Researchers analyzed data from nearly 8,000 people in the ongoing 1970 British Cohort Study of drug use, education and socioeconomic status. The participants' IQ scores were checked at ages 5 and 10 years, and their use of illegal drugs (marijuana, cocaine, uppers, downers, LSD and heroin) was self-reported at ages 16 and 30.
At age 30, about 35 percent of men and nearly 16 percent of women had used marijuana in the previous year, and 8.6 percent of men and 3.6 percent of women had used cocaine during that time. In general, men were twice as likely as women to use drugs.
The investigators found that 30-year-old men who had high IQ scores at age 5 were about 50 percent more likely than those who had low IQ scores to have used amphetamines, ecstasy and several illicit drugs.
At age 30, women who had high IQ scores at age 5 were more than twice as likely to have used marijuana and cocaine as those with low IQ scores.
The study, published in the Nov. 14 online edition of the Journal of Epidemiology and Community Health, found similar associations between high IQ at age 10 and subsequent drug use.
The reasons for the link between high IQ and illegal drug use aren't clear, but highly intelligent people are often open to new experiences and embrace novelty and stimulation, the study authors noted in a journal news release.
Previous research has also shown that highly intelligent children tend to be easily bored and targeted by other children for being different, "either of which could conceivably increase vulnerability to using drugs as an avoidant coping strategy," according to James White of the Centre for the Development and Evaluation of Complex Interventions for Public Health Improvement at Cardiff University in Wales, and a colleague.
While the study uncovered an association between IQ and drug use, it did not prove a cause-and-effect relationship.