Intelligence in youth and all-cause-mortality: systematic review with meta-analysis

Abstract

Background: A number of prospective cohort studies have examined the association between intelligence in childhood or youth and life expectancy in adulthood; however, the effect size of this association is yet to be quantified and previous reviews require updating.

Methods: The systematic review included an electronic search of EMBASE, MEDLINE and PSYCHINFO databases. This yielded 16 unrelated studies that met inclusion criteria, comprising 22,453 deaths among 1,107,022 participants. Heterogeneity was assessed, and fixed effects models were applied to the aggregate data. Publication bias was evaluated, and sensitivity analyses were conducted.

Results: A 1-standard deviation (SD) advantage in cognitive test scores was associated with a 24% (95% confidence interval 23-25) lower risk of death, during a 17- to 69-year follow-up. There was little evidence of publication bias (Egger's intercept = 0.10, P = 0.81), and the intelligence-mortality association was similar for men and women. Adjustment for childhood socio-economic status (SES) in the nine studies containing these data had almost no impact on this relationship, suggesting that this is not a confounder of the intelligence-mortality association. Controlling for adult SES in five studies and for education in six studies attenuated the intelligence-mortality hazard ratios by 34 and 54%, respectively.

Conclusions: Future investigations should address the extent to which attenuation of the intelligence-mortality link by adult SES indicators is due to mediation, over-adjustment and/or confounding. The explanation(s) for association between higher early-life intelligence and lower risk of adult mortality require further elucidation.

Figure 1

Flow diagram of articles selected for systematic review and meta-analysis

Figure 2

Publication rate of longitudinal cohort studies on intelligence in childhood and youth, and all-cause-mortality (n = 27). Circles are shaded to represent country of origin and scaled proportionately to cohort size. One study is missing; its publication precedes 2000

Figure 3

Risk of all-cause-mortality per 1-SD advantage in intelligence test scores (n = 16), in a basic model. Squares mark cohort-specific effect sizes, which are proportional to the statistical weight (i.e. inverse variance), and the diamond indicates the aggregate effect size. Horizontal lines represent 95% CIs. ORs from four studies^,,, are treated as HRs; excluding the two studies with 20–40% risk of death^, made no change to the summary estimate

Figure 4

Funnel plot of HRs and standard errors to assess publication bias. Diagonal lines indicate 95% CIs of the aggregate HR (shown by vertical line) from all studies combined. One study is an outlier of the CI parameters

Figure 5

Risk of all-cause-mortality per 1-SD advantage in intelligence test scores after adjustment for: (a) childhood SES, (b) adult SES and (c) education. Squares mark cohort-specific effect sizes, which are proportional to the statistical weight (i.e. inverse variance), and diamonds indicate the aggregate effect sizes for studies adjusted for each covariate. Horizontal lines represent 95% CIs