Background High intelligence early in life has consistently been associated with decreased mortality, but the mechanisms are still not fully understood. In this cohort study, we examined the association between intelligence in early adulthood and later mortality from natural and unnatural causes taking birth weight, parental socioeconomic position, participants’ own education and body mass index into account.

Methods 13 536 Danish men born in 1953 and 1959–1961 with data from birth certificates and intelligence test scores from conscription were followed until 2009. Information on vital status was obtained from the Civil Registration System. Mortality risks were analysed by the multiple Cox proportional hazards model.

Results The risk of mortality from natural as well as unnatural causes was more than twice as high among men in the lowest scoring intelligence tertile (HR_{natural deaths}=2.24; 1.90–2.65 and HR_{unnatural deaths}=2.67; 2.03–3.53). Adjusting for all covariates attenuated the estimates, but the association remained (HR_{natural deaths}=1.82; 1.48–2.25 and HR_{unnatural deaths}=2.30; 1.63–3.25).

Conclusions In men, intelligence in early adulthood was inversely associated with midlife mortality from natural and unnatural causes. The associations remained after adjustments for a range of covariates.