The question becomes, under what circumstances do individuals of different races mix spontaneously? I believe the answer has to do with finding individuals with whom you share a common goal. If you believe someone can help you achieve you goal it doesn't matter what their race; you share a common goal. If you just want to survive or have a good time, people choose to associate with people who look (act, smell, feel, sound, etc.) like they do. I think this has a lot to do with the perceived ability to predict behavior.

The issue touches all levels of society. The rich mix equally well with anyone who can make them richer. Second and third generation wealthy who are not actively making money do not share this trait but have their own affinities particularly with other who spend large amounts of money. Rich people who achieved their wealth through a segregated activity also may not mix well.

University professors mix when they are focused on their research or sharing intellectual pursuits. Senior business managers mix with anyone who can help them achieve their business goals. Experienced technicians respect skill more than skin color. The military works well enough within units where skill and teamwork is critical. Athletic teams similarly share goals that transcend anything but skill.

The military officers corps is mixed and although there is competition, racism has been ruthlessly rooted out. According to my hypothesis racism would naturally return so there must be constant pressure from above to keep it at bay.

Politicians hardly ever mix well. They mix when it is convenient (when they think it will earn votes). Of course that's how many of them lead their entire lives. They are a reflection of the majority of the voters who don't mix well.

Workers in small companies or mixed groups larger companies mix just fine on the job. There is an old maxim that if you promote a black woman to supervise a group and give her hire/fire capacity, you should expect that group to gradually become all black females (like attracts like).

People in jail don't mix well. These people have no goals that other prisoners can help them with other than survival. When survival is at risk the issue become who can you trust?

The lowest levels of society don't mix well. With no common goals there is little mixing. Competition with each other for jobs, homes, schools, etc. is intense. They just don't share many common interests. Students in school follow their parent's examples.

Races don't socialize together just for fun. They seem to have different ways of having fun. I can't tell you you last time I had a group of Asians over to the house to hang out and listen to some music or watch a sporting event on TV.

Large heterogeneous groups just don't form.