Humans are a puzzling species. On the one hand, we struggle to survive on our own in the wild, often unable to solve basic problems, like obtaining food, building shelters or avoiding predators. On the other hand, human groups have produced innovative technologies, sophisticated languages, and complex institutions that have permitted us to successfully expand into environments across the globe. What has enabled us to dominate such a vast range of environments, more than any other species? The Secret of Our Success lies not in our innate intelligence, but in our collective brains—in the ability of human groups to socially interconnect and to learn from one another.

Drawing insights from lost European explorers, clever chimpanzees, hunter-gatherers, neuroscientists, ancient bones, and the human genome, Joseph Henrich demonstrates how our collective brains have propelled our species’ genetic evolution, and shaped our biology. Our early capacities for learning from others produced many innovations, such as fire, cooking, water containers, plant knowledge and projectile weapons, which in turn drove the expansion of our brains and altered our physiology, anatomy and psychology in crucial ways. Further on, some collective brains generated and recombined powerful concepts, such as the lever, wheel, screw and writing. Henrich shows how our genetics and biology are inextricably interwoven with cultural evolution, and that this unique culture-gene interaction has propelled our species on a unique evolutionary trajectory.

Tracking clues from our ancient past to the present, The Secret of Our Success explores how our cultural and social natures produce a collective intelligence that explains both our species striking uniqueness and odd peculiarities.

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Evolutionary theorists argue that cultural evolution has harnessed various aspects of our evolved psychology to create a variety of different mechanisms for sustaining social norms, including those related to large-scale cooperation. One of these mechanisms, costly punishment, has emerged in experiments as an effective means to sustain cooperation in some societies. If this view is correct, individuals' willingness to engage in the costly punishment of norm violators should be culturally transmittable, and applicable to both prosocial and anti-social behaviors (to any social norm). Since much existing work shows that norm-based prosocial behavior in experiments develops substantially during early and middle childhood, we tested 245 3- to 8-year olds in a simplified third party punishment game to investigate whether children would imitate a model's decision to punish, at a personal cost, both unequal and equal offers. Our study showed that children, regardless of their age, imitate the costly punishment of both equal and unequal offers, and the rates of imitation increase (not decrease) with age. However, only older children imitate not-punishing for both equal and unequal offers. These findings highlight the potential role of cultural transmission in the stabilization or de-stabilization of costly punishment in a population.

Questions about the origins of human cooperation have long puzzled and divided scientists. Social norms that foster fair-minded behavior, altruism and collective action undergird the foundations of large-scale human societies, but we know little about how these norms develop or spread, or why the intensity and breadth of human cooperation varies among different populations. What is the connection between social norms that encourage fair dealing and economic growth? How are these social norms related to the emergence of centralized institutions? Informed by a pioneering set of cross-cultural data, Experimenting with Social Norms advances our understanding of the evolution of human cooperation and the expansion of complex societies.

Editors Jean Ensminger and Joseph Henrich present evidence from an exciting collaboration between anthropologists and economists. Using experimental economics games, researchers examined levels of fairness, cooperation, and norms for punishing those who violate expectations of equality across a diverse swath of societies, from hunter-gatherers in Tanzania to a small town in rural Missouri. These experiments tested individuals’ willingness to conduct mutually beneficial transactions with strangers that reap rewards only at the expense of taking a risk on the cooperation of others. The results show a robust relationship between exposure to market economies and social norms that benefit the group over narrow economic self-interest. Levels of fairness and generosity are generally higher among individuals in communities with more integrated markets. Religion also plays a powerful role. Individuals practicing either Islam or Christianity exhibited a stronger sense of fairness, possibly because religions with high moralizing deities, equipped with ample powers to reward and punish, encourage greater prosociality. The size of the settlement also had an impact. People in larger communities were more willing to punish unfairness compared to those in smaller societies. Taken together, the volume supports the hypothesis that social norms evolved over thousands of years to allow strangers in more complex and large settlements to coexist, trade and prosper.

