FWIW, I don't think the timescales are correct with the longitudinal twin-exercise studies. I don't think anyone would predict that exercise today would have an

FWIW, I don't think the timescales are correct with the longitudinal twin-exercise studies. I don't think anyone would predict that exercise today would have anything (or rather, very little) causal to do with mood 2 years from now. Sustained exercise over the course of a couple of weeks or more might for that period or for maybe a month or two afterward, but I doubt anything more than that. I also am not entirely sure that twin studies are the best way to get at causality in terms of exercise and mood, because the exercise is nonrandom. I think someone might object in the sense that that's the point, but my perspective is that it's still not the same as randomly assigning exercise. There's a lot buried in their acknowledgment that a third factor could still account for the results, as that means a lot given the design. FWIW, there's also evidence in the opposite direction using similar but different logic without twins, but using molecular genetic risk scores: https://www.nature.com/articles/s41398-021-01306-w https://www.nature.com/articles/s41398-021-01306-w "The risk of depression was 22% higher among those at high genetic risk compared with those at low genetic risk (HR = 1.22, 95% CI: 1.14–1.30). Participants with high genetic risk and unfavorable lifestyle had a more than two-fold risk of incident depression compared with low genetic risk and favorable lifestyle (HR = 2.18, 95% CI: 1.84–2.58). There was no significant interaction between genetic risk and lifestyle factors (P for interaction = 0.69). Among participants at high genetic risk, a favorable lifestyle was associated with nearly 50% lower relative risk of depression than an unfavorable lifestyle (HR = 0.51, 95% CI: 0.43–0.60). We concluded that genetic and lifestyle factors were independently associated with risk of incident depression. Adherence to healthy lifestyles may lower the risk of depression regardless of genetic risk." I guess the broader point is to be careful about making claims of BS (or anything else) based on a single study.