Some of us like to think that we define ourselves by what we do, or by what we own, or by what we consume, but in reality we define ourselves by how we define ourselves. We take one half of our composite identity, our social identity, and internalize it; making the other half, our personal identity, into little more than a commentary on our social identity.
We mentally divide people by whether or not they seem to divide people the same ways we do – be it by category, group or what-have-you. We catalog as many categories and divisions as we can find, and whereas our social identity is a matter of positive sorting of these categories (I am: male, white, bicyclist, etc…), we make of our personal identity a negative sorting of the same categories (I am not/I am against those who are: Muslim, French, students, etc…).
Instead of thinking of ourselves and saying “I am me”, we tend to say “I am not her” or “I am not like them”. We pick a group, or groups, to measure ourselves against, play up all of the differences between ourselves and that group – making sure to state those differences in a way that seems favorable to us – and then as further proof of our superiority to them, we accuse them of not “integrating”, of being and remaining to much not us to be trustworthy.