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6. HUMAN COOPERATION

DETRIMENTAL EFFECTS OF SANCTIONS ON HUMAN ALTRUISM

E. Fehr and B. Rockenbach (University of Zurich, CH) discuss human altruism, the authors making the following points:


1) Human societies are characterized by an unprecedented division of labor supported by a myriad of social and economic exchanges and compliance with social norms(1,2). Norm compliance and the exchange of favors, goods and services pervade every aspect of human life. They shape the interactions in families, neighborhoods, schools, firms, markets and politics. The crucial feature of any exchange is that the parties involved have to trust each other. If the exchange partners had doubts about the other's reliability most exchanges would not take place. In fact, much of the economic backwardness in the world can plausibly be explained by a lack of mutual confidence inhibiting cooperation in the production and the exchange of goods and services(3,4). Yet, what ensures that exchange partners trust each other? In modern societies one solution is to conclude a legally enforceable contract that regulates all aspects of the exchange -- in particular, the sanctions imposed on those who breach the contract. If the punishment for breaching the contract is large enough, it is in the self-interest of the involved parties to fulfil their obligations because otherwise they will be heavily punished.

2) For millennia, however, humans have not been able to rely on contracts because there were no impartial courts enforcing voluntary agreements. Even today it is typically not possible to regulate transactions in every detail, and frequently the courts are unable to verify who violated an agreement. Therefore, even in modern societies the overwhelming majority of all social and economic exchanges are not based on complete and legally enforceable contracts but on implicit agreements and social norms lacking explicitly specified sanctions for non-compliance(5). In such situations there are ample opportunities for cheating the exchange partner to one's own advantage. It is the ubiquity of such cheating opportunities that renders altruistic cooperation important. Altruistic cooperators are willing to cooperate, that is, to abide by the implicit agreement, although cheating would be economically beneficial for them. Much recent research indicates that altruistic cooperation is an important behavioral force.

3) However, the prevailing theoretical approaches in biology and economics neglect costly altruistic behavior that does not yield future economic benefits for the altruist. Individuals are supposed to do others a favor only if they can themselves reap direct or indirect future benefits from such others. Likewise, individuals are supposed to cheat on agreements unless they are sufficiently rewarded for compliance or sanctioned for non-compliance. This axiom of self-interested behavior implies that the only remedy for the compliance problem lies in the provision of sufficient rewards or sanctions. Yet, if these incentives for cooperation undermine altruistic behavior the remedy may do more harm than good because it aggravates the compliance problem(5). Influential social scientists have hypothesized that economic incentives might undermine altruism, but so far it has been impossible to provide compelling evidence from uncontrolled field studies.

4) In summary: The existence of cooperation and social order among genetically unrelated individuals is a fundamental problem in the behavioral sciences. The prevailing approaches in biology and economics view cooperation exclusively as self-interested behavior -- unrelated individuals cooperate only if they face economic rewards or sanctions rendering cooperation a self-interested choice. Whether economic incentives are perceived as just or legitimate does not matter in these theories. Fairness-based altruism is, however, a powerful source of human cooperation. The authors demonstrate experimentally that the prevailing self-interest approach has serious shortcomings because it overlooks negative effects of sanctions on human altruism. Sanctions revealing selfish or greedy intentions destroy altruistic cooperation almost completely, whereas sanctions perceived as fair leave altruism intact. The authors suggest these findings challenge proximate and ultimate theories of human cooperation that neglect the distinction between fair and unfair sanctions, and they are probably relevant in all domains in which voluntary compliance matters -- in relations between spouses, in the education of children, in business relations and organizations as well as in markets.

References (abridged):

1. Smith, A. The Wealth of Nations (The Modern Library, New York, 1776)

2. Durkheim, E. De La Division Du Travail Social (Presses Universitaires de France, Paris, 1902)

3. Arrow, K. Gifts and exchanges. Phil. Publ. Affairs 1, 343-362 (1972)

4. Knack, S. & Keefer, P. Does social capital have an economic payoff? Q. J. Econ. 112, 1251-1288 (1997)

5. Bowles, S. Economic Institutions and Behavior (Princeton Univ. Press, Princeton, in the press)

Nature 2003 422:137

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ALTRUISTIC PUNISHMENT IN HUMANS

