Americans just aren't getting it anymore.
The steepest declines have been among highly educated white people, couples in their 50s, partners with children between the ages of 6 and 12, "and those who had not seen a pornographic movie in the last year."
Why the precipitous drop-off? Researchers haven't been able to pin it down, but they have theories.
"Back in the day, we used to think of sex as an entertainment outlet," said study coauthor Brooke Wells, associate professor of human sexuality at Widener University in Chester. "Now that Americans have phones, iPads, and a ton of online content, it could be they just have more vying for their attention."
More clear are the basic social factors underpinning the trend. Fewer people are partnered than two decades ago. For instance, 48 percent of young adults (ages 18 to 29) lived without a significant other in 2005. That rate soared to 64 percent in 2014.
And Americans in committed relationships are having fewer sexual encounters. In 2002, the average U.S. adult had sex 64 times a year. In 2014, the frequency had dropped to 53.
"There have been a number of different explanations for why people are having less sex and having fewer partners than in previous generations," said Zhana Vrangalova, a sex researcher based in New York City. "Some have blamed it on porn. Others blame it on antidepressants, many of which we know can kill libido. It's not one answer. It's a multifaceted phenomenon."
Philadelphia sexologist Timaree Schmit said sex is seen increasingly as a luxury and not a human need.
"People don't prioritize pleasure," Schmit said. "My instinct is that people are having to do more and have less free time and energy for self-care in general, much less sex."
Researchers did check to see if people were devoting so much time and energy to work that it pushed sex by the wayside. "But the study said it's not due to the number of work hours," Vrangalova said.
Wells said it the pervasiveness and variety of sex on TV, film, and mobile devices may be putting a chill on ardor. A glut of titillation can backfire: "It potentially makes it less exciting, because it's something people can see and experience vicariously when they weren't able to do it before."
Though seamy stories of young adults enmeshed in hook-up culture were rife during the last decade, "that culture doesn't really exist now," Wells said. "In our last study, we found large numbers of young adults were not having sex at all. Many more people are identifying themselves as asexual."
Vrangalova, a researcher who studies casual sex and promiscuity, said hook-up apps serve a purpose even if they don't result in meeting someone in the flesh.
'You can get a lot of your needs satisfied just by sexting with people," she said. "You can establish connections ... and get some of your social and emotional needs met without actually having physical sex with them."
Wells said people ask her all the time if she thinks it's a good thing or bad thing if Americans are having less sex.
"There's so much more that we need to know to be able to answer that question. Is it terrible that people aren't connecting or partnering, that happiness is down and depression is up?