People with high intelligence could be less likely to develop schizophrenia, scientists have found, in a study that contradicts previous findings that intelligence can increase a person's risk of mental illness. 

"If you're really smart, your genes for schizophrenia don't have much of a chance of acting," said one of the team and leading psychiatric genetics expert, Kenneth S. Kendler of the Virginia Commonwealth University in the US.

Schizophrenia is an illness that messes with the normal functioning of a person's brain, and it affects 2.4 million adults in the US and almost a quarter of a million Australians. The disorder usually first develops in adolescence, and if not treated, can lead to psychosis, the effects of which include debilitating hallucinations and delusions. People often refer to 'split personalities' when talking about schizophrenia, but this is a myth.

With treatment, some people can recover completely from schizophrenia, and most see at least an improvement in their symptoms. But some people can be affected by the disorder for many, many years. 

According to the university press release, scientists have established that schizophrenia is familial. "Around 1 percent of the general population have schizophrenia, but it occurs in around 10 percent of people who have have a first-degree relative - such as a parent, brother or sister - with the condition."

To find out more about the risk factors, Kendler's team assessed the link between a person's IQ and their risk of developing schizophrenia. 

According to the paper, which was published in The American Journal of Psychiatry, the team used IQ measurements that were taken at the ages of 18 to 20 in 1,204,983 Swedish males born between 1951 and 1975. They then used standardised 'Cox proportional hazard models' to figure out how IQ influences the risk of schizophrenia in both the general population and related pairs of cousins, half-siblings, and full siblings with different IQs, some of which had already been diagnosed with the disorder. 

The entire group was also monitored till 2010, to see who had been diagnosed with schizophrenia up till that point. 

They found that those with a low IQ were more likely to go on to develop schizophrenia than those with a high IQ. And this association was at its strongest in the related pairs that shared a family history of the disorder - the person with the lowest IQ in the pair was more likely to develop schizophrenia than the one with the higher IQ.

"What really predicted risk for schizophrenia is how much you deviate from the predicted IQ that we get from your relatives," Kendler said in the press release. "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."

Of course, it's not just a person's intelligence that influences the risk of developing schizophrenia, and people with high IQs are not immune to the disorder, Kendler was careful to point out. He said those with a lower IQ could see their risk increased due to factors such as drug use at an early age, or the instance of some kind of trauma in childhood. 

The team hopes that the research will help in the diagnosis of the disorder. According to the US National Institute of Mental Health (NIMH), approximately 40 percent of the individuals with schizophrenia can be left untreated in any given year, in many cases because it was left undiagnosed.

"This is not the place for speculative theories about why the still poorly understood genetic pathways of risk for schizophrenia might be more penetrant in individuals with low intelligence than in those with high intelligence," the team concludes in the paper. "If our results are replicated, we can provide one important clue for neuroscientists and molecular geneticists. The changes in brain function that are expressed as low or high intelligence, and that convey sensitivity or resistance to the pathogenic effects of genetic liability to schizophrenia, appear to arise environmentally and will be seen most clearly in close relatives who differ in intelligence."

Source: The Virginia Commonwealth University