[T]he United States ranks second to last among Great Britain, US, France, Canada, and Denmark when it comes to the rate of income improvement over four generations for poor families. [The U.K. was slightly less mobile.] . . . "our country actually has less social mobility and more inequality than most developed countries." . . .
The fact of social mobility is closely tied to facts about social inequality and facts about social class. In a highly egalitarian society there would be little need for social mobility. And in a society with a fairly persistent class structure there is also relatively little social mobility -- because there is some set of mechanisms that limit entry and exit into the various classes.
The analysis in the rest of the linked post isn't very sophisticated or empirical. It omits some of the key mechanisms for social class stability in the United States, like a growing tendency towards assortive marriage (marrying someone of the same social class or education) and hereditary intelligence. It also omits any analysis of important instances of social mobility in U.S. history, like the rise of the black middle class in the wake of the U.S. civil rights movement, the rise up the social ladder of Jewish immigrant communities, or the impact of the G.I. Bill. And, it focuses on inequality in educational opportunities while failing to grapple with the mountain of accumulating evidence that shows that the achievement gap in K-12 education starts much earlier and is much more impervious to being narrowed with simple solutions than most people assume.
But, the fundamental facts spawning the discussion, that income and wealth in the United States are highly concentrated by international standards, and that social mobility is low, are critically important for understanding the American status quo and thinking about what changes to that status quo make sense.