Americans under the age of 45 have found a novel way to rebel against their elders: They’re staying married.

New data show younger couples are approaching relationships differently from baby boomers, who married young, divorced, remarried and so on. Generation X and especially millennials are being pickier about whom they marry, tying the knot at older ages when education, careers and finances are on track. The result is a U.S. divorce rate that dropped 18 percent from 2008 to 2016, according to an analysis by University of Maryland sociology professor Philip Cohen.

Demographers already knew the divorce rate was falling, even if the average American didn’t. Their question, however, was: Why?

One theory is that divorce rates are falling largely because of other demographic changes, especially an aging population. But in Cohen’s analysis of U.S. Census Bureau survey data, even when he controls for factors such as age, the divorce rate over the same period still dropped 8 percent.

“The change among young people is particularly striking,” said Susan Brown, a sociology professor at Bowling Green State University.

Boomers have continued to divorce at unusually high rates, all the way into their 60s and 70s. Cohen’s results suggest this trend, called “gray divorce,” may have leveled out in the past decade, but boomers are still divorcing at much higher rates than previous generations did at similar ages.

However, many poorer and less-educated Americans are opting not to marry at all. They’re living together, and often raising kids together, without tying the knot. And studies have shown these cohabiting relationships are less stable than they used to be.