Twenge said people's age seems to be a critical factor in how much sex they have. "As recently as 2002, the average adult American had sex approximately 64 times a year, but by 2014 that declined to about 53 times a year".
The researchers also found people who worked full time had more sex than those who worked fewer hours, or were unemployed.
"These data show a major reversal from previous decades in terms of marriage and sex", said Twenge.
Twenge has shown in previous studies that millennials are also having fewer partners.
Researchers have found that adults, on average, were having sex seven fewer times annually in the early 2010s compared to the early 1990s, and nine fewer times compared to the late 1990s.
"Older and married people are having sex less often - especially after 2000", Twenge said.
Sounds like you better take a nap, power down your devices - after you block off time for a steamy sex sesh in your calendar of course - maybe light some candles and get busy, America. While some refused to answer the question, or did not know the answer, 94% of participants responded.
For instance, among those who are married or living with partners, they went from having sex 73 times a year in 1990 to about 55 times in 2014 - which is even below the frequency of sexual activity for never-married people, who have sex an average of 59 times a year.More news: Mother And Four Children Killed In Warwick Fire
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.
"While those in their 20s had sex more than 80 times per year, this declined to about 60 times a year by 45 and 20 times a year by 65", according to the study.
One factor behind the decline is today's higher percentage of unpartnered people - who tend to have sex about half as frequently as partnered people. Pornography consumption has increased - since watching porn is a typically a "solitary activity", the "frequency of intercourse is negatively correlated" with the frequency of porn watching.
Among them, the team suggests the rise of social media and other forms of entertainment such as Netflix could be replacing time spent with a partner, while an increase in depression or decrease in marital satisfaction could also be at play. "This is certainly novel and surprising to me", Dr. Sherman says. "That's one of the big things that has changed over the course of time - men and women's gender roles both at work and at home", he said.
Then there's generational differences.
According to a new study released in the Archives of Sexual Behavior, adults in the US are having less sex than they did 25 years ago.
"Are they less happy and thus having less sex or are they having less sex and therefore less happy?"