To unravel the mystery of declining U.S. dynamism—fewer moves, fewer quits, fewer startups—start with most well-documented mystery: why Americans stopped migrating.

Between the 1970s and 2010, the rate of Americans moving between states fell by more than half—from 3.5 percent per year to 1.4 percent. “It’s a puzzle and it’s the one I wish politicians and policy makers were more concerned about,” Betsey Stevenson, a former member of Obama's Council of Economic Advisers, told The New York Times this week. Fewer Americans moving toward the best jobs and starting fewer companies could lead to a less productive economy. On Thursday, the Financial Times reported that productivity “is set to fall in the U.S. for the first time in more than three decades.”

It is impossible to simultaneously and conclusively explain 120 million decisions by 120 million American families. But there are several explanations that economists have offered (and rejected).

Is it because of the Great Recession? No. It would be satisfying to say that the decline in American moxie is directly related to the Great Recession. But the decline in mobility predates the housing crash, and since 2008, the rate of people moving between states declined similarly for both renters and homeowners.

Is the decline in dynamism concentrated in the stricken areas of Appalachia and the Rust Belt, whose struggles some some have tied to the Trump phenomenon? Nope. The decline in dynamism isn’t a regional quirk. It’s happening in every state, particularly in the west, not the Rust Belt.

Technology has changed the mix of jobs and the way that people work: Is automation somehow to blame for the decline in mobility? Nope. States with more workers in routine-intensive tasks, like administrative duties, actually saw smaller declines in labor-market fluidity.

Is it about the rise in dual-earner households, since now, instead of one partner having to a find a new job in a new city, both do? This seems like a smart explanation, except it doesn’t explain much. “Today’s two-paycheck married households are about 46 percent less likely to move across state lines than were their counterparts in the 1980s,” Timothy Noah reported in Washington Monthly. In other words, whatever forces are depressing American mobility aren’t disproportionately affecting dual-earner households.

Has America simply lost its verve as the working force got older? Not really, but this would seem like the obvious explanation, so it’s worth spending a bit more time on why it’s incomplete. Young people are more likely to switch jobs and move around. After all, it's much easier to graduate from Penn State and move to San Diego alone than to raise a family in San Diego and move all four of them to Pennsylvania. If young people are tumbleweeds, adults are like trees: They grow roots, and they tend to stay put. So, as a country ages, it should become less dynamic.