Understanding variation in human fertility: what can we learn from evolutionary demography?

Abstract

Decades of research on human fertility has presented a clear picture of how fertility varies, including its dramatic decline over the last two centuries in most parts of the world. Why fertility varies, both between and within populations, is not nearly so well understood. Fertility is a complex phenomenon, partly physiologically and partly behaviourally determined, thus an interdisciplinary approach is required to understand it. Evolutionary demographers have focused on human fertility since the 1980s. The first wave of evolutionary demographic research made major theoretical and empirical advances, investigating variation in fertility primarily in terms of fitness maximization. Research focused particularly on variation within high-fertility populations and small-scale subsistence societies and also yielded a number of hypotheses for why fitness maximization seems to break down as fertility declines during the demographic transition. A second wave of evolutionary demography research on fertility is now underway, paying much more attention to the cultural and psychological mechanisms underpinning fertility. It is also engaging with the complex, multi-causal nature of fertility variation, and with understanding fertility in complex modern and transitioning societies. Here, we summarize the history of evolutionary demographic work on human fertility, describe the current state of the field, and suggest future directions.

Keywords: evolutionary anthropology; evolutionary demography; fertility.

Figure 1.

Variation in human fertility. (a) Distribution of total fertility rates or mean completed family sizes in 70 natural-fertility populations (solid line) and 70 controlled-fertility populations (dashed line). Note that natural fertility populations are highly variable. In fact, the range in fertility rates observed in natural fertility populations exceeds the difference between the mean fertility rates observed in natural versus controlled settings. Figure redrawn from [, p. 48]. Populations known to have high prevalences of pathological sterility are excluded. (b) A schematic of the ‘Demographic Transition Model’, which describes the typical demographic shifts observed as populations undergo development from a pre-industrial to industrialized economic system. In stage one, total population is low and stable due to high birth rates and high death rates. In stage two, total population rises as death rates fall following improvements in healthcare and sanitation. Birth rates remain high. In stage three, total population is still rising rapidly, but fertility falls, narrowing the gap between birth and death rates. In stage four, population growth stabilizes, balanced by a low birth rate and a low death rate.