2. Reciprocity and social cooperation

The problem of cooperation is one of the classic puzzles of social science and political philosophy. Following a tradition that goes back to Hobbes, social theorists have used the Prisoner's Dilemma to represent the problem of cooperation in a situation where each individual has an incentive to defect from the social contract and free-ride on the fruits of others' labour (Fig. 1). This is the “State of Nature” of classic political philosophy, where no player can trust the others to behave pro-socially.

Figure 1. A Prisoner's Dilemma game. The usual conventions apply: The strategies of Player 1 are represented as rows, and those of Player 2 as columns. The first number in each cell is the payoff of Player 1; the second one of Player 2.

In the Prisoner's Dilemma game two players must choose simultaneously one of two strategies, Cooperate (C) or Defect (D). It is immediately obvious that mutual cooperation (CC) is more efficient than mutual defection (DD). The payoffs of the game, however, are designed in such a way that each player has an incentive to defect, regardless of what the other player does. If the other player cooperates, defection delivers three units of payoff instead of two; if the other defects, it guarantees one unit instead of nothing. But this reasoning should lead both players to defect: In game-theoretic jargon, mutual defection (DD) is the only Nash equilibrium in the one-shot Prisoner's Dilemma.

A Nash equilibrium is a set of strategies (one for each player in a game) such that no one can do better by changing her strategy unilaterally. Nash equilibria are self-sustaining, or self-policing, in the sense that they are robust to individual attempts to gain by deviating from the current strategies (because, quite simply, no such gains are possible). It seems highly desirable that social institutions should be Nash equilibria, for they would be robust to exploitation and the constant threat posed by individual greed. “Cooperate” in the Prisoner's Dilemma is a prototypical rule that would enhance social welfare if generally endorsed by the members of the group. It is not, however, a stable institution, for it is not a Nash equilibrium of this simple game. Although mutual cooperation (CC) is more efficient than mutual defection, it is strictly dominated and will not be played by rational selfish individuals. If the social contract game were a one-shot Prisoner's Dilemma, then a population of rational players would never be able to pull themselves out of the war of all against all.

For many social scientists, the puzzle of cooperation is just an artefact of the peculiar behavioural assumptions of standard economic theory: Surely only selfish economic agents defect in dilemma games, while the rest of us – “the folk” – can do much better than that. But this view is simplistic. Far from being an arbitrary assumption, the self-interest principle is well-rooted in evolutionary theorizing. Indeed, cooperation is in many ways more puzzling from a biological, than from an economic point of view.

“Biological altruism” denotes any behaviour that increases the chance of survival and reproduction of another (genetically unrelated) organism, at the expense of the altruist's direct fitness. Biologists have known for decades that the problem of biological altruism is structurally similar to a social dilemma game in economists' sense (Axelrod & Hamilton Reference Axelrod and Hamilton1981; Dawkins Reference Dawkins1976; Trivers Reference Trivers1971). An organism that does not help but receives help from others will produce on average more offspring, spreading its “selfish” genes more efficiently than its altruistic fellows. Altruists (i.e., organisms playing C-strategies) should be washed out by the forces of natural selection, leaving only self-interested players behind.

But Homo sapiens' spectacular success, in fitness terms, surely has something to do with social cooperation. So the puzzle remains. According to a prominent tradition in economics and biology, the solution lies in the concept of reciprocity. Reciprocity is a human propensity to respond with kindness to kind actions, and with hostility to nasty actions. Its logic is encapsulated in different cultures by Golden-Rule principles such as “Do to others what you would like to be done to you” and “Hurt no one so that no one may hurt you.”

Reciprocity theory bloomed in the 1970s when game theorists and theoretical biologists almost simultaneously began to study the properties of conditional strategies in repeated games.Footnote ² Robert Axelrod's (Reference Axelrod1984) tournaments are perhaps the best-known setting of this kind. Axelrod experimented with artificial players competing in a series of repeated dilemma games. Famously, a strategy called “Tit-for-tat” emerged as the winner in these tournaments. Tit-for-tat is a rudimentary rule of reciprocity, offering cooperation at the outset and then copying whatever move one's partner has made in the previous round. In spite of several limitations (Bendor & Swistak Reference Bendor and Swistak1995; Reference Bendor and Swistak1997; Binmore Reference Binmore1998, Ch. 3), Axelrod's simulations convinced many scholars that reciprocity can sustain cooperation in the long run, and that pairs of reciprocators are more efficient producers of resources than selfish free-riders.

This insight had a precursor in the biological concept of “reciprocal altruism” (Trivers Reference Trivers1971), the idea that what seems altruistic in the short run might actually be self-serving in the long term. Organisms that help others may be indirectly maximizing their own fitness, if their help is going to be reciprocated in the future. To capture the self-serving aspect of cooperation, I classify these approaches under the umbrella of “weak reciprocity” theory, and distinguish them from alternative (“strong”) models that – paraphrasing Trivers – are not designed to “take the altruism out of altruism” (see sect. 3).

Axelrod's (Reference Axelrod1984) and Trivers' (Reference Trivers1971) findings are consistent with a general game-theoretic result known as the folk theorem (Fudenberg & Maskin Reference Fudenberg and Maskin1986; Fudenberg et al. Reference Fudenberg, Levine and Maskin1994). Informally, the folk theorem says that any strategy guaranteeing at least as much as the worst payoff that can be inflicted by the other player is a Nash equilibrium of an indefinitely repeated game. In the repeated Prisoner's Dilemma, a partner who does not reciprocate can be punished by withdrawing cooperation, a mechanism known as “trigger strategy” in game theory. Suppose that in the first round, I play cooperate and you play defect. From the second round, I can “punish” you by defecting, ensuring that your future stream of payoffs is not greater than one unit per period. Because mutual cooperation would guarantee an expected average payoff of two units, you are better off cooperating right from the start. The threat of defection makes mutual cooperation attractive, if the shadow of the future is long enough to make it worthwhile.

The folk theorem carries good and bad news for evolutionary social theory. The good news is that in an indefinitely repeated game, cooperation is sustainable using trigger strategies that punish deviation from cooperative behaviour and cancel the advantages of defection. The bad news is that infinitely many strategies are Nash equilibria of this sort. Tit-for-tat is only one among many equilibria in the infinitely repeated Prisoner's Dilemma. Consider a strategy profile such as “I cooperate on Monday, Wednesday, and Friday, and you cooperate on Tuesday, Thursday, Saturday, and Sunday.” Using the matrix of Figure 1, such a profile delivers an average payoff of 1.71 to me, and 1.28 to you. Because it is better than the worst penalty I can inflict (by withdrawing cooperation) if you do not follow it, it is a Nash equilibrium of the indefinitely repeated game. But like many other strategy profiles, it is not equitable (in many ways, in fact, it is intuitively unfair).

How can we identify, among all the possible equilibria, the ones that will be actually played? Communication can certainly improve coordination among organisms – such as humans – who have the capacity to exchange signals. Moreover, it is possible that selection drives out inefficient signals and their respective equilibria in the long run. The idea is that richer, more productive societies may outperform less efficient ones and replace them by absorption, extinction, or a combination of both. This is known in theoretical biology as the process of group selection. Although it came in disrepute during the 1970s, the idea that selection can operate at group level has been rehabilitated and is now widely used to explain processes of social evolution (Bergstrom Reference Bergstrom2002; Boyd & Richerson Reference Boyd and Richerson1990; Wilson & Sober Reference Wilson and Sober1994). If homogeneous groups of conditional cooperators are more efficient, in the long run they should be able to outperform homogeneous groups of free-riders trapped in suboptimal equilibria. This result, again, holds only under certain restrictive conditions (for group selection to operate, for example, groups must be relatively stable and impermeable to immigrants carrying different traits), but it gives us the beginning of an explanation of the evolution of social cooperation.

3. Strong reciprocity

In weak reciprocity theory withdrawing cooperation is a strategy of self-defence that damages the free-rider but benefits the reciprocator. Weak reciprocity mechanisms therefore appeal to individuals' self-interest (as well as foresight). The folk theorem, for example, does not require that we relax the standard assumptions of self-interest and rationality of neoclassical economic theory. Similarly, Trivers' (Reference Trivers1971) “reciprocal altruism” is not a disinterested form of altruism: A missed opportunity to exploit others' cooperation now, to be sustainable, must be fully repaid by mutual cooperation in the future.

Explaining cooperation by individual self-interest, however, comes at a price. Three conditions limit the application of weak reciprocity mechanisms to a rather narrow set of circumstances: First, the shadow of the future must be long enough, in an objective and subjective sense: the players must not discount future payoffs too heavily, otherwise the temptation to defect will be strong regardless of the future stream of gains from cooperation. Second, the number of players must be small, so that monitoring cooperation is relatively easy, and the withdrawal of cooperation does not damage too many cooperators. Third, information in the group must circulate freely and without error, for otherwise the threat of punishment will be ineffective. When some of these conditions do not apply, the folk theorem holds only for unrealistically high values of the other parameters (Fudenberg et al. Reference Fudenberg, Levine and Maskin1994).

These limitations, according to some critics, make the folk theorem a poor tool for the analysis of social cooperation (Gintis Reference Gintis2006; Reference Gintis2009). Discounting future gains is a well-established fact of human psychology; in large modern societies, moreover, one-shot encounters with unrelated strangers are ubiquitous, and information is rarely transparent. So, the critics argue, we need a kind of reciprocity that is able to sustain cooperation where weak reciprocity cannot reach and folk-theorem mechanisms fail.

