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Stranger Danger

Going to a restaurant is one of my keenest pleasures. Meeting someplace with old and new friends, ordering wine, eating food, surrounded by strangers, I think is the core of what it means to live a civilized life. — Adam Gopnik

After centuries of speculation, the last 25 years has seen a flurry of theoretical advances toward understanding how our species transitioned from a run-of-the-mill hominin living more or less in a state of nature, to entirely reconstructing our environment and living in a “nature” of our own design. While our cousins, the other African apes, use tools, transmit culture, are highly intelligent, and possess rich emotional lives, there remains a large gap between the social culture of the other apes and our own. Although I am accustomed to emphasizing the behavioral similarities between humans and other animals, as I did in my book Not So Different,1 there is no denying that we are peculiar animals. While one could argue, for example, that chimpanzees have the basic toolkit of referential communication, it is obvious that the transition to farming, settled life, and urbanization is nowhere in sight.

In his new tour de force, The Human Swarm: How Our Societies Arise, Thrive, and Fall, biologist Mark Moffett outlines a powerful new thesis that, if correct, could unite several ideas about the origins of behavioral modernity and help explain how we ended up firmly on the path toward civilization and the creation of nation-states. At the risk of oversimplifying what he spends over 400 pages building the case for, the gist of Moffett’s thesis is as simple as it is insightful: at some point in our past we became tolerant of strangers within our own societies. This is indeed unusual. As Moffett writes, “In this final stage of our narrative, humans have taken a path for which there are few parallels in nature.”2

The example that Moffett often gives in his many interviews about the book is that of the café. Because we waltz in and out of coffee shops without a thought, we fail to grasp just how odd this is (and I don’t mean because everyone is buried in their phone or laptop, eschewing in-person conversations in a venue especially deigned to foster them). When we enter the café, we are an unknown person, an interloper, walking into a crowd of persons unknown to us and to each other, a sea of strangers. We consider that experience totally unremarkable, but among almost all other animal species on earth, including the most pro-social ones, the casualness of our café experience is completely unthinkable. As Moffett has put it, “If a chimpanzee walks into a group of stranger chimpanzees, not all of them will leave unscathed.”

Deeper digging reveals that this thesis is anything but simple. To fully comprehend it, we must explore the concept of individual and social identity, in-group/out-group definition and recognition, and cultural signaling and identification. We must also tussle with the dynamics of competition and cooperation, empathy, social cohesion, and, of course, xenophobia and racism. Moffett’s thesis presses on social issues as intractable and timely as immigration, racial justice, and the origin and purpose of warfare. The goal of The Human Swarm is no less ambitious than to redefine the very concept of society and give it a proper biological meaning. Whether or not one agrees with the central thesis, it is hard deny the merits of the book because ideas can only be scientifically tested and refined after they have first been rigorously outlined. Moffett has met that standard and there is a mountain of solid scientific work to contend with in the book.

What Makes a Society

Perhaps surprisingly, The Human Swarm is not a book about human uniqueness. In fact, Moffett draws his inspiration about human societies from studying ants, a trick he learned from his Harvard mentor, E.O. Wilson. The point is not that human societies and ant societies share an evolutionary history, but rather that they share a purpose. The societies (colonies) of a few ant species can grow staggeringly large, trillions of individuals covering thousands of square kilometers, and yet are sharply defined. Distinct ant societies can exist in close proximity but with the strictest borders imaginable. To illustrate the point, Moffett points out that you can relocate an Argentine ant from a kitchen cupboard in San Francisco to a playground in Tijuana and it will be welcomed with open legs and happily go about its work, provided the relocation is within the same society, or “supercolony.” But if a similar ant accidentally ventures across the road into the territory of a different society of Argentine ants, it will be viciously killed on the spot.

Ants define their societies chemically, using molecules called cuticular hydrocarbons. Humans, on the other hand, use a great variety of markers to signal societal memberships. In the modern world, we think of flags, customs, religions, and even accents, but Mesolithic societies undoubtedly had their own markers, as hunter-gatherer societies do today. Jewelry, clothing, body paint, tattoos, other adornments, gestures, and of course languages were certainly in the mix. The important thing is that we, like ants, have a way of immediately knowing who’s in and who’s out. After all, when we walk into the café, it can’t be just any kind of strangers that we’ll be comfortable with.

