Explaining Cross-National Differences in Fertility: A Comparative Approach to the Demographic Shift

Low fertility in modern societies challenges evolutionary social scientists. A comparative analysis of less developed countries investigated various possible predictors of low fertility including urbanization, monogamous marriage, level of participation in the monetary economy (GDP), and low infant mortality. Using a sample of 45 countries broken out by urban and rural location, each of these predictors of total fertility was significant in a regression analysis (r (2) = .76) that controlled for survey year, geographic latitude, sex ratio, and geographic hemisphere. Multivariate prediction estimated that the demographic shift (TFR < 2.6) occurs when infant mortality falls below 33 per thousand, when polygyny reaches zero, and when GDP per capita rises above US$20,508. (The low fertility at high latitudes, reported by Barber, was further found explainable in terms of monogamous marriage). Declining fertility in modern societies evidently constitutes an adaptive response to ecological conditions and is largely predictable from them.