Strong reciprocity theory is the result of collaboration between experimental economists, game theorists, anthropologists, and theoretical biologists interested in the evolution of human cooperation. Samuel Bowles, Herbert Gintis, Ernst Fehr, Robert Boyd, and Peter Richerson are its best-known advocates, but many other social scientists and biologists have contributed to its success (Gintis et al. Reference Gintis, Boyd, Bowles and Fehr2005; Henrich et al. Reference Henrich, Boyd, Bowles, Camerer, Fehr and Gintis2004). The theory departs from the classic approach by modelling “strong reciprocators,” who, unlike weak ones, are not solely concerned about future gains. Strong reciprocators cooperate because they feel it is the right thing to do, and they are ready to punish defectors at a cost. Punishment is not merely withdrawal from cooperation, but involves the subtraction of resources from free-riders. Since taking resources away requires an active effort or risk, punishers pay a fee that is subtracted from their earnings. The act of punishment results in an immediate reduction of welfare both for the punisher and for the punished individual.

Strong reciprocity nevertheless has some important advantages compared to its weak cousin. Costly punishment ensures that defectors do not enjoy the fruits of free-riding. Free-riders, moreover, are punished by strong reciprocators even in one-shot games and when the future is heavily discounted. Strong reciprocity thus can potentially support cooperation even in large groups, where repeated encounters are rare or unlikely, and interactions with strangers are common. Costly punishment changes radically the incentives of free-riders, without affecting the other cooperators in the group.

The logic of punishment, however, takes the form of a “second-order” social dilemma: Sanctions are a public good that benefit all cooperators in the group, but are paid by the punisher only. In principle, everybody would like free-riders to be punished, but would prefer that somebody else do it. A plausible hypothesis is that the second-order dilemma is solved by automatic mechanisms – such as emotions, internalized norms, or social preferences – that bypass strategic considerations and trigger actions that would be avoided by a rational selfish calculator (Frank Reference Frank1988; Hirshleifer Reference Hirshleifer and Dupré1987). But could these mechanisms have survived Darwinian selection?

Simulations suggest that cooperative strategies can evolve in favourable conditions (Bowles & Gintis Reference Bowles and Gintis2004; Boyd & Richerson Reference Boyd and Richerson1992; Boyd et al. Reference Boyd, Gintis, Bowles and Richerson2003; Gintis Reference Gintis2000; Henrich & Boyd Reference Henrich and Boyd2001). These conditions include a certain degree of behavioural homogeneity within groups, trait diversity across groups, and selection mechanisms that grant a higher survival rate to members of more cooperative groups. Notice that the problem of multiple equilibria is more severe for strong reciprocators because costly punishment can support an even wider range of equilibria, including equilibria that are not welfare- or fitness-enhancing for the group (Boyd & Richerson Reference Boyd and Richerson1992). A community, for example, may be prevented from adopting a set of beneficial strategies, simply because they depart from what is considered “correct” behaviour by an aggressive gang of moralistic punishers. In such conditions, evolution arguably must play an even more important role in the process of equilibrium selection, than in classic weak reciprocity models.

4. Costly punishment in the laboratory

The picture of human motives painted by strong reciprocity theory is intuitively appealing – but is it empirically accurate? Since the 1980s the strongest evidence in its favour has come from laboratory experiments, and therefore we will have to examine data and experimental designs in some detail. Although the experimental literature is already large – and constantly growing – it is driven by a set of core results and robust patterns, which are the main focus of this article.

Experimental economists' interest in costly punishment derives from the analysis of a simple bargaining setting known as the Ultimatum Game (Güth et al. Reference Güth, Schmittberger and Schwarze1982). The Ultimatum Game is the simplest sequential bargaining situation that one can think of: Two players have the opportunity to share a sum of money (say, 10 dollars). Player 1 has the advantage of making the first offer. This introduces an important power asymmetry: Player 2 at this point can only accept or reject. If Player 2 accepts, the two players earn the proposed amount; if Player 2 rejects, they walk out with nothing. The unique sub-game perfect equilibrium of the Ultimatum game is for Player 1 to offer as little as possible (one dollar, for example), and for Player 2 to accept because a dollar is better than nothing. The Ultimatum Game in theory should give rise to very inequitable distributions of resources.

When the Ultimatum Game is played for real, however, fair allocations figure prominently. Experiments in North America and West Europe result in average offers between 30% and 40% of the endowment, and a mode at the 50–50 split. Unfair offers (of 30% or less) are rejected about half of the time (Camerer Reference Camerer2003). A common interpretation of this behaviour is in terms of strong negative reciprocity: People are willing to pay a cost to punish offers that they perceive as unfair, even though they are not going to meet the offender ever again. By so doing, they fulfill a useful social function, for unfair players learn what is expected of them, and conform to the prevailing norm in future encounters.Footnote ³

The insight of the Ultimatum Game can be extended to other game-theoretic settings. In a widely cited series of experiments, a group of economists led by Ernst Fehr have studied the effect of punishment on cooperation in public goods and other social dilemma games (e.g., Falk et al. Reference Falk, Fehr and Fischbacher2005; Fehr & Fischbacher Reference Fehr and Fischbacher2004; Reference Fehr, Fischbacher, Gintis, Boyd, Bowles and Fehr2005; Fehr & Gächter Reference Fehr and Gächter2000a; Reference Fehr and Gächter2002; de Quervain et al. Reference de Quervain, Fischbacher, Treyer, Schellhammer, Schnyder, Buck and Fehr2004); this approach was pioneered by Yamagishi (Reference Yamagishi1986) and Ostrom et al. (Reference Ostrom, Walker and Gardner1992). The classic dilemma situation is modified adding a second stage, in which cooperators can punish free-riders and destroy what they have illicitly gained. Punishment comes at a cost, however, in the form of a fee paid by the subjects who voluntarily engage in this sort of policing.

Adding the punishment phase radically changes the game. Take the simplest case of a two-player Prisoner's Dilemma, as shown in Figure 2: The matrix in Figure 2a is turned into a more complex game by the addition of an extra strategy P in the second stage for the cheated player, as in Figure 2b.Footnote ⁴ Suppose, for example, that Row defects while Column cooperates. The outcome of the first stage of the game is (3, 0), but in the second stage Column is given the opportunity to move unilaterally from (3, 0) to (0, –1). This option is strictly dominated, and should not be chosen by a rational self-interested player. Yet, if Player 2 manages to convince Player 1 that she will play P, she will effectively transform the Prisoner's Dilemma into a coordination problem such as that of Figure 2c, where mutual cooperation (CC) is a Nash equilibrium of the game, and a Pareto-efficient one as well.

Figure 2. Transformation of a Prisoner's Dilemma (a) into a coordination game (c), by the addition of a punishment option (b).

This is apparently what happens in standard punishment experiments with public goods games: in spite of the fee, many people are willing to sanction, and their threat is credible enough to raise cooperation to high levels (Fehr & Gächter Reference Fehr and Gächter2000a; Reference Fehr and Gächter2002). This result holds both when the game is played repeatedly by the same players (for a finite number of rounds), and when the membership of the group changes at every round. Costly punishment is administered even in one-shot games (Gächter & Herrmann Reference Gächter and Herrmann2009; Walker & Halloran Reference Walker and Halloran2004) and by “bystanders” or “third parties” – that is, when the potential punishers are not themselves the victims but have merely witnessed exploitative behaviour (Fehr & Fischbacher Reference Fehr and Fischbacher2004) – although in such cases it does not always raise the average level of cooperation.

Recent studies with brain imaging have provided further insights about the psychological and neural mechanisms implicated in such behaviour. Costly punishment seems to be partly triggered by an impulsive negative reaction against injustice (Sanfey et al. Reference Sanfey, Rilling, Aaronson, Nystrom and Cohen2003) and partly motivated by the sheer pleasure of punishing social deviants (de Quervain et al. Reference de Quervain, Fischbacher, Treyer, Schellhammer, Schnyder, Buck and Fehr2004). Building on this evidence, strong reciprocity theorists have argued that reciprocal motives are robust enough to be represented as “social preferences” governing individual behaviour across a variety of decision tasks. Although the formal representation of reciprocity raises a number of difficult technical issues, various models have been proposed in the game theory literature, and probably even more will appear in the future (see Falk & Fischbacher Reference Falk, Fischbacher, Gintis, Boyd, Bowles and Fehr2005 for a survey).

5. Two interpretations of punishment experiments

Costly punishment is robust to replication, a real experimental phenomenon that can teach us something about the mechanics of cooperation. And yet, it is not clear what it does teach, exactly. In this section I will argue that the success of strong reciprocity theory derives in part from equivocating two possible readings of punishment experiments – “narrow” and “wide” – which have different epistemic statuses and implications. While the narrow reading is unobjectionable, it will turn out that the wide one is currently little more than a conjecture. Since its popularity is partly due to its conflation with the narrow (and empirically warranted) interpretation, it is important to distinguish them clearly before we proceed.

According to the narrow interpretation, punishment experiments open an interesting window on psychological motives and reactions to violations of social norms. In a review aimed at advertising punishment experiments among non-economists, for example, Colin Camerer and Ernst Fehr write that “the purpose of this chapter is to describe a menu of experimental games that are useful for measuring aspects of social norms and social preferences” (Camerer & Fehr Reference Camerer, Fehr, Henrich, Boyd, Bowles, Camerer, Fehr and Gintis2004, p. 55). The punishment design seems to be motivated primarily by methodological concerns, rather than by realism. Similarly, according to Fehr and Schmidt,

Under this interpretation, punishment mechanisms are useful methodological devices to observe social preferences. (I use the term “preference” broadly, to cover all sorts of dispositions including desires, emotions, and feelings; on the use of experiments as measurement devices, see Guala Reference Guala2008). This narrow reading is uncontroversial: As far as I am aware, nobody denies that punishment experiments can be used to learn about human attitudes towards cooperation in the lab. But the narrow interpretation does not imply that costly punishment sustains social cooperation in the real world. Costly punishment is just the experimenter's way of turning unobservable attitudes and dispositions (“preferences”) into observable and quantifiable experimental variables.

