Tests of psychometric intelligence (IQ-type tests), introduced about a century ago, have traditionally been used in educational and workplace settings. Only in the past decade has their predictive capacity for non-psychiatric health outcomes been examined in the rapidly evolving field of cognitive epidemiology. The health outcome most commonly examined in relation to people's differences in measured intelligence is all cause mortality. Cohort studies of older people indicate that those with higher intelligence scores experience a lower risk of mortality.1 A problem with examining the association between IQ and mortality in these age groups is the cognition lowering effect of comorbidities, which might explain the apparent protective effect of high intelligence scores against premature mortality.

To address the comorbidity issue, investigators have recently started to link results from intelligence tests taken in early life with adult mortality. Findings are highly consistent—people with a higher IQ in childhood live longer.2^–8 In studies that report comparable statistics,4 6^–8 the hazards ratios for total mortality, comparing the groups with the lowest and …