The human consumption of psychoactive drugs, such as marijuana, cocaine, and heroin, is of even more recent historical origin than the human consumption of alcohol or tobacco, so the hypothesis would predict that more intelligent people use more drugs more frequently than less intelligent individuals.
The use of opium dates back to about 5,000 years ago, and the earliest reference to the pharmacological use of cannabis is in a book written in 2737 BC by the Chinese Emperor Shen Nung. Opium and cannabis are the only “natural” (agricultural) psychoactive drugs. Other psychoactive drugs are “chemical” (pharmacological); they require modern chemistry to manufacture and are therefore of much more recent origin. Morphine was isolated from opium in 1806, cocaine was first manufactured in 1860, and heroin was discovered in 1874.
Given their extremely recent origin and thus evolutionary novelty, the hypothesis would predict that more intelligent individuals are more likely to consume all types of psychoactive drugs than less intelligent individuals. Once again, as with alcohol consumption, the fact that the consumption of psychoactive drugs has largely negative health consequences and few (if any) benefits of any kind is immaterial to the hypothesis. It does not predict that more intelligent individuals are more likely to engage in healthy and beneficial behavior, only that they are more likely to engage in evolutionarily novel behavior. As I point out in an earlier post, more intelligent people are often more likely to do stupid things.
Consistent with the prediction of the hypothesis, the analysis of the National Child Development Study shows that more intelligent children in the United Kingdom are more likely to grow up to consume psychoactive drugs than less intelligent children. Net of sex, religion, religiosity, marital status, number of children, education, earnings, depression, satisfaction with life, social class at birth, mother’s education, and father’s education, British children who are more intelligent before the age of 16 are more likely to consume psychoactive drugs at age 42 than less intelligent children.
The following graph shows the association between childhood general intelligence and the latent factor for the consumption of psychoactive drugs, constructed from indicators for the consumption of 13 different types of psychoactive drugs (cannabis, ecstasy, amphetamines, LSD, amyl nitrate, magic mushrooms, cocaine, temazepan, semeron, ketamine, crack, heroin, and methadone). As you can see, there is a clear monotonic association between childhood general intelligence and adult consumption of psychoactive drugs. “Very bright” individuals (with IQs above 125) are roughly three-tenths of a standard deviation more likely to consume psychoactive drugs than “very dull” individuals (with IQs below 75).
The following graph shows a similar association between childhood intelligence and the latent factor for the consumption of psychoactive drugs among Americans. The data come from the National Longitudinal Study of Adolescent Health. The childhood intelligence is measured in junior high and high school, and the adult drug consumption is measured seven years later, and constructed from indicators for the consumption of 5 different types of psychoactive drugs (marijuana, cocaine, LSD, crystal meth, and heroin). The association is not monotonic, but nevertheless, “normal” (90 < IQ < 110), “bright” (110 < IQ < 125), and “very bright” individuals consume more psychoactive substances than “very dull” or “dull” (75 < IQ < 90) individuals. Once the social and demographic variables are controlled, however, the positive association between childhood intelligence and adult drug consumption is not statistically significant in the American Add Health sample.
People—scientists and civilians alike—often associate intelligence with positive life outcomes. The fact that more intelligent individuals are more likely to consume alcohol, tobacco, and psychoactive drugs tampers this universally positive view of intelligence and intelligent individuals. Intelligent people don’t always do the right thing, only the evolutionarily novel thing.