The wide interpretation of punishment experiments is bolder: Punishment mechanisms are not just measurement devices, but replicate in the laboratory the same processes that support cooperation in the real world. There is no doubt that strong reciprocity theorists interpret their experiments in this wide sense, to support a general account of cooperation based on costly punishment mechanisms. In one of the seminal papers in this literature, for example, Fehr and Gächter claim that “in our view punishment of free-riding also plays an important role in real life” (2000, p. 993). Influential anthropologists Boyd and Richerson add that:

Following the anthropologists' lead, Camerer and Fehr suggest that costly punishment sustains cooperative practices such as food sharing in small groups of hunter-gatherers:

According to Fehr and Fischbacher:

The narrow and wide interpretations of punishment experiments correspond roughly to two levels of validity of experimental results that are sometimes distinguished in the methodological literature in psychology, economics, and biology (Bardsley et al. Reference Bardsley, Cubitt, Loomes, Moffatt, Starmer and Sugden2009; Guala Reference Guala2005; Steel Reference Steel2007). According to this distinction, an experimental result is internally valid when the experimenters have correctly inferred the causal factors or mechanisms that generate data in a particular laboratory setting. Identifying data-generating processes in the lab, however, is rarely the ultimate goal of experimenters in the social sciences. Researchers typically want to find out about variables and processes that play an important role in a class of non-laboratory phenomena of interest (phenomena “in the real world,” as they sometimes put it). The wide interpretation makes the additional claim that experimental results can be extrapolated to explain cooperation in some class of non-laboratory conditions, and so it amounts to an external validity inference.

Notice that the result of every well-designed experiment is valid, trivially, in all non-laboratory circumstances that replicate exactly the experimental conditions. But since the point of running controlled experiments is to create conditions that cannot easily be found in nature, where specific theories can be rigorously tested and new hypotheses investigated, the application of experimental results typically involves an external validity inference or generalization. This generalization requires extra evidence to be sustained, and the quality of this evidence, as we shall see, is very questionable in the case of costly punishment.

6. Experiments in the field

Costly punishment is used explicitly to explain cooperation in large societies, where one-shot encounters are common and information is poor. This may suggest that the punishment story accounts for a real-world phenomenon and is not just the artefact of a peculiar experimental setting. But this conclusion would be too hasty, for disagreement between the weak and strong reciprocity camps begins at the level of the phenomenon to be explained. Critics of the costly punishment story usually hold that one-shot cooperation among strangers in large-scale societies does not take place (except sporadically and unsystematically): The limits of the folk theorem are the limits of spontaneous cooperation. Outside the boundaries of the family, the small circle of a local community, or the long-term relationships we cultivate with business partners, we need other incentives (such as those provided by centralized policing) to prevent exploitation, free-riding, and abuse of power (e.g., Binmore Reference Binmore2005, p. 82). Weak reciprocity theory, in other words, draws different boundaries for spontaneous cooperation, and cannot be blamed (without begging the question) for its presumed “failure” to explain a phenomenon that by its own light may well not exist.

The key source of disagreement, then, is spontaneous cooperation outside the lab. Supporters of strong reciprocity sometimes seem to claim that costly punishment has been observed in the field, which obviously would resolve the issue of validity at once. But such a claim, again, trades on ambiguity. Costly punishment has indeed been observed across subject pools in several developed countries, as well as in Ultimatum and Public Goods experiments run in small-scale societies (Henrich et al. Reference Henrich, Boyd, Bowles, Camerer, Fehr and Gintis2004; Herrmann et al. Reference Herrmann, Thöni and Gächter2008; Marlowe et al. Reference Marlowe, Berbesque, Barr, Barrett, Bolyanatz, Cardenas, Ensminger, Gurven, Gwako, Henrich, Henrich, Lesorogol, McElreath and Tracer2008). But none of these studies investigates behaviour in a natural setting or amounts to a natural field experiment as the term is used in economics.

Harrison and List (Reference Harrison and List2004) distinguish between artefactual and natural field experiments. A natural field experiment successfully manipulates one variable of interest in an environment that is otherwise left as much as possible unaffected by the experimenter. Ideally, the subjects should be unaware that they are participating in an experiment, and select their responses from a menu of strategies that they normally use in their everyday lives. Artefactual experiments, in contrast, differ from conventional laboratory studies only with respect to the sample of subjects, which is drawn from the target population instead of some more convenient pool (e.g., a population of African bushmen, as opposed to university undergraduates, if we are studying cooperation in small-scale societies). The strategic setting and the framing, however, are imposed by design instead of mirroring a realistic decision-making environment (whether the experiments are performed in a university lab or in a hut in the African forest is irrelevant). So-called field experiments with punishment are artefactual in this sense, for they involve situations that are probably quite unfamiliar to the decision makers, and as we shall see, they do not reproduce the full menu of strategies that are available in the dilemmas of cooperation that people face in everyday life. In fact, it would be more appropriate to speak of “experiments in the field” in this case, rather than “field experiments.”

This is not merely a terminological quibble. Artefactual designs raise serious issues for the wide interpretation of punishment experiments. As the terminology suggests, these experiments are more likely to generate experimental artefacts than natural ones. This does not mean, of course, that they are useless. On the contrary, they are extremely helpful because they guarantee a higher degree of control on the environment and allow the elimination of potentially confounding variables that may elude control in the field. It does not mean that experimental phenomena such as costly punishment are somewhat unreal either. A phenomenon may be real and artefactual – a real experimental effect generated in circumstances that do not mirror those naturally found in the natural or social world (Hacking Reference Hacking1988). As we shall see, there are good reasons to believe that costly punishment is a “real artefact” in this sense of the term: Artefactual insofar as it is produced by the specific experimental procedures, but nevertheless real because it does take place in a limited range of (laboratory-like) conditions.

7. Repetition and evolutionary scale

External validity objections can hinder scientific progress when they are meant to raise sceptical doubts about the use of experiments generally. But external validity worries are inescapable and indeed useful when addressed to the specific details of an experimental design, for they help establish the reliability of specific inferences from the laboratory to field settings (cf. Bardsley et al. Reference Bardsley, Cubitt, Loomes, Moffatt, Starmer and Sugden2009; Guala Reference Guala2005; Starmer Reference Starmer1999). It is in the latter spirit that one must ask whether costly punishment is an artefact of the experimental setting that economists implement in their laboratories.

One major external validity problem has to do with scale: Both strong and weak reciprocity models describe behaviour on an evolutionary time-scale and are not primarily intended to capture choices in experimental games that last only for a short time (Binmore Reference Binmore1998; Reference Binmore2005; Ross Reference Ross2006). Of course, there is no reason to expect that what evolves in the long run is similar to what we observe in the short run of experimental games. When people play Ultimatum Games in the laboratory, for example, they may bring with them norms and heuristics that help coordination in their everyday dealings. Such dealings are often in the form of indefinitely repeated games, where egalitarian splits can be sustained by weak reciprocity mechanisms. The behaviour observed in the laboratory thus may be a misapplication, in an unfamiliar setting, of a heuristic rule that works well (and was selected for) in the larger but more familiar games that we play in real life. If the experimental games were repeated long enough, however, out-of-equilibrium strategies would be eliminated by evolutionary forces and learning, until behaviour approaches a rational equilibrium (cf. Binmore Reference Binmore1998; Reference Binmore1999; Reference Binmore2006; Burnham & Johnson Reference Burnham and Johnson2005; Hagen & Hammerstein Reference Hagen and Hammerstein2006; Trivers Reference Trivers2004).

This argument sounds plausible, but unfortunately it is inconclusive. To begin with, it is easy to retort that pro-social behaviour in settings such as the Ultimatum Game is remarkably robust even when the games are repeated for several rounds (e.g., Cooper & Dutcher Reference Cooper and Dutcher2009; Roth et al. Reference Roth, Prasnikar, Okuno-Fujiwara and Zamir1991). The rate of costly punishment has been observed to increase, rather than decrease, after as many as 50 rounds of play (Gächter et al. Reference Gächter, Renner and Sefton2008). If strong reciprocity “misfires” in finitely repeated games, it does so systematically enough to be of theoretical interest for social scientists (Richerson & Boyd & Reference Richerson and Boyd2005, p. 220), since what happens in the very long run is irrelevant for the many short-run games that we play in the lab and in real life. Next, there is evidence that experimental subjects distinguish between one-shot and finitely repeated games, and modulate their strategies accordingly (Fehr & Fischbacher Reference Fehr and Fischbacher2002; Reference Fehr, Fischbacher, Gintis, Boyd, Bowles and Fehr2005; Gintis et al. Reference Gintis, Boyd, Bowles and Fehr2003). To insist that they do not understand the difference between finitely and indefinitely repeated games, as some critics do (e.g., Binmore Reference Binmore1999; Reference Binmore2006), therefore seems arbitrary and unjustified.

These replies are powerful, and the critics of strong reciprocity theory are wrong to insist on this line of argument. From a logical point of view, one can keep asking whether costly punishment would survive hundreds or thousands of repetitions. (How many times can you get angry in an indefinitely repeated Ultimatum Game?) And yet, this challenge in itself does not lead to any new testable proposition: It belongs to the class of sceptical challenges to experimentation that bring the discussion to a halt, unless new evidence is offered in support.

Complemented with new data, in contrast, external validity worries can become a powerful engine for scientific progress – they can be used to make interesting predictions that are tested empirically. It is in this constructive spirit that we must look for field data concerning costly punishment. To assess the wide interpretation of punishment experiments, we must study “richer” situations, where decision makers can choose from the full range of strategies that are customarily available in everyday life. Natural field experiments are richer just in this sense. But since there are no natural field experiments on costly punishment, we ought to look for relevant data elsewhere. The next four sections (8 to 11) review non-experimental evidence that is seldom discussed by theorists on either side of the controversy, that is, ethnographic data from anthropology, a source that is often cited by reciprocity theorists but never analysed in much depth. I shall return briefly to laboratory data in section 12, while section 13 deals with historical evidence concerning common pool institutions.

