For what researchers say is an array of reasons — including technology, heavy academic schedules and an overall slower-motion process of growing up — millennials and now Gen Zers are having less sex, with fewer partners, than their parents’ and grandparents’ generations did. The social isolation and transmission scares of the COVID-19 pandemic have no doubt played a role in the shift. But researchers say that’s not the whole story: The “no rush for sex” trend predates the pandemic, according to a solid body of research.

UCLA has been tracking behavioral trends for years through its annual California Health Interview Survey, the largest state health survey in the nation. It includes questions about sexual activity. In 2021, the survey found, the number of young Californians ages 18 to 30 who reported having no sexual partners in the prior year reached a decade high of 38%. In 2011, 22% of young people reported having no sexual partners during the prior year, and the percentage climbed fairly steadily as the decade progressed.

California adults ages 35 to 50 who participated in UCLA’s 2021 survey also registered an increase in abstinence from 2011 to 2021. But with the percentage of “no sex” respondents rising from 9% to 14% during that time frame, the increase was not as pronounced.

The broader trend of young adults forgoing sex holds true nationally.

The University of Chicago’s General Social Survey — which has been following shifts in Americans’ behavioral trends for decades — found that 3 in 10 Generation Z males, ages 18 to 25, surveyed in 2021 reported having gone without sex the prior year. One in four Gen Z women also reported having had no sex the prior year, according to Jean Twenge, a San Diego State University psychology professor who reviewed the data for her book “Generations.”

In an age where hook-ups might seem as unlimited as a right swipe on a dating app, it’s easy to assume that Gen Z “should be having the time of their lives sexually,” Twenge said.

But that’s not how it’s playing out. Twenge said the decline has been underway for roughly two decades.

She attributed the slowdown in sexual relations most significantly to what she calls the “slow-life factor.” Young people just aren’t growing up as fast as they once did. They’re delaying big milestones such as getting their driver’s licenses and going to college. And they’re living at home with their parents a lot longer.

“In times and places where people live longer and education takes longer, the whole developmental trajectory slows down,” she said. “And so for teens and young adults, one place that you’re going to notice that is in terms of dating and romantic relationships and sexuality.”

A slight majority of 18- to 30-year-olds — about 52% — reported having one sexual partner in 2021, a decrease from 2020, according to the UCLA survey. The proportion of young adults who reported having two or more sexual partners also declined, from 23% in 2011 to 10% in 2021.