Respond to ALL prompt questions:
Zaki & Mitchell (2013).
What evidence do Zaki and Mitchell review that suggests that humans are not instinctively selfish; but rather, are intuitively prosocial? Do you agree or disagree with this argument?
Warneken & Tomasello (2006).
What did Warneken and colleagues find when examining altruistic helping behavior in young children?
Rand & Nowak (2013).
What evidence do Rand and colleagues provide to support the five different mechanimss that may underlie human cooperation?Review
David G. Rand1 and Martin A. Nowak2
Department of Psychology, Department of Economics, Program in Cognitive Science, School of Management, Yale University,
New Haven, CT, USA
Program for Evolutionary Dynamics, Department of Mathematics, Department of Organismic and Evolutionary Biology, Harvard
University, Cambridge, MA, USA
Why should you help a competitor? Why should you
contribute to the public good if free riders reap the benefits of your generosity? Cooperation in a competitive
world is a conundrum. Natural selection opposes the
evolution of cooperation unless specific mechanisms
are at work. Five such mechanisms have been proposed:
direct reciprocity, indirect reciprocity, spatial selection,
multilevel selection, and kin selection. Here we discuss
empirical evidence from laboratory experiments and field
studies of human interactions for each mechanism. We
also consider cooperation in one-shot, anonymous interactions for which no mechanisms are apparent. We argue
that this behavior reflects the overgeneralization of cooperative strategies learned in the context of direct and
indirect reciprocity: we show that automatic, intuitive
responses favor cooperative strategies that reciprocate.
The challenge of cooperation
In a cooperative (or social) dilemma, there is tension between what is good for the individual and what is good for
the population. The population does best if individuals
cooperate, but for each individual there is a temptation to
defect. A simple definition of cooperation is that one individual pays a cost for another to receive a benefit. Cost and
benefit are measured in terms of reproductive success,
where reproduction can be cultural or genetic. Box 1 provides a more detailed definition based on game theory.
Among cooperative dilemmas, the one most challenging
for cooperation is the prisoner’s dilemma (PD; see Glossary),
in which two players choose between cooperating and defecting; cooperation maximizes social welfare, but defection
maximizes one’s own payoff regardless of the other’s choice.
In a well-mixed population in which each individual is
equally likely to interact and compete with every other
individual, natural selection favors defection in the PD:
why should you reduce your own fitness to increase that of
a competitor in the struggle for survival? Defectors always
out-earn cooperators, and in a population that contains
both cooperators and defectors, the latter have higher
fitness. Selection therefore reduces the abundance of cooperators until the population consists entirely of defectors.
For cooperation to arise, a mechanism for the evolution of
cooperation is needed. Such a mechanism is an interaction
structure that can cause cooperation to be favored over
Corresponding author: Nowak, M.A. (email@example.com).
1364-6613/$ – see front matter
ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tics.2013.06.003
defection . These interaction structures specify how the
individuals of a population interact to receive payoffs, and
how they compete for reproduction. Previous work has
identified five such mechanisms for the evolution of cooperation (Figure 1): direct reciprocity, indirect reciprocity,
spatial selection, multilevel selection, and kin selection. It
is important to distinguish between interaction patterns
that are mechanisms for the evolution of cooperation and
behaviors that require an evolutionary explanation (such
as strong reciprocity, upstream reciprocity, and parochial
altruism; Box 2).
In this article, we build a bridge between theoretical work
that has proposed these mechanisms and experimental
work exploring how and when people actually cooperate.
First we present evidence from experiments that implement
each mechanism in the laboratory. Next we discuss why
cooperation arises in some experimental settings in which
no mechanisms are apparent. Finally, we consider the
cognitive underpinnings of human cooperation. We show
Evolutionary dynamics: mathematical formalization of the process of evolution
whereby a population changes over time. Natural selection operates such that
genotypes (or strategies) with higher fitness tend to become more common,
whereas lower-fitness genotypes tend to die out. Mutation (re)introduces
variation into the population. This process can also represent cultural evolution
and social learning, in which people imitate those with higher payoffs and
sometimes experiment with novel strategies.
Evolutionary game theory: combination of game theory and evolutionary
dynamics. There is a population of agents, each of whom has a strategy. These
agents interact with each other and earn payoffs. Payoff is translated into
fitness, and the frequency of strategies in the population changes over time
accordingly: higher-payoff strategies tend to become more common, whereas
lower-payoff strategies tend to die out.
Game theory: mathematical formalization of social interaction and strategic
behavior. A given interaction is represented by (i) a set of players, (ii) the
choices available to each player, and (iii) the payoff earned by each player
depending on both her choice and the choices of the other players. The
prisoner’s dilemma is one such game that describes the problem of
Mechanism for the evolution of cooperation: interaction structure that can
cause natural selection to favor cooperation over defection. The mechanism
specifies how the individuals of a population interact to receive payoffs, and
how they compete for reproduction.
Prisoner’s dilemma: game involving two players, each of whom chooses
between cooperation or defection. If both players cooperate, they earn more
than if both defect. However, the highest payoff is earned by a defector whose
partner cooperates, whereas the lowest payoff is earned by a cooperator
whose partner defects. It is individually optimal to defect (regardless of the
partner’s choice) but socially optimal to cooperate. Box 1 provides further
Public goods game: prisoner’s dilemma with more than two players. In the
public goods game, each player chooses how much money to keep for herself
and how much to contribute to an account that benefits all group members.
