Fertility, mating behavior & group size - a unified empirical theory, hunter-gatherers to megacities

As human groups expand, both fertility and mated ensemble size decline. Monogamy emerges in large societies as cooperation shifts from the shared outcomes of foraging bands to individuated, competitive exchange. This study develops a unified empirical framework linking fertility, mating behavior, and group size ( N ) across human evolution, from hunter-gatherer bands to modern megacities. Using ethnographic, archaeological, and demographic evidence, we define a composite measure of mating behavior ( M ) as mated ensemble size ( N _e ) multiplied by normalized total fertility rate. The results formalize a feedback loop: higher fertility increases N , larger N suppresses fertility, and large- N systems become evolutionarily stable despite sub-replacement reproduction. Then we extend the framework to include the diffusion of polygenic traits that are beneficial to groups but deleterious to individuals . Using a minimal multilevel-selection model derived from the Price and the breeder’s equations, we evaluate how deme size modulates the fixation of such traits under Wright–Fisher drift and island-model exogamy. Results suggest that group-level benefits dominate within an intermediate band of N , producing an inverted-U relationship between group size and trait diffusion. Combined with the fertility– N feedback loop, this implies that modern large- N societies not only depress fertility but may also suppress the evolutionary maintenance of group-beneficial traits. Pronatalist policies may temporarily raise births but ultimately reinforce the fertility-suppressing dynamics of large- N . The findings raise the question of whether new cultural or institutional linkages can restore reproductive viability in complex societies.