I recently read this article on depressive realism which I though was quite interesting. What it basically boils down to is that there's a good bit of scientific evidence that, contrary to the popular belief that depressed people have an unrealistically negative view of the world, they actually have a pretty accurate view of the world. It's all us normal, non-depressed people who have an inaccurate world view, because we consistently hold unrealistically positive opinions. There are several interesting examples cited in the article. The most interesting one for me had to do with eating disorders. Apparently despite the wide-spread notion that people with eating disorders suffer from unrealistic body image, people with such disorders are actually astonishingly good at predicting how attractive random strangers would rate pictures of their bodies. Ordinary people, on the other hand, consistently rate themselves as far more attractive than they actually are. It seems that eating disorders might be caused not by unrealistic body image, but rather by having a body image that's too realistic. Similarly, most mentally-healthy people have far more positive ideas about how other people view them, how important they are, and how much control they have over their own lives than can be objectively supported. Depressed people, on the other hand, tend to rate themselves more accurately. Ordinary people also consistently over-estimate their own competency at performing various tasks, while depressed people usually are able to accurately self-asses their own competency. The implication is that it might be necessary for most people to maintain positive self-delusions in order to stay mentally healthy. Of course, it's not clear whether an inability to maintain such delusions is the cause of the mental illness, or if the mental illness (depression, anorexia, etc) results in the person losing the delusions.