Some women have fantasies of being forced into sex. At first glance, rape fantasies make no sense. Why fantasize about something that in real life would be traumatic, repugnant, and life-threatening?

But on closer examination, such fantasies are not unusual. Many men daydream about getting the girl by rescuing her from a dangerous situation, without the slightest wish to confront armed thugs or be trapped in a fire on the 23rd floor.

Fantasies allow us to "experience" the outer limits of our imaginations safely, with no risk; and for some people, that includes fantasies of coerced sex. In fantasy, everything is permitted and nothing is wrong.

But rape fantasies raise thorny issues. Many women who have them can't shake the feeling that they are abnormal or perverted.

From 1973 through 2008, nine surveys of women's rape fantasies have been published. They show that about four in 10 women admit having them (31 to 57 percent) with a median frequency of about once a month. The actual prevalence of rape fantasies is probably higher because women may not feel comfortable admitting them.

For the latest report, in the Journal of Sex Research, psychologists at North Texas University asked 355 college women: How often have you fantasized being overpowered, forced, raped by a man or woman to have oral, vaginal, anal sex against your will?

Sixty-two percent said they'd had at least one such fantasy. But responses varied depending on the terminology used. When asked about being "overpowered by a man," 52 percent said they'd had that fantasy, the situation most typically depicted in women's romance fiction. But when the term was "rape," only 32 percent said they had had the fantasy. These findings are in the same ballpark as previous reports.

The frequency of rape fantasies varied substantially. Thirty-eight percent of respondents never had them. Of those who did, 25 percent reported such fantasies less than once a year. Thirteen percent had them a few times a year, 11 percent once a month, 8 percent once a week, and 5 percent several times a week. (Twenty-one percent of the respondents said they'd been sexually assaulted in real life.)

Rape fantasies can be either erotic or aversive. In erotic fantasies, the woman thinks: "I'm being forced and I enjoy it." In aversive fantasies, she thinks: "I'm being forced and I hate it." Forty-five-percent of the women in the recent survey had fantasies that were entirely erotic. Nine percent were entirely aversive. And 46 percent were mixed.

Rape or near-rape fantasies are central to romance novels, one of the perennial best-selling categories in fiction. These books are often called "bodice-rippers" and have titles like Love's Sweet Savage Fury, which imply at least some degree of force. In them, a handsome cad becomes so overwhelmed by his attraction to the heroine that he loses all control and must have her, even if she refuses—which she does initially, but then eventually melts into submission, desire, and ultimately fulfillment.

Romance novels are often called "porn for women." Porn is all about sexual fantasies. In porn for men, the fantasy is sexual abundance—eager women who can't get enough and have no interest in a relationship. In porn for women as depicted in romance novels, the fantasy is to be desired so much that the man loses all control, though he never actually hurts the woman, and in the end, marries her.

What do rape fantasies mean? In my opinion, they are no different from any other fantasies. They are neither wrong nor perverted. They imply nothing about one's mental health or real-life sexual inclinations. They just happen, to somewhere around half of women. If you have such fantasies and feel bad about them, I can't tell you how to feel. But I can assure you that you are not alone. Rape or near-rape fantasies are surprisingly common. What do you think?