The value of these societal identifications for humans and ants is that it allows functionally interacting groups to grow very large. Chimpanzees, on the other hand, must get to know every member of its society (called a community), and that becomes a lot to keep track of. Forgetfulness or mistaken identity can be costly, even lethal, and this seems to be the norm among many animals that form societies. Some level of recognition is required and this can put an upper limit on the size that these societies reach. Free from the burden of having to know everyone personally, our societies have swollen to enormous sizes, nearly rivaling those achieved by the Argentine ants. This strongly implies, in my view, that stranger tolerance is a previously underappreciated prerequisite for behavioral modernity. Let’s explore.

Social Intelligence

While ants and humans share peculiarities in the formation and identification of our societies, humans are much more like our fellow apes when it comes to the nature of our social interactions and relationships. Apes have allies, rivals, leaders, friends, competitors, subordinates, lovers, helpers, patrons, and sworn enemies. In fact, each of us maintains hundreds of relationships and they all take on a unique character. Advantageous though this sociality is, it represents a substantial cognitive load. In fact, the pressure to foster nuanced social relationships may have been a key driving force in the expansion of our cognitive capacities. Esther Herrmann and others have proposed the social intelligence hypothesis, which holds that our elaborate sociality was the key evolutionary stimulus of human intelligence, not just because we have intricate relationships but because those relationships can be nested.3 Humans are metasocial. We often exist in groups within groups and we nurture communities within communities, and we do so implicitly, that is, without conscious intention or even awareness.

Placing the social intelligence hypothesis within the context of Moffett’s ideas about stranger tolerance merely establishes the ultimate nested social group, the society. (It is worth noting here that Moffett argues that the society is the primary human group, and that hunter-gatherer societies had few if any long-lasting smaller groups within them.) The phenomenon of anonymous societies augments the social intelligence hypothesis, however, because it serves to reduce the cognitive demand of all this sociality. If we can spot members of our society at a glance, we can extend the number of our social relationships without expanding the brain work it takes to maintain them. It also creates a new and intriguing kind of relationship: the nonthreatening stranger.

As I sit writing this sentence, I am in another such scenario that is unthinkable for any other mammal (for more reasons than one): I am in a train racing through the English countryside surrounded by people I’ve never met and will never meet again, most of whom carry a different passport than I do. And yet, we are sharing this experience in harmony and enjoying fleeting social exchanges, nods and smiles, etiquette and pleasantries, and the occasional discussion of the weather and local sports happenings. Though no valuable information is exchanged and the relationships are ephemeral, to not participate is downright rude and can elicit contempt. I hasten to add that when an older woman dropped some of her things, several of us practically pushed each other out of the way to help her. These fleeting relationships are clearly important to us. It’s no wonder, but also lamentable, that we must work so hard to teach our children to fear friendly strangers.

Cooperation and Competition

At the World Science Festival in New York City this June, Moffett lamented that, “people interested in human social evolution always seem to focus on cooperation, but competition is just as integral to life within societies.” This may indeed be a bias on the part of scientists like myself, too eager to generate uplifting conclusions such as survival of the friendliest in an age that is desperate for them. Indeed, there are very good reasons to suppose that competition was just as important as cooperation in the evolution of our cognitive abilities, and perhaps more so. In fact, the skills needed to empathize and work harmoniously with others are, for the most part, the same skills that are needed to manipulate them. Perspective-taking, interpretation of facial expression and body language, and ability to predict future behavior all come to mind. Richard Byrne has summarized the so-called Machiavellian intelligence hypothesis in stark terms, “…competition with companions is the main biological problem for which increased intelligence is an adaptive solution.”4

Moreover, the notion that cooperation and competition are somehow mutually exclusive, or even in tension, is fallacious. Those with older siblings who mercilessly teased them, while also defending them from the teasing of others, have seen just how fast the cooperation-competition toggle can flip. To advance one’s own position, it is sometimes useful to work with others, and sometimes useful to work against them, either way, one must understand them.

