Happens in every conceivable kind of social group I am aware off. Those in the other classroom, those in the other office, those of the other political stripe,

Happens in every conceivable kind of social group I am aware off. Those in the other classroom, those in the other office, those of the other political stripe, those across the border, those who play on the other team, those who dress differently, those who use Ubuntu, those who code C++, those who prefer cats, or dogs, or whatever, ad nauseam. It happens here on HN, for Gods sake. No, we're not baying for the blood of those guys, but the dislike is there, or the mistrust. And seeing how all kinds of other animals exhibit the exact same pattern, there's no sound reason not to assume we are hardwired for it. In so many cases a silly atavism. And in many others no doubt an essential group mechanism, if nowadays not necessarily for survival, then for internal group cohesion and a basic guard against intruders.