Trends in Cognitive Sciences, August 2013, Vol. 17, No. 8
Trends in Cognitive Sciences August 2013, Vol. 17, No. 8
Box 1. Defining cooperation
Consider a game between two strategies, C and D, and the following
payoff matrix (indicating the row player’s payoff):
C R S
D T P
When does it make sense to call strategy C cooperation and
strategy D defection? The following definition [163,164] is useful. The
game is a cooperative dilemma if (i) two cooperators obtain a higher
payoff than two defectors, R > P yet (ii) there is an incentive to defect.
This incentive can arise in three different ways: (a) if T > R then it is
better to defect when playing against a cooperator; (b) if P > S then it
is better to defect when playing against a defector; and (c) if T > S
then it is better to be the defector in an encounter between a
cooperator and a defector. If at least one of these three conditions
holds, then we have a cooperative dilemma. If none holds, then there
is no dilemma and C is simply better than D. If all three conditions
hold, we have a prisoner’s dilemma, T > R > P > S [6,48].
The prisoner’s dilemma is the most stringent cooperative dilemma. Here defectors dominate over cooperators. In a well-mixed
population, natural selection always favors defectors over cooperators. For cooperation to arise in the prisoner’s dilemma, we need a
mechanism for the evolution of cooperation. Cooperative dilemmas
that are not the prisoner’s dilemma could be called relaxed
cooperative dilemmas. In these games it is possible to evolve some
level of cooperation even if no mechanism is at work. One such
example is the snowdrift game, given by T > R > S > P. Here we find
a stable equilibrium between cooperators and defectors, even in a
If 2R > T + S, then the total payoff for the population is maximized if
everyone cooperates; otherwise a mixed population achieves the
highest total payoff. This is possible even for the prisoner’s dilemma.
The above definition can be generalized to more than two people
(n-person games). We denote by Pi and Qi the payoffs for cooperators
and defectors, respectively, in groups that contain i cooperators and
n–i defectors. For the game to be a cooperative dilemma, we require
that (i) an all-cooperator group obtains a higher payoff then an alldefector group, Pn > Q0, yet (ii) there is some incentive to defect. The
incentive to defect can take the following form: (a) Pi < Qi–1 for i = 1, . . ., n and (b) Pi < Qi for i = 1, . . ., n 1. Condition (a) means that an individual can increase his payoff by switching from cooperation to defection. Condition (b) means that in any mixed group, defectors have a higher payoff than cooperators. If only some of these incentives hold, than we have a relaxed cooperative dilemma. In this case some evolution of cooperation is possible even without a specific mechanism. However, a mechanism would typically enhance the evolution of cooperation by increasing the equilibrium abundance of cooperators, increasing the fixation probability of cooperators or reducing the invasion barrier that needs to be overcome. The volunteer’s dilemma is an example of a relaxed situation . If all incentives hold, we have the n-person equivalent of a prisoner’s dilemma, called the public goods game (PGG) , and a mechanism for evolution of cooperation is needed. that intuitive, automatic processes implement cooperative strategies that reciprocate, and that these intuitions are affected by prior experience. We argue that these results support a key role for direct and indirect reciprocity in human cooperation, and emphasize the importance of culture and learning. Direct reciprocity Indirect reciprocity Spa al selec on Mul -level selec on Kin selec on r TRENDS in Cognitive Sciences Figure 1. The five mechanisms for the evolution of cooperation. Direct reciprocity operates when two individuals interact repeatedly: it pays to cooperate today to earn your partner’s cooperation in the future. Indirect reciprocity involves reputation, whereby my actions towards you also depend on your previous behavior towards others. Spatial selection entails local interaction and competition, leading to clusters of cooperators. Multilevel selection occurs when competition exists between groups and between individuals. Kin selection arises when there is conditional behavior according to kin recognition. 414 Five mechanisms Direct reciprocity Direct reciprocity arises if there are repeated encounters between the same two individuals [2–5]. Because they interact repeatedly, these individuals can use conditional strategies whereby behavior depends on previous outcomes. Direct reciprocity allows the evolution of cooperation if the probability of another interaction is sufficiently high . Under this ‘shadow of the future’, I may pay the cost of cooperation today to earn your reciprocal cooperation tomorrow. The repeated game can occur with players making simultaneous decisions in each round or taking turns . Successful strategies for the simultaneous repeated PD include tit-for-tat (TFT), a strategy that copies the opponent’s previous move, and win–stay lose–shift, a strategy that switches its action after experiencing exploitation or mutual defection . TFT is an excellent catalyst for the emergence of cooperation, but when errors are possible it is quickly replaced by strategies that sometimes cooperate even when the opponent defects (e.g., Generous TFT) . Indirect reciprocity Indirect reciprocity operates if there are repeated encounters within a population and third parties observe some of these encounters or find out about them. Information about Review Box 2. Behavioral patterns versus mechanisms for the evolution of cooperation It is important to distinguish mechanisms for the evolution of cooperation from behavioral patterns that are not themselves mechanisms. Three examples are upstream reciprocity, strong reciprocity, and parochial altruism. Upstream (or generalized) reciprocity refers to the phenomenon of paying it forward, by which an individual who has just received help is more likely to help others in turn. Strong reciprocity refers to individuals who reward cooperation and punish selfishness, even in anonymous interactions with no promise of future benefits. Parochial altruism (or ingroup bias) describes the behavior whereby people are more likely to help members of their own group than members of other groups. None of these concepts explains the evolution of cooperation: adding one or more of these elements to a prisoner’s dilemma will not cause selection to favor cooperation. Instead, these concepts are descriptions of behavior that require an evolutionary explanation. Group selection, spatial structure, or some chance of direct or indirect reciprocity can lead to the evolution of upstream reciprocity [166,167], strong reciprocity [13,39,168], and