Todd Akin, the Republican candidate for senate in Missouri, said in an interview released this weekend that he did 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."

The idea that rape victims cannot get pregnant has long roots. The legal position that pregnancy disproved a claim of rape appears to have been instituted in the UK sometime in the 13th century. One of the earliest British legal texts, Fleta, has a clause in the first book of the second volume stating that:

"If, however, the woman should have conceived at the time alleged in the appeal, it abates, for without a woman's consent she could not conceive."

This was a long-lived legal argument. Samuel Farr's Elements of Medical Jurisprudence contained the same idea as late as 1814:

"For without an excitation of lust, or the enjoyment of pleasure in the venereal act, no conception can probably take place. So that if an absolute rape were to be perpetrated, it is not likely she would become pregnant."

This "absolute rape" is not quite the same as Akin's "legitimate rape". Akin seems to be suggesting that the body suppresses conception or causes a miscarriage, while the earlier idea of Farr relates specifically to the importance of orgasm. Through the medieval and early modern period it was widely thought, by lay people as well as doctors, that women could only conceive if they had an orgasm.

The biological basis for this idea is what the historian Thomas Laqueur has termed the "one sex system". The one sex system suggests that women's reproductive organs are fundamentally based on men's reproductive organs, so the vagina is represented as an inverted penis, the ovaries are testes and so on.