I don’t think it’s an instinct, because it can absolutely be taught.
I encourage my kids to get along with everyone, but at the same time I can see how some of their peers are taught to be racists and other clique behaviours from home by parents who are just like that and don’t even think about it when they pass it on.
But by default, nobody is like that from birth. Babies aren’t racists or afraid of different kinds of people. The fear of others is taught.