It used to be that along with certain tax benefits, one advantage to being married was having more sex than singletons.

That benefit is steadily diminishing, according to a study released Tuesday. Overall, American adults are having less sex than they did a quarter century ago, with married people showing the most dramatic decline of all.

The paper, published in the journal Archives of Sexual Behavior, showed a drop across gender, race, region, education level and work status. One factor is the higher percentage now of unpartnered people, who tend to have less sex than partnered ones. But a major driver is a steady fall in the rate of sexual activity for people who are married or living with partners, which reduces what had been known as the “marriage advantage.”

Using 1989-2014 data from the General Social Survey, the study found that American adults have sex seven to nine fewer times per year than in the 1990s. Back then, Americans on average had sex 60 to 62 times a year, but in the early 2000s the frequency began to slip, and by 2014 it had declined to less than 53 times a year.

When looking only at married people, the drop was even sharper — from around 73 times a year in 1990 to around 55 in 2014 — bringing their frequency of sexual activity below that of never-married people. People in that group have sex an average of 59 times a year.

(Claritza Jimenez/The Washington Post)

At the same time, Americans overall became less coupled. In 1986, 66 percent of American adults were living with a partner; by 2014 only 59 percent were, according to GSS data. People who are not in couples, including those who have been married in the past, tend to have sex half as frequently as people who are, the study said.

The report did not list causes for the decline. But it cited possible factors including increased access to entertainment and social media, a decline in happiness among people age 30 and over, higher incidence of depression, and use of antidepressants associated with sexual dysfunction.

“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,” said Jean M. Twenge, the study’s lead author, who teaches psychology at San Diego State University and wrote “Generation Me,” a book about millennials. “We do know that sexual frequency is linked to marital satisfaction, so overall if you have fewer people having sex, you could have people who are less happy and less satisfied with that relationship.”

The ubiquity of electronic distraction does not help, she added. “People aren’t looking around saying, ‘Hey, it’s 10 o’clock, what are we going to do?’ ”

A major detractor to Americans’ sex lives has been the rising necessity of the two-income family, said Pepper Schwartz, a sociology professor at the University of Washington. “I would say the number one cause for a lack of sex is fatigue,” she said. “You have many more women and men working to create a two-income family to stay middle class or above. . . . People’s minds are occupied with things other than the physical connection, and that has increased in modern life, and especially from the ’80s and ’90s and forward.”

Unsurprisingly the study found a steady decline in frequency of sexual activity as people age, from over 80 times a year for those in their 20s to about 60 times a year by 45 and 20 times a year by 65. But when comparing the same time period in the lives of each generation, the group having sex most often were those born in the 1930s, while those having the least sex were born in the 1990s.

[Why millennials are having less sex than their parents’ generation]

The decline in sexual activity was sharpest among people in their 50s, people with a college degree, people with school-age children, people in the South and those who do not watch pornography. It was less pronounced among younger people, men, nonwhites, people with children under 6, people in the West and those who had watched a pornographic movie in the past year.

As more people put off parenthood until later, the combination of middle age and child-rearing may create a “perfect storm” of sexual infrequency, the study said.

And working parents who spend less time with their children during the week tend to make up for it on weekends, eating into time that would normally go to the couple, Schwartz said.

“What you need for a sex life is energy, focus and time and the right mood,” she said. “If I’ve just run a marathon, is the first thing I want to do have sex? Probably not.”