The Denver Post republished an article by Nicole Ostrow of Bloomberg News that begins, “Young adults who used marijuana as teens were more likely to develop schizophrenia and psychotic symptoms…”
Ostrow claims, “The authors said the study was the first to look at sibling pairs to discount genetic or environmental influence and still find marijuana linked to later psychosis.”
No, the authors did not “discount genetic or environmental influence,” nor did they discount other nongenetic differences between siblings.
Here’s what the study (John McGrath etc., Archives of General Psychiatry) says instead: “Prospective cohort studies have identified an association between cannabis use and later psychosis-related outcomes, but concerns remain about unmeasured confounding variables. The use of sibling pair analysis reduces the influence of unmeasured residual confounding.”
Reducing “the influence of unmeasured residual confounding” is hardly demonstrating the direction of causal flow.
Quite obviously, siblings are quite different from each other, not only genetically but according to their environmental interactions and, of critical importance, in their choices. (Does anyone doubt that “psychosis-related outcomes” are at least in many cases significantly the result of a person’s poor choices?) The most obvious explanation for the study’s findings is that the the siblings with the most problems tended to abuse drugs more. In other words, the drug abuse was a symptom of a person’s problems, not a cause of them.
Notably, the study uses an extremely wide definition of “psychosis-related outcomes” that includes “nonaffective psychosis, hallucinations, and Peters et al Delusions Inventory score.” But marijuana is a hallucinogenic drug. So, in part, the study is claiming, “People who take hallucinogenic drugs tend to suffer hallucinations.” (And it cost how much money to figure that out?)
“Nonaffective psychosis” includes things like poor concentration and mood disorders, which are obvious short-term effects of using the drug as well as reflections of personalities with deeper problems.
And what, you may wonder, is the “Peters et al Delusions Inventory score?”It asks questions like the following:
“Do you ever feel as if people seem to drop hints about you or say things with a double meaning?”
“Do you ever feel as if some people are not what they seem to be?”
Besides the fact that some of these questions are ridiculous and need not indicate psychosis, again, people with more problems tend to abuse drugs more. Big insight, there.
Moreover, the differences associated with marijuana use are relatively small. For example, whereas 26 of 1246 people (two percent) who never used marijuana showed signs of “nonaffective psychosis,” 12 of 310 people (3.9 percent) who had used marijuana for six years or more showed signs. Nintety of 1182 people (7.6 percent) who had never used marijuana showed signs of hallucinations, while 54 or 268 (20 percent) of those who had used marijuana for six years or more showed signs (again, not surprising given that marijuana is a hallucinogenic drug).
The upshot is that a small minority of people who didn’t use marijuana showed “psychosis-related outcomes,” while a somewhat larger minority of people who did use marijuana showed such signs. Again, this is consistent with the idea that people with more problems tend to abuse drugs more.
Now, I do not doubt that abusing marijuana (or any drug) can also contribute to a person’s mental and emotional problems. Certainly drug abuse can reinforce a person’s negative tendencies; I don’t need a costly study to convince me of that. However, it is equally obvious that far and away the major problem is something other than the drug abuse. Mostly, the drug abuse is a symptom of deeper problems, not a cause of them. (Regardless, there are many other good reasons not to use marijuana except perhaps medicinally.)
I’m sure that won’t stop politicians and bureaucrats from citing nonsensical news reports of meaningless studies to stir up more Reefer Madness. (Say, wouldn’t paranoia about the impacts of smoking marijuana count as a “psychosis-related outcome?”)