A correlation is not causation, but this paper (read when screening for a systematic analysis, and rejected from same because of the study design) this is a data matching study that looks at intelligence tests done on conscripts to the army and then matching with suicide using national health data sets. Relatively challenging to do in Scandinavia, impossible, for privacy concerns, to do elsewhere (and conscription has now ended in many other regions of the world).

We found a strong linear association between results of intelligence tests in early adulthood and subsequent suicide in men. The association seems to be mediated in part by educational attainment, which may perhaps influence subsequent job opportunities and income.

Though the large sample size gave us adequate power to investigate associations with suicide across the range of test results, the findings are restricted to men and patterns of association in relation to intelligence testing may differ in women. Also, the lack of detailed information about the men who committed suicide, including possible confounding factors such as drug and alcohol misuse, means we are unable to fully understand possible causal pathways. Exclusion of those with psychiatric disorder at baseline, however, did not greatly influence the strength of the association.

Few previous studies have assessed the association of measures of intelligence with suicide. The 1946 British birth cohort study found that suicide was associated with delayed physical development but not with age at first speech, alertness (age 7 years), or measures of intelligence at age 15 years. There were only 11 suicides in that study and so it lacked power to detect potentially important effects. A study of 43 suicides in Israeli conscripts aged 18-21 years suggested that they were of above average intelligence,4 although patterns of suicide in the context of military service may differ from those in the general population. A study of university students in the United States, however, also found an increased risk of suicide among the offspring of better educated or professional parents.

Suicide rates among Australian army conscripts were strongly associated with a measure of general intelligence, even after adjustment for several other risk markers, but this study was based on only 76 suicides. In a previous study of Swedish conscripts, associations with suicide were attenuated after adjustment for measures of conduct and personality.

Several studies have examined the association of childhood IQ with overall mortality. In two Scottish studies low IQ at age 10-11 years and poor mental ability at age 11 years were strongly associated with all cause mortality. In our analysis the strength of the association of test results with overall mortality was similar to that for suicide, although different pathways probably affect the associations with the specific diseases contributing to all cause mortality.

THis is an association. Associations are not causal. It would require further exploration. The only clinical use is to note that the slower among us may be at higher risk, but this is not that modifiable: moreover the IQ test is a proxy measure.

It would fit with r?K theory, but r/K is a little too much akin to a theory of everything.