Are stepfathers' education levels associated with the intelligence of their stepsons? A register-based study of Norwegian half-brothers

We examined the relationship between the parents’ education levels and the adult intelligence of their children in a population‐based, nationwide sample of Norwegian half‐brothers with different fathers (2,016 pairs of half‐brothers). In a family where the mother has two children with different men, the firstborn child usually lives with the younger child's father during a period of their childhood. This makes it possible to study the non‐genetic effects of paternal education on children's development. Results showed that the education level of the younger half‐brother's father was positively associated with the intelligence score of the older half‐brother. The education level of the older half‐brother's father was not associated with the intelligence score of the younger half‐brother. Firstborn men whose half‐brothers’ fathers had high levels of education had intelligence scores that were 33% (95% confidence interval: 18–47%) of a standard deviation higher than those of firstborn men whose half‐brothers’ fathers had low levels of education, after adjustment for the biological fathers’ education levels, mothers’ education levels, and other background factors. These findings are compatible with the hypothesis that a child's family environment exerts an effect on the cognitive abilities of the child that lasts into adulthood.