Sexy times in the US are declining, according to a new study in the Archives of Sexual Behavior.

American adults reported having nine fewer romps a year in the early 2010s than they did in the late 1990s—dropping from an average of about 62 times a year between 1995 and 2000 to around 53 a year between 2010 and 2014. Researchers saw declines across ages, races, religions, education levels, employment statuses, and regions. They linked the sagging numbers to two trends: an increase in singletons over that period—who tend to have less sex than married or partnered people—plus a slow-down in the sex lives of married and coupled people. But the drivers of those trends are still unclear.

The study is based on data from a long-standing national survey called the General Social Survey (GSS). It involves a nationally representative sample of Americans over 18 years old, surveyed most years between 1972 and 2014. The new study involved responses from 26,620 Americans.

Specifically, researchers found that married people’s annual whoopee frequency dropped from an average of nearly 69 in the 1995-2000 period to just below 56 in the 2010-2014 period. The unmarried saw their lovemaking drop from 54 per year to 51 in the same timeframes. Meanwhile, the number of people without steady partners—married or otherwise—rose from 26 percent of survey respondents in 2006 to 33 percent in 2014. (Questions about steady partnerships were only asked in that time period.)

People who took the biggest hits in the bedroom since the 1990s were those with a college degree (about 15 fewer times a year) and people living in the South (about 13 fewer times a year). Parents of kids older than six also saw big drops (four to nine fewer times a year, depending on their kids’ ages.) And people who skipped pornographic movies also fared poorly, dropping 11 times a year.

It’s unclear what might be driving the trends. The researchers couldn’t link them to increased work hours or increased interest in porn, which, when used solo, could sap sexual potential. Instead, the researchers speculated that driving factors may include declines in happiness, increases in depression, and the availability of distracting digital entertainment in bed—from Netflix to smartphones and social media.

“People aren’t looking around saying, ‘Hey, it’s ten o’clock, what are we going to do?’” Jean Twenge, lead study author and psychologist at San Diego State University, told The Washington Post.

But there may be more complicated factors involved. Twenge and colleagues also noted that people are having children later in life. The double whammy of older age and kids—both nooky-dampers—could be a “perfect storm” to douse the romantic flames, the authors write.

Then there are generational differences. When the researchers controlled for age and time period, they found that people born in the 1990s (Millennials and iGen) are averaging six fewer boinks per year than those born in the 1930s.

Since the data is nationally representative, it mainly applies to heterosexuals, who made up more than 90 percent of respondents. And it might not mean that people are having fewer orgasms, since the survey didn’t ask questions explicitly about solo masturbation.

Still, because the frequency of sex tends to link with well-being, the researchers say the trends are important. Unfortunately, they'll have to push harder to understand them. “Are they less happy and thus having less sex or are they having less sex and therefore less happy? It’s probably some of both,” Twenge told the Post.

Archives of Sexual Behavior, 2017. DOI: 10.1007/s10508-017-0953-1 (About DOIs).