The authors base their findings on data from just under 8,000 people in the 1970 British Cohort Study, a large ongoing population based study, which looks at lifetime drug use, socioeconomic factors, and educational attainment.
The IQ scores of the participants were measured at the ages of 5 and 10 years, using a validated scale, and information was gathered on self reported levels of psychological distress and drug use at the age of 16, and again at the age of 30 (drug use only) .
Drug use included cannabis; cocaine; uppers (speed and wiz); downers (blues, tanks, barbiturates); LSD (acid); and heroin.
By the age of 30, around one in three men (35.4%) and one in six women (15.9%) had used cannabis, while 8.6% of men and 3.6% of women had used cocaine, in the previous 12 months.
A similar pattern of use was found for the other drugs, with overall drug use twice as common among men as among women.
When intelligence was factored in, the analysis showed that men with high IQ scores at the age of 5 were around 50% more likely to have used amphetamines, ecstasy, and several illicit drugs than those with low scores, 25 years later.
The link was even stronger among women, who were more than twice as likely to have used cannabis and cocaine as those with low IQ scores.
The same associations emerged between a high IQ score at the age of 10 and subsequent use of cannabis, ecstasy, amphetamines, multiple drug use and cocaine, although this last association was only evident at the age of 30.
The findings held true, irrespective of anxiety/depression during adolescence, parental social class, and lifetime household income.
“Although most studies have suggested that higher child or adolescent IQ prompts the adoption of a healthy lifestyle as an adult, other studies have linked higher childhood IQ scores to excess alcohol intake and alcohol dependency in adulthood,” write the authors.
Although it is not yet clear exactly why there should be a link between high IQ and illicit drug use, the authors point to previous research, showing that highly intelligent people are open to experiences and keen on novelty and stimulation.
Other research has also shown that brainy children are often easily bored and suffer at the hands of their peers for being different, “either of which could conceivably increase vulnerability to using drugs as an avoidant coping strategy,” explain the authors.