Composition Beats Collapse: Insights from the Bisin–Verdier Model on Endogenous Fertility Reversal

Fertility rates have fallen below replacement in most countries, fueling predictions of demographic collapse and even human extinction. These forecasts overlook a crucial fact: societies are not homogeneous. Using the Bisin–Verdier model of cultural transmission with endogenous fertility and direct socialization, calibrated to U.S. and global religion data, we identify an evolutionary counterforce. Subpopulations with persistently high fertility survive, expand their share, and push the total fertility rate (TFR) upward over time. Even if every country’s TFR reaches a below-replacement level, the persistence of above-replacement groups makes extinction unlikely. Our simulations point to a future of growth with pronounced compositional change—driven above all by high-fertility religious communities—rather than collapse. In particular, in our baseline ten-generation world calibration, Muslims become the largest tradition by share.