8. Costly punishment in small societies

The Leviathan is a relatively recent invention. During most of their evolutionary history, Homo sapiens probably lived in small egalitarian bands without a centralised leadership. The head of each family enjoyed a high degree of autonomy in decision-making, and even the most authoritative men in the band could only persuade – never force – others to follow a certain course of action. In the words of Marshall Sahlins:

The small-scale societies of hunter-gatherers, horticulturalists, and nomadic pastoralists that have been studied extensively by anthropologists are probably the last remnants of these ancient acephalous social orders based on spontaneous cooperation. Although strong reciprocity theorists say that their models explain the emergence and maintenance of cooperation in small egalitarian societies, they provide surprisingly thin evidence in support.

According to Bowles and Gintis (Reference Bowles and Gintis2002, p. 128), for example, “studies of contemporary hunter-gatherers and other evidence suggest that altruistic punishment may have been common in mobile foraging bands during the first 100,000 years or so of the existence of modern humans.” In support of this claim, however, they cite a study (Boehm Reference Boehm1999) that does not endorse a costly punishment account of human sociality. Richerson and Boyd (Reference Richerson and Boyd2005, p. 219) write that “in small-scale societies, considerable ethnographic evidence suggests that moral norms are enforced by punishment.” Among their references, however, one finds only two ethnographic surveys, a laboratory experiment, and a study of dominance that do not support the costly punishment story (cf. Richerson & Boyd Reference Richerson and Boyd2005, p. 280, n. 60).

Most of Richerson and Boyd's (2005) case is, in fact, based on Fehr and Gächter's (2000a; 2002) experiments. Fehr and his colleagues state that “private sanctions have enforced social norms for millennia, long before legal enforcement institutions existed, and punishment by peers still represents a powerful norm enforcement device, even in contemporary Western societies” (Spitzer et al. Reference Spitzer, Fischbacher, Herrnberger, Grön and Fehr2007, p. 185). “The prominent role of such peer punishment” is reported as an established fact, even though their bibliography refers only to a laboratory experiment (Fehr & Gächter Reference Fehr and Gächter2002), an evolutionary model (Boyd et al. Reference Boyd, Gintis, Bowles and Richerson2003), and a survey of ethnographic evidence that – again – does not support a costly punishment account of the evolution of cooperation (cf. Sober & Wilson Reference Sober and Wilson1998, pp. 166–68).

The costly punishment account of cooperation in small societies, then, seems to lack a solid base of ethnographic evidence. This is not surprising, for as we shall see, the available data are scarce. Before we look at the data more carefully, however, it is worth asking what kind of evidence would support the strong reciprocity story. Notice that all the aforementioned quotes tend to conflate costly punishment with punishment in general. But while there is no doubt that sanctions are crucial for the maintenance of social order, it is by no means obvious that they are costly for those who administer them. This is an important point that is often overlooked, or perhaps willingly confused in the literature: The very definition of strong reciprocity calls for evidence of material and costly punishment behaviour in field settings:

More precisely, there are two kinds of cost that are relevant for our purposes. One is the absolute cost, the fee paid by an individual (in material terms) to punish a free-rider. The other one is the relative cost of punishment, that is, the difference between the net benefit of the punisher and the benefit of the other group members who choose not to punish. Absolute and relative costs must be kept separate because they raise different problems for different theoretical perspectives. When sanctions are costly in absolute terms, punishment cannot be explained using models based on self-interested motivation. If the cost is compensated by a positive benefit (to the punisher), in contrast, punishment is consistent with self-interest but potentially problematic from an evolutionary point of view. There may still be a relative cost in fact, and individual selection may work against the punisher (non-punishers may be advantaged in fitness terms, in other words).Footnote ⁵ But it is also possible that the relative cost is nil, because the costs are spread in such a way that everybody carries an equal share of the overall burden. In such a case, punishment would not be selected against within the group.Footnote ⁶

Keeping these concepts in mind, we ought to ask two questions: Does punishment in small-scale societies involve an absolute cost? If so, is the cost borne by a single individual, or is it distributed across group members in such a way as to minimize the relative cost? Answering is not easy, because most of the evidence of punishment in small societies is anecdotal, and quantitative data regarding the frequency, intensity, and effect of material punishment are scarce. Another related problem is that the benefits from punishing a free-rider are often delayed, and even when we observe an immediate cost, we can rarely rule out that it will not be recouped at a later time. The most cooperative and popular members of a group, for example, may have easier access to sexual mates, an incentive mechanism that only bears fruit in the medium-long term of a reproductive cycle (Hawkes Reference Hawkes1993).

Notice that for this reason cooperation in small societies does not constitute a very good test-case for strong reciprocity theory. Most interactions between the members of small societies take the form of an indefinitely repeated game, with relatively high monitoring and circulation of information. These interactions, far from being anonymous as in most laboratory experiments, rely crucially on reputation and trust (Wiessner Reference Wiessner2009). This does not mean that such cooperation should be interpreted by default in weak reciprocity terms, of course; but it does mean that a priori the costly punishment story does not enjoy any advantage over its rival. Because cooperation in small societies is not mysterious or impossible from a weak reciprocity perspective, we ought to know more about the mechanics of coercion as it is described in the ethnographic literature.

9. Sex and death

Christopher Boehm has systematically surveyed and classified the ethnography on punishment and norm-violation. Boehm's (1999) work is the main source of empirical evidence for Sober and Wilson (Reference Sober and Wilson1998), who in turn are widely cited by strong reciprocity theorists in spite of the fact that they do not support a costly punishment account of cooperation. Along the chain of citations Boehm's core message seems to have been lost.

Sanctions are ordered by Boehm (Reference Boehm1999) on a scale that goes from ridicule, gossip, and verbal reproach, up to social ostracism and eventually homicide. Homicide is obviously the harshest and, because of the risk of retaliation, potentially the most expensive form of punishment. In relative terms, however, it is not rare. The view of primitive peoples as largely pacific has been abandoned by anthropologists over the last half-century, as the accumulation of statistical data has revealed a level of endemic violence that is much higher than in most large sedentary societies (e.g., Chagnon Reference Chagnon1988; Knauft Reference Knauft1991). The majority of violent confrontations within the tribe nevertheless are caused by sexual conflict rather than violation of norms of economic cooperation (Knauft Reference Knauft1991), and the punishment of adulterers by jealous husbands accounts for a large share of murders (Chagnon Reference Chagnon1968/1992, p. 187; Lee Reference Lee1979, p. 377; Marlowe Reference Marlowe2010, p. 192).

Following Trivers' (Reference Trivers and Campbell1972) theory of parental investment, adultery can be plausibly modelled as a Prisoner's Dilemma game with fitness payoffs, and jealousy as an adaptive solution. Jealousy is a strong emotion that triggers aggressive behaviour, bypassing complex calculations of cost and benefit that might otherwise deter from the punishment of philanderers. To establish that revenge is systematically costly, however, requires some tricky quantitative analysis. If the probability of getting killed or injured during a fight (i.e., of compromising one's fitness) is lower than the probability of deterring sexual free-riders from sleeping with one's partner in the future, then revenge triggered by jealousy may be advantageous from an evolutionary point of view. Punishment need not be expensive in the long run, for the punisher would recoup the costs – for example, by gaining a reputation of “fierceness” that could promote access to sexual mates in the future.

Unfortunately, the available evidence is mostly qualitative, and only suggestive. Cultures of fierceness seem more common among horticulturalists like the Yanomamö than among mobile hunter-gatherers who can resolve their conflicts by frequent splitting. This seems to point in the direction of weak reciprocity mechanisms that exploit the long horizon of cooperation. But lacking precise data, any reciprocity account of adultery cannot be more than a conjecture.

One point, nevertheless, emerges strongly from the ethnographic literature: The violence that stems from sexual competition, far from contributing to sociality, is actually a major threat to the survival of small societies. Chagnon (Reference Chagnon1968/1992, p. 188) notes, for example, that dyadic club fights among the Yanomamö have a tendency to quickly escalate, and unless the elders are able to control them, they usually result in group fission. There may be a direct causal link between the size of groups, the opportunity to engage in adultery, and the probability of fission, which acts as a powerful limit on social aggregation. Lee (Reference Lee1979, p. 397) similarly claims that “the fear of violence … is a prominent feature of !Kung life,” and the Kalahari bushmen have developed various means to keep violence under control. One of these is simply to live in small groups of tightly related kin.

Because violent punishment hinders, rather than promotes, sociality, several mechanisms are in place to moderate the effects of male aggressiveness. Sexual tensions are often displaced or unacknowledged, and to some extent adultery is simply tolerated. Interestingly for our purposes, punishment is even less common in the case of economic, rather than sexual, free-riding: In her study of “costly” punishment among the Ju/'hoansi, Wiessner (Reference Wiessner2005, p. 134) noticed that “none of the cases with negative outcomes [for the punisher] dealt with regulation of sharing or [economic] free-riding.” Shirkers are for the most part just ignored, an attitude that does not seem to be in any way peculiar to the Ju/'hoansi (see, e.g., Marlowe [2010] on the Hadza).

It is also significant that violent revenge is rarely praised, as one would expect in a society that relies on costly punishment for its survival; on the contrary, the murderer is often considered “polluted” and in need of purification. Sometimes the murderer is ostracized (Mahdi Reference Mahdi1986), and sometimes the killing of a murderer by the victims' relatives is tolerated (a practice that comes very close to an “execution,” in a society without central authority – see Lee Reference Lee1979, Ch. 13). This is very different from the picture painted by strong reciprocity theorists. Far from posing a second-order Prisoner's Dilemma problem, violent acts of revenge risk being far too common in small acephalous societies.

Punishment experiments thus give a misleading appearance of orderly justice to a process that, in most cases, would trigger feuds and eventually degenerate into anarchy and war. In the laboratory this eventuality is typically prevented by design, because in the majority of experiments free-riders cannot revenge the moralistic sanctions they have received. (Recall the empty cells in Figure 2b: in most punishment experiments it is not possible to respond P to P.) But in those few experiments where counter-punishing is allowed, approximately one quarter of the sanctions are revenged. Moreover, the positive effect of strong reciprocity vanishes, causing a reduction of cooperation similar to that observed in experiments without punishment. And on top of that, aggregate payoffs are among the lowest observed in experimental Public Goods games (Denant-Boemont et al. Reference Denant-Boemont, Masclet and Noussair2007; Nikiforakis Reference Nikiforakis2008).

