David P. Tracer

Costly punishment across human societies.

Recent behavioral experiments aimed at understanding the evolutionary foundations of human cooperation have suggested that a willingness to engage in costly punishment, even in one-shot situations, may be part of human psychology and a key element in understanding our sociality. However, because most experiments have been confined to students in industrialized societies, generalizations of these insights to the species have necessarily been tentative. Here, experimental results from 15 diverse populations show that (i) all populations demonstrate some willingness to administer costly punishment as unequal behavior increases, (ii) the magnitude of this punishment varies substantially across populations, and (iii) costly punishment positively covaries with altruistic behavior across populations. These findings are consistent with models of the gene-culture coevolution of human altruism and further sharpen what any theory of human cooperation needs to explain.

Markets, religion, community size, and the evolution of fairness and punishment

A Fair Society Many of the social interactions of everyday life, especially those involving economic exchange, take place between individuals who are unrelated to each other and often do not know each other. Countless laboratory experiments have documented the propensity of subjects to behave fairly in these interactions and to punish those participants deemed to have behaved unfairly. Henrich et al. (p. 1480, see the Perspective by Hoff) measured fairness in thousands of individuals from 15 contemporary, small-scale societies to gain an understanding of the evolution of trustworthy exchange among human societies. Fairness was quantitated using three economic games. Various societal parameters, such as the extent to which food was purchased versus produced, were also collected. Institutions, as represented by markets, community size, and adherence to a world religion all predict a greater exercise of fairness in social exchange. The origins of modern social norms and behaviors may be found in the evolution of institutions. Large-scale societies in which strangers regularly engage in mutually beneficial transactions are puzzling. The evolutionary mechanisms associated with kinship and reciprocity, which underpin much of primate sociality, do not readily extend to large unrelated groups. Theory suggests that the evolution of such societies may have required norms and institutions that sustain fairness in ephemeral exchanges. If that is true, then engagement in larger-scale institutions, such as markets and world religions, should be associated with greater fairness, and larger communities should punish unfairness more. Using three behavioral experiments administered across 15 diverse populations, we show that market integration (measured as the percentage of purchased calories) positively covaries with fairness while community size positively covaries with punishment. Participation in a world religion is associated with fairness, although not across all measures. These results suggest that modern prosociality is not solely the product of an innate psychology, but also reflects norms and institutions that have emerged over the course of human history.

More ‘altruistic’ punishment in larger societies

If individuals will cooperate with cooperators, and punish non-cooperators even at a cost to themselves, then this strong reciprocity could minimize the cheating that undermines cooperation. Based upon numerous economic experiments, some have proposed that human cooperation is explained by strong reciprocity and norm enforcement. Second-party punishment is when you punish someone who defected on you; third-party punishment is when you punish someone who defected on someone else. Third-party punishment is an effective way to enforce the norms of strong reciprocity and promote cooperation. Here we present new results that expand on a previous report from a large cross-cultural project. This project has already shown that there is considerable cross-cultural variation in punishment and cooperation. Here we test the hypothesis that population size (and complexity) predicts the level of third-party punishment. Our results show that people in larger, more complex societies engage in significantly more third-party punishment than people in small-scale societies.

Diagnosis of rickets and reassessment of prevalence among rural children in northern China

Background: Rates of rickets from 15.9 to 26.7% have been reported in China.

The ‘spiteful’ origins of human cooperation

We analyse generosity, second-party (‘spiteful’) punishment (2PP), and third-party (‘altruistic’) punishment (3PP) in a cross-cultural experimental economics project. We show that smaller societies are less generous in the Dictator Game but no less prone to 2PP in the Ultimatum Game. We might assume people everywhere would be more willing to punish someone who hurt them directly (2PP) than someone who hurt an anonymous third person (3PP). While this is true of small societies, people in large societies are actually more likely to engage in 3PP than 2PP. Strong reciprocity, including generous offers and 3PP, exists mostly in large, complex societies that face numerous challenging collective action problems. We argue that ‘spiteful’ 2PP, motivated by the basic emotion of anger, is more universal than 3PP and sufficient to explain the origins of human cooperation.

Culture does account for variation in game behavior

Lamba and Maces critique (1) of our research (2–4) is based on incorrect claims about our experiments and several misunderstandings of the theory underpinning our efforts. Their findings are consistent with our previous work and lead to no unique conclusions.

