Human fertility and fitness optimization

Abstract

Census and other survey data from across the world reveal major differences in fertility rates between the more economically developed and the less economically developed societies. The former are significantly more likely than the latter to feature families of two children or fewer. Multiple regression analysis shows that, among various indicators of "modernization," three (female level of education, female gainful employment, and proportion of physicians in the population) account for 71% of the variation in family size; all three variables have strongly significant, direct, and negative effects on fertility. The paper hypothesizes about the possible evolution of a reproductive psychology toward the two-child family and seeks to explain highly depressed rates of reproduction by reference to both ultimate and proximate factors. In some highly developed countries, zero-child and one-child rates of fertility represent together up to 40% of all ever-married women. The findings stress the importance of systematic research toward establishing the proximate factors that are most likely to facilitate or impede fitness optimization--the importance, that is, of surrounding the optimization principle with the logic and ancillary propositions that will give it a greater and more directive reach.