This is a bit of an oversimplification. Murray refers not to the rich, but the "cognitive elite," who lead lives, if not of economic bounty, at least economic comfort. "Belmont," Mitt Romney's hometown is his synecdoche for it. Then there is the "lower class," which he leaves undefined, but refers generally to those who struggle economically. His symbolic stand-in for this group is "Fishtown," a largely White working class neighborhood in Philadelphia.
You can read about it in Murray's Coming Apart: The State of White America, 1960-2010.
Not surprisingly, this analysis is not a popular with the political left, which wants to pose as being concerned about social polarization and its effects on children while spouting ideas that do exactly the opposite.
Here is Belinda Luscombe in Time magazine, limply trying to soften the hard edges of Murray's analysis, but having to give up in the end:
The gap in the family life of the rich and poor yawns wider that it ever has, and the individuals most hurt by this are, you guessed, it, the children of the poor. The working class have experimented with a new type of family formation that’s not based around the equation of one partner who runs the home front plus one partner who brings in the income both of whom throw in their lot together for the long haul. These new formulations tend not to be as stable, and instability is sub-optimal for kids.This is what the ideas of those who want to redefine the family really do. Read more here.