When Swedish men join the military, they have to get intelligence tests. Researchers in Sweden and the United Kingdom looked at records on military recruits to see if intelligence was linked to suicide risk.

What the researchers wanted to know: What is the relationship between scores on intelligence tests and the risk of suicide?

What they did: The researchers examined data on 987,308 Swedish 18-year-olds who were conscripted between 1968 and 1994. At the time he entered the army, each man was tested on logic and general intelligence, detecting synonyms, visuospatial perception, and technical/mechanical skills. They also looked at education levels of the men and their parents and the parents' socioeconomic level.

What they found: The lower the men scored on any of the tests, the more likely they were to commit suicide. This was especially true of the logic test; men who scored low on that test were three times more likely to commit suicide than men who scored high on the test. Controlling for the men's level of education (by age 25) accounted for some of the relationship, but definitely not al of it. Their parents' education and socioeconomic level had no effect. Also, excluding men who had been diagnosed with a psychiatric disorder by the time they were conscripted didn't have a major effect on results.

What the study means to you: Testing intelligence is highly controversial, so it would be a mistake to read this as the final word on intelligence and suicide. However, it does hint at brain differences in men who later committed suicide. Maybe their brain developed along unusual lines that affected both their scores on the tests and their suicide risk.

Caveats: Nobody knows if the same would be true of women.

Find out more: Basic information on suicide from the National Institute of Mental Health

Read the article: Gunnell, D., Magnusson, P.K.E., and F. Rasmussen. "Low Intelligence Test Scores in 18 Year Old Men and Risk of Suicide: Cohort Study." British Medical Journal. Jan. 22, 2005, Vol. 330, pp. 167–170.

Abstract online: http://bmj.bmjjournals.com