Less than 20 years ago, in 2008, voters in deep-blue California passed a proposition outlawing same-sex marriage. California was not an outlier: Only 40 percent of Americans in 2008 believed same-sex marriage should be legal. Just seven years later, in May 2015, 60 percent did. A month later, the U.S. Supreme Court agreed, legalizing same-sex marriage in all 50 states. That’s about as fast as cultural change gets.

Identities and behaviors changed along with attitudes. For a decade, an increasing number of Americans—especially young adults—identified as lesbian, gay, or bisexual, and more reported same-sex behavior. Recently, however, some of those trends have begun to reverse. But the reversal is not as broad as it first appears: It is mostly due to a steep drop among bisexual young women. Patterns of behavior diverge even more: Women report much less same-sex activity in the last two years, while men report slightly more.

So what’s going on?

When I was writing my book Generations a few years ago, I was curious whether there had been any changes in people’s self-reported sexual orientation and in their sexual behavior. Would more people identify as lesbian, gay, or bisexual (LGB)? And would more people have sex with someone of the same gender?