The theory that
depression is a positive trait, proposed by psychiatrist Nassar Ghaemi, is hardly new. In fact, it is almost fashionable for pop-psych talking heads to try to uncover the evolutionary advantages (yes, advantage, not dis-) of depressive tendencies. I have seen studies suggesting that depressed patients judge reality more accurately, have more empathy, and focus their attention better than non-depressed people. I take them with a grain of salt, especially toward the habitual rush to read "evolutionary advantage" into any pointless and accidental byproducts of this extremely imprecise, unreliable, and non-anthropomorphic process known as "natural selection."
I'm not clinically depressed, yet --- a fact that often surprises myself, because I am an above-average pessimist. And I have a possibly perverse fondness for tragedies and bleak stories. I think I'm right a lot more often than the normal optimists around me, but they disagree. Nevertheless, I have to wonder whether this "healthy people have a slight bias toward optimism" is a biological phenomenon or a cultural one. Could very likely be localized to a few especially self-congratulatory and delusional cultures, but far from universal.