Yeah, the Innocence Project is amazing. It's also extremely scary how many people have been found to be innocent that way...

I'm not sure if any systematic studies have been done to disprove the 'happy, gentle' stereotype. I think it's clear though that some people with retardation are, for some reason or another, more irritable and prone to violence than others, and those ones, due to their retardation, have fewer resources to counteract that disposition. Whether other people with retardation, even the vast majority, have gentle predispositions is totally irrelevant.