"Is it the pot that made him lazy DM?"

“What we need are one-handed scientists!” – Sen Edmund Muskie

The title is a question that is most frequently asked by a parent or close relative of an individual (typically male) who is in the early adult years. Said parent is clearly distressed by the career choices made/not made by their son, grandson or nephew who (they have finally acknowledged to themselves) smokes a lot of dope. Has for years and shows no signs of quitting.
The pot smoker has, of course, turned out to be a disappointment to their relatives in one way or another, typically vocationally. And they ask me, almost pleadingly, frequently with a tinge of self-flagellation, to confirm their suspicion that the pot smoking is at the root of junior’s lack of gumption.
I have to tell them that nobody can satisfactorily answer this question for them. Not me, not science. Not with any confidence of certainty, anyway.

The most insurmountable problem, I explain, is the inability to escape non-random assignment of conditions. Plenty of so-called disappointing career choices out there, right? Cases in which the parents have a certain expectation– be that finishing high school, college, advanced degree or taking a white-collar career arc versus a life of wage labor. Cases in which siblings diverge significantly. This occurs through a host of influences and no-doubt genetic luck, even between sibling pairs. One kid is just the screw up.
And it is not inevitably the case that the alleged screw-up kid is smoking weed. It is also frequently enough the case that the supposedly successful kid(s) in the family are smoking a fair bit of weed themselves, abuse alcohol or whatnot. So no, my dear friend/relative/neighbor/acquaintance, I cannot tell you with any certainty for one particular kid whether or not a specific drug use pattern has “caused” him to disappoint you*.
Statistically, with large samples of people, we might stand to make some general-principle headway. We can look at large numbers of people and say, yes, by your desired measure of “success/failure” the losers smoke more weed than the good upstanding success stories. But that just brings up the next problem…
Familiar to the correlation-is-not-causation triumphalists, we must next address the problem that if one condition/factor/trait tends to consistently cause multiple other conditions/factors/traits to obtain then these secondary caused traits will be correlated with each other.
Suppose, for example, a person exists with an amotivational personality**, whatever that may be. Easy enough to consider that people may just come endowed with different levels of career-related gumption by the time they reach early teens when academic/careerist and drug using traits start to emerge. This will influence all sorts of decisions. Which peer groups they affiliate with, the degree to which they see optional recreational pursuits as conflicting with other goals and simple time-budgeting. So when you see the kid in late teens who has barely escaped high school, won’t go to college and also happens to smoke pot, well, it may not be the pot causing loserdom. It may be the amotivational trait established in early development that causes both the vocational slackage and the pot smoking.
For a given individual it is absolutely impossible to sort out these relationships with any confidence (I say).
At this point, we turn to another phenotype of my taxpaying boss. Typically older, usually a Boomer who has anecdotes a-plenty about the people they’ve known in their lives who smoke a lot of pot. And they absolutely insist that the pot made these folks not just amotivational but out-and-out stupid. I have sympathy for the position. I really do. It is VERY difficult to look at the acute effects of cannabis smoking and the overall phenotype of the dope fans one has known in one’s life and not come to this conclusion. Consequently I have much less success convincing these people of the notion that their college buddy Bob, still smokin’ every day at age 57, might have been a loser regardless. Nevertheless, he might have been.
Of course, once I’ve done this song and dance it is fair to end with the observation that yes, it may very well be the case that chronic cannabis smoking contributed to junior’s lack of gumption, motivation, ambition and success. Despite certain Readers who wish to constantly refer to ancient, essentially anecdotal datasets you can refer to the current modern-science, peer-reviewed literature and put together a story that cannabis smoking itself is likely to have detrimental effects on career success. You can also deploy the caveats I’ve been discussing, bear down on methods on a paper-by-paper basis, and insist that there is no demonstrated effect. At end, however, no matter your reading of the literature you have to admit that we still cannot make any sure conclusions about a specific individual.
It’s fun to talk about how science works, but this is one area where I think I really, really frustrate these poor parents who are desperately seeking answers, even ones that indict themselves, to the question of why their kid is not more successful. They thank me for talking about it….but I am not really convinced that I’ve helped that much.
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*A big problem here lies with the disappointed individual, of course. Frequently enough there is a fair argument to be made that their disappointment in their kid is unjustified. Expressions of upper-middle-class privilege (desire not to see their kid working manual labor for low wages), arbitrary taste distinctions (struggling garage bands and low-level theatre is not respectable) and even rarified snobbery (a PhD is simply not as good as high $$ MD or JD career) are not things that I necessarily credit as legitimate beefs with the kid’s decisions.
**In most cases I try stay away from the more-serious examples, e.g., the fact that affective disorders may be the cause. Anxieties and depressive conditions are frequently associated with drug use, the term “self-medication” is a bit misleading but popular and captures the essence. Unsurprisingly, these disorders can also be associated with lower-than-expected career success. It is a bit difficult to say “Hey, have you considered that your kid might have been depressed at age 11?”. I prefer to know the person pretty well before I’ll get into that can of worms.