FILE - In this Aug. 10, 2012 file photo, Todd Akin, Republican nominee for the Missouri U.S. Senate seat, speaks to reporters after addressing the Missouri Farm Bureau in Jefferson City, Mo. Until this week, Rep. Todd Akin was virtually unknown beyond his district, associated more with his deep religious convictions than any legislative achievements. Long before his comments about women's bodies and "legitimate rape" made him a flashpoint in the fall campaign, this congressional backbencher was a favorite among home-schooling organizations and conservative church groups. (AP Photo/St. Louis Post-Dispatch, Christian Gooden) EDWARDSVILLE INTELLIGENCER OUT; THE ALTON TELEGRAPH OUT

Representative Todd Akin of Missouri made two mistakes when he claimed "From what I understand from doctors, it's really rare. If it's a legitimate rape, the female body has ways to try to shut the whole thing down."

His first mistake was linguistic. The term "legitimate rape" is an oxymoron. "Legitimate rape" does not exist. End of story.

His second mistake was believing the "doctors" who gave him the idea that the female body can "shut the whole thing down." (According to the LA Times, the "doctors" was one Jack C. Wilke, a prominent anti-abortionist physician and past president of the National Right To Life Committee.)

But is there evidence to support this claim? After all, in some species, females do have the ability to rid their body of unwanted semen. Chickens, for example. Researchers at the University of Sheffield studied this phenomenon in flocks of free-ranging chickens. The scientists found most sexual interactions between roosters and hens are coerced. (I was not surprised; I used to raise chickens.) I was surprised, however, to learn that hens had the ability to eject sperm from their reproductive tracts. Further, hens are choosy about it. The researcher found that hens were much more likely to eject the semen of low-ranking "forced copulators" than the rapists at the top of the social hierarchy who presumably have better genes. In spiders, this is called "sperm dumping."

So a biological anti-pregnancy mechanism exists in chickens and spiders that kicks in during forced copulations. Is there any evidence that a similar mechanism exists in human females? Are Representative Akin and Dr. Wilke right?

No. In fact, human females seem to be more likely to get pregnant when raped than when they have consensual sex!

The Evidence That Rape More Often Leads to Conception

The evidence that women are more likely to get pregnant by rape is described in this article in the journal Human Nature by Jonathan and Tiffani Gottschall. Between November 1995 and May 1996, the National Institute of Justice, the National Center for Injury Prevention and Control and the Centers for Disease Control jointly conducted a telephone survey of 8,000 randomly selected American women. Some of the questions dealt with rape and subsequent pregnancy. Four hundred and five of the women indicated they had been raped exactly one time. The Gottschalls used this data to compare the chances of getting pregnant via a rape versus consensual sex.

The Results

Twenty six of the women who had been raped became pregnant -- a pregnancy rate of 6.42 percent. (This increased to 8 percent when adjusted for contraceptive use.)

If you think that this is a low pregnancy rate, you are wrong. Contrary to what most people think, humans are among the most infertile of species. The most widely cited research on this topic was a study of the relationship between pregnancy and the timing of intercourse among women who were trying to get conceive here. Naturally, the researchers found that the odds of pregnancy changed with the women's menstrual cycles. Among the "regular cyclers," a woman's chances of conceiving ranged from a high of 9 percent if they had sex on day 13 of their cycle to 0 percent when they were having their period. Over the entire cycle, however, a woman's chances getting pregnant from one act of intercourse was 3.1 percent. This finding is consistent with other studies of human fertility.

In short, women are at least twice as likely to conceive as a result of rape than by consensual sex.

Why Should Pregnancy Be More Common During Rape?

Why does rape result in increased risk of pregnancy? This remains an open question. The Gottschalls reject some possibilities. These include the idea that rape induces ovulation, (copulation does stimulate the release of an egg in some species), that rapists have more virile sperm, and that rapists possess a special capacity to detect women who are ovulating. They do toss out a couple of conjectures they consider reasonable. The first is that rapists target women who are particularly fertile based on factors such as beauty and age. The second is that women unconsciously "broadcast cues about their ovulatory status that men are capable of registering and interpreting." These hypothetical unconscious cues could be physiological (e.g. body odor, subtle changes in skin tone) or behavioral. To their credit, however, the authors admit that there is little evidence to support their conjectures.

So, sorry Rep. Akin... there is no evidence that human females have a biological mechanism that prevents pregnancy during rape. If anything, it appears that women who are sexually assaulted are MORE rather than less likely to conceive.

Maybe you should start seeing different doctors.

Hal Herzog is professor of psychology at Western Carolina University where he teaches a course in human sexuality. He is author of Some We Love, Some We Hate, Some We Eat: Why It's So Hard To Think Straight About Animals.