Since time immemorial, older generations have fretted over the sexual habits of young people. In today's world, however, elders might just be wondering why young people are having so little sex, according to a new study by San Diego State University psychology professor Jean M. Twenge.

A research team also including Ryne Sherman from Florida Atlantic University and Brooke Wells from Widener University analyzed data from 26,707 respondents to the General Social Survey, a nationally representative survey of U.S. adults that includes members of the current millennial generation and its predecessor, Generation X. The researchers found that today's young people are less likely to have had sex since turning 18.

According to Twenge, author of the book "Generation Me," 15 percent of 20- to 24-year-olds born in the 1990s reported having no sexual partners since age 18, compared to only 6 percent of Generation X'ers when they were young adults. This sexual inactivity stands in stark contrast to the so-called "hookup culture" reportedly pervasive among Millennials: More are not having sex at all, much less hooking up with multiple partners.

"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 a media landscape saturated with reports of collegiate sexual abuse might also contribute to millennials' sexual inactivity compared to previous generations, Twenge continued.

"This generation is very interested in safety, which also appears in their reduced use of alcohol and their interest in 'safe spaces' on campus," she said. "This is a very risk-averse generation, and that attitude may be influencing their sexual choices."

Other factors contributing to fewer millennials having sex could include the widespread availability of pornography, the historically high number of young adults living with their parents, the later age at first marriage, and increased access to instant entertainment online. The researchers published their findings this week in the journal Archives of Sexual Behavior.

Today's teens also appear to be less sexually active. According to the Centers for Disease Control's Youth Risk Behavior Survey, the percentage of U.S. high school students who have ever had sex dropped from 51 percent in 1991 to 41 percent in 2015.

"This generation appears to be waiting longer to have sex, with an increasing minority apparently waiting until their early twenties or later," said Twenge. "It's good news for sexual and emotional health if teens are waiting until they are ready. But if young adults forgo sex completely, they may be missing out on some of the advantages of an adult romantic relationship."