Millennials, or people born in the 1990s, are having much less sex than previous generations did when they were young iStock

Americans are having less sex than they used to, according to data from a national survey of 26,000 people.

The decline is due to married couples today and millennials being less sexually active than previous generations. On average, people reported having sex an average of nine fewer times a year in 2010-14 compared with 1995-1999, according to a paper published in the Archives of Sexual Behaviour.

Part of the decline in sex is that there are more young people not in a relationship - so have less sex (on average) than people in relationships, said Jean Twenge of San Diego State University.

Bucking the trend

"These data show a major reversal from previous decades in terms of marriage and sex," said Twenge. "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."

Twenge has shown in previous studies that millennials are also having fewer partners. Based on their current rate, they are likely to have had 8 partners by the time they reach 45. Baby Boomers, born after the Second World War, had an average of 11 partners by the time they were 45.

"Despite their reputation for hooking up, millennials and the generation after them – known as iGen or Generation Z – are actually having sex less often than their parents and grandparents did when they were young," said Twenge. "That's partially because fewer iGen'ers and Millennials have steady partners."

elderly couple drinking
Baby boomers and people born in the 1930s had more sex when they were young than millennials Istock

Despite not having as much sex as their parents did when they were young, Millennials are still having more sex than anyone else right now. Americans in their 20s had sex about 80 times a year, compared with people in their 60s having sex about 20 times a year.

But it's not all down to millennials. There has also been a significant drop in sex in older couples. Married couples in the US report having sex an average of 16 fewer times a year in 2010-14 compared with 2000-04.

"Older and married people are having sex less often – especially after 2000," Twenge said. "In a previous paper, we found that the happiness of adults over age 30 declined between 2000 and 2014.

"With less sex and less happiness, it's no wonder that American adults seem deeply dissatisfied these days."