From a U.S. News & World Report interview with neuroscientist Michael Gazzaniga:

Do you believe in an insanity defense?

I never have. You know one of the reasons is if you look at schizophrenics for example. Their rate of violent behavior is not above that of the normal population, especially when they're on their medication. So, if that's true, how can you use that as a defense, that they're doing something because they're insane. But the notion of personal responsibility has to do with the fact that people follow rules because they're in a social group and people with these various kinds of disorders can still follow those rules.

What good does the "insanity defense" do? None, according to Gazzaniga—apart from, perhaps, identifying those who need treatment. (Although one would think that the courtroom isn't the optimal place for this sort of thing; maybe judges shouldn't be in the business of handing out diagnoses.) One could also flip this around and ask: What bad does it do? Well, perhaps quite a bit. A good number of prisoners who don't plead insanity, after all, actually do have some sort of mental illness—upwards of one in six, according to Human Rights Watch—and most go shamefully untreated, partly because they were all implicitly deemed "sane" at trial. Indeed, many states now have "guilty but mentally ill" verdicts that often seem to further blur, rather than sharpen, the relevant lines here.

Now, frankly, if a criminal has a mental illness, I don't care if you call him guilty, sort of guilty, not culpable, or Ronald McDonald—all I care about is that he gets treated, which, one would hope, offers the greatest chance for rehabilitation. On the other hand, if the state actually decides to treat these prisoners in the hopes that thence lies rehabilitation, then why is it punishing them in the exact same fashion as the other "sane" prisoners in the first place? According to Gazzaniga, for purposes of retribution: because the mentally ill deserve their punishment just as much as any other prisoner. Something about that doesn't sit well, but I can't quite articulate why. Maybe he's right though, maybe most of those deemed insane really do "choose" not to follow society's rules. Seriously, though?