In a new, long-term study, researchers at Harvard School of Public Health (HSPH) found that children with lower IQs showed an increased risk of developing psychiatric disorders as adults, including schizophrenia, depression and generalized anxiety disorder.

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.