Association between offspring intelligence and parental mortality: a population-based cohort study of one million Swedish men and their parents.

BACKGROUND: An inverse association of IQ with mortality has been observed in previous studies. Analyses of associations between offspring's IQ and parental mortality in biological and non-biological family relations may shed light on genetic and environmental influences. METHODS: In a target cohort of 1,235,375 Swedish men, 931,825 (75%) men had complete data on all variables used. IQ of offspring was measured at age 18 and mothers and father were followed, on average, for 21.2 years and 19.7 years, respectively, with respect to all-cause and cause-specific mortality (cardiovascular disease, coronary heart disease, stroke and diabetes). The analyses were conducted by proportional hazards regression with adjustment for parental occupation, education and income. RESULTS: In adjusted analyses using IQ as a continuous variable over a standard nine-point scale, hazards ratio (HR) for all-cause mortality was 0.96 (95% CI 0.96 to 0.96) for fathers and 0.95 (0.95 to 0.95) for mothers. The corresponding HRs were 0.99 (0.97 to 1.00) for step-fathers and 0.97 (0.95 to 0.99) for step-mothers. In adjusted analyses, HRs for CVD mortality among fathers and mothers were 0.97 (0.96 to 0.97) and 0.94 (0.93 to 0.94) respectively. The corresponding HRs for diabetes mortality were 0.91 (0.89 to 0.92) among fathers and 0.85 (0.83 to 0.87) among mothers. CONCLUSIONS: The associations found in non-biological family relationships suggest shared environmental influences and/or assortative mating. Stronger IQ-mortality associations in biological than non-biological relationships suggest genetic influences. Stronger inverse offspring IQ-parental mortality associations in mothers than in fathers might be due to environmental factors or epigenetic mechanisms.

IQ in early adulthood and later cancer risk: cohort study of one million Swedish men.

BACKGROUND: While several studies have reported an inverse relation between IQ and total mortality rates, little is known about the association, if any, between IQ and disease-specific outcomes, particularly cancer. METHODS: A cohort of 959,540 Swedish men who underwent IQ testing at military conscription at around 19 years of age, and who were followed for incident cancer. Hazards ratios for the relation between IQ and 20 cancer outcomes were computed using Cox regression. RESULTS: During an average of 19.5 years of follow-up, there were 10 273 new cancer cases. IQ showed few associations with the cancer end points studied. There was a suggestion that IQ was positively associated with lung cancer, and inversely related to stomach, oesophageal and liver malignancies, although effects were modest. The only robust gradient was found for IQ in relation to skin cancer (HRper one standard deviation advantage in IQ; 95% confidence interval 1.18; 1.13, 1.24; P value for trend across categories: <0.01), which was attenuated but retained statistical significance after adjustment for indices of socioeconomic position across the life course. CONCLUSIONS: In this large cohort of Swedish men followed into middle age, IQ was related to very few of the cancer outcomes under investigation. This indicates that the recent observation that low IQ is related to increased mortality rates may not be generated by an IQ-cancer gradient. Given that the present analyses are among the first to examine these associations, replication is required.