parochial altruism [122,139,169–171]. such encounters can spread through communication, affecting the reputations of the participants. Individuals can thus adopt conditional strategies that base their decision on the reputation of the recipient [10,11]. My behavior towards you depends on what you have done to me and to others. Cooperation is costly but leads to the reputation of being a helpful individual, and therefore may increase your chances of receiving help from others. A strategy for indirect reciprocity consists of a social norm and an action rule [12–14]. The social norm specifies how reputations are updated according to interactions between individuals. The action rule specifies whether or not to cooperate given the available information about the other individual. Indirect reciprocity enables the evolution of cooperation if the probability of knowing someone’s reputation is sufficiently high. Spatial selection Spatial selection can favor cooperation without the need for strategic complexity [15,16]. When populations are structured rather than randomly mixed, behaviors need not be conditional on previous outcomes. Because individuals interact with those near them, cooperators can form clusters that prevail, even if surrounded by defectors. The fundamental idea is that clustering creates assortment whereby cooperators are more likely to interact with other cooperators. Therefore, cooperators can earn higher payoffs than defectors. More generally, population structure affects the outcome of the evolutionary process, and some population structures can lead to the evolution of cooperation [17,18]. Population structure specifies who interacts with whom to earn payoffs and who competes with whom for reproduction. The latter can be genetic or cultural. Population structure can represent geographic distribution [19,20] or social networks , and can be static [22– 24] or dynamic [21,25–29]. Population structure can also be implemented through tag-based cooperation, in which interaction and cooperation are determined by arbitrary tags or markers [30–32]. In this case, clustering is not literally spatial but instead occurs in the space of phenotypes . Trends in Cognitive Sciences August 2013, Vol. 17, No. 8 Multilevel selection Multilevel selection operates if, in addition to competition between individuals in a group, there is also competition between groups [33–39]. It is possible that defectors win within groups, but that groups of cooperators outcompete groups of defectors. Overall, such a process can result in the selection of cooperators. Darwin wrote in 1871: ‘There can be no doubt that a tribe including many members who . . . were always ready to give aid to each other and to sacrifice themselves for the common good, would be victorious over other tribes; and this would be natural selection.’ . Kin selection Kin selection can be seen as a mechanism for the evolution of cooperation if properly formulated. In our opinion, kin selection operates if there is conditional behavior based on kin recognition: an individual recognizes kin and behaves accordingly. As J.B.S. Haldane reportedly said, ‘I will jump into the river to save two brothers or eight cousins’ . Much of the current literature on kin selection, however, does not adhere to this simple definition based on kin recognition. Instead, kin selection is linked to the concept of inclusive fitness . Inclusive fitness is a particular mathematical method to account for fitness effects. It assumes that personal fitness can be written as a sum of additive components caused by individual actions. Inclusive fitness works in special cases, but makes strong assumptions that prevent it from being a general concept . A straightforward mathematical formulation describing the evolutionary dynamics of strategies or alleles without the detour of inclusive fitness is a more universal and more meaningful approach. This position critical of inclusive fitness, which is based on a careful mathematical analysis of evolution , has been challenged by proponents of inclusive fitness , but without considering the underlying mathematical results . In our opinion, a clear understanding of kin selection can only emerge once the intrinsic limitations of inclusive fitness are widely recognized. Meanwhile, it is useful to remember that no phenomenon in evolutionary biology requires an inclusive fitness-based analysis . Interactions between mechanisms Each of these mechanisms applies to human cooperation. Over the course of human evolution, it is likely that they were (and are) all in effect to varying degrees. Although each mechanism has traditionally been studied in isolation, it is important to consider the interplay between them. In particular, when discussing the evolution of any prosocial behavior in humans, we cannot exclude direct and indirect reciprocity. Early human societies were small, and repetition and reputation were always in play. Even in the modern world, most of our crucial interactions are repeated, such as those with our coworkers, friends, and family. Thus, spatial structure, group selection, and kin selection should be considered in the context of their interactions with direct and indirect reciprocity. Surprising dynamics can arise when mechanisms are combined. For example, direct reciprocity and spatial structure can interact either synergistically or antagonistically, depending on the levels of repetition and assortment . Further 415 Review Experimental evidence in support of the five mechanisms Theoretical work provides deep insights into the evolution of human cooperation. Evolutionary game theory allows us to explore what evolutionary trajectories are possible and what conditions may give rise to cooperation. To investigate how cooperation among humans in particular arises and is maintained, theory must be complemented with empirical data from experiments . Theory suggests what to measure and how to interpret it. Experiments illuminate human cooperation in two different ways: by examining what happens when particular interaction structures are imposed on human subjects, and by revealing the human psychology shaped by mechanisms that operate outside of the laboratory (Box 3). We now present both types of experimental evidence. First we describe experiments designed to test each of the mechanisms for the evolution of cooperation in the laboratory. We then discuss the insights gained from cooperation in one-shot anonymous experiments. For comparability with theory, we focus on experiments that study cooperation using game theoretic frameworks. Most of these experiments are incentivized: the payout people receive depends on their earnings in the game. Subjects are told the true rules of the game and deception is prohibited: to explore the effect of different rules on cooperation, subjects must believe that