ONE of the more significant migrations in recent American history doesn’t involve pioneers heading West, refugees seeking sanctuary, or Joad-like families rambling in search of work. It involves the trajectory of our nation’s most talented citizens, who since the 1970s have been clustering ever more densely in certain favored cities, and gradually abandoning the places in between.

In a mid-2000s piece for The Atlantic, Richard Florida, long a booster of “creative class” conurbations, noted that in 1970 college graduates were distributed pretty evenly around the country, but that three decades later they were much more concentrated. A few regions (the BosWash Northeast, the Bay Area, etc.) were destinations of choice for the well educated, and large swaths of the country emphatically were not. In Washington, D.C., and San Francisco, he noted, half the population had college degrees; for Detroit and Cleveland, the figures were 11 percent and 14 percent.

This migration has happened for understandable personal and professional reasons (said the pundit writing from a coffee shop in northeastern Washington, D.C.), and the dense professional networks it has created have arguably been good for certain kinds of economic dynamism.

But elite self-segregation, and what Charles Murray has dubbed the “coming apart” of the professional and working classes, has also contributed to America’s growing social problems — hardening lines of class and culture, adding layers of misunderstanding and mistrust to an already polarized polity, and leaching brains and social capital from communities that need them most.