NO WAY anyone knows. Although their are biological reasons like those noted in other answers, reliable estimates are not available to make a determination one way or the other.
What was the Question?
Todd Akin's comments were taken out of context. Here is the full text of what Todd Akin said about rape, and the (video link).
Charles Jaco: Okay, so if an abortion can be considered in the case of, say, tubal pregnancy or something like that, what about in the case of rape? Should it be legal or not?
Todd Akin: Well, you know, people always want to try and make that as one of those things: "Well, how do you - how do you slice this particularly tough sort of ethical question?"
It seems to me, first of all, from what I understand from doctors, that's really rare. If it's a legitimate rape, the female body has ways to try to shut that whole thing down. But let's assume that maybe that didn't work or something.
You know, I think there should be some punishment, but the punishment ought to be on the rapist and not attacking the child.
Todd Akin's statement taken out of context seems ridiculous, because it is easy to find a single counter example of a woman who got pregnant from a rape. This isn't even close to what Todd Akin claimed. Todd Akin was arguing that you shouldn't use instances of rape to justify legalizing abortion, because pregnancy due to rape is really rare, he is not saying that pregnancy is never the result of rape. This is clarified in Todd Akin's apology video.
Say it ain't So?
Todd Akins is talking about false accusations when talking about "legitimate" rape. Todd Akins clarified his position on Aug 21, on the Mike Huckabee show (audio available).
You know, Dr. Willke has just released a statement and part of his letter, I think he just stated it very clearly. He said, of course Akin never used the word legitimate to refer to the rapist, but to false claims like those made in Roe v. Wade and I think that simplifies it….. There isn’t any legitimate rapist…. [I was] making the point that there were people who use false claims, like those that basically created Roe v. Wade.
Women do lie about rape. What percentage of rapes are false accusations? Once again, the numbers vary greatly. The US Department of Justice released a report Convicted by Juries, Exonerated by Science places the false accusation percentage between 20%-40% using DNA testing.
Every year since 1989, in about 25 percent of the sexual assault cases referred to the FBI where results could be obtained (primarily by State and local law enforcement), the primary suspect has been excluded by forensic DNA testing. Specifically, FBI officials report that out of roughly 10,000 sexual assault cases since 1989, about 2,000 tests have been inconclusive (usually insufficient high molecular weight DNA to do testing), about 2,000 tests have excluded the primary suspect, and about 6,000 have “matched” or included the primary suspect.
The fact that these percentages have remained constant for 7 years, and that the National Institute of Justice’s informal survey of private laboratories reveals a strikingly similar 26-percent exclusion rate, strongly suggests that postarrest and postconviction DNA exonerations are tied to some strong, underlying systemic problems that generate erroneous accusations and convictions.
(Note: FBI statistics do not differentiate between accusers who lie, make an honest mistake. Neither does it preclude that a rape didn't take place, only that the person accused was innocent.)
False accusations will clearly affect any estimates of legitimate rapes, which will in turn affect any study based upon unsubstantiated claims of rape by the "victim." Todd Akin clarified what he meant when he said "legitimate rape," but lets explore some other possible explanations for what is legally a rape, instead of resorting to believing that Todd Akin must have meant women who weren't asking for it, didn't fight hard enough, or were the victims of war crimes. These cases might not cause a biological response in the victim, because the aren't forcible rape.
- Statutory Rape
- Incapacitated Rape
- Drug/Alcohol Facilitated Rape
32,101? How confident are you?
Finally, we come to the heart of the matter. Is a woman less likely to become pregnant if forcibly raped, than with consensual sex.
How confident should we be that 32,101 pregnancies are the result of rape each year? I wasn't able to access the study online, but you can glean enough information from the abstract to see some problems with the estimate.
- STUDY DESIGN: A national probability sample of 4008 adult American women ...
- Among 34 cases of rape-related pregnancy, ...
