December 1 (HealthDay News) -- Children with lower IQ have an increased risk of developing schizophrenia, depression, generalized anxiety disorder and other psychiatric problems as adults, a new study that spanned more than three decades shows.

Lower childhood IQ was also associated with more persistent depression and anxiety, as well as an increased risk of having two or more psychiatric diagnoses by age 32, say researchers from the Harvard School of Public Health. The findings were published online Monday and in the January print issue of The American Journal of Psychiatry.

The study included more than 1,000 New Zealand children born in 1972. They were initially assessed at age 3 and interviewed and tested on their overall health and behavior multiple times until they reached age 32. Their IQs were assessed at ages 7, 9 and 11, and psychiatric disorders were assessed at ages 18 through 32.

The researchers found no link between lower childhood IQ and substance dependence, simple phobia, panic disorder, or obsessive-compulsive disorder.

While the actual mechanism that increases the risk of certain psychiatric disorders in people with a lower IQ as children isn't known, there are a number of possible explanations, study author Karestan Koenen, an assistant professor of society, human development and health at Harvard, and colleagues wrote in a university news release.

Lower childhood IQ may indicate a difference in brain health that makes a person more vulnerable to certain disorders, the researchers suggested. Or it may be that people with lower childhood IQ are less able to cope with the complex challenges of modern life, and therefore may be more vulnerable to developing mental health problems.

The findings may help improve treatment of patients with psychiatric disorders.

"Since individuals with persistent and multiple mental disorders are more likely to seek services, cognitive ability may be an important factor for clinicians to consider in treatment planning," Koenen said in the news release. "For example, individuals with lower cognitive ability may find it harder to follow instructions and comply with treatment regimens. By taking clients' cognitive ability into account, clinicians may improve treatment outcome."

The study may also prove useful in prevention efforts.

"Educators and pediatricians should be aware that children with lower cognitive ability may be at greater risk of developing psychiatric disorders. Early detection and intervention aimed at ameliorating mental health problems in these children may prevent these problems from carrying over into adulthood," Koenen said.

SOURCE: Harvard School of Public Health, news release, Dec. 1, 2008