So there are probably good reasons why decentralised, spontaneous material punishment is so rare outside the laboratory. In modern states decentralised sanctioning is explicitly forbidden by law, and anti-social behaviour is curtailed in ways that minimize the risk of feuds. Retaliation is controlled by imposing a monopoly of state violence, and the cost of punishment is recouped by compensating “professional punishers” (e.g., policemen). In small societies, apart from cases of sexual conflict, homicide is used occasionally to resolve political issues, such as the rise of a bullying chief (Boehm Reference Boehm1999). However, it is typically administered by a coalition against an individual – that is, in a way that resembles the centralised punishment typical of large-scale modern societies. The formation of coalitions and coordinated punishment is an important mechanism that is beginning to attract the attention of reciprocity theorists, so I will come back to it later (in section 13). Before that, it will be instructive to explore other mechanisms that sustain cooperation in small societies where costly material punishment is rarely administered.

10. Gossip and symbolic sanctions

Homicide and overt physical aggression account for only a fraction of punishment episodes reported by ethnographers of small societies. When justice is not administered centrally, violations of norms are mostly dealt with by means of sanctions that affect the material welfare of the recipient only indirectly, and at the same time impose little or no costs on those who administer them. Some critics of strong reciprocity theory have rightly pointed out that the evolution of higher cognitive capacities in humans has brought as a side-effect a dramatic reduction in the cost of anti-social sanctioning (Binmore Reference Binmore2005, pp. 82–84; Ross Reference Ross2006, pp. 65–67). Going down Boehm's (1999) list, in fact, it is clear that most sanctions do not fit neatly the definition of costly punishment. Take verbal reproach and ridicule, for example. The process of symbolic punishment is quite different from that of material punishment: Whereas the former is non-invasive, the latter is not. While the latter encourages physical aggression, the former does it on a much smaller scale. And although inflicting material punishment is likely to infringe upon individual rights that regulate the life of a group (e.g., property rights), symbolic punishment does not.

In experiments, subjects are even willing to pay a fee to administer symbolic punishment (Carpenter et al. Reference Carpenter, Daniere and Takahashi2004). Although this confirms that they have a strong motivation to manifest disapproval of norm violations, it is not clear that any fee has to be paid in real life. The ethnography of norm regulation emphasizes that gossip and reproach are low-cost strategies. “Spreading the word” usually takes the form of spontaneous gossiping, the chit-chat that accompanies most activities of nomadic foragers (see, e.g., Marshall Reference Marshall1961; Dunbar Reference Dunbar1996/1998). The primary function of this constant flow of information is the necessity to sustain trust and monitor others' activities, as in folk-theorem accounts of repeated cooperation. Even when it is used as a sanctioning device, however, gossip is a collective endeavour – an important point to which I will return later – and certainly nothing like an individualistic initiative that requires considerable investment of time or the subtraction of resources from other profitable activities. “Speaking up first” against a norm violator is often cited as a costly act in the strong reciprocity literature because of the risk of retaliation, but there are very cheap ways of circulating information and forming coalitions against individual group members.

In her in-depth study of the Chaldean community in modern Detroit, Natalie Henrich reports that direct reproach is used only to sanction relatively minor violations of social norms (such as garbage recycling), whereas serious issues are always dealt with by “behind-your-back” gossip (Henrich & Henrich Reference Henrich and Henrich2007, pp. 147–50). Because of its potentially destructive effect on reputation, gossip is a very powerful enforcement mechanism and is particularly feared by Chaldeans, with the added advantage of protecting the punishers from the wrath of their target.

Polly Wiessner (Reference Wiessner2005) has made a systematic attempt to find evidence of costly punishment in the field, using ethnographic evidence collected among the bushmen of Botswana. Most of the punishment she reports is purely symbolic in character. Wiessner's conclusion is cautiously favourable to strong reciprocity theory, based on her estimate that 8% of observed punishment episodes had negative consequences for the punishers. Her definition of “negative consequence,” however, is very broad, including cases like severed social relations and the loss of a group member through ostracism, which do not fit the proper definition of costly punishment. Wiessner does not distinguish between absolute and relative costs, but her discussion of the data suggests that both are very low in the case of economic dilemmas of cooperation. Even the risk of retaliation is extremely low: Physical confrontation, as a matter of fact, occurs in only 2% of the episodes recorded by Wiessner and never results in serious injuries (Wiessner Reference Wiessner2005, p. 132). All in all, in a sample of 171 episodes, the statistical incidence of material cost for the punishers is close or equal to zero.

11. How pygmies punish free-riders

The next big step in the scale of sanctions reported by anthropologists is ostracism. Although descriptions of specific episodes are rare in the literature, ostracism figures prominently in Gurven's (Reference Gurven2004) recent survey of the ethnographic record on food sharing, and experimental evidence confirms its efficacy in laboratory settings (Cinyabuguma et al. 2004; Page et al. Reference Page, Putterman and Unel2005). Ostracism can be very damaging in material terms. Even though ostracized individuals or families usually join other groups, they lose ties with their kin and the protection that the latter provide. Among the Yanomamö studied by Chagnon (Reference Chagnon1968/1992), for example, leaving one's group entails leaving one's garden, and being dependent on the hosts for food for several months (the guests usually pay a “rent” in terms of women). Still, ostracism does not have to be costly: the exclusion of an individual or clan from the tribe usually takes place in such a way that no individual punisher has to bear the full “cost” of it. Ostracism can be so low-cost that it is often preferred to verbal reproach, especially in highly mobile societies: In such cases, it is not even necessary to expel the offender from the group – it is easier for the group to move elsewhere:

To give an idea of how low-cost ostracism works, I will recount an episode reported by Colin Turnbull (Reference Turnbull1961) in his classic ethnography of the Mbuti pygmies in central Congo. Hunting is for the Mbuti a highly cooperative enterprise, involving all adult tribe members. Women work as beaters – they scare animals with screams and noises, pushing them towards an area of the forest that has been closed down using a line of nets. Once an animal is trapped in a net, it is speared by the nearest hunter who then “owns” the meat and is entitled to allocate it among the members of the hunting party, usually keeping the best parts for his own family. This technique requires the participation of several hunters, who must position themselves in an arc so as to close down a large area of the forest and act in concert to prevent the animals from escaping.

The band studied by Turnbull comprised several hunters, including a family headman named Cephu who was not well-liked and was already gossiped about in the group. Perhaps for this reason, Cephu occupied a peripheral location in the hunters' formation. This clearly put him at a disadvantage, since animals are more likely trapped in the middle sector of the line, and the hunters who occupy this sector end up with the largest share of the meat. On one particular occasion, the group had already killed a couple of preys when Cephu decided to abandon his position and, unseen, place his net in front of the other hunters. This is a typical free-riding strategy in a social dilemma game: By changing location, Cephu increased the probability that the next animal caught in the trap would be speared by him, but at the same time he reduced the probability that an animal would be captured by the group at all.

On this particular occasion, Cephu's strategy was successful – he killed the first animal fleeing from the beaters – but did not go undetected. As Turnbull tells the story (1961, pp. 97–101), Cephu immediately became the victim of moralistic aggression by the whole group. While returning to the camp, several hunters began criticizing his conduct behind his back, with some of the youngsters ridiculing and insulting him amidst generalised laughter. This quickly escalated into a criticism of Cephu's anti-social behaviour in general, until an emergence meeting was called to resolve the matter once and forever. After a lame attempt to find an excuse, Cephu eventually tried to assert his right to occupy a better location in the line of nets, by virtue of his “chief” status. At this point, one of the other headmen simply and quietly invited him to leave the group, if he was too good and important to stay with the others on equal terms. This was sufficient to end the discussion. Here is Turnbull's description of subsequent events:

Although this is clearly a case of material punishment, the punishment was certainly not very costly. First, the group made it clear that Cephu's conduct was considered unacceptable. The oral criticism was not just aimed at Cephu but was also for the benefit of the other members of the group, who were reassured about the balance of power. Then punishment was administered by a coalition against an individual (or a small clan) who would have no chance to counter-punish, and had no interest in escalating conflict.

Another interesting point is that the free-rider was punished by taking away his illicit gain. But, pace strong reciprocity theory, no wealth was destroyed, because the other families consumed what Cephu had caught. And even Cephu's punishment turned out to be not so harsh after all: Once peace had been restored, one member of the main group took some food to Cephu's hut to feed him and his family. At that point all animosity seemed to be gone, and Cephu participated in the feast with the rest of the group (Turnbull Reference Turnbull1961, p. 101). (Cephu's clan, to be sure, abandoned the group later, to join another group of Mbuti hunters.)

Ostracism, as already mentioned, is described only rarely at this level of detail. Nevertheless, Cephu's story is representative of other episodes of moralistic aggression and ostracism reported in the anthropological literature (e.g., Briggs Reference Briggs1970; Boehm Reference Boehm1999, Ch. 3; for a survey, see Baumard Reference Baumard2010b). It shows that even cases that seem favourable (e.g., because they involve the subtraction of material resources) do not actually fit well with the explanatory framework of strong reciprocity theory. The expression “costly punishment” turns out to be a misnomer, because the punishment is inflicted in such a way as to keep both absolute and relative costs close to nil. Given the difficulty of obtaining a precise quantitative measurement, of course, one cannot rule out the costly punishment story with certainty. But it is fair to say that there is currently no evidence that cooperation is sustained by strong negative reciprocity in small societies. And whatever evidence there is, it rather points in the direction of cheap mechanisms like ostracism and coalitional punishment.

