It’s a common phrase, but people who dislike psychiatry are correct to point out that “normal” behavior is also caused by “chemical imbalances”. That’s just basic metaphysical materialism!

I don’t agree with them that “there’s no such thing” as mental illness, but I do agree that we medicalize trivial variations too quickly, and what counts as ordered or disordered depends on social context, and we can’t just assume the social context is ordered.