E. Fehr and S. Gaechter (University of Zurich, CH) discuss human altruism, the authors making the following points:

1) Throughout evolution, crucial human activities like hunting big game, sharing meat, conserving common property resources, and warfare constituted a public good. In situations like these, every member of the group benefits from the "good", including those who did not pay any costs of providing the good. This raises the question of why people regularly participate in costly cooperative activities like warfare and big-game hunting(1-4). Several theories have been proposed to explain the evolution of human cooperation. The theory of kin selection(5) focuses on cooperation among individuals that are genetically closely related, whereas theories of direct reciprocity focus on the selfish incentives for cooperation in bilateral long-term interactions. The theories of indirect reciprocity and costly signaling show how cooperation in larger groups can emerge when the cooperators can build a reputation. Yet these theories do not readily explain why cooperation is frequent among genetically unrelated people, in non-repeated interactions, when gains from reputation are small or absent.

2) Punishment provides a solution to this problem. If those who free ride on the cooperation of others are punished, cooperation may pay(3). Yet this "solution" begs the question of who will bear the cost of punishing the free riders. Everybody in the group will be better off if free riding is deterred, but nobody has an incentive to punish the free riders. Thus, the punishment of free riders constitutes a second-order public good. The problem of second-order public goods can be solved if enough humans have a tendency for altruistic punishment, that is, if they are motivated to punish free riders even though it is costly and yields no material benefits for the punishers.

3) The authors examined the question of whether humans engage in altruistic punishment and how this inclination affects the ability of achieving and sustaining cooperation. A total of 240 students participated in a "public goods" experiment with real monetary stakes and two treatment conditions: punishment and no punishment.

4) In summary: Human cooperation is an evolutionary puzzle. Unlike other creatures, people frequently cooperate with genetically unrelated strangers, often in large groups, with people they will never meet again, and when reputation gains are small or absent. These patterns of cooperation cannot be explained by the nepotistic motives associated with the evolutionary theory of kin selection and the selfish motives associated with signaling theory or the theory of reciprocal altruism. The authors demonstrate experimentally that the altruistic punishment of defectors is a key motive for the explanation of cooperation. Altruistic punishment means that individuals punish, although the punishment is costly for them and yields no material gain. The authors demonstrate that cooperation flourishes if altruistic punishment is possible, and breaks down if it is ruled out. The evidence indicates that negative emotions towards defectors are the proximate mechanism behind altruistic punishment. These results suggest that future study of the evolution of human cooperation should include a strong focus on explaining altruistic punishment.

References (abridged):

1. Smuts, B. B., Cheney, D. L., Seyfarth, R. M., Wrangham, R. W., & Struhsaker, T. T. (eds) Primate Societies (Univ. Chicago Press, Chicago, 1987)

2. Richerson, P. & Boyd, R. in Ideology, Warfare and Indoctrinability (eds Eibl-Eibesfeldt, I. & Salter, F.) 71-95 (Berghan Books, New York, 1998)

3. Sober, E. & Wilson, D. S. Unto Others: The Evolution and Psychology of Unselfish Behavior (Harvard Univ. Press, Cambridge, Massachusetts, 1998)

4. Boyd, R. & Richerson, P. in Evolution and Culture (ed. Levinson, S.) (MIT Press, Cambridge, Massachusetts, in the press)

5. Hamilton, W. D. Genetical evolution of social behavior I and II. J. Theor. Biol. 7, 1-52 (1964)

Nature 2002 415:137

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THE PUZZLE OF HUMAN COOPERATION

D.D. Johnson et al (Harvard University, US) discuss human cooperation, the authors making the following points:

1) Humans often defy rational-choice theory by cooperating in simple dilemma games(1), a paradox that has been explained by theories of kin selection(2), reciprocal altruism(3) and indirect reciprocity (reputation)(4). Fehr and Gaechter (2002) claim that human cooperation remains an evolutionary puzzle because people will cooperate with genetically unrelated strangers, often in large groups, with people whom they may not meet again, and without any gain in reputation ("strong reciprocity")(5) -- that is, when existing theories do not seem to apply. However, the authors argue that those theories are rejected for the wrong reasons and that the paradox may therefore be imaginary. This has implications for whether punishment is crucial to promoting cooperation(5).

2) First, strong reciprocity is not necessarily a puzzle because altruistic tendencies need not reflect contemporary selective pressures. Rather, they may reflect motivations that evolved during the past 5 7 million years, in situations that were very different from those presupposed in the puzzle (small groups with relatedness greater than random, individuals well known to each other, interactions likely to be repeated, and people organized in hierarchies). The puzzle disappears if human brains apply ancient tendencies to cooperate that persist in newer environments, even if they are maladaptive (heuristic rules that violate expected utility often make sense for common tasks in our evolutionary history).

