When a woman is raped, she will almost always release a chemical from her brain that prevents conception taking place.
What is the evolutionary advantage here?
Cite please. Sounds like nonsese to me.
I would also like to see some kind of cite for this, I’ve never heard of this before, on the contrary, many people who are pro-choice(me included, but I don’t want to start a debate, there are plenty of those already) use rape as a specific example of a good case for an abortion. As well as this type of pregnancy being the plot device for the overwhelming majority of movies on the Lifetime channel.
I, too, have never heard of this. I have heard, however, that the hormones released under extreme stress (adrenalin, cortisol, etc.) can disrupt the menstrual cycle sometimes. Perhaps that is the genesis of this rumor. In any case, this mechanism would not prevent pregnancy if the egg (or the hormone which causes its release) was already free in the body at the time of the rape.
There is a good evolutionary reason for a woman not to want to have a rapist’s kid. Children require a huge investment of time and resources that would be better spent on a child whose father would help raise it. Because of the size of the investment, a lot of female animals choose their mates carefully, and, among those where the father helps raise the offspring, ability to provide is a common criterion. Rapists tend not to hang around and help. But if there’s a mechanism in humans that specifically causes a woman to reject a rapist’s sperm (as opposed to mechanisms like cervical mucus thinning that help sperm from someone who gives her an orgasm), it doesn’t seem to work very well.
The premise itself is false. It is a cruel myth. Re-inforced a few years ago by one of our more moronic Senators, who claimed that “if the woman’s juices don’t flow, pregnancy cannot take place”. Is statement was pure nonsense.
Qadgop, MD
Bah.
This is, quite simply, not true. Not one even little bitty bit. Not even “factoid” status, like “Yaks have pink milk.” You might as well ask us:
Agreed, the OP is odd. There is no such mechanism.
But why not? The evolutionary advantage would be huge, for the reasons cited. Or is rape uncommon enough that such a mechanism doesn’t have enough selective pressure to drive it?
He’s not the only one…and a doctor yet! 
http://www.ncinsider.com/insider/1998/october/insd1009.html
"An eye surgeon running for U.S. Senate in Arkansas said it was rare for women to get pregnant from rape because of physiological changes during attacks that can block conception. An adrenalin rush triggered by fear causes hormonal changes that block a woman’s ability to conceive during a violent attack, Republican Fay Boozman said. He denied a newspaper report that he attributed the phenomenon to “God’s little protective shield.”
His remarks sparked a heated response from Democrats and some doctors and was reminiscent of the reaction when state Rep. Henry Aldridge, R-Pitt, said several years ago that women can’t get pregnant from rape because the “juices” don’t flow. Boozman, a state senator running to succeed Democratic U.S. Sen. Dale Bumpers, said he would support an abortion ban that makes exceptions for rape and incest because such pregnancies rarely occur. His Democratic opponent Blanche Lincoln, a former congresswoman who left in 1996 to raise twins, said Boozman’s remark “shows a real lack of sensitivity.” Two obstetricians said there was no scientific basis for Boozman’s claim that it is rare for women to become pregnant from rape."
Never heard of it, but I’ll give a WAG just in case it is true:
Maybe nature thinks men who don’t get women to multiply with voluntarily must have “bad” genome (if they had not, they wouldn’t have to rape), so you better don’t get their children.
[sigh]
There’s no “in case it’s true”. It’s not true. Period.
Think about the basic premise: That a woman can only get pregnant if she consents to sex, or has an orgasm, so the “juices flow”. What utter BS. Women have been getting pregnant for thousands of years, with or without consenting to sex, with or without an orgasm, with or without their “juices flowing”. All those Victorian women who “closed their eyes and thought of England”–they didn’t have any trouble getting pregnant. I mean, I didn’t notice that there was a huge drop in the birth rate for the Western world for the years from about 1840 to 1960. :rolleyes:
Rape victims have also been getting pregnant for thousands of years. If rape victims “rarely get pregnant because they release a brain chemical that prevents conception”, then why in the world did we have that huge controversy a while back about a “morning after” contraceptive pill for rape victims? I mean, if their brain chemicals were going to do the job, why go through all the hassle of research and development, and the bad publicity from anti-abortion activists? Why would we even need to deal with the problem?
The Catholic Church wouldn’t even need to address this, the whole issue would be moot, if a rape victim’s body could take care of it herself “almost always”. It would be a minor side issue in American life, on a par with the issue of “what to do about people on life support”, not a mainstream front-page hot-button issue. The reason it IS a mainstream hot-button issue is precisely because rape victims DO get pregnant, and “frequently”, not “rarely”.
I mean, geez, people, it’s just a dumb thing to say, on a par with, “You can’t get pregnant the first time you do it”, or “You can’t get pregnant if you do it standing up”.
Okay? 
Well, traits don’t just happen because they might have some benefit. You need a random non-fatal mutation in a female that could continue breeding - and it would have to be strong enough to select amongst a population to gain a foothold.
Well, duh, it’s clearly because they’re Satanists that want to kill little 16-celled babies.