the rules really apply. Finally, interactions are typically anonymous, often occurring via computer terminals or over the internet. This anonymity reduces concerns about reputational effects outside of the laboratory, creating a baseline from which to measure the effect of adding more complicated interaction structures. Box 3. How behavioral experiments inform evolutionary models Experiments shed light on human cooperation in different ways . One type of experiment seeks to recreate the rules of interaction prescribed by a given model. By allowing human subjects to play the game accordingly, researchers test the effect of adding human psychology. Do human agents respond to the interaction rules similarly to the agents in the models? Or are important elements of proximate human psychology missing from the models, revealing new questions for evolutionary game theorists to answer? Other studies explore behavior in experiments in which no mechanisms that promote cooperation are present (e.g., one-shot anonymous games in well-mixed populations). By examining play in these artificial settings, we hope to expose elements of human psychology and cognition that would ordinarily be unobservable. For example, in repeated games, it can be self-interested to cooperate. When we observe people who cooperate in repeated games, we cannot tell if they have a predisposition towards cooperation or are just rational selfish maximizers. One-shot anonymous games are required to reveal social preferences. The artificiality of these laboratory experiments is therefore not a flaw, but can make such experiments valuable. It is critical, however, to bear this artificiality in mind when interpreting the results: these experiments are useful because of what they reveal about the psychology produced by the outside world, rather than themselves being a good representation of that world. 416 Direct reciprocity Over half a century of experiments  demonstrate the power of repetition in promoting cooperation. Across many experiments using repeated PDs, people usually learn to cooperate more when the probability of future interaction is higher [49–55] (in these games, there is typically a constant probability that a given pair of subjects will play another round of PD together). Repetition continues to support cooperation even if errors are added (the computer sometimes switches a player’s move to the opposite of what she intended) , which is consistent with theoretical results [9,56]. More quantitatively, theoretical work using stochastic evolutionary game theory (modeling that incorporates randomness and chance) finds that cooperation will be favored by selection if TFT earns a higher payoff than the strategy Always Defect (ALLD) in a population in which the two strategies are equally common (when TFT is risk-dominant over ALLD) . More generally, as the payoff for TFT relative to ALLD in such a mixed population increases, so too does the predicted frequency of cooperation. Here we show that this prediction does an excellent job of organizing the experimental data: across 14 conditions from four papers, the fraction of cooperators is predicted with R2 = 0.81 by the extent to which the probability of future interaction exceeds the risk dominance threshold (Figure 2). This is one of numerous situations in which stochastic evolutionary game theory  successfully describes observed human behavior [58–61]. 0.9 First period coopera on exploration of the interactions between mechanisms is a promising direction for future research. Trends in Cognitive Sciences August 2013, Vol. 17, No. 8 0.8 0.7 0.6 Key: Dal Bo  Dreber et al.  Dal Bo and Fréche e  Rand et al. [SSRN] 0.5 0.4 0.3 0.2 0.1 0 –0.4 –0.3 –0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 ‘Shadow of the future’: w – (T + P – S – R)/(T – S) TRENDS in Cognitive Sciences Figure 2. Repetition promotes cooperation in the laboratory. The frequency of cooperative strategies in various repeated prisoner’s dilemma (PD) experiments is plotted as a function of the extent to which future consequences exist for actions in the current period. Specifically, the x-axis shows the amount by which the continuation probability w (probability that two subjects play another PD round together) exceeds the critical payoff threshold (T + P – S – R)/(T – S) necessary for tit-for-tat (TFT) to risk-dominate always defect (ALLD). In a population that is 1/2 TFT and 1/2 ALLD, w < (T + P – S – R)/(T – S) means that ALLD earns more than TFT; w = (T + P – S – R)/(T – S) means that TFT and ALLD do equally well; and the more w exceeds (T + P – S – R)/(T – S), the more TFT earns compared to ALLD. The y-axis indicates the probability of cooperation in the first round of each repeated PD game (cooperation in the first period is a pure reflection of one’s own strategy, whereas play in later periods is influenced by the partner’s strategy as well). Data are from [52–54] and [Rand, D.G., et al. (2013) It’s the thought that counts: the role of intentions in reciprocal altruism, http://ssrn.com/abstract=2259407]. For maximal comparability, we do not include the treatments from  with costly punishment, or the treatments from Rand et al. (http://ssrn.com/abstract=2259407) with exogenously imposed errors. Owing to variations in experimental design, subjects in different experiments had differing lengths of time to learn. Nonetheless, a clear increasing relationship is evident, both within each study and over all studies. The trend line shown is given by y = 0.93x + 0.40, with R2 = 0.81. Review Repetition promotes cooperation in dyadic interactions. The situation is more complicated, however, if groups of players interact repeatedly . Such group cooperation is studied in the context of the public goods game (PGG) , an n-player PD. The PGG is typically implemented by giving each of n players an endowment and having them choose how much to keep for themselves and how much to contribute to the group. All contributions are multiplied by some constant r (1 < r < n) and split equally by all group members. The key difference from the two-player PD is that in the PGG, targeted interactions are not possible: if one player contributes a large amount while another contributes little, a third group member cannot selectively reward the former and punish the latter. The third player can choose either a high contribution, rewarding both players, or a low contribution, punishing both. Thus, although direct reciprocity can in theory stabilize cooperation in multiplayer games, this stability is fragile and can be undermined by errors or a small fraction of defectors . As a result, cooperation almost always fails in repeated PGGs in the laboratory [65–67]. Does this mean that mechanisms other than direct reciprocity are needed to explain group cooperation? The answer is no. We must only realize