IQ linked to schizophrenia risk

By Nikki Withers, medwireNews Reporter

The risk for developing schizophrenia is significantly affected by an individual’s IQ, show results of a study of over 1 million Swedish men.

Researchers found that each 1 point decrease in IQ score was associated with a 3.8% increase in the risk of developing schizophrenia.

“The observed IQ-schizophrenia association does not, to any appreciable degree, appear to result from declines in intelligence in individuals undergoing an insidious onset of schizophrenia at the time of testing”, note Kenneth Kendler (Virginia Commonwealth University, Richmond, USA) and co-authors in The American Journal of Psychiatry.

Contrary to previous research, which suggests an association between “genius” and schizophrenia, the current study found no evidence of a link between these two factors. “Risk for schizophrenia in our highest IQ category was lower than that in the next highest group”, the researchers explain.

The findings were based on information from 1,204,983 Swedish males who were born between 1951 and 1975 and who had their IQ score tested between the age of 18 and 20 years. Schizophrenia was assessed by hospital diagnosis until 2010.

The team observed a negative monotonic relationship between IQ and schizophrenia, beginning with a relatively steep slope in the low IQ range and then a decline in slope as IQ increased.

Although it has been suggested that the IQ–schizophrenia relationship may be affected by genetic and environmental factors, when the researchers performed co-relative analyses they found no significant effect. “Within pairs of relatives with differing IQs, the association between intelligence and schizophrenia was as strong as in the general population”, they say.

Finally, Kendler’s team analysed the joint effects of genetic liability to schizophrenia and IQ on the risk for schizophrenia. They report that, in the lower IQ range, large differences in risk were observed in individuals with varying levels of genetic liability. However, at higher IQs, the impact of genetic liability on risk for schizophrenia decreased substantially and nearly disappeared at the highest IQ level.

The researchers note that the IQ–genetic liability interaction arose largely from IQ differences between close relatives.

They conclude: “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.”

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