> A similar finding is that kids who smoke marijuana are likely to experience serious mental health issues later on -- that's indisputable. The problem is the same as in this case -- people assume a cause-effect relationship that isn't supported by the evidence.

This is one of those topics where people will set an impossibly high standard for evidence and then use that to reject everything.

We do actually know that heavy marijuana use is associated with increases of mental health conditions. We know that discontinuing heavy use can result in improvements in depression scores over time (following the withdrawal periods which, yes, exists despite decades of people claiming it doesn’t). We know that the increased availability of high concentration THC products has made it worse.

Yet if you want to set your threshold for evidence so high that nothing short of a large scale RCT will answer the question you can ignore all of this, because such an RCT will never be produced for obvious ethical reasons.

This pattern is common in topics where people simple don’t want to believe a casual relationship might exist. I knew a guy who started believing that there isn’t any strong evidence that flossing or even twice-daily brushing were important because he didn’t think the science proved causation that flossing and brushing improved dental health. He believed it was just correlation and people who were healthier in general were also flossing and brushing, coincidentally. His breath was terrible and he later abandoned those ideas when the dental problems undeniably arrived.

> Yet if you want to set your threshold for evidence so high that nothing short of a large scale RCT will answer the question you can ignore all of this, because such an RCT will never be produced for obvious ethical reasons.

Yes, but this means existing studies can't be relied on to justify a conclusion that would have required the impossible study. Saying that a study isn't practical can't be used to justify making a policy decision based on circumstantial evidence.

My use of marijuana studies is meant only to show how bad science steers public policy, not to advocate for its use. There are other, better reasons to avoid that drug.

> This pattern is common in topics where people simple don’t want to believe a casual relationship might exist.

This is why science exists. But if the science cannot be carried out, then we have no right to draw conclusions that would require actual science to be legitimate.

This is moving away from the original topic, which is the kind of sloppy science common in modern psychology, where people wave their hands instead of their scientific results.

> I knew a guy who ...

A red herring, but I think you knew that.