The life of Ferdinand Tonnies, German sociologist and political activist, is a testament to the dramatic social changes which swept across Europe in the 19th and 20th centuries. Born in 1855 to a wealthy farmer at the northern fringe of the German empire, Tonnies spent his childhood working on the family estate, sharing close quarters with parents, siblings, and servants, and living in a traditional, face-to-face community of neighbors and extended family. But late in his childhood, young Tonnies’ life transformed when his father uprooted his family to the bustling market town of Husum in a career shift from farming to banking. In this new world of commerce and cosmopolitan society, young Tonnies personally excelled at school and began a life-long journey which further distanced him from his rural roots and propelled him to academic appointments in some of Europe’s most ascendant metropoli.

These divergent experiences—the stable, face-to-face community of his childhood and the increasingly anonymous and commercial social worlds he inhabited as an adolescent and adult—served as exemplars for the two primary forms of sociality Tonnies outlined in his later theoretical work. The first social form, Gemeinschaft or community, was the face-to-face life of his rural youth where people worked together for collective goals and where individual wishes were subordinated to those of the group. The second form, Gesellschaft or civil society, turned this relationship between individuals and the group on its head. Exemplified by modern nation-states, corporations, and voluntary clubs, Tonnies’ civil society existed to serve its members needs and wishes—not the other way around. As opposed to Gemeinschaft’s emphasis on collective goals and personal relationships, Gesellschaft was built on individual rights and responsibilities and impersonal exchanges of money, goods, and services.

Tonnies’ distinction between community and civil society profoundly shaped 20th century social science and inspired anthropologists, sociologists and cross-cultural psychologists to explore how different social groups organized themselves according to these different principles. Often framing the distinctions in different terms, such as collectivism vs. individualism, embeddedness vs. autonomy, or particularism vs. universalism, these investigators discovered striking and reliable cross-population differences in how people endorse a group’s interests over their own goals, how people value personal relationships over impartial rules with strangers, and how they define themselves in terms of their social relationships rather than their own individual qualities and accomplishments. Researchers have also found that a basic element of Tonnies’ Gemeinschaft—in-group loyalty—hitchhikes with a suite of other “traditional” values and behaviors. Notably, people who are more concerned about loyalty are also more likely to value obedience to authority, to conform to group norms and to avoid impure, unnatural, unchaste and ungodly acts. Thus, in describing the social change he experienced in 19th century Germany, Tonnies identified what appears to be a general and wide-reaching dimension of human social life which far transcended his own biography.

For most of the 20th century, scholars focused largely on the consequences of these individual and societal differences in collectivism. How does growing up in a highly embedded or collectivist community make one more likely to favor the interests of one’s group or one’s personal relationships when faced with tough decisions? How does the balance of traditional community and civil society in a nation shape its economic and political atmosphere? And how does having a more collectivist outlook shape one’s views toward people from other groups? Cross-cultural psychologists, political scientists, and anthropologists have made great progress in answering these questions, showing how either a group- or individual-orientation shapes our views of others, our treatment of strangers and outsiders, and our solutions to ethical dilemmas such as lying to maintain social harmony. These differences also creep into basic cognition and appear to affect how we incorporate contextual and situational factors in our perceptions of ourselves and our world.

But where do these differences come from? Why do some societies appear more grounded in Gemeinschaft or Gesellschaft than others? Are these differences simply the result of arbitrary historical accidents, with some societies—such as those in East Asia—following different cultural paths than others—such as those in Northern Europe? Or do these differences arise from more general processes of adaptation and change? With mounting evidence for these cognitive and behavioral differences, a number of evolutionary social scientists have begun to tackle these deeper questions, proposing and exploring evolutionarily-informed hypotheses for their origins. How might our evolutionary history have shaped an organism capable of exhibiting such cross-population variation in social attitudes? How might these cross-population differences reflect an adaptive fit with local conditions? And what are the mechanisms that generate and perpetuate such cross-population variation? In contrast to tired debates about whether evolutionary approaches are appropriate for the study of such human differences, these inquiries have led to emerging (and frankly more interesting) debates within evolutionary social science about which specific evolutionary processes might have played a role in generating them.

In 2008, a team of biologists and psychologists, including Randy Thornhill, Corey Fincher, Mark Schaller and Damian Murray, made one of the early systematic forays into this new territory. Their parasite stress theory grew from the observation that infections are a powerful evolutionary force and that many of our social behaviors may have been selected because they prevent the spread of these deadly pathogens. The parasite stress theory has since been used to explain variation in a dizzying array of human behaviors, including warfare, conformity, religiosity, interstate trade, food preferences, and even homicide. However, most relevant for Tonnies’ distinction between Gemeinschaft and Gesellschaft, the theory also made explicit predictions about avoiding interactions with outsiders and focusing inward on one’s local group and relationships. The explanation was highly intuitive. In situations of high pathogen risk, avoiding outsiders prevented exposure to new and deadly pathogens. However, in situations of low pathogen risk, people with such in-group preferences missed opportunities for learning new skills, finding mates, and exchanging novel goods. Thus, the theory proposed that humans had evolved a psychological system—a behavioral immune system—which was dedicated to detecting the presence of pathogens in the environment and that would become more collective when faced with greater pathogen risk and more individualistic when pathogen risk declined.

