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BLOOMINGTON, Ind. — A study by Indiana University researchers found that sexual frequency is on the decline in the United States, consistent with the rest of the world.

“Our study adds to a growing body of research that has reported on declines in sex,” said Tsung-chieh “Jane” Fu, a research associate at the IU School of Public Health-Bloomington, in a press release. “The declines in partnered sexual activity seen in our study are consistent with findings from studies in the U.K., Australia, Germany and Japan.”

Debby Herbenick, a professor of sexual and reproductive health at the School of Public Health co-led the study with Fu. She had said the decreases in sexual engagement are likely to be caused by several different factors.

“The decreases are not easily explained by a single shift, such as health status, technology, access to pornography or stress,” Herbenick said in a press release. “There are likely multiple reasons for these changes in sexual expression, and we need more research to understand how these changes may be related to changes in relationships, happiness and overall well-being.”

Herbenick said that the study of sexual frequency is particularly important in light of how the COVID-19 pandemic impacted relationships. Declining sexual activity among adults has several consequences for human health and fertility, which have only been worsened by pandemic restrictions.

The study, published in the Archives of Sexual Behavior, is the first to include a comprehensive assessment of “diverse sexual behaviors.” They had obtained their information from U.S. participants between the ages of 14 to 49 from the years 2009 to 2018 in a confidential, nationally representative survey conducted online. A total of 1,647 adolescents between ages 14 to 17 and 7,055 adults between ages 18 to 49 were included in the survey.

“The National Survey of Sexual Health and Behavior includes detailed data on a variety of sexual behaviors, so we could examine more precisely whether declines in vaginal intercourse might be explained by increases in other sexual behaviors, such as oral sex,” Herbenick had said. “However, we found that was not the case. Rather, we found that from 2009 to 2018, fewer adults engaged in a range of partnered sexual activities. We were also surprised to find that, among adolescents, both partnered sex and solo masturbation had declined.”

When comparing adult participants in the 2009 survey to adults in the 2018 survey, the 2018 group were “significantly more likely” to report no penile-vaginal intercourse within the prior year. Participants were also significantly less likely to report getting engaged in any other sexual behaviors examined in the study, such as oral or anal sex.

“More studies are needed to understand if this decline is associated with the emergence of other types of sexual activities in recent years, such as the adverse impact of what some people call aggressive or rough sex,” Fu said.

The study’s findings also noted a decline in sexual frequency among adolescents, according to Herbenick. In 2009, 28% of young men and 49% of young women had reported participating in neither solo masturbation nor partnered sexual behavior. In 2018, that has increased to 43% of young men and 74% of young women.

“Many studies haven’t included those under age 16 or 18, so our study expands what we know about younger adolescent behavior and how we think about adolescent sexual development,” Herbenick said.

Researchers note that a number of societal and cultural changes may have been affecting sexual behavior among young people, including widespread internet access, decreased alcohol use, increased conversations about consent and more young people identifying as LGBT, including asexual identities.

While the current findings will help inform sexual health researchers, clinicians and educators, Herbenick said she hopes the study will lead to new investigations into areas such as how people are feeling about their sexual lives and how those feelings may shape their choices about sex.