12. Cheap versus costly punishment in the lab

So why do people engage in costly punishment so enthusiastically in the laboratory? A plausible answer is that costly punishment is usually the only way for them to manifest their disappointment, and in any case punishers are protected by anonymity and by the rules of the experiment. But when they are given other options, subjects' behaviour changes: A handful of experiments have explored and compared the effects of different sanctioning techniques, ranging from purely symbolic (reproach) to purely material punishment. Evidence regarding the efficacy of symbolic sanctions is mixed, with some studies suggesting that reproaches backed by material punishment work best (cf. Janssen et al. Reference Janssen, Holahan, Lee and Ostrom2010; Masclet et al. Reference Masclet, Noussair, Tucker and Villeval2003; Noussair & Tucker Reference Noussair and Tucker2005). If they are given the opportunity to choose, subjects prefer to support cooperation using a mix of symbolic communication, weak reciprocity, and the last-resort threat of material punishment (e.g., Ostrom et al. Reference Ostrom, Walker and Gardner1992; Rockenbach & Milinski Reference Rockenbach and Milinski2006; Ule et al. Reference Ule, Schram, Riedl and Cason2009; Xiao & Houser Reference Xiao and Houser2005).

Most of these experiments, however, still ignore the problem of feuds and the anti-social effect of counter-punishment. There are to date only a couple of experimental studies that combine alternative ways of incentivising cooperation – including costly punishment – with the threat of counter-punishment. Nikiforakis and Engelmann (Reference Nikiforakis and Engelmann2010) find that strategies that could trigger lengthy feuds are avoided in the laboratory, and Dreber et al. (Reference Dreber, Rand, Fudenberg and Nowak2008) show that in such circumstances people prefer to implement cheap strategies (i.e., withdraw cooperation) rather than costly punishment. This is sensible, because in the aggregate, feuds destroy more resources than they help create.

But even ignoring the problem of counter-punishment, “costly” punishment works only if it costs relatively little. Above a cost/impact ratio of 1:3, sanctions do not increase cooperation significantly (Egas & Riedl Reference Egas and Riedl2008; Nikiforakis & Normann Reference Nikiforakis and Normann2008; Ohtsuki et al. Reference Ohtsuki, Iwasa and Nowak2009), and even low-cost punishment does not necessarily improve aggregate payoffs – in fact, it often reduces them. Although punishment pushes the rate of cooperation up, it also destroys resources. Janssen et al. (Reference Janssen, Holahan, Lee and Ostrom2010) report a strong positive effect on total revenue when communication is allowed, and when it is matched with punishment, but not with punishment alone. Clearly this is deeply problematic, given the strong reciprocity theorists' emphasis on group selection.

An exception to this body of results is the discovery by Gächter et al. (Reference Gächter, Renner and Sefton2008) that costly sanctions can raise average earnings when the horizon of cooperation is very long (50 rounds). But notice that the game in this experiment involves repeated interactions with the same subjects, and efficiency increases because the long horizon makes the use of punishment almost unnecessary (there is more punishment in the final round than in the early part of the game, in fact). So, to sum up, costly punishment alone does not seem to be an efficient solution to social dilemmas in the laboratory, precisely in those conditions – such as one-shot interactions with strangers – where, according to strong reciprocity theorists, it would be most needed.

13. How common pool institutions sustain cooperation

I have discussed the ethnography of small societies in some detail because the behavioural scientists who are unfamiliar with this literature may be misled to believe that costly punishment is an established anthropological fact. But anthropology is not our only source of evidence concerning decentralised cooperation in the field, and small societies are neither the only nor the primary domain of application of strong reciprocity theory. Economic historians have studied extensively the spontaneous emergence of institutions for the management and preservation of public goods in complex societies. These studies emphasize that successful cooperative institutions solve social dilemma problems in ways that have little to do with costly punishment. Rather, they tackle the problem by removing the obstacles that prevent non-costly mechanisms from functioning.

We have a remarkable array of cases that can be brought to bear on this issue. I will briefly illustrate one example – the evolution of the Carte di Regola studied by Marco Casari (Reference Casari2007) in Northern Italy – that is representative of many similar institutions which have emerged spontaneously in different historical periods and in different parts of the world (see Ostrom Reference Ostrom1990). All of them, as we shall see, have an important feature in common: They artificially create the conditions that, according to weak reciprocity accounts, make cooperation possible, but that for various reasons were naturally unavailable in the given circumstances.

The Carte di Regola, or “charters,” are ancient written codes used by communities in the Trentino region in the northeast of Italy to regulate the exploitation of common pastures. The Carte were progressively introduced from 1200 until 1800, when they were eventually abolished by Napoleon. The charters were spontaneously adopted by single villages rather than imposed from above, and were aimed at preventing the over-exploitation of communal fields – a specific instance of Prisoner's Dilemma (or “common pool” problem) that has been studied in depth by historians (since McCloskey Reference McCloskey1972). Using a database of more than two hundred villages, Casari (Reference Casari2007) has shown that the charters had a common structure and were aimed at removing precisely the obstacles that prevented weak reciprocity mechanisms from functioning well, even in isolated villages such as those in the Italian Alps. The Carte, to put it differently, made the application of (something like) the folk theorem possible.

A first set of charter rules enhanced the stability of local communities, by locking existing members in and preventing the entrance of opportunistic outsiders. This was done mainly by forbidding the sale of communal field rights (hence increasing the cost of leaving), and by requiring a supramajority consensus for the admission of new members. The only costless way of transmitting rights, then, was via inheritance through the head of the family, a mechanism that extended the horizon of cooperation across future generations and turned a finitely repeated game into an indefinitely repeated game.

A second function of charters was to set up and regulate the monitoring of inside and outside users of the fields. The monitoring system was organized by the community and involved designated guards who could impose fines on free-riders. The guards could not inflict physical punishment (which remained under state jurisdiction), and were incentivised by retaining a third of the fine. Reports of transgressions by community members were also incentivised in a similar way. Instead of letting the punishers bear the cost of monitoring, the Carte thus introduced mechanisms that alleviated the costs, and even made sanctioning a lucrative activity.

Nevertheless, the historical record reveals that fines were rarely imposed on insiders, but were mostly collected from trespassers (Casari Reference Casari2007, p. 210). This could be because the rate of compliance was in fact very high inside each village, or because symbolic sanctions (like verbal reproach and gossip) were preferred when a member of the community was involved. Circulation of information and record-keeping were facilitated by holding regular meetings, with mandatory attendance for all community members. A special local court settled disputes among insiders and resolved ambiguous cases.

The case of Trentino's charters shows how the three main problems of folk-theorem mechanisms (infinite horizon, information, and costs) are solved by institutional design. Where these problems did not exist – or existed on a smaller scale – local villages were slower to adopt a charter, if they did adopt one at all. Smaller villages and communities in the most remote valleys of Trentino, for example, were less likely to adopt a charter than larger villages and communities in accessible and difficult-to-monitor locations (see Casari Reference Casari2007, pp. 209–13).

The Carte are absolutely typical from this respect: Elinor Ostrom (Reference Ostrom1990), winner of the 2009 Nobel Memorial Prize, has identified the same features of Trentino's charters in a series of case studies spanning several centuries and countries across six continents. Stable membership, monitoring incentives, graduated fines, exclusion of outsiders, and conflict-resolution mechanisms figure in her list of key factors that make institutions for collective actions viable and robust across time. “In all known self-organized resource governance regimes that have survived for multiple generations, participants invest resources in monitoring and sanctioning the actions of each other so as to reduce the probability of free riding” (Ostrom Reference Ostrom2000, p. 138). But the punishers are rewarded materially, and material damage is inflicted only rarely on the members of the community. Most of the work is done by creating a long-term prospect for cooperation, and by the extensive use of symbolic sanctions.

Because Ostrom's work is sometimes cited by strong reciprocity theorists in support of their theses (see, e.g., Gintis et al. Reference Gintis, Boyd, Bowles and Fehr2005), it is worth spending a few words on the implications of her work. Emphasis on the costs of punishment and the second-order dilemma these raise is indeed central in the common pool literature. The costs this literature refers to, however, are rather different from those modelled in strong reciprocity models of cooperation. Whereas Ostrom (Reference Ostrom1990) emphasises the cost of setting up common pool institutions, strong reciprocity theorists focus on the ongoing cost of inflicting punishment. These two problems are quite different and should be kept distinct.

Institutions such as the Trentino charters are in some respects more similar to national states in the way in which they administer sanctioning, than to the uncoordinated mechanisms of (most) punishment experiments. Once a coordinated punishment mechanism is in place, the cost of running it (and of implementing sanctions on a daily basis) largely takes care of itself. Common pool institutions avoid the problems caused by systems of uncoordinated punishment in which everyone decides on their own (arbitrarily and idiosyncratically) when and how to punish, with the potential for feuds that follows (Casari & Plott Reference Casari and Plott2003). These advantages make coordinated punishment institutions remarkably robust and resilient across time.

Experiments performed by Yamagishi (Reference Yamagishi1986) and Gürerk et al. (Reference Gürerk, Irlenbusch and Rockenbach2006) have shown that subjects prefer and tend to migrate towards institutions with coordinated punishment; and a recent modelling exercise (Boyd et al. Reference Boyd, Gintis and Bowles2010) backs up the insight that these institutions may enjoy an evolutionary advantage compared to systems of uncoordinated sanctions. Boyd and co-workers propose a model in which the cost of punishment is inversely proportional to the number of punishers, and players can condition their decision on the size of the coalition. They show that under a plausible range of parameters cooperation and punishment can evolve. On the experimental side, Casari and Luini (Reference Casari and Luini2009) report higher levels of cooperation in Public Goods games when the decision to punish is supported by a coalition rather than by individual subjects. Part of the reason is that coordinated punishment tends to reduce individual attempts at anti-social punishment and revenge. Another advantage of real-life coordinated punishment is that it requires communication among peers, a factor that has a well-known positive effect on cooperation (Ostrom Reference Ostrom1990; Ostrom et al. Reference Ostrom, Walker and Gardner1992). Communication in turn brings legitimacy – the punishment is perceived as just because it is consensual – and lack of legitimacy is probably a major cause of failure of externally imposed sanctions (Cardenas et al. Reference Cardenas, Stranlund and Willis2000).

