Flexibility in reproductive timing in human females: integrating ultimate and proximate explanations

Abstract

From an ultimate perspective, the age of onset of female reproduction should be sensitive to variation in mortality rates, and variation in the productivity of non-reproductive activities. In accordance with this prediction, most of the cross-national variation in women's age at first birth can be explained by differences in female life expectancies and incomes. The within-country variation in England shows a similar pattern: women have children younger in neighbourhoods where the expectation of healthy life is shorter and incomes are lower. I consider the proximate mechanisms likely to be involved in producing locally appropriate reproductive decisions. There is evidence suggesting that developmental induction, social learning and contextual evocation may all play a role.

Figure 1.

Illustrative model of the predicted response of AFB to changes in the costs and benefits of delaying childbearing. Assuming that benefits (solid line) accrue linearly with every year's delay, and that costs (dotted line) increase exponentially as years of possible reproduction run out, then selection favours an age of onset which balances costs and benefits (the vertical line). Increasing the benefit of each year's delay, for example through labour force participation becoming more productive, moves the optimum to a later age (b versus a). Increasing the costs of delay, for example through an increase in mortality rate, moves the optimal age earlier (c versus a).

Figure 2.

(a) Relationship between female LE and average AFB first across 116 countries; (b) Relationship between average female income (2001 US$ PPP) and average AFB across 116 countries. After Low et al. [2]. Sources as described in text.

Figure 3.

(a) Mean AFB against female LE and (b) mean gross weekly income (UK£) for contemporary England. Data points represent groups of neighbourhoods classified on the basis of socioeconomic deprivation. For sources see text.

Figure 4.

AFB observed (points) and predicted by the simple rule ‘begin childbearing at such an age that you can on average expect to be in good health until your oldest grandchild is five’ (line), for English neighbourhoods divided into deciles according to the index of multiple deprivation. For sources, see text.