Resolves positively if, by my estimation, there is convincing evidence that cold showers are an effective treatment for depression by 2030. There is currently some evidence- theoretical and empirical- for an effect, but there is a paucity of large studies specifically on the effect of showers on depression.

I will consider the effect of cold showers demonstrated if there are at least two peer-reviewed experimental studies random allocation to conditions showing a statistically significant (or Bayesian equivalent) effect on depression which is, in my view, large enough to be clinically significant.

Especially given the low cost of the treatment, the bar to being considered clinically significant is not especially high- for example, if 30% of participants fell below the cutoff for depression in the treatment condition, and only 20% in the control condition, I would consider that clinically significant.

Although multiple controlled studies in a peer-reviewed journal are the most likely route to a positive resolution, I might accept other evidence at my discretion- for example, a single study large and rigorous enough that I consider it decisive on its own.

I will not bet in this market.