The Evolutionary Demography of the Fertility-Mortality Quasi-Equilibrium

Abstract

A close-to-equilibrium relationship between levels of fertility and mortality has characterized most of the history of the human species. On average, women have given birth to two reproductive offspring, plus a small fraction. This quasi-equilibrium is in part the effect of neurobiological and life history characteristics that enhance reproductive success. The latter include cultural factors, age at sexual maturity, fecundity, family size, duration of the reproductive period, age-specific probabilities of survival, and epigenetic rules that guide response to changing environmental conditions. Among such rules, the authors hypothesize a ''two-child psychology.'' Its basic operative mechanisms seem to be: (1) a neurobiological capacity to respond to certain environmental stimuli useful to gauge probabilities of offspring survival, and (2) a quest for creature comforts. The greater the perceived probability of offspring survival within a population, the more intense the two-child psychology. The greater the quest for creature comforts, the keener and more widespread the two-child psychology.