To sum up: Strong reciprocity theorists view punishment as local, costly, and uncoordinated. The empirical literature instead reports mainly the emergence of local, cheap, and coordinated punishment institutions. Both solutions to the dilemma of cooperation differ in part from the traditional imposition of external sanctions administered by the state, and both can be seen as raising second-order social dilemma problems. However, they also have rather different properties and should not be treated as if they were identical: The devil, in institutional design as in almost everything else, is very much in the details.

14. Models and policies

Having presented the bulk of the argument, I now turn to an obvious objection that can be raised against it. Lacking precise quantitative data, throughout this article, I have referred rather liberally to “cheap,” “low-cost,” and “no-cost” punishments as if they were the same thing. Undoubtedly, however, a small cost is still a cost, and for this reason alone, strong reciprocity theory can legitimately claim an advantage over its main rival.

This objection is far from trivial, and it raises important issues concerning the use of models and evidence in the social sciences. Part of my reluctance to speak of zero costs comes from the current lack of data concerning the cost-benefit ratio of punishment. And lacking precise data – on benefits especially – one should not rush to conclusions as soon as a small positive cost is detected. Nevertheless, I want to argue that even small but positive net costs would constitute too slender a basis to claim a victory for strong reciprocity theory.

The debate between weak and strong reciprocity theorists takes place in the context of an old controversy on the use of rational choice models – especially models based on narrow self-interest – in social policy. As Bowles and Gintis point out,

Using models to inform the design of institutions is a special activity that calls for special criteria of appraisal. The value of a policy-oriented model lies less in its descriptive accuracy than as a guide to effective action. This is particularly important in light of the well-known fact that all models simplify and betray reality in some respects. But while simplifications in one dimension ought to be exchanged for increased descriptive or predictive accuracy in some other dimension when we do pure science, simplifications ought to lead to good advice when policy-making is concerned.

How do weak and strong reciprocity fare in this respect? Costly punishment experiments are often accompanied – as in the preceding quotation – by suggestions that self-interest, long-term horizon, and information matter less than traditionally assumed by economists and biologists. But this suggestion is misleading. As Ostrom and others have emphasized, the opposite is likely to be true: Individual costs are crucial obstacles in the way to cooperation and must be kept low; uncoordinated punishment is dangerous and fragile; the shadow of the future and the circulation of information matter enormously. All these insights follow directly from weak reciprocity accounts of cooperation, in spite of the fact that its models – and their implications, like the folk theorem – are almost certainly false. False theories can still provide useful advice at the level of application.

Seen in this light, the issue of low- versus zero-cost punishment loses much of its importance. Perhaps gossip, ostracism, and verbal reproaches are a bit costly, and gene-culture co-evolution has helped humans overcome this little hurdle on the path towards sociality. Be that as it may, a theory of low-cost punishment would have relatively little practical interest for applied social science. Its advice for the policy-maker would be almost indistinguishable from that of weak reciprocity theory: Pay attention to individual costs; keep them low or make sure they are recouped later; extend the horizon of cooperation; and circulate information as much as possible. All these precepts were well known before the discovery of costly punishment in the laboratory, and the rise of strong reciprocity theory has only increased the risk that social scientists may forget about them.

15. Concluding remarks: Reciprocity without costly punishment

In this article I have argued the following:

1. 1. Two interpretations of costly punishment experiments – narrow and wide – are usually conflated by strong reciprocity theorists.

2. 2. Only the narrow interpretation is supported by experimental data, while the wide interpretation requires field evidence about the mechanisms that sustain cooperation in the wild.

3. 3. Contrary to often-repeated claims, there is no evidence in the anthropological literature that costly material punishment is used in small acephalous societies, except in the regulation of sexual conflict.

4. 4. On the contrary, there is a lot of evidence that revenge is a major cause of dissolution of social ties.

5. 5. Economic cooperation in the small societies studied by anthropologists is usually supported by low-cost or no-cost mechanisms such as verbal criticism, ostracism, and coalitional punishment.

6. 6. The robust common pools institutions studied by historians and institutional economists foster cooperation by recouping the costs of punishment, extending the horizon of cooperation, and circulating information among group members, as implied by weak reciprocity accounts of cooperation.

It is important to clarify that the evidence summarized in the preceding list does not refute the claim that Homo sapiens has evolved other-regarding (“social”) preferences, or that punishment is an important mechanism for the enforcement of social norms. What it does challenge is the claim that social preferences are expressed via costly sanctions that sustain cooperation in a broad range of experimental and field situations. The weak point of strong reciprocity theory is not its analysis of individual motivation (Dubreuil Reference Dubreuil2010; Rosas Reference Rosas2008), but its narrow focus on artificial environments in which uncoordinated costly punishment has a beneficial effect on sociality. Strong reciprocity may well play a key motivating role in the creation of institutions – such as systems of collective monitoring and coordinated sanctioning – that foster cooperation, without triggering the negative side-effects of uncoordinated punishment.

In my view, the lack of support for the costly punishment account of cooperation is not to be celebrated. We would all like to have the best of both worlds: social cooperation in a large, diverse society without the burden of a centralized policing apparatus. But the evidence that cooperation can be sustained by decentralised costly punishment in the field is scant. Logically speaking, of course, we cannot rule out that in some cases costly punishment can sustain cooperation. However, while there is extensive evidence of spontaneously evolved institutions aimed at eliminating the cost of sanctioning, disregarding costs and relying on uncoordinated punishment would be very risky at the level of institutional design.

It is also worth stressing that lack of confirmation is not due to lack of scientificity. On the contrary, the rise of costly punishment is a good example of how the combination of rigorous theorizing with ingenious experimental data can foster quick progress in the social sciences. The moral to be drawn is that models and experiments can only take you so far, and the time has come for reciprocity theory to change gears and seek the test of historical and field data. This step was taken a long time ago in the investigation of related topics such as mutual insurance and collusion, and it is important to keep in mind that laboratory data – no matter how useful – cannot ultimately replace the evidence collected in the field.

Finally, nothing said in this article challenges the idea that strong positive reciprocity may be an important ingredient of human sociality. An adequate discussion of the other half of strong reciprocity would require a separate paper, but it will suffice to say that the prospects of positive reciprocity look brighter at first sight. Robust support comes from surveys (Andreoni et al. Reference Andreoni, Erard and Feinstein1998; Fong Reference Fong2001), laboratory (e.g., Berg et al. Reference Berg, Dickhaut and McCabe1995; Burlando & Guala Reference Burlando and Guala2005; Fehr et al. Reference Fehr, Kirchsteiger and Riedl1993; Fischbacher et al. Reference Fischbacher, Gächter and Fehr2001), and natural field experiments (Frey & Meier Reference Frey and Meier2004; Shang & Croson Reference Shang and Croson2009).

This asymmetry of support is probably not an accident, and may reflect profound differences in the psychology of cost-processing. In the technical sense of economic theory, replying to a cooperative move with cooperation (instead of free-riding) in a one-shot dilemma game is equal to incurring a cost. Through the lens of the theory, positive reciprocity appears theoretically identical to negative reciprocity, for in both cases the agents are willing to pay a “fee” to reciprocate. But it is not obvious that positive and negative reciprocity are governed by the same psychological mechanisms. It is well known that the perception of gains and losses is biased by framing effects, and that missed opportunities are processed differently from directly incurred costs (e.g., Borges & Knetsch Reference Borges and Knetsch1997; Kahneman et al. Reference Kahneman, Knetsch and Thaler1991). Psychological evidence on loss aversion suggests that we should be more reluctant to pay a fee to sanction nasty actions, than to miss an opportunity to profit at somebody else's expense. And it is possible that the evolutionarily ancient neural circuits that trigger negative reciprocity feelings work quite separately from the networks that support trust and positive reciprocity in the human brain (although the evidence is still contradictory and inconclusive; see, e.g., Tom et al. Reference Tom, Fox, Trepel and Poldrack2007; Yacubian et al. Reference Yacubian, Gläscher, Schroeder, Sommer, Braus and Büchel2006).

Far from constituting an indictment of the strong reciprocity programme, then, the data call for a re-orientation away from its current obsession with costly punishment. More effort should be made in investigating how non-costly sanctions, backed up by adequate institutional scaffoldings, may be used to sustain positive reciprocity in a variety of real-world settings (as in, e.g., Boyd et al. Reference Boyd, Gintis and Bowles2010; Rustagi et al. Reference Rustagi, Engel and Kosfeld2010). The policy implications of this insight are important enough to justify further investment in this research programme. But we should accept that accounts of cooperation based on costly, uncoordinated policing are not backed up by the empirical evidence collected so far.

