There’s a puzzling paradox in anthropology: The larger a population gets, the better protected it should be from random fluctuations in births, deaths, and environmental conditions that could cause it to collapse. And yet, the archaeological record is rife with examples of large civilizations that have collapsed, like the Roman and Egyptian empires.
Historical records show that populations are protected by increasing size, but not as strongly as mathematical models predict, says SFI External Professor Marcus Hamilton, who is also an anthropologist at the University of Texas at San Antonio. So why is it that large populations sometimes falter and fail?
Because as societies grow, their members start to cooperate on life-sustaining activities like farming and hunting, Hamilton’s recent research, published in Cambridge Prisms: Extinction, suggests. Cooperation can be hugely beneficial because it spreads out risk, like a form of insurance. But it also means members no longer behave as individual units. Rather, their fates become intertwined.
Imagine, for example, that two neighboring villages are accustomed to trading food, and then one village’s crops get destroyed by pests. The unaffected village will also suffer because they’re accustomed to getting part of its food supply from the affected village. Cooperation can make a population of 400 people act like only 20 when it comes to how susceptible they are to shocks, the new research showed. “That’s a massive decrease in robustness,” Hamilton says.
Cooperation also increases opportunities for diseases to spread. When Europeans introduced diseases into Indigenous groups in North America, for instance, they spread quickly because those groups frequently traded with each other. The consequences were devastating.
To uncover this downside to cooperation, Hamilton and his collaborator, anthropologist Robert Walker from the University of Missouri, used census data from recently contacted Indigenous groups in Brazil. The Brazilian government collects these counts as part of its efforts to protect Indigenous populations. Insight into isolated societies is rare, making the dataset uniquely powerful for answering questions about how population sizes fluctuate.
The researchers don’t yet know whether the results extend to large, globally integrated societies like the United States. But Hamilton thinks there’s a general truth underlying the findings that relates to many fields: “The larger a population, the more structure it has. And all that structure solves certain kinds of problems, but it also creates a kind of fragility,” he says.
Conservation biologists, for example, could benefit from this new understanding of how societies live and die. When reintroducing animal species into the wild, they may need to reintroduce a larger population than they’d expect if the species is cooperative. “It’s a new way of thinking about processes that lots of scientists have been working with for a very long time,” Hamilton says.
Read the study "Demographic synchrony increases volatility and extinction risk in human populations" in Cambridge Prisms: Extinction. DOI: 10.1017/ext.2026.10013