Scientists out of Widener University in Pennsylvania have come to a rather astonishing conclusion: Americans are having far, far less sex than we did 25 years ago. And the researchers say they think they understand why, and what can be done about the issue.

A study of 30,000 U.S. adults showed that the number of 18 to 29 years old who are not living with a partner jumped from 48 percent in 2006 to 64 percent in 2014, and the average rate of sex was 64 times a year in 2002, declining to 53 times per year by 2014. However, sex activity remained steady among those without partners compared to married couples or those who lived together.

Researchers blamed two factors for the phenomenon: an increasing number of people in steady relationships, and a decline in sexual frequency among such people.

“These data show a major reversal from previous decades in terms of marriage and sex,” said Jean M. Twenge, the study’s lead author and professor of psychology at San Diego State University. “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.”

“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.”