He had a group of 110 volunteers ride on the trains and record their moods during the journey. The result was that psychological distress increased when people became a minority within the carriage regardless of what race the volunteer was.
In other words, people of all races felt discomfort being a minority:
Participants' negative mood heightened as the ratio of people from different ethnic backgrounds aboard the train increased, regardless of their own race and after controlling for various factors, such as an individual's personality, familiarity with metro trains and perceived safety of the surrounding neighborhoods.
This suggests that it is kinder and wiser to allow people to continue to live within their own ethnic groups. It is evidence as well that the "white privilege" theory of ethnic solidarity is false, as members of all ethnic groups, and not just whites, feel more comfortable when they are part of an ethnic majority.