Demographic synchrony increases volatility and extinction risk in human populations.

Cooperation is widely viewed as a stabilizing force because it pools risk and buffers against environmental uncertainty. Demographic theory predicts that volatility in per-capita population growth declines rapidly with population size, implying that larger populations should be more stable. Yet human societies of all sizes eventually collapse. Here, we propose a general mechanism that helps explain this pattern. We show that cooperation generates demographic synchrony-correlated survival and reproductive outcomes among individuals-which inflates aggregate volatility. Using more than 200 longitudinal population records from indigenous societies in the Brazilian Amazon, we find that volatility declines with population size substantially slower than predicted under demographic independence. This pattern implies pervasive within-population correlation, such that census populations behave demographically as if composed of far fewer independent modules. We formalize this process with a synchrony-extended stochastic model in which shared environmental exposure, social coupling, and spatial overlap generate demographic correlations that erode the stabilizing effects of scale. We term this dynamic the cooperation-synchrony paradox: the same institutions that reduce risk for individuals can increase vulnerability to population-wide shocks. Further, we propose this is likely a general mechanism across species, where cooperation stabilizes locally but generates systemic fragility by reducing effective demographic dimensionality.