I think ( from my research and experience) that it can also be caused by nutritional imbalances/deficiencies. I don't know if you can separate this from the brain though, since the brain needs the nutrients to function properly. I do feel that starving the brain of the nutrients that it needs is the real cause of mental problems-not some genetic thing we can't avoid.

I know that if you are deficient in magnesium you can become depressed, paranoid, irritable, and worse. It has been linked to schizophrenia as well.