Honest, evidence-based answers to questions about anal sex are difficult to come by. You’d think we would know more about a behavior that’s become a common part of Americans’ sex lives – one that, for all its potential pleasures, remains among the riskiest sex acts when it comes to spreading sexually transmissible infections (STI) including HIV. Yet there is strikingly little scientific research on anal sex. The list of what we don’t know about anal sex is far longer than the list of what we do. This makes it difficult for sex educators to feel truly confident in answering people’s very real and important questions.

This is also what made the recent University of Zagreb study so valuable. They surveyed more than 2,000 women ages 18 to 30 about their experiences with anal sex. Building on limited early research about anal pain among men who have sex with men, the researchers asked about women’s experiences with pain. This was critical because, as much as we often talk about anal sex possibly hurting, and lubricant possibly minimizing pain or discomfort during anal sex, there is almost no research on women’s experiences of anal sex. One exception is a study that I conducted with my research team at Indiana University in which we gave six different lubricants to more than 2,400 women and asked them to use them during their masturbation, vaginal sex and/or anal sex activities. Among our interests was whether using a lubricant helped to make sex – including anal sex – more pleasurable, more satisfying and less painful (it did).

The Zagreb team found that about half of women (49 percent) stopped their first experience of anal intercourse because it was too painful to continue – not surprising considering 52 percent of women report not even using lubricant when they first had anal sex! An additional 17 percent of women also experienced pain or discomfort during their first anal sex, but didn’t stop their partner. Only about one-quarter of women said their first experience with anal sex was pleasant.

That said, nearly two-thirds tried anal sex again (hopefully this time with lubricant), continuing on another occasion. Those women who found it positive, pleasurable and pain-free were more likely to try it again. About 9 percent of women who had anal sex at least twice in the past year said that they experienced pain every single time. Based on what I know about women who experience pain during vaginal intercourse, my guess is that chronic pain during anal sex is even more common – perhaps hovering in the 10-15 percent range – once the women who actively avoid it because it always hurts are taken into account.

This 9 percent figure is important. It tells us that a similar proportion of women experience pain consistently during anal sex as experience pain consistently during vaginal penetration. That’s right: Somewhere around 10 percent of women experience pain during vaginal intercourse or even during daily activities like sitting down or riding in the car. The 9 percent number is also close to the 10-14 percent range that’s been identified as the proportion of men who have sex with men who experience pain during anal sex. And though the Zagreb study asked women what sense they made of their pain (most blamed themselves or their sexual practices, suggesting their pain was linked to not feeling fully relaxed, inadequate anal foreplay, or not using sufficient lubricant), the fact is that we still don’t know clinically what’s causing their pain.

It may be that, like the vagina and vulva, the anuses of some women and men respond to touch or penetration in painful ways and for unknown reasons. It may be that some of these women and men have skin disorders, such as lichen sclerosus, which can affect genital skin (including anal skin), increasing the likelihood of discomfort, pain or tearing. Certainly lack of information and education is at the root of some people’s pain, but it’s probably not the primary cause for everyone. Some women and men do everything “right” – they use gobs of lubricant, they start out slowly, relax, communicate well with their partner, avoid desensitizing or numbing gels/creams – and yet it still hurts. Do they have an underlying medical condition that’s contributing to the pain? Wonky nerve receptors that scream in pain rather than perceive penetration as neutral or pleasurable? We don’t know.

In case you’re wondering, we also don’t know much about the long-term effects of anal intercourse. Certainly enough people have been having anal sex over enough generations that if anything were seriously dangerous about anal sex, we would know it by now. But as for questions about how regular anal sex, rough anal sex or insufficiently lubricated anal sex might ultimately affect the likelihood of a woman experiencing rectal prolapse or of a woman or man experiencing various anal or rectal health issues, we don’t know because no one has studied these kinds of things. It’s 2012 and pretty much all we know about anal sex is that lots of people have tried it, there’s a higher degree of risk for STI/HIV transmission (compared to vaginal sex or oral sex), many people have found it painful on occasion, many people also find it pleasurable sometimes, and about one in 10 women and men experience pain during anal sex on a regular basis. Much of the research involving HPV and anal cancer is focused on men who have sex with men – which is needed — even though more women in the U.S. have received anal sex than the number of men who have received anal sex. That’s not to say that anal cancer isn’t important to study among men – it very much is the case – but women get anal cancer, too, and we need to know more about risk and protective factors (related: check out this I Have Butt What? blog by a brave anal cancer survivor named Michelle).

Knowledge gap, anyone?

Even though most people who have had anal sex engage in it only occasionally, anal sex is a fairly common practice. And if people are going to engage in sexual behavior, then they deserve enough information to help make that behavior as safe, pleasurable and satisfying as possible. To do so, science has to catch up and taboos have to dissipate enough so that more people feel comfortable talking about it and sharing their experiences.

Debby Herbenick, PhD, MPH is co-Director of The Center for Sexual Health Promotion at Indiana University’s School of Public Health-Bloomington, a sexual health educator at The Kinsey Institute, and author of five books about sex and love. Her most recent is Sex Made Easy: Your Awkward Questions Answered for Better, Smarter, Amazing Sex (Running Press, 2012).

Check out this TEDx Talk by Debby Herbenick  “Why Your Bed is the Ultimate Treehouse”