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

TLDR

Modern low fertility is theorized to maximize intergenerational socioeconomic inheritance and potentially increase long‑term fitness. The study investigates how fertility and socioeconomic position affect descendant success over four generations in a Swedish cohort. The authors analyze data from 14,000 Swedish individuals born 1915–1929 to assess descendant outcomes across generations. Low fertility combined with high socioeconomic position boosts descendant socioeconomic success across four generations but does not enhance reproductive success, revealing a trade‑off between socioeconomic advantage and biological fitness and underscoring the role of fertility in sustaining intergenerational inequality.

Abstract

Adaptive accounts of modern low human fertility argue that small family size maximizes the inheritance of socioeconomic resources across generations and may consequently increase long-term fitness. This study 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 during 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 maximizes long-term fitness. This indicates a conflict in modern societies between behaviours promoting socioeconomic versus biological success. This study also makes a methodological contribution, demonstrating that the 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 can have important long-term effects on the persistence of social inequalities across generations.