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While the topic of sex is less taboo than it was a generation ago, that doesn't necessarily mean people are having more of it.

According to a new study published in the journal Archives of Sexual Behavior today, Americans who were married or living together had sex 16 fewer times per year in 2010-2014 compared to 2000-2004.

The survey also found that overall, Americans had sex about nine fewer times per year in 2010-2014 compared to 1995-1999.

The study is based on data collected from the General Social Survey, a nationally representative sample of more than 26,000 American adults asked about their sexual behavior since 1989.

"These data show a major reversal from previous decades in terms of marriage and sex," said Jean M. Twenge, the study's lead author and professor of psychology at San Diego State University. "In the 1990s, married people had sex more times per year than never-married people, but by the mid-2000s that reversed, with the never-married having more sex."

According to Twenge, author of the book "Generation Me," a critical factor appears to be birth cohort, with later-born generations having sex less often than those born earlier in the 20th century.

In an earlier study, Twenge and co-authors Ryne Sherman at Florida Atlantic University and Brooke Wells at the Center for Human Sexuality Studies at Widener University, found that Millennials had fewer sexual partners than their Generation X predecessors.

"Despite their reputation for hooking up, Millennials and the generation after them (known as iGen or Generation Z) are actually having sex less often than their parents and grandparents did when they were young," said Twenge. "That's partially because fewer iGen'ers and Millennials have steady partners."

Age also appears to play a significant role. People in their 20s have sex more than 80 times per year, declining to 60 times per year by age 45, and 20 times per year by age 65. Each year after the peak of sexual frequency at 25, sexual frequency declines 3.2 percent.

"Older and married people are having sex less often—especially after 2000," Twenge said. "In a previous paper, we found that the happiness of adults over age 30 declined between 2000 and 2014. With less sex and less happiness, it's no wonder that American adults seem deeply dissatisfied these days."

Blame might be placed on the busy lives of more working parents, but the research didn't bear that out, said Twenge.

Instead, those who worked more hours actually had sex more often, as well.

More information: Jean M. Twenge et al, Declines in Sexual Frequency among American Adults, 1989–2014, Archives of Sexual Behavior (2017). Journal information: Archives of Sexual Behavior Jean M. Twenge et al, Declines in Sexual Frequency among American Adults, 1989–2014,(2017). DOI: 10.1007/s10508-017-0953-1