The present register-based study investigated the influence of familial factors on the association of IQ with educational and occupational achievement among young men in Denmark. The study population comprised all men with at least one full brother where both the individual and his brothers were born from 1950 and appeared before a draft board in 1968–۱۹۸۴ and 1987–۲۰۱۵ (N = 364,193 individuals). Intelligence was measured by Børge Priens Prøve at age 18. Educational and occupational achievement were measured by grade point average (GPA) in lower secondary school, time to receiving social benefits at ages 18–۳۰, and gross income at age 30. The statistical analyses comprised two distinct statistical analyses of the investigated associations: A conventional cohort analysis and a within-sibship analysis in which the association under investigation was analysed within siblings while keeping familial factors shared by siblings fixed. The results showed that an appreciable part of the associations of IQ with educational and occupational achievement could be attributed to familial factors shared by siblings. However, only the within sibling association between IQ and GPA in lower secondary school clearly differed from the association observed in the cohort analysis after covariates had been taken into account.

Introduction

Intelligence test scores have been shown to correlate positively with educational and occupational achievement in numerous observational studies (Gottfredson, 2003; Neisser et al., 1996; Strenze, 2007). The strongest correlations are typically found for educational achievement with correlation coefficients of 0.5–۰٫۶ (Deary & Johnson, 2010; Roth et al., 2015; Strenze, 2007), followed by occupational level and income with correlation coefficients of 0.4–۰٫۵ and 0.2–۰٫۴, respectively (Gottfredson, 2003; Schmidt & Hunter, 2004; Strenze, 2007). However, because of the observational study designs, it cannot be ruled out that part of the observed associations between intelligence and educational and occupational achievement is explained by unmeasured familial factors operating in childhood. Some of these familial factors are possible to measure and to take into account in statistical analyses. For instance, parental socioeconomic position is often included as a confounder due to its association with offspring intelligence and educational and occupational achievement (Hegelund, Flensborg-Madsen, Dammeyer, & Mortensen, 2018). Other relevant factors, such as parental intelligence and parental psychiatric history, are not taken into account because information on these factors are not available. Additionally, unknown factors might also explain some of the observed associations. Studies of twins and non-twin sibling pairs have shown that both shared genetic factors and shared environmental factors are in fact responsible for part of the observed associations between intelligence and educational and occupational achievement (Johnson, McGue, & Iacono, 2007; Lichtenstein & Pedersen, 1997; Rowe, Vesterdal, & Rodgers, 1998).