A new study conducted by a San Diego State University psychologist shows U.S. couples are having less sex now than in the 1990s and 2000s, and its younger generations who are cooling down the American boudoir.

Married or cohabited couples had sex 16 fewer times per year in 2010-2014 compared to 2000-2004 and nine times fewer than in the late 1990s, Science Daily reported Tuesday.

“[A] critical factor appears to be birth cohort, with later-born generations having sex less often than those born earlier in the 20th century,” the website added.

“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,” researcher and study author Jean Twenge said, Science Daily reported. “That’s partially because fewer iGen’ers and Millennials have steady partners.”

The study, titled “Declines in Sexual Frequency among American Adults, 1989–2014” and published in the journal Archives of Sexual Behavior, can be purchased online here.