that group interactions do not occur in a vacuum, but rather are superimposed on a network of dyadic personal relationships. These personal, pairwise relationships allow for the targeted reciprocity that is missing in the PGG, giving us the power to enforce group-level cooperation. They can be represented by adding pairwise reward or punishment opportunities to the PGG. (Box 4 discusses costly punishment in repeated twoplayer games). After each PGG round, subjects can pay to increase or decrease the payoff of other group members according to their contributions. Thus, the possibility of targeted interaction is reintroduced, and direct reciprocity can once again function to promote cooperation. Numerous laboratory experiments demonstrate that pairwise reward and punishment are both effective in promoting cooperation in the repeated PGG [65–70]. Naturally, given that both implementations of direct Box 4. Tit-for-tat versus costly punishment The essence of direct reciprocity is that future consequences exist for present behavior: if you do not cooperate with me today, I will not cooperate with you tomorrow. This form of punishment, practiced by TFT in pairwise interactions, via denial of future reward is different from costly punishment; in the latter case, rather than just defecting against you tomorrow, I actually pay a cost to impose a cost on you [54,65–67,84,172–175]. The following question therefore arises: what is the role of costly punishment in the context of repeated pairwise interactions? A set of behavioral experiments revealed that costly punishing in the repeated PD was disadvantageous, with punishers earning lower payoffs than non-punishers. This was because punishment led to retaliation much more often than to reconciliation . Complementing these observations are evolutionary simulations that revealed similar results: across a wide range of parameter values, selection disfavors the use of costly punishment in the repeated PD . Similar results were found in an evolutionary model based on group selection : even a minimal amount of repetition in which a second punishment stage is added causes selection to disfavor both punishment and cooperation because of retaliation. Trends in Cognitive Sciences August 2013, Vol. 17, No. 8 reciprocity promote cooperation, higher payoffs are achieved when using reward (which creates benefit) than punishment (which destroys it). Rewarding also avoids vendettas [54,71] and the possibility of antisocial punishment, whereby low contributors pay to punish high contributors. It has been demonstrated that antisocial punishment occurs in cross-cultural laboratory experiments [72–74] and can prevent the evolution of cooperation in theoretical models [75–78]. These cross-cultural experiments add a note of caution to previous studies on punishment and reward in the PGG: targeted interactions can only support cooperation if they are used properly. Antisocial punishment undermines cooperation, as does rewarding of low contributors [Ellingsen, T. et al. (2012) Civic capital in two cultures: the nature of cooperation in Romania and USA, http://ssrn.com/abstract=2179575]. With repetition and the addition of pairwise interactions, cooperation can be a robust equilibrium in the PGG, but populations can nonetheless become stuck in other, less efficient equilibria or fail to equilibrate at all. Taken together, the many experiments exploring the linking of dyadic and multiplayer repeated games demonstrate the power of direct reciprocity for promoting largescale cooperation. Interestingly, this linking also involves indirect reciprocity: if I punish a low contributor, then I reciprocate a harm done to me (direct reciprocity) as well as a harm done to other group members (indirect reciprocity ). Further development of theoretical models analyzing linked games is an important direction for future research, as is exploring the interplay between direct and indirect reciprocity in such settings. Indirect reciprocity Indirect reciprocity is a powerful mechanism for promoting cooperation among subjects who are not necessarily engaged in pairwise repeated interactions. To study indirect reciprocity in the laboratory, subjects typically play with randomly matched partners and are informed about their choices in previous interactions with others [80,81]. Most subjects condition their behavior on this information: those who have been cooperative previously, particularly towards partners who have behaved well themselves, tend to receive more cooperation [80–89]. Thus, having a reputation of being a cooperator is valuable, and cooperation is maintained: it is worth paying the cost of cooperation today to earn the benefits of a good reputation tomorrow. Figure 3 provides quantitative evidence of the value subjects place on a good reputation by linking PD games with a market in which reputation can be bought and sold . It has also been shown that reputation effects promote prosocial behavior outside of the laboratory. Field experiments find that publicizing the names of donors increases the level of blood donation  and giving to charity . It was also shown that non-financial incentives involving reputation outperformed monetary incentives in motivating participation in an energy blackout prevention program in California  and the sale of condoms on behalf of a health organization in Namibia [Ashraf, N. et al. (2012) No margin, no mission? A field experiment on incentives for pro-social tasks, Harvard Business School Working Paper]. 417 Review Coopera on (A) Trends in Cognitive Sciences August 2013, Vol. 17, No. 8 Key: 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 Reputa on using ‘Standing’ social norm No reputa on 10 20 30 Period Trading price in market for reputa on (B) 16 Key: Social norm: 14 Standing Puni ve standing 12 10 8 6 4 2 0 –5 0 5 10 15 20 –10 Theore cal value of good reputa on 25 TRENDS in Cognitive Sciences Figure 3. Formal reputation systems make cooperation profitable. (A) In a series of randomly shuffled PDs without reputation, cooperation decays over time. In the reputation condition, however, cooperation is maintained at a high rate. Here, subjects are assigned a label of ‘good’ or ‘bad’ in each round, depending on their behavior. The social norm referred to as ‘standing’ is used: Cooperating gives a good reputation and defecting gives a bad reputation, except when a good player meets a bad player; in this case, the good player must defect to obtain a good reputation. (B) Cooperation is costly, but you can benefit from the good reputation you receive if it increases the chance that others will cooperate with you in the future. Thus, the more people in a particular group are inclined to cooperate with those with a good reputation, the greater the value of having a good reputation in that group. Allowing people to buy