In two upcoming papers Oskar Burger and colleagues are furthering our understanding of fertility behaviour, how it changes, and what it might do in the future. Both papers address the theme of the the historical reduction in the number of births per woman (so called “demographic transition”). This began in 19th Century England and is ongoing worldwide.
In the first paper, Oskar and Daniel Hruschka of Arizona State University examine how the variation in fertility changes during a general overall decline. Many investigations have looked at changes in the average fertility behaviour, and have usually focused on national or county-level data. Hruschka and Burger however analyse individual-level data from the Demographic and Health Survey, looking at fertility data on women from 92 low- and middle- income countries. They show that a great deal of the variation among individuals is due to chance rather than to measurable individual differences. This means that chance might have more to do with fertility outcomes than characteristics such as education or wealth.
In the second paper, Oskar and John DeLong of the University of Nebraska, address the pressing topic of projecting population sizes of the future. They specifically question a common assumption that once a population begins the fertility decline, that the process is irreversible. This assumption is based on basic observations of population history, but Burger and DeLong point out that such an assumption is suspect from an evolutionary point of view. They provide five propositions based on evolutionary and ecological principles that suggest fertility might increase in the future. The five propositions are based on the following: 1) genetic change occurring from natural selection; 2) the difficulty of maintaining cultural norms that are at odds with evolutionary pressure, especially across an uncertain and complex future; 3) the adjustments of institutions that lower the costs of childbearing; 4) the influence of wealth inequality on both culture and the distribution of energy; and 5) the problem of meeting the escalating demands of a growing modernized and affluent population.