I guess it's because first world middle/upper-middle class are the most anxious of all social strata, because they have to constantly struggle with maintaining

I guess it's because first world middle/upper-middle class are the most anxious of all social strata, because they have to constantly struggle with maintaining their social standings and that their children would not fall down, which is extremely hard, mostly because in first world middle/upper middle class kids have to compete with all other children in the same or lesser social standings and they usually don't have competitive edge. If u.s. society is a caste system, where upper middle class kids don't have to compete with middle class kids, and middle class kids don't have to compete with middle lower class people, where each class has its cultural/religious/socioeconomic/political moat to keep lower class people from getting in, people in general would not feel that level of anxiousness.