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.
First, you’d assume that all measured differences represent “abnormalities” among cannabis users: for example, both increased gray-matter density in some brain regions and decreased gray-matter density in other brain regions. You’d ignore the fact that your study hadn’t found any functional differences between the two groups. You’d use the loaded term “abnormalities” rather than the neutral term “differences” in the title of your paper. Second, even if the journal reviewers made you put in some weasel-words about not being able to make any causal inference (because the study was purely correlational, with neither a controlled intervenion nor a longitudinal design) you’d still put out a press release that makes it sound as if you’d found something very scary, “a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences,”: (remember, no actual bad consequences of any kind were found). Pretending that the findings to “casual” cannabis user would require that you gloss over how extreme your sample was: an average age of onset of just over 15 (very young exposure is known to be correlated with higher risks) and cannabis use of a minimum of a joint a week and an average of 11 joints a week. (The median cannabis user consumes once a month; once a week – the minimum in this study – puts someone in the top quartile, while 11 joints a week would put someone in the top 15%.) Instead, you’d describe your findings as applying to “recreational” or “light-to-moderate” cannabis use. And just in case anyone had missed the point, you’d claim that “further study of marijuana effects are needed to help inform discussion about the legalization of marijuana.” (And of course you’d get away with it, though an alcohol researcher who tried to put in a plug for higher alcohol taxes in a paper on alcohol-related violence would almost certainly get slapped down.)
It’s entirely possible, though not yet demonstrated, that chronic heavy cannabis use causes undesirable changes in brain structure and function. Even if it doesn’t, spending a good chunk of your waking hours zonked seems to me like a bad idea no matter what the zonking agent is, and that’s true in spades for adolescents, who may be unable to make up missed opportunities for both formal and social learning.
So I agree entirely that preventing a large increase in the number of people at the top of the cannabis-use distributionm and the number of mid-teens initiating to more than very occasional use are important policy goals, and that, as we go about legalizing cannabis, we ought to design the tax and regulation systems to serve those goals. I’ve made myself unpopular among extreme legalization advocates – not to mention the people hoping to cash in on an increase in substance use disorder – by saying so, and by making fun of the What-Me-Worry approach to the problem (typified by the claim that legalization will reduce underage use because legal vendors won’t sell to kids). But the way this study was written up demonstrates that neither side in the culture war has a monopoly on b.s., and I think we’re all entitled to complain especially bitterly when the b.s. is supported by our tax dollars.
[See also Maiz Szalavitz and Jacob Sullum. Sullum points to Lior Pachter, a computational biologist, who gets down in the statistical weeds before concluding that this is “the worst paper I’ve read all year.”]
Update The lead author on the paper seems to be innocent of any political purpose. The scare quotes in the press release come from the last-named author, who is also the most senior person on the team.