Lead author Karestan Koenen of the Harvard School of Public Health said study participants were members of the Dunedin Multidisciplinary Health and Development Study, involving children born in 1972-1973 in Dunedin, New Zealand. At the initial assessment at age 3, the study had 1,037 children.

The participants were also interviewed and tested on their overall health and behavior at ages 5, 7, 9, 11, 13, 15, 18, 21, 26 and at age 32. IQs were assessed at ages 7, 9 and 11.

Psychiatric disorders were assessed at ages 18 through 32 in interviews by clinicians who had no knowledge of the subjects' IQ or psychiatric history.

The study, published online ahead of print in the January issue of The American Journal of Psychiatry, found lower childhood IQ predicted increased risk of schizophrenia, depression and generalized anxiety disorder. Those with lower childhood IQ also had more persistent depression and anxiety and were more likely to be diagnosed with two or more disorders in adulthood, the study said.

Koenen said no association was found between lower childhood IQ and substance dependence disorders, simple phobia, panic disorder or obsessive-compulsive disorder.