Since that time, two of the theory’s main proponents, Corey Fincher and Randy Thornhill, have expanded the theory to argue that in-group loyalty does more than simply prevent the spread of pathogens. It also provides important aid when one gets sick. This theoretical extension, based on the in-group loyalty and support provided by collectivism is no longer limited to the avoidance of parasites. Indeed, the same reasoning can apply to ensuring aid against any basic threat to survival and reproduction—including food and water shortages and personal feuds. Indeed, the ability of in-groups to provide reliable aid in times of need opens up the possibility that collectivism is not simply a response to pathogen stress, but rather to all kinds of threats. According to this alternative, and more general perspective, when people can reduce the risk of individual threats through cooperation and collective action, they will build cooperative ties and invest in groups which help them do this. This material insecurity hypothesis is closely aligned with earlier non-evolutionary theories proposed by political scientists, including Ronald Inglehart and Pippa Norris. However, it draws from behavioral ecology and niche construction theory to posit that humans have been selected as master builders and exploiters of their social niches. Importantly, human social environments are highly variable, ranging across mobile foraging bands, stable farming communities, and urban cityscapes in a modern nation-state. Each of these social environments provides humans with a different set of options and tools for building their social niches. In the case of mobile foragers, the key social resources may be band members and personal ties with outsiders. In stable farming communities, such as those of Tonnies’ youth, the main social resources may have been family, neighbors, and the local Lutheran congregation. In modern welfare states, expansive social insurance and legal systems may both alleviate the need for local support and permit new far-flung forms of impersonal social interaction and market exchange. Thus, the suite of available social resources and institutions can dramatically shape the trade-offs faced by people and how people ultimately cultivate their social worlds. In situations where state-level institutions meet basic needs, provide social opportunities, and mitigate major risks, people will be less likely to value in-groups. Meanwhile, in situations where in-groups provide the basic form of reliable social insurance, people will be more collectivist. In both cases, the available suite of social institutions and opportunities plays a big role in influencing people’s decisions, and thus the material insecurity hypothesis could also be termed the institutional hypothesis.

A third evolutionary hypothesis for such differences is grounded in life history theory—a theory of how organisms divide limited time and resources across fitness-related goals, such as mating, parenting, growth, and survival. One prediction of life history theory is that when faced with an uncontrollable and uncertain future, an organism’s best bet is to reproduce early and avoid future investments—what some scholars have called a ‘fast’ strategy. Conversely, when the world is stable enough that one can reap big future rewards from current investments, it may make sense to follow a ‘slower’ strategy—delaying reproduction in favor of other investments in growth, existing offspring, or future prevention of threats. Humans are already at the ‘slow’ end of life history strategies, but depending on the environment, they may live faster or slower. Indeed, researchers have identified clusters of behaviors which appear to follow this fast-slow dimension, with individuals at the fast end more likely to start sex earlier, to become pregnant as a teen, to contract sexually transmitted diseases, to avoid long-term pair bonds, and to shirk on parental care. Evolutionary psychologists have also applied this line of reasoning to other kinds of investments, including helping in-group members, cultivating long-term bonds, and involvement in religion. If these social investments have relatively delayed returns, one would also expect individuals with faster life histories to avoid them, favoring instead an asocial strategy. Thus, according to this theory, in situations of high uncontrollable risk, we would expect people to invest even less in their social niches, devote less time and effort to long-term relationships, express less loyalty to in-groups, and be less religious. I will call this the asocial life history hypothesis.

Even though these three hypotheses all derive from evolutionary thinking, they make different predictions about when we should see greater investment in and loyalty to one’s local group and when we should see individuals acting, well, as autonomous individuals. According to parasite stress theory, we should see higher levels of collectivism in situations of higher pathogen stress, but not in response to other kinds of material insecurity. According to the material insecurity or institutional hypotheses, we should expect people to exhibit less collectivism and in-group loyalty when state-level institutions successfully meet people’s basic needs and guard against material threats. Finally, the asocial life history hypothesis suggests that in situations where people have fewer opportunities to control risk, for example, in situations of greater mortality and economic uncertainty, they will avoid investments in anything but their own immediate reproduction (and thus eschew commitments to relationships and other social investments).

