Childhood intelligence in relation to major causes of death in 68 year follow-up: prospective population study

Abstract

Objectives To examine the association between intelligence measured in childhood and leading causes of death in men and women over the life course.Design Prospective cohort study based on a whole population of participants born in Scotland in 1936 and linked to mortality data across 68 years of follow-up.Setting Scotland.Participants 33 536 men and 32 229 women who were participants in the Scottish Mental Survey of 1947 (SMS1947) and who could be linked to cause of death data up to December 2015.Main outcome measures Cause specific mortality, including from coronary heart disease, stroke, specific cancer types, respiratory disease, digestive disease, external causes, and dementia.Results Childhood intelligence was inversely associated with all major causes of death. The age and sex adjusted hazard ratios (and 95% confidence intervals) per 1 SD (about 15 points) advantage in intelligence test score were strongest for respiratory disease (0.72, 0.70 to 0.74), coronary heart disease (0.75, 0.73 to 0.77), and stroke (0.76, 0.73 to 0.79). Other notable associations (all P<0.001) were observed for deaths from injury (0.81, 0.75 to 0.86), smoking related cancers (0.82, 0.80 to 0.84), digestive disease (0.82, 0.79 to 0.86), and dementia (0.84, 0.78 to 0.90). Weak associations were apparent for suicide (0.87, 0.74 to 1.02) and deaths from cancer not related to smoking (0.96, 0.93 to 1.00), and their confidence intervals included unity. There was a suggestion that childhood intelligence was somewhat more strongly related to coronary heart disease, smoking related cancers, respiratory disease, and dementia in women than men (P value for interactions <0.001, 0.02, <0.001, and 0.02, respectively).Childhood intelligence was related to selected cancer presentations, including lung (0.75, 0.72 to 0.77), stomach (0.77, 0.69 to 0.85), bladder (0.81, 0.71 to 0.91), oesophageal (0.85, 0.78 to 0.94), liver (0.85, 0.74 to 0.97), colorectal (0.89, 0.83 to 0.95), and haematopoietic (0.91, 0.83 to 0.98). Sensitivity analyses on a representative subsample of the cohort observed only small attenuation of the estimated effect of intelligence (by 10-26%) after adjustment for potential confounders, including three indicators of childhood socioeconomic status. In a replication sample from Scotland, in a similar birth year cohort and follow-up period, smoking and adult socioeconomic status partially attenuated (by 16-58%) the association of intelligence with outcome rates.Conclusions In a whole national population year of birth cohort followed over the life course from age 11 to age 79, higher scores on a well validated childhood intelligence test were associated with lower risk of mortality ascribed to coronary heart disease and stroke, cancers related to smoking (particularly lung and stomach), respiratory diseases, digestive diseases, injury, and dementia.

Fig 1 Derivation of Scottish Mental Survey 1947 (SMS1947) analytic sample. Most of those with missing census dates had migrated or joined armed forces for whom records contained no last known GP registration date. *Analytic sample represents 92% of survey. †46.0% of those with vital status had died, which is approximate to Scottish population statistics for those aged 75-80. ‡Includes 5381 who emigrated abroad (2857 men); 3391 cancelled from GP registration (1383 men); 454 armed forces recruits (411 men); 96 residents of Northern Ireland or Isle of Man (52 men)

Fig 2 Association between intelligence (Moray House test score) at age 11 and major causes of death (age and sex adjusted hazard ratios and 95% confidence intervals) to age 79 in Scottish Mental Survey 1947 (SMS1947)

Fig 3 Hazard ratios (95% confidence intervals) for association between 1 SD higher score in intelligence test score at age 11 and cause of death to age 79 in 65 765 people in Scottish Mental Survey 1947 (SMS1947). Effect sizes are adjusted for sex and age at cognitive testing

Fig 4 Association between intelligence (Moray House test score) at age 11 and deaths from specific cancers (age and sex adjusted hazard ratios and 95% confidence intervals) to age 79 in Scottish Mental Survey 1947 (SMS1947)

Fig 5 Hazard ratios (95% confidence intervals) for association between 1 SD higher score in intelligence at age 11 and cause of death by cancer type to age 79 in Scottish Mental Survey 1947 (SMS1947). Effect sizes are adjusted for age at intelligence testing and sex, with exception of ovarian and breast cancer (women only) and prostate cancer (men only). *Non-smoking-related cancers (all others are smoking related)