Your statement, “Individuals often want the opposite of what they have learned to accept as the norm,” borders on contradictory. If they wanted the opposite then that means they do not accept it. I’m also not sure where you were trying to go with this statement. Is ethnic and cultural diversity the norm that they oppose? Because the rest of your comment seems to imply that it is not.

I’m not sure the propensity towards violence or forced segregation is as high as you claim in western civilization, particularly the United States. Most of the West seems to have recognized the potential benefits of diversity, so long as it within a “common ground” such as the legal order. The tendency to prefer homogeneity of ethnicity and culture is based on the initial increase in transaction costs that will lower the prosperity, inconvenience, or even cause some trauma to the community that is trying to integrate a new culture. These initial costs, if not weighed against potential future benefits, have often led to the attempted purging of outside cultures in order to maintain the status quo. This is due to a lack of education on the benefits experienced by communities who do make the effort to integrate diversity. However, forced integration will have much worse consequences than forced segregation. As you have noted, when the community does not accept that they will benefit from the integration it leads to violence and the attempt to “eradicate” or resegregate. With forced segregation, while it is not optimal, it is much less likely to lead to violence and as the differing parties learn more about each other on their own time they will most likely see the benefits of a cooperative relationship and probably eventual integration to some degree.

On the other hand, some homogeneity in culture and behavior is beneficial to a community. Accepted norms, shared traditions, mutual understandings, similar use of language, shared belief in a common legal order, etc. all leads to lower transaction costs in the interactions between individuals. As these become more diverse more costs (money, time, psychic)are accrued trying to understand each other and come to terms that satisfy both parties.

Finding the balance between ostracizing individuals for arbitrary reasons and still maintaining some homogeneity of culture can not be established by any “logical” lines in the sand; instead, the balance must be found through social evolution, ie “trial and error” and imitation of successful communities.