"This is part of a general theme of later maturation that's been pretty well-documented," said Jean Twenge, lead researcher of the new study and author of the book "Generation Me."
Just as young adults are now less likely to have jobs and get married and are more likely to live with their parents, part of this sexual trend may have something to do with economic realities, she said. Still, other factors might also explain these results.
"There's the possibility that technology has something to do with this," Twenge said. If you're spending more time texting with your friends and less time in person, she explained, you might have fewer opportunities to "hook up." Or, more simply, since "there are more ways to entertain yourself," sex is less important, being just one of many possibilities on a growing list.
For the study, Twenge and her colleagues used data from the General Social Survey, which was conducted between the years 1989 and 2014. The survey essentially asked all the right questions for the purposes of this study, explained Ryne Sherman, co-author of the paper and a psychologist at Florida Atlantic University.
The nationally representative survey includes demographic information about each respondent, allowing Twenge and Sherman to compare differences in sexual activity across lines of gender, race, education level, region and religious service attendance. Among a hundred or so queries about a variety of topics, the survey asks people direct questions about their sexual partners.
This was crucial, said Twenge, also a professor of psychology at San Diego State University. "We could compare people at the same age ... and we were looking at no sexual partners compared to having any sexual partners."
What they discovered was that young adults today -- millennials (born starting in the 1980s) and iGen (born in the '90s) -- are less likely to be sexually active compared with young adults from Generation X -- those born in the 1960s and '70s. They also discovered that levels of sexual inactivity increased for women more than men, whites more than blacks, those who did not attend college more than those who did, and those who lived in the east more than those in the west.
In terms of getting a late start having sex, millennials most resembled those born during the 1920s.
Twenge and Sherman were not very surprised by their results. A separate study of changing sexual attitudes
they published last year suggested as much.
"It's also consistent with the Centers for Disease Control and Prevention data," said Martin Monto, a sociology professor at the University of Portland who is not linked to the current study. The most recent CDC data
(PDF) on teen sexual behavior found that the percentage of high school students who have had sex plummeted from 54% in 1991 to 41% in 2015.
"A lot of that drop happened pretty recently," Twenge commented.
Based on the latest statistics, Twenge observed that this generation appears to be waiting longer to have sex, while an increasing minority wait even longer -- until their early 20s or later.
"It's a good study; the data is excellent: It's consistent with other data, so it's pretty solid," said Monto, whose own 2014 study
involving data from the General Social Survey found that young adults between 18 and 25 did not report "more sexual partners since age 18, more frequent sex, or more partners during the past year" than Generation X respondents.
They're commonly billed as the "hookup" generation, but this gives a false impression of millennials, explained Monto. "The term 'hookup' is entirely ambiguous," he said, and since it is "basically a nebulous term that could mean anything," it has led to a misunderstanding of what's actually going on today.
Overall, both Monto's and Twenge's results suggest a win for the young adults who are not emotionally ready for a romantic relationship. The pressure is off everyone. However, "humans hit their sexual peak in their early 20s," noted Twenge, so presumably there are many who are ready for and want a romantic relationship, but they simply have fewer opportunities.
Finally, there's the issue of safety. "Mine was the first generation that came of age when sex could equal death," Twenge said, noting that the AIDS crisis occurred during Gen Xers' formative years. "Thankfully, that abated somewhat," she said, though now "there's a new concept about emotional safety." As an example, she mentioned trigger warnings, in which content (primarily online) is flagged as potentially disturbing.
"This generation is much more concerned about safety on both a physical and an emotional level," Twenge said.