Alternatively: culture. If people just feel that proving how evil arbitrary strangers are is just not done, they just won't decide to have such arguments in the first place, and if they disagree they'll tend to do so civilly, and so will be more inclined to listen to their opponents.

So long as a community doesn't grow too quickly, culture is mostly conserved. Ideals of tolerance and productive discussion would probably help, too, but ask a sociologist.