There's probably a strong selection bias here though. If one's trying the insanity defense it's either a.) they're insane and really didn't know what was going

There's probably a strong selection bias here though. If one's trying the insanity defense it's either a.) they're insane and really didn't know what was going on, or b.) they have no other defense and are going for a hail mary. People in both camps are very unlikely to receive shortened sentences or early parole.