Blame it on boredom, peer abuse, curiosity, or anything else, but it appears that smarter children tend to get into illegal drugs later in life. An analysis of data from the 1970 British Cohort Study, which tracked 8,000 people over more than 30 years from their childhood onwards, looked at their IQ when they were young and then experimentation with drugs later in life.
While it's more than a little dubious testing the IQ of kids when they're 5- and 10-year olds, what the research found is that kids who were marked as having a high IQ were much more likely to dabble in drugs. Men were 50% more likely to have used amphetamines, ecstasy, and several illicit drugs, and women were more than twice as likely to have used cannabis and cocaine when compared to their low IQ counterparts.
Personally, I'm tempted to link this to a desire for new experiences and mental stimulation, but with studies such as this there are always societal factors that come into play, like which drugs were seen as socially acceptable in 1986 Britain as opposed to the year 2000, and what people would have had access to.