Roughly 40 percent of American millennials have four-year college degrees, and if there’s one thing these highly educated young people have liked to do over the last 15 years, it’s move to big cities.

Researchers find they (well, we) have accounted for more than half the population increase in “close-in” urban neighborhoods in the country’s largest metro areas since 2010, and they credit our migration (and our taxes) with accelerating urban revival. We don’t have to guess as to why: Millennials like diverse, walkable environments with good public transit and bike lanes. They like the rich cultural amenities, including bars, restaurants, and concert venues. And they like the higher-paying work opportunities available.

All this might make you think millennials have moved to cities permanently. But as they get older, the number of urban children has continued to drop. Lower birth rates are part of the story, but economists say the strong correlations with population shifts strongly suggest that “out-migration” of cities explains a big portion of the loss. In other words, millennials now in their mid-30s and 40s with young kids have started decamping for suburbs to raise their families.

Some older adults nod smugly, seeing these suburban migration patterns as proof that there was never any meaningful difference between their preferences and that of millennials at all. Millennials did not start the trend of moving to cities in one’s 20s: Plenty of baby boomers and Gen X moved to urban areas in young adulthood, and then back to the suburbs to raise a family once they coupled up and needed more space.