3) Accordingly, kin selection, reciprocal altruism and indirect reciprocity need not explain why altruism fails to conform to rationality theory today; rather, they explain why it became ingrained in our brains in the past. Today, humans distinguish and favor kin, or select partners with whom to repeat games, but this does not conflict with those explanations (it supports their legacy in our evolution). The cost of errors may be high but they are recent, so it is unclear whether selection has been strong (or fast) enough to realign humans' strategic behavior specifically to deal with modern problems -- evidence for systematic deviation from rational solutions indicates that it has not. Such deviations occur across cultures(1), suggesting an ancient and/or common origin, rather than a fine-tuning to varied modern circumstances.

4) Second, the punishers in laboratory experiments such as Fehr and Gaechter's are anonymous5, so potential extra costs resulting from retaliation (of any sort) by victims are ruled out. Anonymity is unrealistic among early human groups: vigilantes would have to confront defectors to punish them, which incurs risk, and punishment among group members gives rise to grudges and reprisals, which undermine future cooperation. Although groups may be willing to punish individual defectors, people in one-on-one situations may not accept the personal cost of punishment (for example, they are often unwilling to intervene in criminal acts or to testify in trials for fear of retaliation).

5) Third, the problem remains of what prevents the occurrence of second-order free-riders, who cooperate for the public good but defect from bearing the cost of punishment. Fehr and Gaechter's results suggest that this is not a problem, as a core of people willingly incur personal costs to administer punishment, motivated by anger (although it is unclear whether they would act on it if they were not anonymous).

References (abridged):

1. Henrich, J. et al. Am. Econ. Rev. 91, 73-78 (2001)

2. Hamilton, W. D. J. Theor. Biol. 7, 1-52 (1964)

3. Trivers, R. L. Q. Rev. Biol. 46, 35-57 (1971)

4. Alexander, R. D. The Biology of Moral Systems (Hawthorne, Aldine, New York, 1987)

5. Fehr, E. & Gaechter, S. Nature 415, 137-140 (2002)

Nature 2003 421:911

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REPLY TO JOHNSON ET AL: THE PUZZLE OF HUMAN COOPERATION

E. Fehr and S. Gaechter (University of Zurich, CH) reply to the previous comments of D. Johnson et al, the authors making the following points:

1) The claim by Johnson et al (2003) that human cooperation in social-dilemma games violates rational-choice theory is not justified(1). If people have altruistic aims, altruistic behavior is a rational means by which to achieve their proximate goals. From an evolutionary viewpoint, we need to explain why humans are often altruistic by strong reciprocity(2-4). Although kin selection, reciprocal altruism and indirect reciprocity explain relevant forms of human cooperation(5), they do not ultimately explain strong reciprocity.

2) Kin selection would account for strong reciprocity if human behavior were driven by rules that do not distinguish between kin and non-kin. But humans, like other primates, distinguish cognitively and behaviorally between the two(5), and generally feel stronger emotions towards kin. Likewise, reciprocal altruism could account for strong reciprocity if humans' behavioral rules did not depend on the probability of future interactions with potential opponents. But humans can distinguish long-term partners from people with whom future interaction will be less likely ("strangers"), and will cooperate more if they anticipate that interaction will be frequent. Emotional responses may also be stronger towards a long-term partner than towards a "stranger" (unpublished results of the authors).

3) Reputation-based ultimate theories could account for strong reciprocity if our behavioral rules did not depend on our actions being observed by others. However, if reputation formation is ruled out, cooperation breaks down, whereas it flourishes if subjects gain in reputation.

4) Early humans whose behavior was fine-tuned to respond to kin or non-kin, partners or strangers, and gaining in reputation, probably had an evolutionary advantage because, contrary to common belief, they faced interactions where the probability of future encounters was sufficiently low as to make defection worthwhile. Ethnographic evidence indicates that humans had many encounters with individuals with whom they had little future interaction. In addition, the costs of mistakenly treating unrelated individuals as kin, or treating strangers as partners, were high -- for instance, a lack of vigilance with strangers could be fatal. Because of these costs, individuals who could adjust their behavior to suit the their opponent's characteristics had greater fitness. The problem with any theory claiming that strong reciprocity is maladaptive in modern circumstances is that individuals understand the risks of exploitation in interactions with non-kin and strangers, and behave accordingly. An evolutionary explanation of strong reciprocity is needed that does not assume that individuals are maladapted(2,3).

