Childhood socioeconomic status does not explain the IQ-mortality gradient

- Higher IQ predicts longer lifespan. Norwegian register data were used to assess confounding from childhood social class.
- Estimating the IQ-mortality gradient with family-fixed effects avoids confounding from childhood factors shared by siblings.
- The IQ-mortality gradient is present within families, and childhood social class is not a substantial confounder.

BackgroundCognitive 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.MethodsNorwegian 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).ResultsA 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.ConclusionsThe IQ-mortality gradient is not due to confounding from family background or childhood SES. Higher IQ-scores are associated with substantially reduced mortality risk within a modern welfare-state setting, and the relationship was largely stable across a 30-year period.