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Humanity’s Group Size Problem

Paul R. Ehrlich—

Many of our problems seem traceable to Homo sapiens being a small-group animal, most comfortable in collections of under 150 people or so, the so-called Dunbar’s number. It was proposed by anthropologist Robin Dunbar based on studies of primate brain size and group size. That’s roughly the maximum size of most hunter-gatherer groups, as it is today of typical groups of colleagues, lengths of Christmas card lists, and so on.18

We’re now a species trying to get “comfortable” in groups of thousands, millions, or in some peoples’ minds, billions. And we’re clearly often doing a lousy job of it. Religion is one way we’ve found to develop in-group versus out-group distinctions that can make our perceived groups smaller, as are race, gender, patriotism, political parties, soccer team support, corporate loyalties, fraternities, sororities, and on and on. Many of these groups go far beyond Dunbar’s number and could be a rich research field for social scientists interested in the causes, connections, and consequences of group-size variation. Almost all of these entities carry the same seeds of believing in myths, failing to have as much as possible an evidence-based world view, and promoting intergroup dissension and even violence.

So as a species, we apparently evolved genetically and culturally to live in groups of 50 to 150 people.19 That means that for most of modern Homo sapiens’s hundreds of thousands of years of history we associated mostly with close relatives with the same general appearance, same language, same genes, same environment, and same culture. Such limitations still show up in our societies.20 No doubt this is a major reason we have both a fascination with diversity (is it novelty?) and a problem with diversity (the others?). Sadly, we don’t have wide discussions of topics like “Is there an optimal level of diversity for a given society?” or “Considering the uneven global distribution of resources, and the virtual absence of serious efforts to reduce global inequities, are borders ethical?” It is, of course, still not clear whether any sustainable social system can be devised for a small-group animal like Homo sapiens, struggling to live in groups of millions and even billions.

In summary, the biggest question is: How to get from where we are to where we want to go? That means the first task is to get a substantial portion of society to understand humanity’s current situation and to recognize the growing barriers we’ve created to our own sustainability. Then, if we can agree we want to create a peaceful and equitable future world where everyone has a reasonable level of well-being, we need to change the direction of our efforts, discourse, and institutions to make determining how best to respond to the human predicament society’s collective primary goal. This is obviously an extremely daunting challenge, demanding revision of many of the most basic assumptions of today’s maladaptive cultures.21 Once that’s generally agreed upon, if it can be, the next goal is even tougher—making the changes. But it has long seemed to me that the very least we should do is try. Nothing is more impractical for humanity than not developing a massive and clever response and instead plunging on to experience a ghastly future.

From Life: A Journey through Science and Politics by Paul R. Ehrlich. Published by Yale University Press in 2023. Reproduced with permission.

Paul R. Ehrlich is cofounder of the field of coevolution and a pioneer in alerting the public to the problems of overpopulation, depletion of natural resources, and deterioration of the environmental systems that support humanity. His previous books include Humanity on a Tightrope: Thoughts on Empathy, Family, and Big Changes for a Viable FutureThe Dominant Animal: Human Evolution and the EnvironmentHuman Natures: Genes, Cultures, and the Human Prospect, and the bestselling The Population Bomb. He lives in Palo Alto, CA.

18 There is a literature in which some argue that hunter-gatherer groups were mostly smaller or, conversely, that people may feel comfortable with two hundred or more. Who cares whether the number could instead be seventy-five or two hundred?

19 R. I. M. Dunbar, “Neocortex Size as a Constraint on Group Size in Primates,” Journal of Human Evolution 20 (1992): 469–93.

20 Bruno Goncalves, Nicola Perra, and Alessandro Vespignani, “Validation of Dunbar’s Number in Twitter Conversations,” arXiv preprint arXiv:1105.5170 (2011); Barry Wellman, “Is Dunbar’s Number Up?,” British Journal of Psychology 103 (2012): 174–76; Robin Dunbar, How Many Friends Does One Person Need? Dunbar’s Number and Other Evolutionary Quirks (London: Faber and Faber, 2010).

21 Paul R. Ehrlich and Daniel T. Blumstein, “The Great Mismatch,” BioScience 68 (2018): 844–46.

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