The research provides insight into how IQ and schizophrenia interact and suggest that intelligence is an important moderator in the development of the mental disorder.
“If you’re really smart, your genes for schizophrenia don’t have much of a chance of acting,” said first author Kenneth S. Kendler, M.D., professor of psychiatry and human and molecular genetics in the Department of Psychiatry, VCU School of Medicine. Kendler added that low IQ is one of a wide array of risk factors for schizophrenia. Like unaffected individuals, people with schizophrenia vary widely in their intelligence.
In the largest study of IQ and schizophrenia to date, researchers assessed IQ at ages 18-to-20 of more than 1.2 million Swedish males born between 1951 and 1975. Schizophrenia-related hospitalization was tracked for 24 years until 2010.
“What really predicted risk for schizophrenia is how much you deviate from the predicted IQ that we get from your relatives,” Kendler said. “If you’re quite a bit lower, that carries a high risk for schizophrenia. Not achieving the IQ that you should have based on your genetic constitution and family background seems to most strongly predispose for schizophrenia.”
Kendler said environmental factors that diminish IQ, including intrauterine experience, childhood trauma or early drug use, could contribute to the increased risk.
However, a high IQ does not eliminate the risk of schizophrenia, and well-known figures in popular culture, such as math prodigy John Nash whose story was made famous in the film “A Beautiful Mind,” have demonstrated that brilliant and highly creative individuals can suffer from schizophrenia.
“The question is, might we see some upward bump at that high level of intelligence where really brilliant people have increased risk for the disease and we show no such trend,” Kendler said.
The study, which is titled “IQ and Schizophrenia in a Swedish National Sample: Their Causal Relationship and the Interaction of IQ with Genetic Risk,” was published in the American Journal of Psychiatry’s online journal AJP in Advance on Nov. 7. It will be published again in an upcoming print issue of the journal.
Kendler, director of the Virginia Institute for Psychiatric and Behavioral Genetics at VCU, collaborated with Lund University researchers Henrik Ohlsson, Ph.D.; Jan Sundquist, M.D., Ph.D.; and Kristina Sundquist, M.D., Ph.D. Sundquist and Sundquist also have affiliations with the Stanford Prevention Research Center at the Stanford University School of Medicine.
The study was jointly funded by National Institute of Mental Health grant RO1 MH083094, the Swedish Research Council, the ALF project grant in Lund, Sweden, and the Swedish Research Council for Health, Working Life, and Welfare.