It appears that Boyz II Men, plus the entire golden age of baby-making music, is wasted on millennials. More young adults born in the 1980s and 1990s are choosing not to have sex, according to the results of a new study published in the Archives of Sexual Behavior.

In fact, they’re twice as likely to be virgins, compared to GenXers—people born in the 1960s and ’70s—when they were the same age.

“A few years ago, I would have been very surprised at this result,” says study author Jean Twenge, a psychology professor at San Diego State University and author of the book Generation Me. It certainly seems surprising, given Twenge’s recent data that same-sex hookup rates have doubled in the U.S. and young Americans are more sexually free and less traditional. But other recent research, and some from the CDC, all points in this direction: that young adults these days have fewer sexual partners and are starting to have sex later.

Twenge and her team wondered if people who weren’t having any sex at all were contributing to this sexless trend. So they analyzed data from a nationally representative group of 26,707 adults, focusing on GenX’ers, Millennials and the generation after them, which some people call iGen. Fifteen percent of young adults ages 20-24 who were born in the 1990s said they had no sex partners since age 18, while just 6% of GenX’ers admitted the same when recalling the romps of their early twenties.

How, for all the handwringing over today’s hookup culture, can this be true? And why are more young adults pushing off sex? They’re impossible questions to answer without conducting a randomized controlled trial (which you just can’t do in this area) so researchers can only speculate. But Twenge has some compelling ideas. “There’s been a general pattern over the last few generations for people to become adults or cross adult milestones later: things like getting married, having a child, settling into a stable career, buying a house,” she says. An extended adolescence used to mean that people were starting to have sex but putting off marriage and kids. “Now, it seems Millennials and iGen are putting off everything—including sex,” she said.

Another intriguing explanation is that Millennials and the generation after them have grown up with a strong emphasis on safety. “That may potentially impact their sexual behavior, if they’ve gotten the message that you can get sick or even die from sex,” Twenge said. The safer-is-better state of mind is reflected elsewhere as well: underage drinking is at an all-time low, and protests on college campuses often call for safe spaces and safer sex.

It could be, too, that young adults are increasingly living on their phones. “If you’re on your phone communicating with people more often, and not seeing your peers in person as much, that could lead to less sex,” Twenge says. And while phones may be helping some people get laid, dating apps like Tinder likely leave out a whole segment of the population—perhaps the one not having sex. “Maybe if someone who isn’t physically attractive meets someone in a bar, they can charm the other person with their engaging personality and humor,” Twenge explains. “On Tinder, it’s a picture.”

More young adults are likely choosing to forgo sex for a range of reasons. But the trend is clear: kids just aren’t growing up as fast as they used to.

Write to Mandy Oaklander at mandy.oaklander@time.com.