We traced 2230 (79.9%) of the 2792 subjects tested in schools within Aberdeen city in 1932. On 1 January 1997, 646 men had died, 507 were alive, 247 could not be traced, and 27 were known to have moved from Scotland between 1987 and 1997; 438 women had died, 594 were alive, 315 could not be traced, and 18 had moved from Scotland between 1987 and 1997.
Subjects who died before 1 January 1997 had a significantly lower mean IQ at age 11 years than subjects who were alive or untraced (table ). This effect was also seen when men and women were analysed separately. Overall, untraced subjects had childhood IQs similar to those of subjects who were still alive.
Mean (SD) IQ at age 11 years for subjects who were dead, alive, untraced, and migrant on 1 January 1997
A Cox regression analysis including all traced subjects (alive, dead, and moved out of Scotland) showed that IQ at age 11 years on 1 June 1932 was significantly related to survival up to age 76 years on 1 January 1997 (P<0.0001, table ). While gathering data for a follow on report, we rechecked all deceased people to obtain death certificate details. These checks discovered that 39 people whom we had coded as dead (based on community health index information or notes available in the General Register Office) had no death certificate available. Their mean IQ was 108.0 (SD=10.3). Inclusion of these people as dead leads to a small underestimate of the true effect of IQ on survival. To illustrate this, we reassigned the 39 from the dead to the untraced category and redid the main univariate Cox proportional hazards regression analyses. The change in survival expectancy showed slightly stronger associations as follows: from 0.9847 to 0.9840 for all subjects, from 0.9887 to 0.9883 for men, and from 0.9775 to 0.9765 for women. We prefer to let the more conservative estimates stand.
Results of Cox proportional hazards regression used to predict age at death from IQ scores at age 11 and overcrowding
The influence of childhood IQ on survival was weaker in men than in women. This could be due to the effect of the second world war on death rates in men. Figure shows that women with a high childhood IQ had a consistently better average survival expectancy than women with low childhood IQ. However, for men with a high IQ, survival suddenly drops during the second world war and does not catch up and improve on that in men with low childhood IQ until later in life.
Probability of survival at ages 12-76 years for women and men in highest and lowest quarters for IQ score at age 11
The implications of the Cox regression analyses can be shown by comparing the mean probabilities of people of different childhood IQ levels being alive on 1 January 1997. When subjects with 1 SD difference in childhood IQ are compared, the chances of those with the lower IQ being alive on 1 January 1997 are 79% for all subjects (95% confidence interval 75% to 84%), 71% for women (64% to 78%), and 83% for men (76% to 89% including only those alive on 1 January 1950). If the IQ difference is 2 SD—for example, 85 v 115—the relative mean chances of survival for those with the lower IQ compared with those with the higher IQ are 63% for all subjects (56% to 71%), 51% for women (42% to 61%), and 68% for men (58% to 80%).
Overcrowding in the childhood school's catchment area was significantly related to survival when all subjects were included (table ). The effect was significant in men but not in women when the sexes were analysed separately. Overcrowding was significantly correlated with childhood IQ score (r=−0.22, P<0.001); children with higher ability scores tended to live in catchment areas with less overcrowded homes. Controlling for overcrowding hardly altered the association between childhood mental ability and survival. Details of these results are available from the authors.
Complete data on IQ at age 11, father's occupation, overcrowding in the school's catchment area, and age at death were available for 722 subjects. IQ correlated with age at death (0.18, P<0.001), overcrowding (−0.22, P<0.001), and father's occupational category (−0.20, P<0.001; more professional occupations have lower numbers). Father's occupational category correlated significantly with overcrowding (0.09, P=0.01) but not age at death (−0.02, P>0.05). Overcrowding was not significantly correlated with age at death (0.02, P>0.05). The correlation between IQ at age 11 and age at death after father's occupation and overcrowding were controlled for was 0.19 (P<0.001).
We used the EQS structural equation modelling program competitively to test models of these data that did and did not assume direct effects of IQ, father's occupation, and overcrowding on age at death. The best fitting model conceptualises IQ at age 11 as a mediating variable between social factors and age at death (fig ). Models which assumed direct effects of the available social factors on age at death and those which assumed social factors as mediators between IQ and age at death had unacceptable fit statistics.
Figure 2 Best fitting structural equation model of associations among paternal occupation, overcrowding, IQ at age 11, and age at death up to 76 years. All parameter estimates are significantly greater than zero. Squaring the parameter weights gives the variance (more ...)