Tolerance of strangers puts a different twist on competition and cooperation, one that is harder to model because most other primates display hostility to interlopers. When working with monkeys or apes, zookeepers usually must introduce new members very slowly, allowing them to become familiar and tolerant of each other before allowing unrestricted, unsupervised contact. Even with the upmost care, tragic conflicts are not uncommon. When we contrast this with how easily groups of newly introduced humans work together, it is obvious that we are in new territory with our group dynamics. New territory requires new skills.

It’s hard to say for certain how tolerance of strangers and the development of anonymous societies affected the evolution of our social intelligence, but it’s not hard to imagine a potential connection. Understanding and working with strangers poses additional challenges than cooperating with those we know well. Competing against them seems even more so. Then again, those that know us best can often most easily manipulate us. Parsing the intricacies of cooperation and competition with strangers versus intimates is a topic that warrants study.

Self-Domestication and Reduced Aggression

One of the most intriguing theoretical frameworks to emerge from the study of human society is that of self-domestication. In summary, this theory holds that there exists a common suite of phenotypes shared by domesticated animals and that humans exhibit as well, at least in part. This is an old idea, obliquely referred to by Darwin himself but more fully articulated by Franz Boaz in 1901.5 A diverse set of domesticated animals, from dogs to goats, display characteristic anatomical changes in the ears, face, cranium, tails, and skin. These are accompanied by behavioral changes including docility and neotany (persistently juvenile behavior).6 This has been most dramatically demonstrated in the domesticated foxes of Russia.7

Geneticists have even identified a common set of genetic changes in domesticated animals compared to their non-domesticated relatives.8 Collectively, the so-called domestication syndrome applies surprisingly well to humans compared to our recent ancestors. Consider our reduced nose, brow, and teeth. In fact, the cranio-facial feminization observed in our ancestors has been specifically linked to a drop in average circulating testosterone.9 Self-domestication even predicts the ~10% reduction in cranial capacity that we observe in the fossil record of the last million years.

The most salient feature of self-domestication for the construction of human societies is docility, the reduction of aggression. This was explored by Michael Shermer in his 2004 book The Science of Good and Evil and more recently by Richard Wrangham in his 2019 book The Goodness Paradox.10 While we do not have ready comparisons as we do with dogs and wolves and domesticated foxes and wild ones, it is widely accepted that our species experienced a steady decline in interpersonal and intergroup violence as we transitioned from the archaic human form to the behaviorally modern one, as documented in Steven Pinker’s 2011 book The Better Angels of Our Nature.11

A reduction in general aggression fits nicely with the tolerance of strangers, but it is not clear in which direction the causal arrow points, if at all. A major problem is that aggression and violence seem to bring such obvious adaptive benefits in humans and many other animals. Further complicating this tension, or perhaps resolving it, is Richard Wrangham’s articulation of the two distinct modes of human aggression: proactive and reactive.11 Sometimes called predatory attack, proactive aggression refers to the willful, planned—we might even say premeditated—assault on conspecifics driven by particular interests and toward clear goals. This form of aggression seems not to have waned in our species, and urbanization and statehood probably intensified it.

Reactive aggression, on the other hand, is a violent response to an imminent threat, real or perceived, even when that treat is nonviolent in nature. Separately, both Wrangham and Christopher Boehm have argued that our species has seen a steady decline in reactive aggression,13 and Boehm developed this further into a theory about our moral origins.14 Put another way, our species has seen a marked increase in self-restraint. Our responses to nonviolent threats tend to be less impulsive and more deliberative than that of other apes, at least on our good days, and this has almost certainly contributed to the decline of interpersonal violence. Importantly, while this observed drop in reactive aggression seems to flow naturally from self-domestication and/or a drop in testosterone, it does not strictly depend on either of those theories being correct.