Innovative and ambitious, Experimenting with Social Norms synthesizes an unprecedented analysis of social behavior from an immense range of human societies. The fifteen case studies analyzed in this volume, which include field experiments in Africa, South America, New Guinea, Siberia and the United States, are available for free download on the Foundation’s website.

Anthropologists have documented substantial cross-society variation in people's willingness to treat strangers with impartial, universal norms versus favoring members of their local community. Researchers have proposed several adaptive accounts for these differences. One variant of the pathogen stress hypothesis predicts that people will be more likely to favor local in-group members when they are under greater infectious disease threat. The material security hypothesis instead proposes that institutions that permit people to meet their basic needs through impartial interactions with strangers reinforce a tendency toward impartiality, whereas people lacking such institutions must rely on local community members to meet their basic needs. Some studies have examined these hypotheses using self-reported preferences, but not with behavioral measures. We conducted behavioral experiments in eight diverse societies that measure individuals' willingness to favor in-group members by ignoring an impartial rule. Consistent with the material security hypothesis, members of societies enjoying better-quality government services and food security show a stronger preference for following an impartial rule over investing in their local in-group. Our data show no support for the pathogen stress hypothesis as applied to favoring in-groups and instead suggest that favoring in-group members more closely reflects a general adaptive fit with social institutions that have arisen in each society.

In suggesting that new nations often coalesce in the decades following war, historians have posed an important psychological question: Does the experience of war generate an enduring elevation in people's egalitarian motivations toward their in-group? We administered social-choice tasks to more than 1,000 children and adults differentially affected by wars in the Republic of Georgia and Sierra Leone. We found that greater exposure to war created a lasting increase in people's egalitarian motivations toward their in-group, but not their out-groups, during a developmental window starting in middle childhood (around 7 years of age) and ending in early adulthood (around 20 years of age). Outside this window, war had no measurable impact on social motivations in young children and had only muted effects on the motivations of older adults. These war effects are broadly consistent with predictions from evolutionary approaches that emphasize the importance of group cooperation in defending against external threats, though they also highlight key areas in need of greater theoretical development.

Cooperation between nonrelatives is common in humans. Reciprocal altruism is a plausible evolutionary mechanism for cooperation within unrelated pairs, as selection may favor individuals who selectively cooperate with those who have cooperated with them in the past. Reciprocity is often observed in humans, but there is only limited evidence of reciprocal altruism in other primate species, raising questions about the origins of human reciprocity. Here, we explore how reciprocity develops in a sample of American children ranging from 3 to 7.5 years of age, and also compare children's behavior to that of chimpanzees in prior studies to gain insight into the phylogeny of human reciprocity. Children show a marked tendency to respond contingently to both prosocial and selfish acts, patterns that have not been seen among chimpanzees in prior studies. Our results show that reciprocity increases markedly with age in this population of children, and by about 5.5 years of age children consistently match the previous behavior of their partners. (C) 2013 Published by Elsevier Inc.

Human populations differ reliably in the degree to which people favor family, friends, and community members over strangers and outsiders. In the last decade, researchers have begun to propose several economic and evolutionary hypotheses for these cross-population differences in parochialism. In this paper, we outline major current theories and review recent attempts to test them. We also discuss the key methodological challenges in assessing these diverse economic and evolutionary theories for cross-population differences in parochialism.

Humans are an exceptionally cooperative species, but there is substantial variation in the extent of cooperation across societies. Understanding the sources of this variability may provide insights about the forces that sustain cooperation. We examined the ontogeny of prosocial behavior by studying 326 children 3-14 y of age and 120 adults from six societies (age distributions varied across societies). These six societies span a wide range of extant human variation in culture, geography, and subsistence strategies, including foragers, herders, horticulturalists, and urban dwellers across the Americas, Oceania, and Africa. When delivering benefits to others was personally costly, rates of prosocial behavior dropped across all six societies as children approached middle childhood and then rates of prosociality diverged as children tracked toward the behavior of adults in their own societies. When prosocial acts did not require personal sacrifice, prosocial responses increased steadily as children matured with little variation in behavior across societies. Our results are consistent with theories emphasizing the importance of acquired cultural norms in shaping costly forms of cooperation and creating cross-cultural diversity.