Lifehistory Trade-Offs Influence Women's Reproductive Strategies

Abstract

Objective: In a UK national census sample, women from the upper and lower socioeconomic (SES) classes achieve parity in completed family size, despite marked differences in both birth rates and offspring survival rates. We test the hypothesis that women adopt reproductive strategies that manipulate age at first reproduction to achieve this.

Methods: We use a Monte-Carlo modeling approach parameterized with current UK lifehistory data to simulate the reproductive lifehistories of 64,000 individuals from different SES classes, with parameter values at each successive time step drawn from a statistical distribution defined by the census data.

Results: We show that, if they are to achieve parity with women in the higher socioeconomic classes, women in lower socioeconomic classes must begin reproducing 5.65 years earlier on average than women in the higher SES classes in order to offset the higher class-specific mortality and infertility rates that they experience. The model predicts very closely the observed differences in age at first reproduction in the census data.

Conclusions: Opting to delay reproduction in order to purse an education-based professional career may be a high risk strategy that many lower SES women are unwilling and unable to pursue. As a result, reproducing as early as possible may be the best strategy available to them.

Keywords: Contingent decisions; Lifehistory; Reproductive decisions; Socioeconomic class; Women.

Fig. 1

Flow chart for lifetime reproductive output simulation model. The model calculates an individual female’s net lifetime reproductive output as a function of her socioeconomic class, taking into account the risks of class-specific mortality for both the mother and each successive offspring conceived. Offspring survival is calculated to age 24 years. Fecundity, spontaneous abortion and mortality rates are based on national rates for England and Wales for the decade ending in 1991 given by the UK’s Office of National Statistics (ONS). For simplicity, all women are assumed to achieve menarche at age 15 years and menopause at age 45 years

Fig. 2

Mean completed family size predicted by the simulation model for women from different social classes who start reproduction at different ages. For each age at first reproduction and each social class, the reproductive lifehistories of 1000 women were simulated using a model parameterized with the equations given in Table 1

Fig. 3

Cumulative percentage of actual age at first birth for women of social (SES) classes I/II and IV/V in England and Wales in 1970–2000. Mean age at first birth is indicated by the dotted lines down from the 50th centile line: 23.6 years for class IV/V versus 27.6 years for class I/II. Source: ONS (2001)