I'd just like to point out a piece of beautiful writing from Mind Hacks which highlights the problems facing psychiatry as a diagnostic science. I have tried to make this point myself, but Vaughn has done it better:
"Genes are defined on the neurobiological level, while psychiatric diagnoses are defined on the phenomenological level - in other words, verbal descriptions of behaviour, or verbal descriptions of what it is like to have certain mental states.
"There is no guarantee, and in many people's opinion, probably no likelihood, that these 'what it is like' descriptions actually clearly demarcate distinct processes at the biological level.
"It's a bit like classifying people as heavy metal fans if they have five or more heavy metal albums."
Go and read the rest of it. It will help greatly with your understanding of the problems facing the mental health professions.
Link to Mind Hacks article: Mental Illness: In with the intron crowd