References (abridged):

1. Gintis, H. Game Theory Evolving (Princeton Univ. Press, 2000)

2. Henrich, J. & Boyd, R. J. Theor. Biol. 208, 79-89 (2001)

3. Gintis, H. J. Theor. Biol. 206, 169-179 (2000)

4. Fehr, E. & Gaechter, S. Nature 415, 137-140 (2002)

5. Silk, J. B. Am. Anthropol. 82, 799-820 (1980)

Nature 2003 421:912

VOLUNTEERING AS RED QUEEN MECHANISM FOR COOPERATION IN PUBLIC GOODS GAMES

C. Hauert et al (University of Vienna, AT) cooperation in humans, the authors making the following points:

1) Public goods are defining elements of all societies. Collective efforts to shelter, protect, and nourish the group have formed the backbone of human evolution from prehistoric time to global civilization. They confront individuals with the temptation to defect, i.e., to take advantage of the public good without contributing to it. This is known as Tragedy of the Commons, Free Rider Problem, Social Dilemma, or Multiperson Prisoner's Dilemma -- the diversity of the names underlines the ubiquity of the issue (1-5).

2) Theoreticians and experimental economists investigate this issue by public goods games, which are characterized by groups of cooperators doing better than groups of defectors, but defectors always outperforming the cooperators in their group. In typical examples, the individual contributions are multiplied by a factor (r) and then divided equally among all players. With (r) smaller than the group size, this is an example of a social dilemma: Every individual player is better off defecting than cooperating, no matter what the other players do. Groups would therefore consist of defectors only and forego the public good. For two-player groups, this is the prisoner's dilemma game. In this case, cooperation based on direct or indirect reciprocation can get established, provided the probability of another round is sufficiently high. But retaliation does not work if many players are engaged in the game, because players intending to punish a defector can do so only by refraining from cooperation in subsequent rounds, thereby also punishing the cooperators in the group.

3) If players are offered, after each round, the option of fining specific coplayers, cooperation gets firmly established. This happens even if punishment is costly to the punisher and if players believe that they will never meet again. But such fining, or alternatively rewarding, requires that players can discriminate individual defectors. Although reward and punishment must be major factors in human cooperation, the authors draw attention to a simpler mechanism. It consists in allowing the players not to participate, and to fall back on a safe "side income" that does not depend on others. Such risk-averse optional participation can foil exploiters and relax the social dilemma, even if players have no way of discriminating against defectors. In this public goods game, the drop-out option allows groups to form on a voluntary basis and thus to relaunch cooperation again and again. But each additional player brings a diminishing return and an increased threat of exploitation. As in the land of the Red Queen, "it takes all the running you can do, to keep in the same place." Individuals keep adjusting their strategies but in the long run do no better than if the public goods option had never existed. On the other hand, voluntary participation avoids the deadlock of mutual defection that threatens any public enterprise in larger groups.

4) In summary: The evolution of cooperation among nonrelated individuals is one of the fundamental problems in biology and social sciences. Reciprocal altruism fails to provide a solution if interactions are not repeated often enough or groups are too large. Punishment and reward can be very effective but require that defectors can be traced and identified. The authors present a simple but effective mechanism operating under full anonymity. Optional participation can foil exploiters and overcome the social dilemma. In voluntary public goods interactions, cooperators and defectors will coexist. The authors demonstrate that this result holds under very diverse assumptions on population structure and adaptation mechanisms, leading usually not to an equilibrium but to an unending cycle of adjustments (a Red Queen type of evolution). Thus, voluntary participation offers an escape hatch out of some social traps. Cooperation can subsist in sizable groups even if interactions are not repeated, defectors remain anonymous, players have no memory, and assortment is purely random.

