In 2011, economists from the Federal Reserve and the University of Notre Dame issued a working paper called “Internal Migration in the United States.” In it, they concluded that “internal migration has fallen noticeably since the 1980s, reversing increases from earlier in the century.” In other words, Americans are moving less than they used to.

In that paper, and in research since, it’s been shown that the decline in migration holds up across the board, from high-school and college graduates to dropouts. Wealthier people are moving less than they used to, and so are poorer people. Migration from both distressed areas and prosperous areas has declined.

Researchers have resisted coming to any definite conclusions about what underlies this decreased mobility. As Derek Thompson wrote for The Atlantic last year, so far there’s no good one-size-fits-all explanation. There’s also no consensus about what this lessening of internal migration means for the American economy.