and sell reputations in a market can be used to assess whether people explicitly understand the value of a good reputation. As is shown here, there is a strong positive correlation between the theoretical value of a good reputation in a given group and the equilibrium trading price in the market (each circle represents one group, with size proportional to the total number of trades in the market). This positive relationship exists using both standing and an alternate norm in which two players with a bad reputation must defect with each other to regain a good reputation. Data reproduced from . Indirect reciprocity relies on peoples’ ability to effectively communicate and distribute reputational information. Not surprisingly, people spend a great deal of their time talking to each other (gossiping) about the behavior of third parties [85,93]. In addition to this traditional form of transmitting reputational information, the internet has dramatically expanded our ability to maintain large-scale reputation systems among strangers. For example, online markets such as eBay have formalized reputation systems in which buyers rate sellers. As predicted by indirect reciprocity, there is a large economic value associated with having a good eBay reputation . Similarly, business rating websites such as Yelp.com create a global-level reputation system, allowing people without local information to reliably avoid low-quality products and services, and creating economic incentives for businesses to earn good reputations [Luca, M. (2011) Reviews, reputation, and revenue: the case of Yelp.com, Harvard Business School NOM Unit Working Paper]. 418 A fascinating question that these studies raise is why people bother to leave evaluations at all. Or, even when people do provide information, why be truthful? Providing accurate information requires time and effort, and is vital for reputation systems to function. Thus, rating is itself a public good . However, indirect reciprocity may be able to solve this second-order free-rider problem itself: to remain in good reputation, you must not only cooperate in the primary interactions but also share truthful information. Exploring this possibility further is an important direction for future research. Enforcement poses another challenge for indirect reciprocity. Withholding cooperation from defectors is essential for the reputation system to function. However, doing so can potentially be damaging for your own reputation. This is particularly true when using simple reputation systems such as image scoring , which is a first-order assessment rule that only evaluates actions (cooperation is good, defection is bad). However, it can apply even when using more complex reputation rules whereby defecting against someone with a bad reputation earns you a good reputation: if observers are confused about the reputation of your partner, defecting will tarnish your name. Here we suggest a possible solution to this problem. If players have the option to avoid interacting with others, they may shun those in bad reputation. Thus, they avoid being exploited while not having to defect themselves. Such a system should lead to stable cooperation using even the simplest of reputation systems. Another interesting possibility involves intermediation: if you employ an intermediary to defect against bad players on your behalf, this may help to avoid sullying your reputation. Consistent with this possibility, experimental evidence suggests that the use of intermediaries reduces blame for selfish actions [96,97]. We expect that researchers will explore these phenomena further in the coming years, using theoretical models as well as laboratory and field experiments. Finally, there is evidence of the central role of reputational concerns in human evolution. Infants as young as 6 months of age take into account others’ actions toward third parties when making social evaluations [98,99]. This tendency even occurs between species: capuchin monkeys are less likely to accept food from humans who were unhelpful to third parties . Humans are also exquisitely sensitive to the possibility of being observed by third parties . For example, people are more prosocial when being watched by a robot with large fake eyes  or when a pair of stylized eye-spots is added to the desktop background of a computer . In the opposite direction, making studies double-blind such that experimenters cannot associate subjects with their actions increases selfishness . Spatial selection Unlike direct and indirect reciprocity, experimental evidence in support of spatial selection among humans is mixed. (There is good evidence for spatial selection in unicellular organisms ). Experiments that investigate fixed spatial structures typically assign subjects to locations in a network and have them play repeatedly with their neighbors. Cooperation rates are then compared to a Review 0.8 Coopera ve players control in which subjects’ positions in the network are randomly reshuffled in each round, creating a well-mixed population. As in theoretical models, subjects in these experiments are usually given a binary choice, either cooperate with all neighbors or defect with all neighbors; and are typically presented in each round with the payoff and choice of each neighbor. However, unlike the models, cooperation rates in these experiments are no higher in structured than in well-mixed populations [106–110]. Various explanations have been advanced for this surprising set of findings. One suggestion is that subjects in laboratory experiments engage in high rates of experimentation, often changing their strategies at random rather than copying higher-payoff neighbors . Such experimentation is analogous to mutation in evolutionary models. High mutation rates undermine the effect of spatial structure: when players are likely to change their strategies at random, then the clustering that is essential for spatial selection is disrupted . Without sufficient clustering, cooperation is no longer advantageous. Another explanation involves the way in which subjects choose which strategy to adopt. Theoretical models make detailed assumptions about how individuals update their strategies, and whether network structure can promote cooperation depends critically on these details . It is possible that human subjects in the experimental situations examined thus far tend to use update rules that cancel the effect of spatial structure . A related argument involves the confounding of spatial structure and direct reciprocity that occurs in these experiments . Subjects in the experiments know that they are interacting repeatedly with the same neighbors. Thus, they can play conditional strategies, unlike the agents in most theoretical models. Because players must choose the same action towards all neighbors, players in these