People with a high IQ may be less likely to develop schizophrenia than those with a low IQ.
The researchers, including first author Dr. Kenneth S. Kendler of the Virginia Commonwealth University, say their findings challenge past studies indicating that people who are intelligent are more likely to be mentally ill.
"If you're really smart, your genes for schizophrenia don't have much of a chance of acting," says Dr. Kendler.
Schizophrenia is a disabling brain disorder that affects around 2.4 million adults in the US. Onset of the condition usually occurs in early adolescence, and it is characterized by hallucinations, delusions, abnormal thoughts and agitated body movements.
The exact causes of schizophrenia are unclear, but scientists have established that the disorder is familial; around 1% of the general population have schizophrenia, but it occurs in around 10% of people who have have a first-degree relative - such as a parent, brother or sister - with the condition.
In this study, Dr. Kendler and colleagues set out to assess the association between IQ and subsequent schizophrenia risk among the general population and those have a genetic predisposition for the disorder.
Not achieving predicted IQ 'most strongly predisposes for schizophrenia'
The team assessed the IQ at ages 18-20 years of 1,204,983 Swedish males who were born between 1951 and 1975.
The researchers used Cox proportional hazard models to calculate how IQ influences schizophrenia risk among the general population and among cousin, half-sibling and full-sibling pairs, some of which had a relative with the condition.
Up until 2010, the team monitored any schizophrenia-related hospital admissions among the participants.
The results of the analysis revealed that individuals with a low IQ were more likely to develop schizophrenia than those with a high IQ. This relationship was strongest among participants with a family history of the disorder; among cousin, half-sibling and full-sibling pairs, the individual with the lowest IQ was at highest risk for schizophrenia.
Commenting on the team's findings, Dr. Kendler says:
"What really predicted risk for schizophrenia is how much you deviate from the predicted IQ that we get from your relatives.
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."
He adds that high risk of schizophrenia associated with low IQ may be influenced by environmental factors that lower intelligence, such as early drug use or childhood trauma.
Dr. Kendler stresses, however, that their findings do not suggest that people with high intelligence are unable to develop schizophrenia. He uses the example of American mathematician John Nash. As portrayed in the 2001 movie A Beautiful Mind by actor Russell Crowe, Nash won a Nobel Memorial Prize in Economic Sciences despite having schizophrenia.
"The question is," Dr. Kendler adds, "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?"
In October last year, Medical News Today reported on a study claiming a child's later-life intelligence is not influenced by parenting and is more dependent on genetics.