For the cad (if the game isn't iterated), always. Otherwise, I think people start defecting when they suspect everyone else is. If you'll be shot walking by windowsills, you might as well try to get a pie out of it.

In a society with uniform behavior any variance "would attract strong disapproval," but that's mostly a function of its uniformity, isn't it? Both cooperative people and cads would prefer a society where people cooperate, because both stand to benefit from counterparties cooperating. That was my original point at least.