Authority, mostly. Despite what most people think (or, if we're being honest, fantasize), the majority of cases of nonce actions are not the results of someone pulling your kid off the street into a windowless, white van. It's someone with a level of unquestioned, unchallenged authority who are able to abuse their power. Teachers, religious figureheads, medical experts, and anyone else that can order people "beneath" them.

Authority, mostly.