There are two diametrically opposed forces at work in human evolution. Biology wants diversity, that's why we have sex; to stir up the gene pool. Diversity increases the chances that at least some of our species will survive a catastrophic event.
Social pressure wants uniformity. My tribe is good, other tribe is bad. Those like me should be protected, those that are different should be destroyed. I think that that is the real basis of racism, sexism, homophobia, religious and all other bigotries. What I find weird, and it always made me feel strange in high school, is that we teach "Hate the others" to our children. We call it a "Pep Rally!" I remember when we were expected to cheer for our athletes because they were us, and boo and vilify the athletes from the school across town, because they were them! (Hell, half my friends went to the school across town, I didn't wish them ill.)
How can we teach this kind of competition to our children and then ask "can't we all just get along?"
I don't have the athletics gene, I don't do sports. I do find it interesting that my friends all have a favorite team, though the members of their team are all mercenaries and only play for that team because it offered them the most money, or if its a college team; because it offered the best scholarship with benefits deal. But, nevertheless, they are our team, they are good, and the other teams suck. And every Monday, half my friends are happy because their team won by beating a team that sucks, and the other half are pissed because their team lost to a team that sucks.