I predict that we will eventually come to know of a few dozen different nutrient deficiencies, lifestyle habits, childhood developmental factors, neurological d

I predict that we will eventually come to know of a few dozen different nutrient deficiencies, lifestyle habits, childhood developmental factors, neurological defects, neurotic thought patterns, and other random things that collectively account for nearly all cases of what we now call "depression". These explanations are not widely pursued because there are no practical and rigorous experiments to verify them (no ethical way to control psychological experiments) and they don't lead to practical solutions (society does not have the wherewithall to carry out the remedies). However, as this story illustrates, "we don't know" is never an acceptable answer when we demand from science a solution to an immediate and dire problem. The confidence of experts is fairly constant over time, while only their degree of correctness improves.