The WEIRDEST People in the World is a seminal, far-ranging contribution to the study of cultural evolution that deepens our understanding of why what McCloskey (Reference McCloskey2007) calls the “Great Enrichment” began in Western Europe around 1800. It should be of particular interest to business ethicists, as it offers an empirically grounded theory of the conditions that make possible impersonal markets and mutually beneficial exchange among strangers in complex economies. I start by reviewing the book’s main thesis concerning how marriage and family structures—particularly the prevalence of cousin marriage—have profound effects on a population’s behavior. I conclude by showing how Henrich’s book advances the scholarly discussion that goes back to Adam Smith of the moral basis of impersonal market exchange.

Joseph Henrich is one of the originators of the WEIRD acronym. Psychological studies, typically conducted on university students, target that subset of the human population that is Western, educated, industrialized, rich, and democratic. Researchers often treat the results of these studies as offering key insights into general human nature, when in fact they describe WEIRD minds shaped by recently formed and still uncommon cultural institutions. Among several other features, these minds tend to be individualistic, value adherence to impartial moral principles and the rules of impersonal institutions, and display an unusual willingness to cooperate with strangers (56).

Henrich’s book traces the origins of the WEIRD mind back to policies the Roman Catholic Church imposed on Western Europe in the Middle Ages. These policies, part of what Henrich calls the “Marriage and Family Program,” prohibited marriage to blood relatives and affinal kin, and they encouraged property ownership and inheritance by personal testament. These changes to marriage and family customs, he argues, resulted in the breakdown of kin-based social networks. Individuals needed to be highly mobile to find marriage partners, making them more willing to create mutually beneficial relationships with strangers and to work to distinguish themselves from the crowd, often doing this in packed cities. Henrich argues that this gradual shift in the organization of European social life, which unfolded over a millennium, explains much global psychological variation today, both within Europe and in its offshoots.

Much of his evidence that WEIRD peoples are, indeed, weird consists of noting strong correlations between general behavior and a region’s Kinship Intensity Index (KII), which measures rates of cousin marriage, among other factors. Henrich’s data come from dozens of countries worldwide and even reveal disparities within specific countries, such as Italy, which has significant regional variation in the prevalence of cousin marriage. Taken one by one, these correlations do not establish much. A single strong correlation is not enough to establish a causal claim, as hidden variables can always be driving the connection, and many of the correlations Henrich notes are of moderate or even weak strength. But the cumulative effect of dozens of these correlations builds a persuasive case that the dissolution of kin networks causes lasting changes in psychology and behavior.

People from countries with a high KII tend to display the following behaviors. They are more distrustful of strangers and adherents to other religions (206). They show greater willingness to give false testimony in court (208). They contribute less on average to public goods games (211). With WEIRD people, introducing the threat of punishment generally increases contributions to these games. Not so with players from places with the most intensive kinship: for them, adding punishment lowers contributions further by introducing “revenge cycles,” where individuals punish those who contribute more than themselves (218–19). Diplomats from countries with intensive kin networks accumulate more parking tickets than those from countries with weak kin networks, showing disregard for local rules when they are immune from punishment (215). Additionally, countries with higher KII scores display lower levels of analytical thinking in tests that differentiate analytic from holistic thinking styles (222). In short, kinship network intensity is correlated with a suite of behaviors that are non-WEIRD. Throughout, Henrich treats these behavioral differences as evidence of underlying psychological variation. But these behaviors are rarely observed in psychology studies, which tend to be conducted using subjects from regions with weaker kinship intensity.

This has important implications for business ethics scholarship that uses results from social science research. Ultimatum games, Asch conformity experiments, or studies that assume the “Big Five” dimensions of personality do not reveal universal truths. Instead, they offer insight into the behavior of populations with low KII scores, which provide most university students who are subjects for social science research. Business ethicists should be wary of generalizing from these results when theorizing about non-WEIRD market behavior.

