Today's young people may be going off sex, having intimate partners less often after turning 18 than the previous generation - and with hours spent online a possible cause.
No parental warnings were needed to achieve this turnaround in promiscuity. The researchers are putting it down to the high numbers who still live at home - and ready access to pornography over the internet.
Spending time online also has a powerful influence, said Prof Jean Twenge of San Diego State University, who led a survey of 26,700 people in the US to gauge changes in the levels of sexual activity among people born in the 1960s-70s and those born in the 1990s.
Those involved were respondents to the General Social Survey, a nationally representative survey of US adults.
The older group surveyed were designated "Generation Xers" and the younger as "Millennials" in her study, published on Tuesday in the journal Archives of Sexual Behaviour.
It showed 15 per cent of 20- to 24-year-olds born in the 1990s reported having no sexual partners since turning 18. Only 6 per cent of the older Generation Xers reported having no sexual partners after 18.
Online technology and social media, which increasingly can facilitate the forming of personal relationships, seems also to be having the opposite effect, Prof Twenge said.
“Online dating apps should, in theory, help Millennials find sexual partners more easily,” she said. “However, technology may have the opposite effect if young people are spending so much time online that they interact less in person, and thus don’t have sex.”
Concerns over personal safety and media reportage of sexual abuse could also contribute to reduced sexual activity. This was seen in reduced use of alcohol and access to safe spaces on university campuses, Prof Twenge said.
“This is a very risk-averse generation, and that attitude may be influencing their sexual choices,” she said.
Teens today seem to be continuing the tendencies of Millennials, the researchers add.
US studies on teen risk behaviour shows the percentage of US high school students who had sex fell from 51 per cent in 1991 to 41 per cent in 2015.