Low fertility increases descendant socioeconomic position but reduces long-term fitness in a modern post-industrial society

Low fertility increases descendant socioeconomic position but reduces long-term fitness in a modern post-industrial society
Anna Goodman et al.; 2012


Abstract

Adaptive accounts of modern low human fertility argue that small family size maximises the inheritance of socioeconomic resources across generations and may consequently increase long-term fitness. This paper explores the long-term impacts of fertility and socioeconomic position (SEP) on multiple dimensions of descendant success in a unique Swedish cohort of 14,000 individuals born 1915-1929. We show that low fertility and high SEP predict increased descendant socioeconomic success across four generations. Furthermore, these effects are multiplicative, with the greatest benefits of low fertility observed when SEP is high. Low fertility and high SEP do not, however, predict increased descendant reproductive success. Our results are therefore consistent with the idea that modern fertility limitation represents a strategic response to the local costs of rearing socioeconomically competitive offspring, but contradict adaptive models suggesting that it maximises long-term fitness. This indicates a conflict in modern societies between behaviours promoting socioeconomic versus biological success. This paper also makes a methodological contribution, demonstrating that number of offspring strongly predicts long-term fitness and thereby validating use of fertility data to estimate current selective pressures in modern populations. Finally our findings highlight that differences in fertility and SEP have important long-term effects on the persistence of social inequalities across generations.

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"We do, however, find strong support for the prediction that fertility limitation in modern societies enhances descendant socioeconomic success. Thus, our results indicate that reproductive behaviours that promote biological success (i.e. long-term genetic fitness) are in conflict with those that promote descendant socioeconomic success in modern populations. Specifically, we find that both low parental fertility and high parental SEP independently predict higher schoolmarks, educational level and income, and this is generally true in male and female descendants alike ... . Moreover, these associations persist up to at least the great-grandchild generation, reflecting the advantage of starting one’s own offspring on a favourable socioeconomic trajectory.
We also demonstrate for the first time a multigenerational interaction between SEP and fertility, such that the socioeconomic benefit of low fertility was especially large in groups that already had high SEP. This finding adds to a number of recent studies indicating that demographic modernization is associated with increased socioeconomic pay-offs to fertility limitation for the wealthiest families ... ."