cooperate with others and to punish those who violate the norms of cooperation, at personal cost, even when it is implausible to expect that these costs will be repaid. We present evidence supporting strong reciprocity as a schema for predicting and understanding altruism in humans. We show that under conditions plausibly characteristic of the early stages of human evolution, a small number of strong reciprocators could invade a population of selfregarding types, and strong reciprocity is an evolutionary stable strategy. Although most of the evidence we report is based on behavioral experiments, the same behaviors are regularly described in everyday life, for example, in wage setting by firms, tax compliance, and cooperation in the protection

Abstract: In recent years a large number of experimental studies have documented the existence of strong reciprocity among humans. Strong reciprocity means that people willingly repay gifts and punish the violation of cooperation and fairness norms even in anonymous one-shot encounters with genetically unrelated strangers. We provide historical and experimental evidence suggesting that ultimate theories of kin selection, reciprocal altruism, costly signaling and indirect reciprocity do not provide satisfactory evolutionary explanations of strong reciprocity. The problem of these theories is that they can rationalize strong reciprocity only if it is viewed as maladaptive behavior whereas the evidence suggests that it is an adaptive trait. Thus, we conclude that alternative evolutionary approaches are needed to provide ultimate accounts of strong reciprocity. 1 This paper is part of a research project on strong reciprocity financed by the Network on Economic Environments and the Evolution of Individual Preferences and Social Norms of the MacArthur Foundation. Fehr and Henrich In recent years a large body of evidence has emerged from laboratory experiments indicating that a substantial fraction of people willingly repay gifts and punish the violation of cooperation and fairness norms, even in anonymous one-shot encounters with genetically unrelated strangers (see,

Those who contribute to a public good sometimes experience punitive sentiments toward others. But is the system that produces these sentiments an adaptation and, if so, which collective action problem was it designed to solve? Prior results from experimental economics show that acts of free riding are sometimes punished; that punishment deters free-riding; and that the risk or actuality of punishment recruits higher levels of cooperation in a joint effort. This suggests that one function of punitive sentiments could be to recruit labor for collective actions. However, adaptations designed to cause participation in collective actions could not have evolved unless there were some mechanism that protected those who participated from having lower fitness than non-participating free riders. Therefore, a second possible function of punishment could be to eliminate or reverse fitness differentials that favor free rider designs over participant designs. To map the computational structure of this motivational adaptation (and hence identify its specific function) requires data that relate an individual's circumstances to his or her desire to punish. Herein we report such data. The results indicate that the computational system that regulates one's level of punitive sentiment in collective action contexts is functionally specialized for removing the fitness advantage enjoyed by free riders, rather than for labor recruitment or other functions. Results also support the hypothesis that a separate pro-reward motivational system exists that appears designed to handle the problem of labor recruitment. Rational choice counterexplanations for punitive sentiments were considered, but eliminated on the basis of the evidence.

commitment, conscientiousness, costly signaling theory, equilibrium selection, emotion, empathy, ethics, evolutionary psychology, fitness indicators, genetic correlations, good genes, good parents, good partners, human courtship, kin selection, kindness, individual differences, intelligence, mate choice, mental health, moral virtues, mutation load, mutual choice, person perception, personality, reciprocal altruism, sexual fidelity, sexual selection, social cognition, virtue ethics “Human good turns out to be the activity of the soul exhibiting excellence.” Aristotle (350 BC) Moral evolution theories have emphasized kinship, reciprocity, group selection, and equilibrium selection. Yet, moral virtues are also sexually attractive. Darwin suggested that sexual attractiveness may explain many aspects of human morality. This paper updates his argument by integrating recent research on mate choice, person perception, individual differences, costly signaling, and virtue ethics. Many human virtues may have evolved in both sexes through mutual mate choice to advertise good genetic quality, parenting abilities, and/or partner traits. Such virtues may include kindness, fidelity, magnanimity, and heroism, as well as quasi-moral traits like conscientiousness, agreeableness, mental health, and intelligence. This theory leads to many testable predictions about the phenotypic features, genetic bases, and social-cognitive responses to human moral virtues. A

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Informal sanctions are a major determinant of a society's social capital because they are key to the enforcement of implicit agreements and social norms. Yet, little is known about the driving forces behind informal sanctions. We systematically examine the determinants of informal sanctions by a large number of experiments. Our experiments allow us to identify the relative importance of three major potential factors: (i) strategic sanctioning for selfish reasons, (ii) non-strategic sanctions driven by spitefulness, and (iii) non-strategic sanctions that are driven by the violation of fairness principles. In addition, the observed sanctioning patterns provide insights into the relevance of different fairness principles.

The classic prisoner’s dilemma model of game theory is modified by introducing occasional variations on the options available to players. Mutation and selection of game options reliably change the game matrix, gradually, from a prisoner’s dilemma game into a byproduct mutualism one, in which cooperation is stable, and ‘‘temptation to defect’’ is replaced by temptation to cooperate. This result suggests that when there are many different potential ways of interacting, exploring those possibilities may make escape from prisoner’s dilemmas a common outcome in the world. A consequence is that persistent prisoner’s dilemma structures may be less common than one might otherwise expect.

Aggregated phenomena in social sciences and economics are highly dependent on the way individuals interact. To help understanding the interplay between socio-economic activities and underlying social networks, this paper studies a sequential prisoner’s dilemma with binary choice. It proposes an analytical and computational insight about the role of endogenous networks in emergence and sustainability of cooperation and exhibits an alternative to the choice and refusal mechanism that is often proposed to explain cooperation. The study focuses on heterogeneous equilibriums and emergence of cooperation from an all-defector state that are the two stylized facts that this model successfully reconstructs.

We show that in a society of self-interested agents "intolerance" may provide arbitrary genes with stability, even if these genes are detrimental to the owner agent. The intolerant society model is based on a repressive behavior: each agent, in fact, may attack any other, determining symmetric penalties. An agent who is intolerant attacks any other agent who behaved differently from itself in the immediate past. When the frequencies of the intolerant gene and of the arbitrary gene are suciently high, agents owning both genes are advantaged with respect to the others. Since the arbitrary gene may code for a cooperative behavior that is not beneficial to the individual fitness directly, this model may be used to explain the stability of costly cooperative behaviors in societies of egoistic agents, a crucial step for the emergence of higher levels of complexity from lower ones.