I'm not going to offer any facts here, just a sneaking suspicion.
We keep hearing people fuss about our legal system for the way it supposedly hands punishments out unequally. Hey, I'm all for justice. But it seems like some areas get a lot more attention than others.
For instance, does anyone think that psychiatry is so standardized and evenhanded that it can define "insanity" in a legally appropriate way? Surely there can be no justice based on something so arbitrary. So in light of that, why do we tolerate an insanity defense?
Being short of time, I have done no research - maybe I'll get back to this later, and as always feedback is welcome. I'll just note that it has always struck me that insanity as determined by courts correlates directly with your wealth and with your likelihood of receiving the death penalty.