Intelligence in young adulthood and cause-specific mortality in the Danish Conscription Database - A cohort study of 728,160 men

- A cohort of 728,160 men was followed from young adulthood until late mid-life.
- An inverse intelligence-mortality association was found (HRper1SDdecrease = 1.28 95% CI = 1.27-1.29).
- Similar associations were found for most main categories of natural and external causes.
- The intelligence-mortality associations were stronger in more recent birth cohorts.

An inverse association has been reported between early life intelligence and all-cause mortality. The aim of this study was to investigate whether this well-established association differed according to the underlying cause of death and across different birth cohorts. The associations between young adult intelligence and mortality from natural and external causes were investigated in the Danish Conscription Database (DCD), which is a cohort of more than 700,000 men born 1939-1959 and followed in Danish registers from young adulthood until late mid-life. Young adult intelligence was inversely related to all-cause mortality with a 28% higher risk of dying during the study period per 1 standard deviation (SD) decrease in intelligence test score (HR = 1.28 95% CI = 1.27-1.29). The strength of the observed inverse associations did not vary much across main groups of natural and external causes with the exception of the associations for mortality from respiratory diseases (HR = 1.61 95% CI = 1.55-1.67) and homicide (HR = 1.65 95% CI = 1.46-1.87) which were more pronounced compared to the rest. Moreover, for skin cancer mortality, each SD increase in intelligence test score was associated with a small increase in mortality risk (HR = 1.03 95% CI = 1.01-1.15). Furthermore, the association between intelligence and mortality was stronger for those born 1950-1959 compared to those born 1939-1949 for almost all natural and external causes of death.