Lower IQ was also linked to psychiatric disorders that were more persistent and an increased risk of having two or more diagnoses at age 32.
The study participants were members of the Dunedin Multidisciplinary Health and Development Study, a cohort of 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, when 96 percent of the original cohort participated. 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 researchers used IQ as a marker of a concept called cognitive reserve, which refers to variation between people in their brain's resilience to neuropathological damage.
They found that lower childhood IQ predicted an increased risk of a variety of adult mental disorders.
"Lower childhood IQ predicted increased risk of schizophrenia, depression, and generalized anxiety disorder. Individuals 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," said lead author Karestan Koenen, assistant professor of society, human development, and health at HSPH.
No link was found between lower childhood IQ and substance dependence disorders, simple phobia, panic disorder or obsessive-compulsive disorder.
The mechanism through which lower childhood IQ might lead to increased risk of adult anxiety disorders is not known, but the authors suggest some possible explanations.
They write that lower childhood IQ might reveal a difference in brain health that makes an individual more vulnerable to certain mental disorders.
Another possible mechanism is stress - individuals with lower childhood IQs are less equipped to deal with complex challenges of modern daily life, which may make them more vulnerable to developing mental disorders.
The findings may be helpful in treating individuals with mental health disorders.
"Lower childhood IQ was associated with greater severity of mental disorders including persistence over time and having two or more diagnoses at age 32," said Koenen.
The study will be published online December 1, 2008 and in the January print issue of The American Journal of Psychiatry.