Effects of marijuana on young adults

I was interested to read the article by Peter Fried and colleagues,1 particularly since funding for such long-term, prospective research is increasingly hard to find. The results of this study over the years have been interesting and important. However, I am concerned on 2 counts.

First, it is universally accepted that intelligence is multifaceted. Current research proposes as many as 9 different “intelligences,” including kinesthetic, interpersonal, emotional and motivational, as well as the more traditionally understood verbal and spatial functions. To use IQ difference scores as a dependent variable to measure the effects of anything on the complexity of human cognitive functioning is at best woefully simplistic. At worst, it perpetuates the dangerous and misguided myth that human intelligence can be encapsulated in 1 score — a belief that is abused and misused by many professionals and lay people alike. This “single score” notion not only misrepresents the whole field of modern psychometrics in the area of intellectual assessment but also ignores how marijuana and other drugs may affect some aspects of cognitive functioning but not others. The investigation of any such differential effects would be more challenging, but also far more useful and enlightening.

Second, assuming no error occurred on the table outlining the characteristics of marijuana user groups, the authors chose not to discuss a highly significant result. In the current heavy-user group, prenatal exposure to marijuana was very significantly higher than in all other groups (11.6 joints per week v. 1.4, 1.4 and 1.5 in the non-user, light user and former-user groups, respectively; p < 0.001). This suggests that individuals who were exposed to marijuana in utero are more than 10 times more likely to become heavy users in late adolescence and early adulthood. Surely this is a major finding that should be highlighted and addressed?

Maggie Mamen Psychologist Centrepointe Professional Services Nepean, Ont.