[bitwize]

"Mental illness" alone is not a defense. The defense is insanity, which term in law means that your mental condition was such that you had no way of knowing what you did was wrong. If you're schizophrenic, say, and you kill someone, but you are compos mentis enough to understand that you killed a person and that is wrong, you don't get to plead insanity. If, on the other hand, you were under the delusion that you were killing an alien or a demon or something, then a plea of insanity might work.

I know that this is Hackernews, and the perspective that there's no such thing as responsibility for our actions, because we are all products of our biology and environment; and hence no such thing as evil, is popular here. But whether and how mental illness absolves, or does not absolve, you from criminal culpability is a fairly rigorous area of law.

[Lord-Jobo]

I think a lot of people have a hysterically warped perspective on the insanity defense because of American legal/police media.

People don't understand how near impossible it is to deploy successfully in trial, in the US.

[bitwize]

Indeed. As I understand it (ianal), insanity is a positive defense, meaning that you have to prove to the judge/jury in court that you were insane, i.e., that you were functionally incapable of distinguishing right from wrong at the moment you committed the crime. This is because a mens rea, or guilty mind, is necessary to establish guilt and thus criminal culpability, in addition to the actus reus or the criminal deed itself; you have to be in a state where you know, or could/should have known, not to do what you did. Proving a negative, especially when it comes to this, is extremely difficult. You will have to undergo an extensive psychiatric evaluation by a psychiatrist and they have to come to the very strong conclusion that you were well and truly out of your mind at the time of the commission of the crime. Otherwise, just the evidence that you did the crime is enough to establish guilt.

Oh, and if you somehow successfully plead insanity, you will spend quite possibly the rest of your life in a mental institution instead of prison, which isn't much better.

And yes, it's good to remember that police procedurals are to criminal justice what Hackers (1995) is to hacking: a lot of stuff is distorted to make good cinema/television. In the UK, judges on cop shows use gavels (that is strictly an American custom); how can we expect them to get the details of rare events like an insanity defense right?