The social and psychological costs of punishing Adams, Gabrielle S. and Mullen, Elizabeth Graduate School of Business, Stanford University, Stanford, CA 94305. gsadams@stanford.edu http://www.stanford.edu/~gsadams emullen@stanford.edu http://elizabeth.mullen.socialpsychology.org/ Proximate and ultimate causes of punishment and strong reciprocity Barclay, Pat Department of Psychology, University of Guelph, East Guelph, Ontario, N1G 2W1, Canada. barclayp@uoguelph.ca http://www.uoguelph.ca/nacs/page.cfm?id=229 The restorative logic of punishment: Another argument in favor of weak selection Baumard, Nicolas Philosophy, Politics, and Economics Program, University of Pennsylvania, Philadelphia, PA 19104. nbaumard@gmail.com https://sites.google.com/site/nicolasbaumard/Home Reciprocity and uncertainty Bereby-Meyer, Yoella Department of Psychology, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel. Yoella@bgu.ac.il www.YoellaBerebyMeyer.com Costs and benefits in hunter-gatherer punishment Boehm, Christopher Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089. cboehm1@msn.com The punishment that sustains cooperation is often coordinated and costly Bowles, Samuel, Boyd, Robert, Mathew, Sarah and Richerson, Peter J. Santa Fe Institute, Santa Fe, NM 87501. samuel.bowles@gmail.com http://www.santafe.edu/~bowles; Department of Anthropology, University of California–Los Angeles, Los Angeles, CA 90095. rboyd@anthro.ucla.edu http://www.sscnet.ucla.edu/anthro/faculty/boyd/ smathew@ucla.edu http://smathew.bol.ucla.edu/Site/Home.html; Department of Environmental Science and Policy, University of California–Davis, Davis CA 95616. pjricherson@ucdavis.edu http://www.des.ucdavis.edu/faculty/Richerson/Richerson.htm Weak reciprocity alone cannot explain peer punishment Casari, Marco Department of Economics, University of Bologna, Piazza Scaravilli 2, 40126 Bologna, Italy. marco.casari@unibo.it http://www2.dse.unibo.it/casari/ In medio stat virtus: Theoretical and methodological extremes regarding reciprocity will not explain complex social behaviors Civai, Claudia and Langus, Alan SISSA/ISAS–International School for Advanced Studies, via Bonomea, 265, 34136 Trieste, Italy. civai@sissa.it http://www.sissa.it/cns/index.html alanlangus@gmail.com Examining punishment at different explanatory levels dos Santos, Miguel and Wedekind, Claus Department of Ecology and Evolution, Biophore, University of Lausanne, 1015 Lausanne, Switzerland. miguel.dossantos@unil.ch http://www.unil.ch/dee/page57244.html claus.wedekind@unil.ch http://www.unil.ch/dee/page21538.html Retaliation and antisocial punishment are overlooked in many theoretical models as well as behavioral experiments Dreber, Anna and Rand, David G. Department of Economics, Stockholm School of Economics, 113 83 Stockholm, Sweden. anna.dreber@hhs.se http://sites.google.com/site/annadreber/; Program for Evolutionary Dynamics, Harvard University and Department of Psychology, Harvard University, Cambridge, MA 02138. drand@fas.harvard.edu http://www.people.fas.harvard.edu/~drand/ Gossip as an effective and low-cost form of punishment Feinberg, Matthew, Cheng, Joey T. and Willer, Robb Department of Psychology, University of California, Berkeley, Berkeley, CA 94720-1650. matthewfeinberg@berkeley.edu http://sites.google.com/site/matthewfeinbergpsychology/; Department of Psychology, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada. joeycheng@psych.ubc.ca; Department of Sociology, University of California, Berkeley, Berkeley, CA 94720-1980. willer@berkeley.edu http://willer.berkeley.edu/ Blood, sex, personality, power, and altruism: Factors influencing the validity of strong reciprocity Ferguson, Eamonn and Corr, Philip Personality and Social Psychology Group, School of Psychology, University of Nottingham, Nottingham, NG7 2RD, United Kingdom. eamonn.ferguson@nottingham.ac.uk http://www.psychology.nottingham.ac.uk/staff/ef/home.html; School of Social Work and Psychology, and Centre for Behavioural and Experimental Social Science (CBESS); University of East Anglia., Norwich NR4 7TJ, United Kingdom. P.Corr@uea.ac.uk http://www.ueapsychology.net/differential-psychology-pg14.html In the lab and the field: Punishment is rare in equilibrium Gächter, Simon School of Economics, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom. simon.gaechter@nottingham.ac.uk http://www.nottingham.ac.uk/Economics/people/simon.gaechter The social structure of cooperation and punishment Gintis, Herbert and Fehr, Ernst Santa Fe Institute, Santa Fe, NM 87501. hgintis@comcast.net; Department of Economics and Laboratory for Social and Neural Systems Research, University of Zurich, CH-8006 Zurich, Switzerland. efehr@iew.unizh.ch Is strong reciprocity really strong in the lab, let alone in the real world? Güney, Şule and Newell, Ben R. School of Psychology, University of New South Wales, Sydney, NSW, 2052, Australia. s.guney@unsw.edu.au http://www.psy.unsw.edu.au/profiles/phd/sguney.html ben.newell@unsw.edu.au http://www2.psy.unsw.edu.au/Users/BNewell/ Understanding the research program Henrich, Joseph and Chudek, Maciej Department of Psychology and Department of Economics, University of British Columbia, Vancouver, B.C., V6T 1Z4, Canada. joseph.henrich@gmail.com; Department of Psychology, University of British Columbia, Vancouver, B.C., V6T 1Z4, Canada. maciek@interchange.ubc.ca http://www2.psych.ubc.ca/~henrich/home.html Social preference experiments in animals: Strengthening the case for human preferences Jensen, Keith Research Centre for Psychology, School of Biological and Chemical Sciences, Queen Mary, University of London, London E1 4NS, United Kingdom. k.jensen@qmul.ac.uk http://www.sbcs.qmul.ac.uk/staff/keithjensen.html The strategic logic of costly punishment necessitates natural field experiments, and at least one such experiment exists Johnson, Tim Department of Political Science, Stanford University, Stanford, CA 94305; Atkinson Graduate School of Management, Willamette University, Salem, OR 97301. timj@stanford.edu www.stanford.edu/~timj Altruistic punishment: What field data can (and cannot) demonstrate Nikiforakis, Nikos Department of Economics, University of Melbourne, 3010 Victoria, Australia. n.nikiforakis@unimelb.edu.au http://www.economics.unimelb.edu.au/staff/nikosn/ Experiments combining communication with punishment options demonstrate how individuals can overcome social dilemmas Ostrom, Elinor Workshop in Political Theory and Policy Analysis, Indiana University, Bloomington, IN 47408. ostrom@indiana.edu http://www.indiana.edu/~workshop/ Importing social preferences across contexts and the pitfall of over-generalization across theories Pisor, Anne C. and Fessler, Daniel M. T. Department of Anthropology, University of California, Santa Barbara, CA 93106-3210. pisor@umail.ucsb.edu http://www.uweb.ucsb.edu/~pisor/; Department of Anthropology and Center for Behavior, Evolution, and Culture, University of California, Los Angeles, Los Angeles, CA 90095-1553. dfessler@anthro.ucla.edu http://www.sscnet.ucla.edu/anthro/faculty/fessler Culture: The missing piece in theories of weak and strong reciprocity Read, Dwight Department of Anthropology, University of California, Los Angeles, Los Angeles, CA 90095. dread@anthro.ucla.edu http://www.anthro.ucla.edu/people/faculty?lid=886 Towards a unified theory of reciprocity Rosas, Alejandro Department of Philosophy, Universidad Nacional de Colombia, Bogotá, Colombia, and Konrad Lorenz Institute (KLI) for Evolution and Cognition Research, 3422 Altenberg, Austria. arosasl@unal.edu.co http://www.docentes.unal.edu.co/arosasl/ Special human vulnerability to low-cost collective punishment Ross, Don University of Cape Town, School of Economics, University of Cape Town, Private bag, Rondebosch, 7701 Cape Town, South Africa. don.ross@uct.ac.za http://uct.academia.edu/DonRoss Strong reciprocity is not uncommon in the “wild” Runciman, W. G. Sociology, Trinity College, Cambridge CB2 1TQ, United Kingdom. wgr@wgrunciman.u-net.com http://www.trin.cam.ac.uk/index.php7pageid=538 Lab support for strong reciprocity is weak: Punishing for reputation rather than cooperation Shaw, Alex and Santos, Laurie Department of Psychology, Yale University, New Haven, CT 06511. Alex.Shaw@yale.edu Laurie.Santos@yale.edu https://sites.google.com/site/alexshawyale/ Altruistic punishment as an explanation of hunter-gatherer cooperation: How much has experimental economics achieved? Sugden, Robert School of Economics, University of East Anglia, Norwich NR4 7TJ, United Kingdom. r.sugden@uea.ac.uk https://www.uea.ac.uk/eco/People_old/Faculty/rsugden Punishing for your own good: The case of reputation-based cooperation Tennie, Claudio Max Planck Institute for Evolutionary Anthropology, 04013 Leipzig, Germany. tennie@eva.mpg.de www.claudiotennie.de What we need is theory of human cooperation (and meta-analysis) to bridge the gap between the lab and the wild Van Lange, Paul A. M., Balliet, Daniel P. and IJzerman, Hans Department of Social and Organizational Psychology, VU University, Amsterdam, 1081 BT Amsterdam, The Netherlands. pam.van.lange@psy.vu.nl http://www.paulvanlange.com dp.balliet@psy.vu.nl h.ijzerman@psy.vu.nl http://h.ijzerman.googlepages.com The social costs of punishment van den Berg, Pieter, Molleman, Lucas and Weissing, Franz J. Theoretical Biology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, 9700 CC Groningen, The Netherlands. pieter.van.den.berg@rug.nl l.s.molleman@rug.nl f.j.weissing@rug.nl http://www.rug.nl/fmns-research/theobio When the strong punish: Why net costs of punishment are often negligible von Rueden, Christopher R. and Gurven, Michael Integrative Anthropological Sciences, Department of Anthropology, University of California, Santa Barbara, Santa Barbara, CA 93106-3210. vonrueden@umail.ucsb.edu http://sites.google.com/site/chrisvonrueden/home gurven@anth.ucsb.edu http://www.anth.ucsb.edu/faculty/gurven/ Perspectives from ethnography on weak and strong reciprocity Wiessner, Polly Department of Anthropology, University of Utah, Salt Lake City, UT 84108. wiessner@soft-link.com http://www.anthro.utah.edu/faculty/wiessner.html Strong reciprocity is real, but there is no evidence that uncoordinated costly punishment sustains cooperation in the wild Guala, Francesco Department of Economics, University of Milan, 20122 Milan, Italy. francesco.guala@unimi.it http://users.unimi.it/guala/index.htm