shows the relations between IQ score and the study covariates. Because of the large sample size, all correlations were highly statistically significant but mostly of low magnitude. Men with higher IQ scores had greater educational attainment, were less likely to have parents in unskilled occupations or to be in unskilled occupations themselves in later life, and were taller than those who had lower scores. Men with a higher IQ tended to be slightly older at conscription, and they had a lower body mass index. There was little evidence of a consistent relation between IQ and blood pressure.
presents the relations between IQ and risk of death from all types of unintentional injury, plus the 2 most common causes of such deaths: road injury and poisoning. During a mean follow-up period of 22.6 years, there were 5,415 deaths from all unintentional injuries combined. Average age at censoring was 40.5 years (range: 17.5–54.0). As indicated previously, in an earlier follow-up of the present cohort we reported on the relation of IQ with all deaths due to injury (n
3,954) alone (29
). For the purposes of comparison with individual injury types, we report on this same association here based on a longer follow-up with more such deaths.
Men with a lower IQ had an increased risk of dying from all unintentional injury: for a standard deviation decrease in IQ, the age-adjusted hazard ratio was 1.32 (95% confidence interval (CI): 1.27, 1.35). There was a gradient in risk across the range of IQ scores (P-trend < 0.001). Compared with those in the highest IQ category, men in the lowest-scoring IQ category had a death rate from unintentional injury more than twice as high: 2.34 (95% CI: 2.13, 2.56). When we adjusted separately for indicators of early-life socioeconomic position (height and parental social class), body mass index, and blood pressure, these effect estimates were unchanged. Adjustment for later-life occupational social class (available for a subgroup of men) had only a modest attenuating impact on the effect estimates. However, adding education (also available for a subgroup of the men) to the multivariable model led to marked attenuation of IQ-injury gradient. Following simultaneous control for all potential covariates, the hazard ratios per standard deviation decrease in IQ without and with adjustment for education were 1.31 (95% CI: 1.26, 1.36) and 1.06 (95% CI: 1.02, 1.11), respectively.
For deaths ascribed to road injury and poisoning, we again found an increase in risk with a decrease in IQ, with a particularly large effect for mortality due to poisoning. Thus, compared with those in the highest IQ category, men in the lowest category had a death rate from poisoning almost 6 times higher: the age-adjusted hazard ratio was 5.82 (95% CI: 4.25, 7.97). For a standard deviation decrease in IQ, the age-adjusted hazard ratios were 1.79 (95% CI: 1.64, 1.96) for death due to poisoning and 1.29 (95% CI: 1.25, 1.34) for death due to road injury. With the exception of education, these risk estimates changed little following individual adjustment for other covariates and in the multivariable model controlling simultaneously for all covariates except education. Further control for education effectively flattened the IQ–road injury relation: the hazard ratio for a standard deviation decrease in IQ was 1.01 (95% CI: 0.94, 1.07). In the analyses featuring poisoning as the outcome of interest, we also found marked attenuation after adding education to the multivariable model, although statistical significance at conventional levels was retained (hazard ratio (HR)
1.31, 95% CI: 1.18, 1.46).
As indicated, because of a low number of deaths for less common injury types, we have presented the injury outcomes in relation to 4 categories of IQ. For the 3 most commonly occurring outcomes—all injuries combined, road injury, and poisonings—there were sufficient cases to conduct analyses across all 9 original IQ categories, depicted in . These illustrations essentially confirm the results given in the tables: age-adjusted IQ-injury associations were incremental across the 9-point IQ range, and controlling for covariates had only a moderate impact, except in the case of education.
Figure 1. IQ and selected injury mortality (N=1,116,442), Sweden, 1969–2004. Gray bars, age adjustment; black bars, full adjustment (without education); white bars, full adjustment (with education). Referent: highest-scoring IQ group (category (more ...)
depicts the relation of IQ with death due to falls, fire, and drowning. These analyses revealed dose-response relations with IQ similar to those described above. Although, as previously, statistical significance was retained when education was added to the regression model, a nonlinear IQ-drowning association became apparent—the only departure from a dose-response gradient in these analyses. Risk of death from types of injury other than the above categories (n
1,431) was more weakly related to IQ than in most of the other analyses (age-adjusted HR for a standard deviation decrease in IQ
1.17, 95% CI: 1.11, 1.23), an association that was lost after multiple adjustment for covariates that included education (HR
0.96, 95% CI: 0.89, 1.04) (results not shown but available from the corresponding author upon request).
When we used the alcohol consumption data available for a subgroup of the study participants (n
33,319) in which there were 190 deaths due to injury, we found an age-adjusted inverse association between IQ and all injuries combined (HRper 1 standard deviation decrease in IQ
1.44, 95% CI: 1.26, 1.65). This result was essentially unchanged when alcohol consumption was added to the model (HR = 1.41, 95% CI: 1.23, 1.62). Similar observations were made when road injury was the mortality endpoint (80 deaths). There were too few deaths from other injury types for separate analysis. We also adjusted for smoking as a marker of risk-taking behavior, and the hazard ratios again changed little.
In the original sample (N
1,116,442), 66,224 men were diagnosed with a psychiatric illness at conscription. Although IQ was inversely associated with psychiatric illness (odds ratioper 1 standard deviation decrease in IQ
1.84, 95% CI: 1.82, 1.85), adding this variable to the age-adjusted model had very little impact on the magnitude of the relation between IQ and any of the injury outcomes under investigation. The largest attenuation was seen for risk of death from poisonings, for which, after further adjustment for psychiatric illness, the age-adjusted hazard ratio for mortality per standard deviation decrease in IQ fell only from 1.79 (95% CI: 1.64, 1.96) to 1.64 (95% CI: 1.52, 1.80). In the case of other types of accidents and all accidents combined, adjustment for psychiatric illness attenuated the hazard ratio by, at most, 3%.
Finally, we examined the extent to which IQ “explained” socioeconomic inequalities in injuries, that is, the degree to which it accounted for the relative change in risk of parental and own occupational social class with injury mortality. For brevity in this paper, we show the age-adjusted relation for 3 selected injury mortality outcomes with socioeconomic position, before and after control for IQ (). As expected, socioeconomic disadvantage was related to an elevated rate of deaths due to road injury, poisonings, and all injuries combined. Controlling for IQ led to marked attenuation of these gradients such that the percentage reduction in the hazard ratio ranged between 19% and 86%.
Hazard Ratios for the Relation of Indicators of Socioeconomic Position With Mortality From Unintentional Injury—All Combined, Road Injury, and Poisoning, Sweden, 1969–2004