The IQ-mortality gradient is present within families, and childhood social class is not a substantial confounder.

Abstract

Background Cognitive ability correlates with mortality risk, but confounding from childhood social class has been a persistent concern. While studies controlling for indicators of childhood social status report limited attenuation of coefficients, important parental and family factors are likely to vary substantially within social class.

Methods Norwegian administrative register data with high-quality intelligence scores measured at age 18–19 for the large majority of males in the 1962–1990 birth cohorts (n = 720,261) were used to assess the IQ-mortality gradient using progressively stronger controls for childhood social class in Cox proportional hazard and linear probability models. A family-fixed effects specification avoids confounding from any family or childhood characteristics fixed over time within families (e.g., childhood socio-economic status, parenting style, and neighborhood environment).

Results A large difference in mortality risk is evident across Norwegian males: We find that the mortality risk of the lowest ability bracket, relative to that of the median bracket, is 2.31 (Confidence Interval (CI): 2.12, 2.52, p < 0.0005), declining to 0.64 (CI: 0.56, 0.73, p < 0.0005) for the highest ability bracket. Estimated differences are similar in linear probability models with and without controls for birth year and parental SES, in Cox models with birth year and parental SES controls, and in a linear probability model with family-fixed effects.