If you were a neuroscientist and discovered that there were, on average, measurable anatomical differences between (1) the brains of 20 young right-handed people who smoke an average of a little more than 11 joints per week and had been consuming cannabis for a little more than 6 years and (2) the brains of an age-sex-chirality-matched group, and that those differences persisted even after controlling for alcohol consumption and were accentuated among those who use more cannabis, you’d probably say, “Hmmm … that’s interesting. I wonder what it means.”
After all, it might mean:
1. That using cannabis at that level causes changes in the brain.
2. That something else correlated with cannabis use – for example, use of other illicit drugs – causes changes in the brain.
3. That something about having that kind of brain makes cannabis use more attractive to people to have it than it is to people who don’t.
4. That the brain differences and the cannabis-use differences between the two groups are the product of some unknown third factor.
If, on further study, it were to turn out that the differences were the result of cannabis use, then you might ask, “What are the consequences – if any – of those differences?” After all, various learning tasks are known to change brain anatomy: London cabbies, who are required to learn London in astonishing detail, have larger-than-normal anterior and posterior hippocampi, and visual artists also have brains that are structurally different from non-artists.
Now, if you’d started out with already-known measured deficits in cannabis users and found brain changes independently known to lead to such deficits, that last question wouldn’t be so hard to answer. Of course, that’s hard, because the most recent meta-analysis found “no evidence for enduring negative effects of cannabis use” on neurcognitive function. (The Dunedin 8-IQ-point finding is about people with chronic substance use disorder.)
Overall then, if you were that neuroscientist, you’d write a paper saying “We studied cannabis users and non-users and found the following brain differences. Here’s the next study we plan to do, addressing the questions of causation and possible impact.”
That’s assuming that your goal was informing your readers about the content of your findings. If instead you wanted to score points in the culture wars, push your political agenda, and perhaps please your sponsors at the National Institute on Drug Abuse and the Office of National Drug Control policy, you’d behave differently. Continue Reading…