I would go further i think MelanieR, i think that if the predisposition is strong enough even what would be thought of as a relatively "normal" childhood can be enough to tip the balance, whereas without that predisposition there would probably need to be more extreme trauma to bring on an absence of empathy, etc.

So if a person were pre-disposed simply not abusing might not be anywhere near enough to prevent the switch being flicked.

Bx