Americans in the "Greatest Generation," or those born between roughly 1901 and 1924, slept with an estimated average of three partners during adulthood, according to a study published Tuesday in the Archives of Sexual Behavior. Baby Boomers have picked up the pace, averaging about 11 partners. Generation X has nearly kept up with 10.

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People don't stop having sex at any particular age, of course. Researchers calculated projections based on historic sexual patterns across generations. To simplify: Twenge's group compared the average number of sexual partners among 25-year-old Boomers to, for example, today's 25-year-olds.

So, why do Millennials appear to be more selective than previous generations? We don't know for sure. One theory: While this generation brought attention to the idea of "friends with benefits," said Twenge, who led the research and wrote Generation Me, they actually may be having "sex within a smaller circle of people," and not doing the serious dating that might actually lead to more partners.

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Millennials also tend to become sexually active later than Gen X. Between 2006 and 2008, 11 percent of teenage girls and 14 percent of teenage boys reported having sex before age 15 -- compared to 19 percent and 21 percent in 1995, when Gen Xers would be coming of age, the Guttmacher Institute found.

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The reasons that millennials may be delaying sex could be practical, Twenge said. Today's young people are generally aware of risks of sexually transmitted infections and unplanned pregnancies, thanks to sex education and the Internet. Recent CDC data suggest there are more than 110 million total STIs among American men and women.

But millennials seem to be less judgmental about other people's sex lives. Sixty-two percent think there's nothing wrong with sex before marriage, according to Twenge's calculations, and 56 percent have no issue with same-sex relationships.

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That may stem from a cultural tendency to embrace individuality, research shows. “They’re more likely to say, “I’m going to do my thing," Twenge said, "and 'you do your thing.'”

Social forces may also shape Millennial attitudes on sex. For starters, fewer twenty-somethings are tying the knot.

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Last year, the U.S. marriage rate reached a 93-year low. "With more Americans spending more of their young adulthood unmarried," the researchers wrote, "they have more opportunities to engage in sex with more partners and less reason to disapprove of non-marital sex."

Overall, adult acceptance of premarital sex increased from 42 percent in 2000 to 58 percent in 2012, an all-time high. Approval of same-sex relations, meanwhile, has more than tripled from 13 percent in 1990 to 44 percent in 2012, another all-time high.

There's an important takeaway from these findings: Understanding a generation's bedroom habits can better inform policy discussions about contraception, abortion and sex education.

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“In 2015, we don’t have our heads in the sand anymore,” Twenge said. “The majority of Americans now believe there’s nothing wrong with sex before marriage. We shouldn't say, ‘Let’s just teach abstinence,’ because that seems increasingly untenable.”