Nov. 14, 2011 -- Brainy kids -- especially girls -- may be more likely to experiment with marijuana, cocaine, and other illicit drugs when they grow up, according to a new report.

In the study of close to 8,000 people, those who had high IQs when they were aged 5 and 10 were more likely to use certain illicit drugs at age 16 and at age 30.

The findings appear online in the Journal of Epidemiology and Community Health.

Exactly why having a high IQ at age 5 or 10 may encourage future drug use is not fully understood. But researchers have a theory. "People with a high IQ have also been found to be more open to new experiences," says study researcher James White, PhD, in an email.

White is a research associate at the Center for Development and Evaluation of Complex Interventions for Public Health Improvement at Cardiff University, Wales.

The research has been mixed on how a high childhood IQ affects behavior in adulthood, White says.

"Previous studies have found high childhood IQ is associated with mostly healthy behaviors in adult life, such as having a healthy diet, being physically active, and not smoking," he says. "However, other studies have found high childhood IQ is linked to excess alcohol intake and alcohol dependency in adult life."