- The national rape-related pregnancy rate is 5.0% per rape among victims of reproductive age ... and estimated 32,101 pregnancies
As I said, my local library didn't have online access to the study, so I can't verify their exact numbers or how they arrived at 5.0%. I was able to find secondary sources that explained some of the numbers from the study:
In their methods they called their study the National Women’s Study, and say that they interviewed 4008 women in the first wave. Each wave had fewer women. They said they noted 616 instances of rape from the 3031 respondents in wave three. How then, do they then say:
“Analysis of the National Women’s Survey raw data (without statistical weighting required for determining representative population estimates) indicates that there were 34 cases of rape-related pregnancy. A total of 30 women reported one rape-related pregnancy and two additional women reported two rape-related pregnancies. Of the 34 cases 21% occurred when the victim was aged 12 to 15 years, 27% occurred among women aged 16 and 17, and 52% occurred after age 18.”
This study is extrapolating estimates for the USA reproductive female (1996) (page 44): 67,047,000 women age (12-45), with only 616 rapes resulting in 34 pregnancies from a sample of 4008-3031 women. Using a handy dandy online Confidence Interval calculator (Confidence Level 95%, Sample Size: 616, Population: 3031, Percent: 5 = CI: 1.54). This represents a slightly greater than 30% margin of error. (Note: Hopefully I am using the right (someone correct me if I am wrong) sample size 616, not 34. Since I think the sample is 616 women with 5% (34 women) answering "Yes" to the quality of becoming pregnant as a result of the rape. If I was supposed to use 34, then the CI=7.29 which would make the margin of error greater than the estimated 5% pregnancy rate)
One of the authors of the study is Dean G. Kilpatrick, should know better (I'm just an amateur scientist). He should be more cautious when using figures that have a >30% margin of error. He is at least aware of other measures of rape, since he wrote a white paper Making Sense of Rape in America. One of the data sources used to measure rape listed on page 3 is National Violence Against Women Survey (NVAWS), which has a nice little chart (page 7) showing estimated standard error. The NVAWS also notes:
Estimates with RSEs [Ratio Standard Error] that exceed 30 percent were deemed unstable and were not tested for statistically significant differences between or among groups. These estimates have been identified in the tables and should be viewed with caution.
I guess Kilpatrick wasn't cautious, since he uses a similar figure in the white paper (rape was 0.27%) that the NVAWS cautions about (page 14, Exhibit 14 & 15):
Because annual rape victimization estimates are based on responses from only 24 women and 8 men who reported having been raped, they should be viewed with caution.
Update: I have included another secondary reference for the Johnathan Gottschall estimate provided in the accepted answer (Reference 7), that challenges the 6.4% rape pregnancy rate claimed in that study.
The Gottschalls’ article looked at the National Violence Against Women survey, which polled 8,000 women. Of the 405 rape victims in the poll who fit the studies’ methodology, about 6.4 percent of them became pregnant. The Gottschalls think the best studies show consensual sex resulting in pregnancies about 3 percent of the time. Why the gap?
Using data from the (National Violence Against Women Survey) own Relative Standard Error chart, you will note that this gives an RSE of >30%, which means that Gottshall ignored the NVAWS recommendations about making statistical comparisons regarding subsets with such high RSE. In the politico article (linked above as the Update: secondary ref), Jonathan Gottschall even admits:
But Gottschall did warn that methodological problems mean the numbers “aren’t carved in a stone.”
From the major studies listed in Kilpatrik's whitepaper (see each national studies overview and methodological limitations), their doesn't exist a reliable estimate the chances of pregnancies from rape, and without a reliable rape pregnancy rate, a comparison cannot be made to consensual sex leading to pregnancy. Not only that, but the studies (and consensual sex) estimates usually cannot account for confounding factors that might affect the outcome. The Rape Abuse and Incest National Network does a good job of breaking these down.
When taken in context, it is clear that legal abortions for rape victims are very rare (Atkins actual claim). Without reliable estimates for the number of rapes, or resultant pregnancies from rape, it is impossible to tell if a woman is less likely to get pregnant after being raped than having consensual sex (the [possible] claim that Atkins says he gets from doctors).