Commenter Freddiemac asks me whether the vicious pack behavior displayed by girls is nature or nurture. Given its universality, and how young it appears, I'd bet mostly nature with an able assist from the surrounding culture. I expect this also explains the visceral pleasure that most women get from gossip, which most men really don't seem to enjoy nearly so much--the perhaps sad truth is that I feel closer to my female friends when we have gotten through a really good round of "what's wrong with everyone else". Though I don't actually find what seems to be the male equivalent, "who's winning the cocktail party?", any more attractive.
But saying "nature" doesn't tell us the thing is inevitable. Lots of behaviors are natural, like rape and murdering strangers, that we struggle mightily to overcome--and mostly succeed. Even if my gender has a preprogrammed tendency to self-define through the people we can exclude from the group, we can rise above that. Feminists who use the phrase "anti-feminist" to describe anyone who disagrees with them are choosing to view the world as composed of two mutually exclusive groups: feminists, us; and the bad people who have not joined the group and are therefore our sworn enemies. They are choosing, too, the nastiness that tends to result from giving into our baser primate instincts.