Once again, tolerance of strangers fits perfectly within this paradigm. In a transition from a suspicious or even violent reaction toward strangers to a tolerant one when those strangers are identified as part of our society, or members of a friendly or at least non-threatening foreign society, a logical first step is self-restraint of the violent impulses. Selection might then have diminished those instincts if they were altogether unhelpful in the grand scheme of survival.

Group Selection (or not)

Speaking of selection, the precise mode in which tolerance of strangers emerged and spread through our species is currently obscure. Moffett posits that the first step must have been the emergence of societal identity markers in the remote past. Given the emphasis on social interactions and intra- and intergroup conflict, theorists will surely be divided on the selection mechanisms at play. To wit, the majority of evolutionary biologists reject the existence of non-kin group selection as a major evolutionary force, sometimes in quite unforgiving terms.15 Yet, the influence of group selection within a larger paradigm called multi-level selection has seen a growing list of defenders, most prominently David Sloan Wilson and E. O. Wilson. As Wilson and Wilson have put it, “Selfishness beats altruism within groups. Altruistic groups beat selfish groups.”16

The progression of human social structures, however, is at least as much a matter of cultural evolution as genetic evolution.17 Humans have become master accumulators of cultural knowledge and there can be little doubt that the adaptive features that flow from cultural forces have overpowered biological adaptation since at least the emergence of behavioral modernity itself. Even hardline gene-centric evolutionists like Richard Dawkins have noted how the depth of human culture has turned evolution on its head through the creation of memes. Human populations no longer wait around for happy genetic accidents in order to drive adaptation. We employ cultural solutions to conquer various environmental challenges from climate to securing food. We are the ultimate niche constructors and the world that we have created is intricately social. The theory of cultural group selection combines these phenomena without reliance on group selective forces on the genetic level.18 Nonetheless, mechanisms of selection will remain controversial for some time, at least in part due to our inability to model human social forces either in simulations or in other species.

When pressed on the question of selection mechanism, Moffett responded that he is “almost unique among biologists” to not be concerned with that question. Instead, his interests focus squarely on spelling out what societies are and how they function. The Human Swarm deftly sidesteps the controversy by allowing that we could arrive at large anonymous societies through either a gene-centric or multi-level selection model. Indeed, the Argentine ants have clearly achieved this through their genes alone and everyone agrees that cooperation can emerge purely through its benefits for individuals. Either way, tolerance of strangers and the construction of large societies places human cultural evolution into warp drive. As barriers to the free exchange of ideas and innovations come down, the richness of the cultural milieu explodes.

Size Matters

As intriguing as tolerance of strangers is per se, the real value of this phenomenon is that it would eventually allow our populations to grow exponentially. Without having to personally know everyone in order to functionally interact with them, our ancestors could form societies that grew without bounds, not unlike the Argentine ants, resources permitting. Moreover, in a species in which cultural evolution is now firmly in the driver’s seat, group size has profound effects. Studies have shown that the size of a human society has direct bearing on the depth of its cultural toolkit. Each individual plays a role in harboring, transmitting, and even contributing to the pool of shared ecological wisdom. This is most dramatically shown when populations collapse to very small size and the reservoir of cultural knowledge collapses with it.

The Human Swarm: How Our Societies Arise, Thrive, and Fall (book cover)

This effect of group size is intensely, though not uniquely, human. We have giant brains, yes, but so do plenty of other animals in which this effect is more modest. The key is that we have evolved into what we might call obligate social learners. There is very little that any of us truly learns on our own. Almost everything we know, both as individuals and as a society, we learned in a social context, which includes books, articles, demonstrations, higher learning, and so on. Very few of us can identify even one single skill that we developed in isolation.

Further still, we don’t just learn from close affiliates. We all have gained valuable lessons and insights from strangers and even from our enemies. Odds are pretty good that you, dear reader, do not know me from Adam, and yet I humbly hope that you have learned something from the exchange we are currently engaged in. And so it goes with almost all of our social interactions. As Adam Rutherford writes in the closing passage of The Book of Humans, “We accumulate knowledge and we pass it on… By teaching, we created ourselves, an animal that, together, is more than the sum of its parts.”19 END


The ideas herein were honed through incredibly insightful discussions, and disagreements, with Adam Rutherford (over too many whiskeys), David Sloan Wilson (with too many acronyms), and, of course, Mark Moffett (at too many restaurants). My gratitude to them should not be construed as an endorsement of any of the contents of this essay.

