Americans are having less sex than they did in the past two decades, and couples who are married or live together reported the largest drop in frequency, researchers said Tuesday.
Americans living as couples had sex 16 fewer times per year in 2010-2014 compared to 2000-2004, said the survey, based on a nationally representative sample of more than 26,000 American adults asked about their sexual behavior since 1989.
On the whole, Americans had sex about nine fewer times per year in 2010-2014 compared to 1995-1999.
"These data show a major reversal from previous decades in terms of marriage and sex," said lead author Jean Twenge, a 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."
While the survey did not delve into the reasons for this trend, researchers said it was not because people are working more than ever.
In fact, people who reported working more hours actually had sex more often than people who worked less.
Twenge said people's age seems to be a critical factor in how much sex they have.
Younger generations, such as millennials, tend to have sex less often than their grandparents did when they were young, in part because fewer of them have steady partners.
An earlier study by Twenge and co-authors found that millennials had fewer sexual partners than their Generation X predecessors.
Sexual frequency tends to peak around age 25, then declines 3.2 percent per year after that, said the study.
In general, 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, it added.
"In a previous paper, we found that the happiness of adults over age 30 declined between 2000 and 2014," said Twenge.
"With less sex and less happiness, it's no wonder that American adults seem deeply dissatisfied these days."
The study appeared in the Archives of Sexual Behavior.