Abstract
Human populations in many countries have undergone a phase of demographic transition, characterized by a significant reduction in fertility at a time of increased resource availability. Typically, the reduction in fertility is preceded by a reduction in mortality and a consequent increase in population density. Various theories have been proposed to account for the demographic transition process, including maladaptation, increased parental investment in fewer offspring, and cultural evolution. Aspects of cultural evolutionary processes in relation to demographic transitions have been studied with mathematical models. The reason for the observation that fertility reduction tends to be preceded by a decline in mortality, however, remains poorly understood. Using a variety of mathematical modeling approaches, we show that the cultural selection of low fertility traits crucially depends on the population death rate: if mortality is relatively high, the trait fails to spread. If mortality is reduced, the trait can spread successfully, thus offering an explanation for the observed trends. Computer simulations can reproduce the central characteristics of the demographic transition process, including significant changes in reproductive behavior within only 1-3 generations. A model tracking the continuous evolution of reproduction rates through “errors” in the cultural transmission process predicts fertility to fall below replacement levels if death rates are sufficiently low. This can potentially explain the very low ideal family sizes in Western Europe.