Cooperation is widely understood to stabilize populations by pooling risk and buffering individuals against environmental uncertainty.Classical demographic theory predicts that volatility in per-capita population growth declines with the square root of population size, implying that large populations should be inherently stable.Yet collapses occur repeatedly across cooperative systems, including human societies.Here we propose a general mechanism that helps resolve this puzzle.We show that cooperation introduces demographic synchrony, correlated survival and reproductive outcomes among individuals that inflates aggregate volatility by reducing the effective number of independent demographic units within populations.Using more than 200 time series of indigenous populations from the Brazilian Amazon, we demonstrate that volatility scales as (r) N -1/4 rather than the classical (r) N -1/2 predicted under independence.This deviation implies substantial within-population correlation, such that a community of hundreds behaves demographically as if composed of far fewer independent modules.We formalize this process in a synchrony-extended stochastic model, showing how shared environmental exposure, social coupling, and spatial overlap generate correlations that erode otherwise stabilizing effects of scale.We term this dynamic the cooperation-synchrony paradox: the same institutions that buffer individuals against everyday risk simultaneously synchronize demographic outcomes, increasing vulnerability to large-scale shocks.By linking human demography with ecological synchrony theory, our results reveal a general constraint on cooperative organisms.Scale alone does not guarantee resilience when demographic fates are coupled.Cooperation stabilizes locally but can generate systemic fragility by reducing effective demographic dimensionality.
Hamilton et al. (Tue,) studied this question.