Another important distinction between these different theories is the feedbacks they posit. Parasite stress and asocial life history theories both assume a relatively fixed environment to which individuals adapt. The material insecurity or institutional hypothesis, on the other hand, assumes positive co-evolutionary feedbacks, whereby the existence of beneficial institutions can lead people to make further investments in those institutions to render them even better investments in the future. Thus, the material insecurity or institutional hypothesis can lead to novel social trajectories which are not purely driven by initial conditions—such as parasite levels—in a setting.

In the last five years, a number of papers have assessed these different hypotheses by examining how they account for observed variation in in-group loyalty and collectivism across human populations. Initial reports by Corey Fincher and Randy Thornhill indicated that variation across both countries and U.S. States was highly consistent with a dedicated response to pathogens. However, more work in the last year has raised questions about these findings. In a recently published paper in Human Nature, Elizabeth Cashdan and Michael Steele assessed the hypothesis with more than 70 ethnographically documented cultures and found no relationship between pathogen load and either loyalty to one’s local community or to one’s ethnic group.

In two recent papers, colleagues and I also have re-analyzed the country- and state-level data originally used to support the pathogen stress hypothesis. We arrived at different conclusions. In a paper published this summer in PLOS ONE, Joe Henrich and I re-analyzed the cross-national data used by Fincher and Thornhill. We found first that the associations observed by Fincher and Thornhill were driven exclusively by differences across six major world regions, but not across countries within those regions. This raises serious questions about the generalizability of the hypothesis to smaller population scales (e.g. countries), and also about the ability of such a small sample size (N = 6!) to truly compare the parasite stress hypothesis with other plausible hypotheses. In that paper, we also assessed the material insecurity or institutional hypotheses, by estimating how in-group loyalty decreases when government services improve. We found strong, consistent support for this hypothesis within world regions and even when controlling for pathogen stress.

In another paper in Evolution and Human Behavior, Joe Hackman and I also re-examined the state-level data used in prior studies to support the pathogen stress hypothesis for family ties and religiosity—two components of collectivism. Once again we found big, apparently significant, effects created artificially by a few major populations. But instead of just a sample size of six, the effective sample size was only two! Specifically, all of the effects of pathogen stress on family ties and religious involvement were due to differences between two populations—non-hispanic whites and blacks. Indeed, when we considered these groups separately, we did not find any of the cross-state associations with pathogen stress. It is possible that the differences between U.S. whites and blacks are due to differing exposure to pathogens, but many other plausible hypotheses could also explain these racial differences, including more than three centuries of differential access to effective government and institutions and greater exposure to various forms of uncontrollable risk. We hope to explore this possibility in future work.

These findings raise serious questions about the ability of infections to explain cross-population variability in collectivism. The cross-national findings also suggest that the material security and institutional hypothesis is still in the running. Remarkable in both datasets is the way that an asocial life history hypothesis fails completely. Specifically, in situations of lower life expectancy and among populations with faster life histories, people showed both greater in-group loyalty and greater religiosity. Both of these findings go against a life history hypothesis where avoiding social investment is the assumed short-term option. However, the consistency and strength of these findings suggest an important pattern worth further exploration. They also suggest we may need to reconsider whether avoiding social investment is a smart move (and thus a realistic theoretical default) in early human societies. A human life history theory which more clearly outlines how different social investments provide varying degrees of short-term and long-term benefits may help resolve this puzzle. Fortunately, such a socially-informed human life history theory is beginning to emerge with the work of a number of anthropologists, including Hillard Kaplan, Kim Hill, and Carol Worthman.

The only definitive statement I can make about these current investigations is that it is an exciting time for the evolutionary study of human sociality. I have outlined only three of what I hope will be many more evolutionary hypotheses for extant variation in collectivism in the coming years. We are also reaching the limit of existing datasets to discriminate between these three (and other) hypotheses. Hopefully, creative and motivated researchers will find novel longitudinal datasets at other population scales to provide new tests and comparisons.

I also cannot say with certainty what Tonnies would have thought of all these developments, especially given his early critiques of Darwinian models of human society. That said, he was an ardent social activist, and he would likely have wanted to know how these different theories could inform efforts to build a better world. In a recent target article about the parasite stress hypothesis in Behavioral and Brain Sciences, several commentators argued that the theory pointed to the positive social consequences of reducing parasites. These benefits include theoretical reductions in ethnocentrism, ethnic conflict, parochialism, and corruption. Depending on the theory, similar arguments could be made about the social benefits of improving social welfare or reducing basic forms of material insecurity. However, even before any of these theories were proposed, large international organizations have been working for more than half a century toward these goals, and it’s not clear that these theories will increase their resolve. Indeed, these organizations have been quite successful at reducing the burden of infectious diseases, controlling other risks to human life, and improving social welfare in the last century. If, as these diverse theories propose, collectivism and in-group loyalty declines in the face of any of these changes, then we may want to ask instead (as Tonnies did), whether such theoretically inexorable transitions away from collectivism and in-group loyalty are an unalloyed good, and how we can still use the best of collectivism and group loyalty to add meaning to our lives.