About the Author

Dr. Nathan H. Lents is Professor of Biology at John Jay College of the City University of New York. He also maintains The Human Evolution Blog and hosts the science podcast This World of Humans. He is the author of Not So Different: Finding Human Nature in Animals and Human Errors: A Panorama of Our Glitches, from Pointless Bones to Broken Genes.

  1. Lents, N. H. 2016. Not So Different: Finding Human Nature in Animals. Columbia University Press. (https://
  2. Moffett, M.W. 2019. The Human Swarm: How Our Societies Arise, Thrive, and Fall. Basic Books.
  3. Herrmann, E., Call, J., Hernández-Lloreda, M. V., Hare, B., & Tomasello, M. 2007. “Humans Have Evolved Specialized Skills of Social Cognition: The Cultural Intelligence Hypothesis.” Science, 317(5843), 1360–1366. (
  4. Byrne, R. 1996. “Machiavellian Intelligence.” Evolutionary Anthropology, 5(5), 172–180.
  5. Boas F. The Mind of Primitive Man. (Rev. ed.). New York: 1938; 122–44.
  6. Wilkins, A. S., Wrangham, R. W., & Fitch, W. T. 2014. “The ‘Domestication Syndrome’ in Mammals: A Unified Explanation Based on Neural Crest Cell Behavior and Genetics.” Genetics, 197(3), 795–808.
  7. Trut, L., Oskina, I., & Kharlamova, A. 2009. “Animal Evolution During Domestication: The Domesticated Fox as a Model.” Bioessays, 31(3), 349–360.
  8. Theofanopoulou, C., Gastaldon, S., O’Rourke, T., Samuels, B. D., Martins, P. T., Delogu, F., … Boeckx, C. 2017. “Self-Domestication in Homo sapiens: Insights from Comparative Genomics.” PloS one, 12(10), e0185306. doi: 10.1371/journal.pone.0185306
  9. Cieri, R. L., Churchill, S. E., Franciscus, R. G., 9 Tan, J., Hare, B., Athreya, S., … & Wrangham, R. 2014. “Craniofacial Feminization, Social Tolerance, and the Origins of Behavioral Modernity.” Current Anthropology, 55(4), 000–000.
  10. Shermer, Michael. 2004. The Science of Good and Evil. New York: Henry Holt. Wrangham, Richard. 2019. The Goodness Paradox. New York: Pantheon.
  11. Pinker, S. 2012. The Better Angels of Our Nature: Why Violence Has Declined. Penguin Group USA.
  12. Wrangham, R. W. 2018. “Two Types of Aggression in Human Evolution.” Proceedings of the National Academy of Sciences, 115(2), 245–253.
  13. Boehm, C. 2009. Hierarchy in the Forest: The Evolution of Egalitarian Behavior. Harvard University Press.
  14. Boehm, C. 2012. Moral Origins. New York: Basic Books.
  15. Pinker, S. 2015. “The False Allure of Group Selection.” The handbook of evolutionary psychology, 1–14. (
  16. Wilson, D. S. 2015. Does Altruism Exist?: Culture, Genes, and the Welfare of Others. Yale University Press.
  17. Danchin, E., Giraldeau, L. A., Valone, T. J., & Wagner, R. H. 2004. “Public Information: From Nosy Neighbors to Cultural Evolution.” Science, 305(5683), 487–491.
  18. Henrich, J. 2004. “Cultural Group Selection, Coevolutionary Processes and Large-Scale Cooperation.” Journal of Economic Behavior & Organization, 53(1), 3–35.
  19. Rutherford, A. 2018. The Book of Humans: The Story of How We Became Us. Hachette UK.

This article was published on July 2, 2019.

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