References (abridged):

1. W. D. Hamilton, Biosocial Anthropology, R. Fox, Ed. (Malaby, London, 1975), pp. 133-153

2. G. Hardin, Science 162, 1243 (1968)

3. R. L. Trivers, Q. Rev. Biol. 46, 35 (1971)

4. J. Maynard Smith, E. Szathm ry, The Major Transitions in Evolution (Freeman, Oxford, UK, 1995)

5. K. G. Binmore, Playing Fair: Game Theory and the Social Contract (MIT Press, Cambridge, MA, 1994)

Science 2002 296:1129

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COMPETITION AMONG COOPERATORS: ALTRUISM AND RECIPROCITY

Peter Danielson (University of British Columbia, CA) discusses altruism, the author making the following points:

1) Sober and Wilson (1) note that the behavior labeled "cooperation" by evolutionary game theorists is the same as that discussed in the evolution of altruism literature. Indeed, altruism/cooperation is "the central theoretical problem of sociobiology" (1). Naturally, the main question is how is altruism/cooperation possible among agents selected by competitive evolutionary processes? But this possibility question can be misleading. For example, both Axelrod (2) and Gauthier (3) advanced the discussion of these issues by insisting that there are situations in which cooperative agents responsive to the behavior or disposition of their opponents do as well or better than straightforwardly competitive agents. Yet each defended a single cooperative strategy -- Tit for Tat and Constrained Maximization, respectively -- that is not a unique equilibrium. Neither attended to the competition between cooperative strategies and both neglected less "nice" alternative strategies.

2) In earlier work (5), the author extended Gauthier's model to address competition among cooperators, finding that what the author called "reciprocal cooperators", who demand discriminatory responsiveness from cooperative partners, exploit some more tolerant cooperators, and thereby supplant nicer Constrained Maximizers. More generally, the combination of evolution, altruism, and reciprocity need not result in populations of equal, optimal, and tolerant cooperators. Most generally, because we are inclined to favor cooperation, we need methods that challenge our intuitive biases. Evolutionary simulation can be a good test because evolution builds in strong competitive pressure. But simulations can easily confirm biases, unless we allow the generator to range quite widely, to include sometimes counterintuitive possibilities.

3) In summary: It has been argued that neither self-interest nor altruism explains experimental results in bargaining and public goods games. Subjects' preferences appear also to be sensitive to their opponents' perceived altruism. Sethi and Somanathan (2001) provide a general account of reciprocal preferences that survive under evolutionary pressure. Although a wide variety of reciprocal strategies pass this evolutionary test, Sethi and Somanthan conjecture that fewer are likely to survive when reciprocal strategies compete with each other. The author develops evolutionary agent-based models to test their conjecture in cases where reciprocal preferences can differ in a variety of games. The author confirms that reciprocity is necessary but not sufficient for optimal cooperation. The author explores the theme of competition among reciprocal cooperators and displays three emergent organizations: racing to the "moral high ground", unstable cycles of preference change, and, when reciprocal mechanisms are implemented, hierarchies resulting from exploiting fellow cooperators. The author suggests that if reciprocity is a basic mechanism facilitating cooperation, we can expect interaction that evolves around it to be complex, non-optimal, and resistant to change.

References (abridged):

1. Sober, E. & Wilson, D. S. (1998) Unto Others: The Evolution and Psychology of Unselfish Behavior (Harvard Univ. Press, Cambridge, MA)

2. Axelrod, R. M. (1984) The Evolution of Cooperation (Basic Books, New York)

3. Gauthier, D. (1986) Morals by Agreement (Oxford Univ. Press, Oxford)

4. Binmore, K. (1994) Game Theory and the Social Contract: Playing Fair (MIT Press, Cambridge, MA)

5. Danielson, P. (1992) Artificial Morality: Virtuous Robots for Virtual Games (Routledge, London)

Proc. Nat. Acad. Sci. 2002 99:7237

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THE LONG-TERM BENEFITS OF HUMAN GENEROSITY IN INDIRECT RECIPROCITY

C. Wedekind and V.A. Braithwaite (University of Edinburgh, UK) discuss human generosity, the authors making the following points:

1) Among the theories that have been proposed to explain the evolution of altruism [1 5] are direct reciprocity and indirect reciprocity. The idea of the latter is that helping someone or refusing to do so has an impact on one's reputation within a group. This reputation is constantly assessed and reassessed by others and is taken into account by them in future social interactions. Generosity in indirect reciprocity can evolve if and only if it eventually leads to a net benefit in the long term.

2) The authors demonstrate that the key assumption of indirect reciprocity is met. The authors let 114 students play for money in an indirect and a subsequent direct reciprocity game. The authors found that although being generous, i.e., giving something of value to others, had the obvious short-term costs, it paid in the long run because it builds up a reputation that is rewarded by third parties (who thereby themselves increase their reputation). A reputation of being generous also provided an advantage in the subsequent direct reciprocity game, probably because it builds up trust that can lead to more stable cooperation.

