(Reuters) - The long-discredited notion that rape victims cannot become pregnant - a claim that pushed Republicans to repudiate one of their own U.S. Senate candidates on Monday - dates back centuries to when human reproduction was hardly understood.
But the medieval theory has surfaced in 21st century political discourse as a result of the U.S. abortion wars.
Writers from the Middle Ages and modern politicians alike have based their arguments on the idea that a trauma of the magnitude of rape can shut down the body's reproductive system.
The combination of misunderstanding and cherry-picked science even led some to conclude that a woman who says she was raped yet becomes pregnant must have been lying about the attack. Modern proponents of the claim repeat it despite empirical research showing that rape victims are at least as likely to become pregnant as women who have consensual sex, and possibly more likely.
Representative Todd Akin, the Republican candidate for the U.S. Senate in Missouri, spurred new outrage on the subject when he told a St. Louis television station he does not support abortion for rape victims because "if it's a legitimate rape, the female body has ways to try to shut that whole thing down."
Akin, a member of the House science committee, apologized on Monday for his statement, calling it "ill conceived" and "wrong." Senior Republicans scrambled to distance themselves from the comments a week before the party holds its presidential nominating convention in Florida.
The claim that rape is unlikely to lead to a pregnancy has "no biological plausibility," said Dr. Barbara Levy, vice president for health policy at the American Congress of Obstetricians and Gynecologists. The claim is "not grounded in any physiology or scientifically valid data."
Akin is not alone in his view about rape and pregnancy, however. It dates at least to medieval times, when a 13th century English legal tome called Fleta asserted that pregnancy was prima facie evidence against a charge of rape, "for without a woman's consent she could not conceive."
A 19th century book, "Elements of Medical Jurisprudence" by Samuel Farr, said that conception is unlikely "without an excitation of lust, or the enjoyment of pleasure in the venereal act." That reflected the common notion that pregnancy requires a woman, like a man, to reach orgasm during intercourse.
Both early references were noted by The Guardian newspaper in a blog post on Monday.
In fact, "human ... female orgasm is not necessary for conception," explained a 1995 paper in the journal Animal Behaviour, one of many studies reaching the same conclusion.
In more modern times, the rape-pregnancy claim seems to have been linked to the fact that stress can decrease fertility.
"Mental stress can temporarily alter the functioning of your hypothalamus - an area of your brain that controls the hormones that regulate your menstrual cycle," explains the Mayo Clinic in a publication about infertility. "Ovulation and menstruation may stop as a result."
But the stress that reduces fertility is the chronic kind that occurs over months or years, not the acute trauma of a rape.
"A woman who is raped at a vulnerable time in her menstrual cycle is as likely to conceive and retain a pregnancy as a woman who was voluntarily attempting pregnancy," said ACOG's Levy. "There's absolutely no validity to any sort of theory that the trauma related to rape - or to any thing else for that matter - would shut down ovulation that has already begun."
Physicians and researchers had long thought that conception occurs when sperm encounter an already-waiting egg. Recent research has shown that in fact sperm do the waiting, remaining in the woman's uterus or fallopian tubes until an egg is released from the ovaries.
Although the trauma of rape might impair a woman's fertility months or years later, said Levy, "you're not going to interrupt something (like the release of an egg) that's already started."
Numerous studies support that. In a 1996 study in the American Journal of Obstetrics & Gynecology, researchers surveyed 4,008 American women for three years. Among women in their prime reproductive years, 12 to 45, 5 percent of rapes resulted in pregnancy, mostly among adolescents. One-third "did not discover they were pregnant until they had already entered the second trimester," the researchers found, concluding that "rape-related pregnancy occurs with significant frequency."
It may occur with greater frequency than after consensual sex. Indeed, evolutionary psychologists - who seek to explain human behavior by imagining what actions might have helped our ancient ancestors survive and reproduce - say the reason rape has been so endemic throughout history is precisely because it often leads to pregnancy: men who commit that crime, goes the argument, were more likely to have progeny, passing along their "rape genes" to the next generation.
While the explanation for rape has been discredited, the fact that rape often leads to pregnancy has not been. In a 2003 study in the journal Human Nature, researchers found that 6.4 percent of rapes in the hundreds of women they surveyed caused pregnancy; that compares to a rate roughly half that with consensual intercourse. In Mexico, rape crisis centers have reported that some 15 percent of rapes cause pregnancy.
The rate may be high because rape victims are less likely to be using contraception at the time of the crime than are women in a relationship, who can also choose to forego sex during fertile periods in their reproductive cycle if they do not want to conceive.
Sharon Begley reported from New York and Susan Heavey from Washington; Editing by Doina Chiacu and Michele Gershberg