One criticism of the book is that Henrich’s central causal thesis has limited explanatory power. Henrich argues that the Church’s program of marriage and family reform resulted in the gradual spread of WEIRD traits across Europe (236). But those reforms worked alongside several other factors—including geographic and agricultural ones—to erode kinship network intensity. As evidence that the Church’s reforms were the dominant factor explaining kinship variation, Henrich cites economist Jonathan Schulz’s difference-in-difference analysis of medieval Europe as a whole (226n41), but we have seen that KII scores are sometimes only weakly correlated with specific WEIRD traits. Even within the space of the former Carolingian Empire, which was longest under bishopric influence, there is significant variation in responses to the “generalized trust question,” which measures impersonal social trust (47). And Scandinavian countries like Sweden score highest in Europe on certain WEIRD traits, including patience (38) and interpersonal trust (45), while also having several hundred years’ less exposure than the Carolingian regions to the Church’s marriage reforms. Henrich’s causal thesis of Church exposure may be the best explanation of KII variation in Europe, but it does not fully explain European variation in the WEIRD behaviors that are conducive to economic prosperity.

Nevertheless, the book should convince readers that there is a significant relationship between kinship network intensity and WEIRD mentalities. Appreciating these correlations enriches our understanding of the conditions that make mutually beneficial exchange possible. This, I believe, is where the book has the most to offer business ethicists.

Strong kin-based networks make it harder for individuals to engage in mutually beneficial trade. Lower levels of impersonal trust mean that individuals have reduced confidence that strangers will adhere to impartial moral norms. This leads to a tendency to see the world in zero-sum terms: any benefits to my trading partner must come at my expense, and my benefits are his losses. Zero-sum thinking leads to reduced levels of trade with strangers, meaning that people are exposed to fewer innovations. Over time, this significantly reduces the pace of technological advancement.

In contrast, weaker kin-based social networks help impersonal markets flourish. Trust in strangers reinforces an impersonal prosociality that privileges the best business partners or clients over family or tribal connections. Seeing anonymous others as sources of value encourages positive-sum thinking and creates a setting where “the greatest respect goes to those who succeed by their own talents and hard work while still being fair, honest, and impartial” (294).

Henrich argues that effective markets require these norms to be in place to some degree. But markets can also strengthen these norms. On this point, he cites a study by economist Devesh Rustagi, who ran variations on public goods games with the Oromo people in Ethiopia. First, Rustagi found that the Oromo showed higher rates of conditional cooperation the closer they lived to market towns. Second, those who displayed cooperative behavior in games also showed a higher propensity to cooperate in voluntary organizations to restrict logging to prevent deforestation (298–99). Henrich sees the study as evidence that market integration—measured as proximity to market towns—produces greater compliance with the impersonal norms of voluntary associations. Markets promote virtues like fairness and impartiality, even as they require those traits to function well.

The transition to WEIRD norms comes with significant costs. Henrich acknowledges that the shift to WEIRD psychologies can lead to widespread feelings of loneliness, increasing the chance of committing suicide (426). The economic prosperity that erosion of kinship networks makes possible is not an unmitigated good, and Henrich’s analysis does not address the serious trade-offs at stake.

Business is about mutually beneficial exchange. Adam Smith ([1776] Reference Smith1981, 25), in The Wealth of Nations, leaves open whether the disposition to exchange is “one of those original principles of human nature” or “the necessary consequence of the faculties of reason and speech.” Henrich presents us with a third option: our disposition to engage in mutually beneficial trade with strangers on fair terms is a contingent product of cultural learning. It is not a skill that can easily be taught to a new generation; rather, this disposition is the result of the gradual coevolution of our cultural institutions and our psychologies. This coevolutionary process does not act on genes. Henrich argues that our psychologies are shaped by nongenetic factors like learning to read, which dramatically affects neural development. Other biological factors driving this process include male’s reduced testosterone levels in monogamous marriage, compared to non-WEIRD polygynous men. Rituals, too, play a role: he speculates that the cultural importance of structuring time around precise clocks reinforced virtues of prudence, diligence, and punctuality, which are necessary for managing complex business enterprises.

This review has only scratched the surface of Henrich’s book, which offers a sweeping historical narrative investigating how culture causes economic prosperity. That narrative draws on a range of scholarly disciplines to understand the contingent process through which Westerners became the kind of moral creatures who cooperate and trade at global scale. Anyone interested in the conditions that make impersonal, mutually beneficial markets possible has much to learn from this book.