The Sunbelt is growing, the Rust Belt is dying, and the only thing keeping expensive coastal cities afloat is international immigration, as American-born residents flee their escalating housing prices.

That pretty much sums up the conventional wisdom about the recent growth and decline of U.S. cities. And that conventional wisdom was only reinforced last month when the Census Bureau released its latest figures on population growth for America’s metropolitan areas. Nine of the top 10 counties with the largest numeric increase in population last year were in the Sunbelt, with the one exception being King County, where Seattle is located.

But the conventional wisdom masks a deeper trend: America’s geography continues to be reshaped by a polarized pattern of socioeconomic sorting. This process is driven by a selective population shift of the most affluent, the best-educated, and the young to expensive coastal metros like the San Francisco Bay Area, Los Angeles, Seattle, and the New York–Boston–Washington corridor, with the less affluent and less educated flowing into cheaper Sunbelt metros, and the even less advantaged trapped in Rust Belt areas.

That is the basic pattern documented in a new analysis by urban economist Issi Romem, who charts the socioeconomic status of the domestic migrants to and from America’s largest metro areas between 2005 and 2016. Romem finds a selective class-based sorting of Americans. Those moving to expensive coastal metros, according to his analysis, have significantly higher incomes and higher levels of education than those moving out, and are considerably younger.