Slate Star Codex points out that rates of suicide and depression are weirdly terrible metrics for how good a society is. I wonder if some of that is a definitional effect. Depression is more or less defined as occurring for no reason. If you have a reason for sleeping poorly and feeling unable to do everyday things (e.g. fibromyalgia), you’re diagnosed with that instead. As society gets worse, people who were chemically destined to be depressed are given reasons to be sad, and so stop contributing to the depression statistics.
This is related to but slightly distinct from the idea that depressed people are less likely to commit suicide when conditions are objectively miserable than when they are good because bad conditions leave room for hope in a way good conditions don’t. That is about individuals specific reaction to their depression. My hypothesis is about how the number “% depressed” is measured.
Of course, my suggestion doesn’t account for increased suicide rate. The expectations hypothesis does account for that. One other factor I think may be in the mix is coping mechanisms. Before the AIDS cocktail, someone noticed AIDS patients actually got better when co-infected with another virus. The reason turned out to be interferon, an intercellular signal to ramp up anti-viral defenses. HIV didn’t trigger it, or didn’t trigger it enough, but when another virus did the resulting interferon protected from HIV as well as the original virus. Maybe external bad events trigger coping mechanisms in a way depression doesn’t, and they incidentally fight depression. This could be true even if “coping mechanism” just means disassociating until things get better.