In small hunter-gatherer bands or subsistence farming villages, it's pretty easy to keep track of just how cooperative your neighbors are. But when groups grow to encompass thousands and eventually millions of strangers, a Big-Brother-in-the-Sky can watch how your fellow citizens behave when you can't. And even better, Sky Big Brother can punish them with eternal damnation if they swindle you. One big downside is that groups have different Sky Big Brothers, which means that "the same mechanisms involved in ingroup altruism may also facilitate outgroup antagonism." In other words, kill the infidels!