experiments cannot target their reciprocity (like in the PGG). Thus, a tendency to reciprocate may lead to the demise of cooperation. Here we offer a possible alternative explanation. Theoretical work has provided a simple rule for when a fixed network structure will promote cooperation: cooperation is only predicted to be favored when the PD benefit-to-cost ratio exceeds the average number of neighbors in the network . In most of the experiments on fixed networks to date, this condition is not satisfied. Thus, it remains possible that fixed networks will actually succeed in promoting cooperation for the right combinations of payoffs and structure. Exploring this possibility is an important direction for future study. In contrast to these negative results using static networks, dynamic networks successfully promote cooperation in the laboratory (Figure 4) [113–116]. In these experiments, subjects can make or break connections with others and the network evolves over time. This dynamic nature allows subjects to engage in targeted action via ‘link reciprocity’: players can choose to sever links with defectors or make links with cooperators. The importance of dynamic assortment based on arbitrary tags has also been demonstrated in laboratory experiments using coordination games: associations between tags and actions emerge spontaneously, as does preferential interaction between players sharing the same tag . Trends in Cognitive Sciences August 2013, Vol. 17, No. 8 0.6 0.4 Well-mixed popula on Fixed network Dynamic network Key: 0.2 0 1 2 3 4 5 6 7 Round 8 9 10 11 TRENDS in Cognitive Sciences Figure 4. In behavioral experiments, dynamic social networks can promote cooperation via link reciprocity. The fraction of subjects cooperating in a multilateral cooperation game is shown (cooperation entailed paying 50 units per neighbor for all neighbors to gain 100 units). In the well-mixed condition, the network was randomly shuffled in every round. In the fixed network condition, subjects interacted with the same neighbors in each round. In the dynamic network condition, 30% of player pairs were selected at random, and one of the two players could unilaterally update the connection (i.e., break an existing link or create a link if none existed before). Data reproduced from . More generally, there is substantial evidence that social linkages and identity are highly flexible. Minimal cues of shared identity (such as preference for similar types of paintings, i.e., the minimal groups paradigm) can increase cooperation among strangers . Alternatively, introduction of a higher-level threat can realign coalitions, making yesterday’s enemies into today’s allies [119,120]. Such plasticity is not limited to modern humans: many early human societies were characterized by fission–fusion dynamics, whereby group membership changed regularly . The development of evolutionary models that capture this multifaceted and highly dynamic nature of group identity is a promising direction for future work. Models based on changing set memberships [27,122] and tagbased cooperation [30–32] represent steps in this direction. Finally, studies examining behavior in real-world networks also provide evidence of the importance of population structure in cooperation. For example, experiments with hunter–gatherers show that social ties predict similarity in cooperative behavior . A nationally representative survey of American adults found that people who engage in more prosocial behavior have more social contacts, as predicted by dynamic network models . There is also evidence that social structure is heritable , as is assumed in many network models. In sum, there is evidence that spatial selection is an important force in at least some domains of human cooperation. However, further work is needed to clarify precisely when and in which ways spatial selection promotes cooperation in human interactions. Multilevel selection In the laboratory, multilevel selection is typically implemented using interaction structures in which groups compete with each other. For example, two groups play a PGG and compete over a monetary prize: the group with the larger total contribution amount wins, and each member of that group shares equally in the prize. Thus, the incentive to defect in the baseline PGG is reduced by the potential gain from winning the group competition, although 419 Review Box 5. In-group bias is not necessarily evidence of selection at the level of the group Some might argue that the ubiquitousness of in-group bias is proof that multilevel selection played a central role in human evolution. Ingroup bias, or parochial altruism, is a behavioral pattern whereby people cooperate more with members of their own group than with out-group members [118,119,177,178]. It is true that multilevel selection and inter-group conflict can lead to in-group bias [139,169]. However, other mechanisms can also give rise to ingroup bias. Spatial selection can lead to the evolution of in-group bias via set-structured interactions or tag-based cooperation [30,121,171]. Reciprocity can also favor in-group bias. For example, in the context of direct reciprocity, it seems likely that the probability of future interaction is greater for in-group than for out-group members. Given this, it could be adaptive to play cooperative strategies such as TFT with in-group members but to play ALLD with out-group members. Similarly, in the context of indirect reciprocity, information about the behavior of out-group members may be less accurate or detailed . Thus, the presence of in-group bias in human psychology can be explained by different mechanisms and does not necessarily indicate multilevel selection. defection is typically still the payoff-maximizing choice. Numerous such experiments have shown that competition between groups increases cooperation substantially [126– 131]. Furthermore, just phrasing the interaction as a competition between groups, without any monetary prize for winning, also increases cooperation [130,132]. Experience with real-world intergroup conflict also increases cooperation [133,134]. (Note that although the prevalence of in-group favoritism may seem to indicate a psychology shaped by intergroup conflict, such bias can also be explained by other mechanisms; Box 5). In sum, there is ample evidence that intergroup competition can be a powerful force for promoting within-group cooperation. Critics of multilevel selection argue that empirically, the conditions necessary for substantial selection pressure at the group level were not met over the course of human history : concerns include low ratios of between-group to within-group variation because of factors such as migration and mutation/experimentation, and the infrequency of group extinction or lethal inter-group warfare. The laboratory experiments discussed