Breastfeeding structure as a test of parental investment theory in Papua New Guinea.

Evolutionary parental investment theory predicts that parents invest preferentially in offspring best able to translate investments into fitness payoffs. It has also been proposed that where the reproductive prospects of offspring are directly correlated with parental investment and variance in fertility is higher for males than females, parents in better condition should bias investment toward males while those in poorer condition should bias investment toward females. Lactation is arguably among the costliest forms of investment expended by mothers and is thus expected to be allocated in ways consistent with fitness payoffs. Quantitative data collected among 110 Papua New Guinean mother‐infant pairs during 470 h of focal follows on nursing frequency and duration and responses to infant demands by maternal and offspring characteristics are presented to provide empirically‐based descriptions of infant care and tests of evolutionary parental investment theory. Results indicate that mothers show very high levels of investment in offspring. However, although breastfeeding in developing countries is often characterized as on‐demand, fussing and crying by infants were only attended to with breastfeeding about 30% of the time. Contrary to expectations of parental investment theory that parents should invest less in poorer quality offspring, mothers increased investment in offspring in poorer condition. The expectation that mothers in better condition would bias investment toward male offspring was also not supported; better nourished mothers biased investment toward female offspring. This study illustrates how infant feeding data may be used for testing larger evolutionary questions such as those derived from parental investment theory. Am. J. Hum. Biol. 2009.

Models of decision-making and the coevolution of social preferences

We would like to thank the commentators for their generous comments, valuable insights and helpful suggestions. We begin this response by discussing the selfishness axiom and the importance of the preferences, beliefs, and constraints framework as a way of modeling some of the proximate influences on human behavior. Next, we broaden the discussion to ultimate-level (that is evolutionary) explanations, where we review and clarify gene-culture coevolutionary theory, and then tackle the possibility that evolutionary approaches that exclude culture might be sufficient to explain the data. Finally, we consider various methodological and epistemological concerns expressed by our commentators.

Market Integration, Reciprocity, and Fairness in Rural Papua New Guinea

In order to test the proposition that performance in bargaining experiments is significantly affected by degree of monetarization, market integration, and relative westernization, a one-shot Ultimatum Game was conducted during the months of June and July 1998 in two villages in a rural region of Papua New Guinea: Anguganak (where the people speak Au) and Bogasip (where they speak Gnau). Although the villages are located in close proximity to one another and are relatively homogeneous culturally, and both subsist using a mixture of foraging and horticulture and have an elaborate system of exchange relationships, they are distinguished by their average degree of exposure to and integration in a cash-based economy, as well as their degree of education (both are greater in Anguganak). The different sections of the chapter provide: an ethnographic account of the two villages; a description of the experimental methods employed; a presentation and analysis of the results in terms of various indicators of wealth and market integration; and a discussion of the implications of the results. The level of offers made in the Ultimatum Game data combined for Anguganak and Bogasip were between those in western industrialized populations and the Machiguenga of Peru. There was some indication that variability in the level of market integration between the two village populations may have influenced the results, although they appeared to be equally influenced by local beliefs on reciprocity, generosity, and indebtedness, and an unfamiliarity with impersonal transactions.

Reply to van Hoorn: Converging lines of evidence

We agree with the comments by van Hoorn (1) on our critique (2): testing causal hypotheses about human behavior is a challenge (1, 3). Making progress requires specifying alternative hypotheses and then testing these hypotheses using diverse and converging lines of evidence. We have defended the hypothesis that social norms, which culturally coevolved with the institutions of large-scale societies including markets, influence economic decision-making. This hypothesis emerged from a larger set that we developed both at the outset of our project and as we went along. Our interdisciplinary team’s initial list of hypotheses included the idea that experimental games might spark an innate reciprocity module that would yield little variation across populations. We also considered the hypothesis that group-level differences might result from individual differences in wealth or income. Nevertheless, what emerged in the data in our first project was (i) substantial variation among 15 populations, (ii) a strong correlation with market integration, and (iii) little relation to individual-level economic or demographic variables. Not satisfied with our first effort, we sampled 10 new populations, replicated these findings with improved protocols (developed based on critiques of the Phase I), and then extended them to two additional experimental games. Along the way, we have explored alternative hypotheses using measures of genetic relatedness, social network position, anonymity manipulations, and framing tools. To our knowledge, no other existing hypotheses can better account for the observed patterns of variation.