[PrimalDual]

Perhaps this has to do with the increasing heterogeneity of American society. The US probably looked more like Sweden, Denmark and Japan in the past. People may feel more compelled to partake in civil engagement and similar things like helping your neighbors out if you have the same background. At the very least you must trust the members of your community to play fair. Homogeneity comes at a cost but it's definitely useful to be able to assume correctly that you are a reasonable model for other people in your community.