> But you know if you really think that mental illness depends on brain
> chemistry, you have to explain why it has gotten so much worse.
It might well be that _all_ mental states depend on brain chemistry. The assumption that mental illness has actually "gotten so much worse" (one plausible counter hypothesis: we just diagnose it a lot more these days) doesn't contradict this thesis; any plausible physicalist or supervenience view of mental states would allow that (at least some) changes in the external environment are tracked by changes in brain chemistry.
-- Luke