above do not address these concerns: in these studies, the interaction structure is explicitly constructed to generate group-level selection. Instead, anthropological and archaeological data have been used to explore when the conditions necessary for multilevel selection have been satisfied in human history, either at the genetic [37,38] or cultural  level. Kin selection Perhaps surprisingly, kin selection is the least-studied mechanism for human cooperation. Research on humans largely focuses on cooperation between non-kin. In part this is because cooperation between related individuals is seen as expected and therefore uninteresting. Furthermore, humans cooperate with unrelated partners at a much higher rate than for other species, and thus non-kin cooperation is an element of potential human uniqueness. There are also substantial practical hurdles to studying kin selection in humans. The effect of kinship is difficult to measure, because relatedness and reciprocity are inexorably intertwined: we 420 Trends in Cognitive Sciences August 2013, Vol. 17, No. 8 almost always have long-lasting reciprocal relationships with our close genetic relatives. Nonetheless, understanding the role of kinship in the context of human cooperation is important. Parents helping children is not an example of kin selection, but rather straightforward selection-maximizing direct fitness. Kin selection, however, may be at work in interactions between collateral kin (family members who are not direct descendants). In this context, some scholars have investigated the cues used for kin recognition. For example, in predicting self-reported altruistic behavior, an interaction has been found between observing your mother caring for a sibling (maternal perinatal association, MPA) and the amount of time spent living with a sibling (co-residence) : MPA is a strong signal of relatedness, and thus co-residence does not predict altruism in the presence of MPA. In the absence of MPA (e.g., if you are a younger sibling who did not observe your older siblings being cared for), however, coresidence does predict altruism. This interaction suggests that co-residence is used as an indication of relatedness, rather than only as an indication of the probability of future interaction. More studies on this topic are needed, in particular the development of experiments that tease apart the roles of kinship and reciprocity. Progress in this area would be aided by theoretical developments combining evolutionary game theory and population genetics . Cooperation in the absence of any mechanisms How can we explain cooperation in one-shot anonymous laboratory games between strangers? Such cooperation is common , yet seems to contradict theoretical predictions because none of the five mechanisms appears to be in play: no repetition or reputation effects exist, interactions are not structured, groups are not competing, and subjects are not genetic relatives. Yet many subjects still cooperate. Why? Because the intuitions and norms that guide these decisions were shaped outside the laboratory by mechanisms for the evolution of cooperation. How exactly this happens is a topic of debate. There are two dimensions along which scholars disagree: (i) whether cooperation in one-shot interactions is explicitly favored by evolution (through spatial or multilevel selection) or is the result of overgeneralizing strategies from settings in which cooperation is in one’s long-run self-interest (due to direct and indirect reciprocity); and (ii) the relative importance of genetic evolution versus cultural evolution in shaping human cooperation. On the first dimension, one perspective argues that multilevel selection and spatial structure specifically favor altruistic preferences that lead to cooperation in oneshot anonymous settings [38,39,139]. Thus, although laboratory experiments may not explicitly include these effects, they have left their mark on the psychology that subjects bring into the laboratory by giving rise to altruism. The alternative perspective argues that direct and indirect reciprocity were the dominant forces in human evolution. By this account, selection favors cooperative strategies because most interactions involve repetition or reputation. Because cooperation is typically advantageous, we internalize it as our default behavior. This Review Trends in Cognitive Sciences August 2013, Vol. 17, No. 8 cooperative predisposition is then sometimes overgeneralized, spilling over into unusual situations in which others are not watching [103,140]. In this view, cooperation in anonymous one-shot settings is a side effect of selection for reciprocal cooperation, rather than an active target of selection itself. Note that in both views, evolution gives rise to people who are truly altruistic and cooperate even when there are no future benefits from doing so: the disagreement is over whether or not that altruism was directly favored by selection or is a byproduct of selection in non-anonymous interactions. Turning to the second dimension, all of the mechanisms for the evolution of cooperation can function via either genetic or cultural evolution. In the context of cultural evolution, traits spread through learning, often modeled as imitation of strategies that yield higher payoffs or are more common . It has been argued by some that multilevel selection promotes cooperation through genetic evolution , whereas others posit an important role of culture [38,142–144]. The same is true for reciprocity. We might have genetic predispositions to cooperate because our ancestors lived in small groups with largely repeated interactions [140,145]. Or we might have learned cooperation as a good rule of thumb for social interaction, because most of our important relationships are repeated and thus cooperation is typically advantageous, as per the ‘social heuristics hypothesis’  [Rand, D.G. et al. (2013) Intuitive cooperation and the social heuristics hypothesis: evidence from 15 time constraint studies, http://ssrn.com/ abstract=2222683]. Thus one’s position in this second area of debate need not be tied to one’s belief about the first. Figure 5. Automatic, intuitive responses involve reciprocal cooperation strategies. (A) In a one-shot public good game, faster decisions are more cooperative. Thus, it is intuitive to cooperate in anonymous settings. Data reproduced from . (B) In a repeated prisoner’s dilemma, faster decisions are more cooperative when the partner cooperated in the previous round, and are less cooperative when the partner did not cooperate in the previous round. Thus, it is intuitive to reciprocate in repeated settings. Analysis of data from  and the no-error condition of . For visualization, we categorize decisions made in Purchase answer to see full attachment
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