Swedish men born 1951–1967: Relative to men with IQ 100, the group with the lowest category of cognitive ability have 0.56 fewer children; men in the highest category have 0.09 more children

Cognitive ability and fertility among Swedish men born 1951–1967: evidence from military conscription registers. Martin Kolk and Kieron Barclay. Proceedings of the Royal Society B: Biological Sciences, May 8 2019. https://doi.org/10.1098/rspb.2019.0359

Abstract: We examine the relationship between cognitive ability and childbearing patterns in contemporary Sweden using administrative register data. The topic has a long history in the social sciences and has been the topic of a large number of studies, many reporting a negative gradient between intelligence and fertility. We link fertility histories to military conscription tests with intelligence scores for all Swedish men born 1951–1967. We find a positive relationship between intelligence scores and fertility, and this pattern is consistent across the cohorts we study. The relationship is most pronounced for the transition to a first child, and men with the lowest categories of IQ scores have the fewest children. Using fixed effects models, we additionally control for all factors that are shared by siblings, and after such adjustments, we find a stronger positive relationship between IQ and fertility. Furthermore, we find a positive gradient within groups at different levels of education. Compositional differences of this kind are therefore not responsible for the positive gradient we observe—instead, the relationship is even stronger after controlling for both educational careers and parental background factors. In our models where we compare brothers to one another, we find that, relative to men with IQ 100, the group with the lowest category of cognitive ability have 0.56 fewer children, and men with the highest category have 0.09 more children.

1. Introduction

A paradox of human behaviour in industrialized societies is that high socioeconomic status is usually negatively associated with reproductive success. This is puzzling from an evolutionary perspective in which high status is assumed to give greater access to partners as well as enhanced ability to support offspring [1–3], which was also the case in pre-industrial societies, and has likely been true throughout Homo sapiens pre-historic past [4]. It is also puzzling from an economic perspective because children are a major expenditure that should be more affordable for those with more resources [5]. The typically observed negative relationship has been described as a central problem in the evolutionary study of human behaviour [2]. The relationship between cognitive ability and fertility is an important dimension of this puzzle. For more than a century most studies have found that higher cognitive ability is associated with lower reproductive success (e.g. [2,6,7]), despite the fact that individuals with high cognitive ability achieve substantially higher socioeconomic success than individuals with lower cognitive ability [8], and both men and women rate intelligence as a desirable feature in a potential mate [9]. Furthermore, it has been suggested that the evolution of high cognitive ability in Homo sapiens is attributable to positive selection on intelligence, as higher intelligence facilitated greater social interaction capabilities, which in turn led to greater reproductive success [10,11]. Empirical evidence suggests that the link between socioeconomic success, likely associated with high cognitive ability, and reproductive success was positive in a wide variety of pre-industrial societies [4,12]. By contrast, the empirical evidence for the relationship between socioeconomic status and fertility over the past two centuries is ambiguous, with most studies reporting a negative association. In this study, we revisit this research question, applying a rigorous statistical treatment to high-quality population data to study the relationship between cognitive ability and fertility in contemporary Sweden.