References (abridged):

1. Hamilton W.D. (1963) The evolution of altruistic behavior. Am. Nat., 97:354-356

2. Wilson D.S. The Natural Selection of Populations and Communities. (1980) Menlo Park, CA: Benjamin-Cummings Press

3. Simon H.A. (1990) A mechanism for social selection and successful altruism. Science, 250:1665-1668

4. Wilson D.S. and Dugatkin L.A. (1997) Group selection and assortative interactions. Am. Nat., 149:336-351

5. Riolo R.L., Cohen M.D. and Axelrod R. (2001) Evolution of cooperation without reciprocity. Nature, 414:441-443

Current Biology 2002 12:1012

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REPUTATION HELPS SOLVE THE "TRAGEDY OF THE COMMONS"

M. Milinski et al (Max Planck Institute of Limnology Ploen, DE) discuss the tragedy of the commons, the authors making the following points:

1) The problem of sustaining a public resource that everybody is free to overuse -- the "tragedy of the commons"(1-5) -- emerges in many social dilemmas, such as our inability to sustain the global climate. Public goods experiments(4), which are used to study this type of problem, usually confirm that the collective benefit will not be produced. Because individuals and countries often participate in several social games simultaneously, the interaction of these games may provide a sophisticated way by which to maintain the public resource. Indirect reciprocity, "give and you shall receive", is built on reputation and can sustain a high level of cooperation, as shown by game theorists

2) The authors demonstrate, through alternating rounds of public goods and indirect reciprocity games, that the need to maintain reputation for indirect reciprocity maintains contributions to the public good at an unexpectedly high level. But if rounds of indirect reciprocation are not expected, then contributions to the public good drop quickly to zero. Alternating the games leads to higher profits for all players. As reputation may be a currency that is valid in many social games, the authors suggest their approach could be used to test social dilemmas for their solubility.

References (abridged):

1. Hardin, G. The tragedy of the commons. Science 162, 1243-1248 (1968)

2. Dawes, R. Social dilemmas. Annu. Rev. Psychol. 31, 169-193 (1980)

3. Berkes, F., Feeny, D., McCay, B. J. & Acheson, J. M. The benefits of the commons. Nature 340, 91-93 (1989)

4. Ledyard, J. O. in Handbook of Experimental Economics (eds Kagel, J. H. & Roth, A. E.) 111-194 (Princeton Univ. Press, Princeton, 1995)

5. Hardin, G. Extensions of "the tragedy of the commons". Science 280, 682-683 (1998)

Nature 2002 415:424

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EVOLUTION OF INDIRECT RECIPROCITY BY IMAGE SCORING

M.A. Nowak and K. Sigmund (University of Oxford, UK) discuss cooperative behavior, the authors making the following points:

1) Darwinian evolution has to provide an explanation for cooperative behavior. Theories of cooperation are based on kin selection (dependent on genetic relatedness)(1,2), group selection(3-5), and reciprocal altruism. The idea of reciprocal altruism usually involves direct reciprocity: repeated encounters between the same individuals allow for the return of an altruistic act by the recipient.

2) The authors present a new theoretical framework, which is based on indirect reciprocity and does not require the same two individuals ever to meet again. Individual selection can nevertheless favor cooperative strategies directed towards recipients that have helped others in the past. Cooperation pays because it confers the image of a valuable community member to the cooperating individual.

3) The authors present computer simulations and analytic models that specify the conditions required for evolutionary stability of indirect reciprocity. The authors demonstrate that the probability of knowing the "image" of the recipient must exceed the cost-to-benefit ratio of the altruistic act. The authors propose that the emergence of indirect reciprocity was a decisive step for the evolution of human societies.

References (abridged):

1. Hamilton, W. D. The evolution of altruistic behavior. Am. Nat. 97, 354-356 (1963)

2. Hamilton, W. D. The genetical evolution of social behavior. J. Theor. Biol. 7, 1-16 (1964)

3. Williams, G. C. Group Selection (Aldine-Atherton, Chicago, (1971)

4. Eshel, I. On the neighborhood effect and evolution of altruistic traits. Theor. Popul. Biol. 3, 258-277 (1972)

5. Wilson, D. S. & Sober, E. Reintroducing group selection to the human behavioral sciences. Behav. Brain Sci. 17, 585-654 (1994)

Nature 1998 393:573

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