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Millennial women in the U.S. are Housewifes On Call Driver Side Job Day and Nightwaiting longer to start having sex, putting them more on par with 20-somethings from the Baby Boom-era than Generation X, a new study has found.

By delaying sex, young women today are breaking a decades-long pattern that saw American men and women embracing similar sex lives, Jean Twenge, the study’s lead author, told Mashable.

SEE ALSO: 4 smart ways to use the internet to educate yourself about sex

The study's findings fit with a trend toward millennials delaying steps associated with adulthood, such as getting married, and having a more individualistic outlook than their Gen X peers have, said Twenge, who teaches psychology at San Diego State University and authored “Generation Me,” a book about millennial culture.

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All the single ladies. Credit: Moment Editorial/Getty Images

"Millennials are taking longer to grow into adulthood,” she said.

Her latest study, published this week in the journal Archives of Sexual Behavior, found that overall, more millennials -- born in the 1980s and 1990s -- reported having no sexual partners compared to Gen X’ers born in the 1960s and 1970s.

Millennial women, in particular, saw the largest increase in sexual inactivity compared to their Generation X peers. Some 5.4 percent of millennial women said they’d had no sexual partners, up from 2.3 percent of Gen X women.

Twenge and her colleagues considered “no sexual partners” to indicate that survey respondents were either virgins, or had not had sex at least once since turning 18.

This means the study did not look at people who were temporarily single and abstaining from sex for a period of, say, six months or a year.

Researchers also looked most narrowly at the 20-to-24-year-old age group, referring to millennials born in the 1990s and Gen X’ers born in the 1960s.

Among those groups, about 15 percent of millennials said they were sexually inactive, compared with just 6 percent from Generation X, according to the study.

Researchers used data from the General Social Survey, a nationally representative sample of nearly 27,000 American adults.

Let's just be friends. Credit: Getty images

A similar trend toward waiting is happening in America’s high schools. Among 9th through 12th graders, 59 percent said they were virgins in 2015, up from 46 percent of students in 1991, according to the U.S. Centers for Disease Control and Prevention's Youth Risk Behavior Survey.

In Twenge’s study, women saw the biggest and most significant change among all of the demographic segments.

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Men saw a much smaller shift, with 1.9 percent of millennial men saying they were sexually inactive, up from 1.7 percent in Generation X.

Twenge said the gender gap likely has to do with women’s “erotic plasticity” -- a term coined by social psychologist Roy Baumeister in 2000 that suggests women adjust their sex lives to reflect the cultural or social norms of the time, more so than men.

“Women’s sexuality is more malleable depending on the situation and circumstance,” Twenge said, adding that men have tended to lose their virginity around the same age throughout generations.

“Men and women’s sexual behavior had become more similar with Generation X, and now it’s becoming more diverted again,” said Twenge.

Columbia University grads hold up inflated condoms. Credit: AFP/Getty Images

In the 1950s, for instance, women tended to wait until they married to start having sex. The median age for women marrying, and the average age for becoming sexually active, was about 20-years-old in both cases.

As American culture grew more relaxed around premarital sex, and as the average age for marriage notched up a few years, the link between getting married and having sex loosened.

Millennials are narrowing the gap between the first time you have sex and the first time you get married

By the 1990s, the average age for losing one’s virginity was 16-years-old, Twenge and her colleague Brooke Wells found in a previous paper.

“Most of that change happened for women instead of men. Men and women became similar [in sexual behaviors] up to Generation X,” Twenge said.

Now, however, the genders are once again diverging.

So why are U.S. millennial women decreasing their sexual activity, if they face less pressure than ever to tie the knot or wait until marriage to have sex?

Twenge suggested a few key reasons.

First, millennials on the whole are putting off major steps associated with adulthood: getting married, buying a house, moving out of their parent’s house and, it seems, starting a sexual relationship.

Via Giphy

“Millennials are narrowing the gap between the first time you have sex and the first time you get married. Everything is getting put off until later,” the psychologist said.

Second, the press release accompanying the study cites personal safety concerns as a possible reason for millennials' sexual inactivity, given reports of on-campus sexual assault cases.

And third, American culture at large is growing more individualistic, with people encouraged to explore their own wants and needs and focus less on social norms.

“That can mean having premarital sex and hooking up a lot, but it can also mean being asexual, or choosing to wait to have sex,” Twenge said.

“There’s more acceptance for all different kinds of choices.”