One clue about who is at risk for developing schizophrenia appears to come from a person’s IQ before symptoms of the illness appear. For years, data have indicated that low IQ correlates with the illness, but researchers have been unsure about how and why the two are linked.

One theory is that intelligence (as measured by IQ tests) is affected by some of the same genes that influence the development of schizophrenia. Both intelligence and schizophrenia risk appear to come from genetic factors, indicating a strong familial link. Recent genetic studies have found that some of the genes, when hit by mutation, can increase risk for schizophrenia and, in different people, also increase risk of intellectual disability. Given these facts, the question becomes: Is the link to be attributed entirely to genetic factors?

A new study published online November 7th in The American Journal of Psychiatry says no, and supports the theory that factors in the environment also contribute to the IQ-schizophrenia link. Researchers drew from Swedish national databases to conduct the largest-ever study of the IQ-schizophrenia connection. The team was led by Kenneth Kendler, M.D., of Virginia Commonwealth University. He is a two-time NARSAD Distinguished Investigator grantee and a member of the Foundation’s Scientific Council.

The team obtained IQs measured in more than 1.2 million Swedish men, born between 1951 and 1975, at the time they enlisted for compulsory military service. Dr. Kendler’s colleagues, Jan Sundquist, M.D., Ph.D., and Kristina Sundquist, M.D., Ph.D., matched this same group of men against 2010 hospital records to see who among them was later diagnosed with schizophrenia.

“A robust interaction was seen between genetic liability to schizophrenia and IQ in predicting schizophrenia risk,” the team reported. Specifically, they found that genetic susceptibility for schizophrenia had a much stronger impact on risk of illness among men if their IQ was low rather than high. The converse was also true: men with genetic risk factors for schizophrenia were less likely to have developed the illness if their IQ was high. In broad numerical terms, the relationship between IQ and the risk of schizophrenia was a 3.8 percent increase in risk for every one-point decrease in IQ. This finding confirmed previous studies.

Even though many genetic links have been established between IQ and schizophrenia, the team was open to the notion that environmental factors also play a role in the relationship. In fact, they reported, “[Our] results were most consistent with the hypothesis that some environmental risk factors not shared with close relatives have an impact on brain functioning in a way that both lowers IQ and predisposes to risk for schizophrenia.”

For now, the team’s findings about environmental factors in schizophrenia are only suggestive and need to be studied further.

Read the paper.