Higher Childhood IQ Linked to Later Drug Use

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A report in the Journal of Epidemiology & Community Health (abstract here) shows a positive association between IQ at five years old and subsequent drug use as an adult. This follows other research that showed the same effect for alcoholism.

The study took data collected in the UK since 1970 to match up children who tested above average on IQ tests with adult drug use. Interestingly, although the higher a child tested, the more likely they were to use drugs later, there was a difference between the sexes. Males showed the effect with amphetamines, ecstacy and polydrug (more than three different substances) use, while the link in females was to marijuana and cocaine.

The relationship is notable, but, like many population studies, more needs to be determined. For example, the study cannot assert that being smart makes you take drugs. Demonstrating an association doesn’t rise to the level of cause and effect. That would be an error.

One famous example of this type of error comes with the association between ice cream sales and murders in New York City. When ice cream is selling better, murders go up. Does ice cream cause violence? Nope. The hidden variable is that higher temperatures (hot summers) increases both ice cream sales and murders. In the case of this study, we do not yet know whether there is a direct causative relationship or some hidden variable not yet determined.

Although it is likely that news reports will inflate the relationship between childhood IQ and drug use, the authors of the study are careful to word it this way: Conclusion: High childhood IQ may increase the risk of illegal drug use in adolescence and adulthood.” It’s the careful placement of “may” in that statement that will probably be ignored by the press.

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