“The data also shows that 85 percent of young people in their sample are sexually active in the last 12 months,” he said. “The vast majority of American youth are sexually active, and that’s the reality we need to take seriously.”

The paper, part of a series of three reports that Dr. Twenge and her colleagues have published or plan to publish about young people’s sexual behavior, relied on the General Social Survey, a nationally representative survey of American adults that has been taken regularly since 1972. (The researchers used annual and biennial surveys taken from 1989 to 2014.)

It found that about 6 percent of young adults — defined as being between the ages of 20 and 24 — who were born between 1965 and 1969 reported having no sexual partners after age 18. By contrast, 15 percent of young adults born between 1990 and 1994 reported having no sexual partners after turning 18.

The increase in sexual inactivity was far more notable among women than men, and was only significant for those without a college education, the paper said. The phenomenon was not observed among survey participants who had attended college.

The trend of more sexless lives was also nonexistent among black Americans, according to the paper, which did not break out information about other racial groups. The trend was “larger and significant” among those who attended religious services; it was present but “not significant” among those who did not.

The paper’s findings were echoed by the national Youth Risk Behavior Survey, conducted by the Centers for Disease Control and released in June, which found that the number of sexually active high schoolers had decreased from 1991 to 2015.