There are a lot of emotions commonly associated with sex: love, happiness, excitement, maybe even relaxation. But for many women, one sexual feeling that comes to mind is a darker one: fear.

In a recent study, Debby Herbenick, a professor and sex researcher at the Indiana University School of Public Health, found that nearly a quarter of adult women in the United States have felt scared during sex. Among 347 respondents, 23 described feeling scared because their partner had tried to choke them unexpectedly. For example, a 44-year-old woman wrote in that her partner had “put his hands on my throat to where I almost couldn’t breathe.”

Sex can involve consensual choking, but that’s not what’s going on here, as Herbenick explained to an audience during a panel at Aspen Ideas: Health, which is co-hosted by the Aspen Institute and The Atlantic. Instead, “this was clearly choking that no one had talked about it and it got sprung on somebody,” she said. Many sexual-assault cases among students at her university now center around nonconsensual choking. According to her research, 13 percent of sexually active girls ages 14 to 17 have already been choked.

The reason such young kids know about such a violent sexual act is likely porn, said Dan Savage, a sex columnist and the host of Savage Lovecast, who was also on the panel. And that’s not the only disturbing change that might be attributable to porn, added Kate Julian, a senior editor at The Atlantic and the author of a recent magazine cover story on sexual behavior among young people. For her story, she talked with many women who said their male partners seemed to be taking a cue from what they had seen in porn, pounding away or penetrating then anally when they weren’t ready.