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What Do You Do When There is No Best Dataset? A follow-up on pregnancy and rape statistics


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Trigger warning: discussion of violence against women.

 

I’ve received a number of questions over email and Twitter about the statistics I mention at the end of my last post. Here is what I wrote:

“So, rape and consensual sex have the same pregnancy rate. This means that of those 64,080 rapes in 2004-2005, minus the 15% of rapes that are of children under the age of 12 which gets us to 54,468 rapes of almost all reproductively-aged women, somewhere between 1,689 (3.1%) to 2,723 (5%) pregnancies from rape could have occurred in that year alone. Somewhere around half of those women were probably using some form of hormonal contraception, so let’s hope the numbers are even lower. Unfortunately, access to emergency contraception is still a challenge for rape survivors who go to hospitals, particularly Catholic hospitals, to receive treatment.”

The question is: why did I not use the Holmes et al (1996) estimate that there are 32,000 pregnancies from rape each year? Why use the RAINN stats which are clearly more conservative?

Both of these sets of numbers – the Holmes et al and the RAINN – are estimates. It’s worse than that. The RAINN values are estimates from the 2005 National Crime Victimization Survey, which I then estimated contained 85% reproductively-aged adults from an estimate of the percentage of rape survivors that are children (15%) from the 2004 National Crime Victimization Survey (also on the RAINN site). I then had to arrive at a range based on two different studies about the rate of pregnancy from a single act of intercourse (Holmes et al versus Wilcox et al). I’m not wedded to these numbers, it just seemed like a good place to start.

The Holmes et al estimates from their pool of 3031 adult women from the third wave of their research, that there is a 5% pregnancy rate from rape. The reason I didn’t use their extrapolation out to 32,000 pregnancies a year was that I found the discussion section a little unclear, and I didn’t quite understand how they arrived at that number. 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.”

Do you see how I’m confused? I thought there were 20 instances of rape-related pregnancy, and here it says 34. I think they are maybe referring to their first citation: Kilpatrick DG, Edmunds CN, Seymour AK. Rape in America: a report to the nation. Charleston (SC): The National Victim Center and the National Crime Victims Research and Treatment Center at the Medical University of South Carolina, 1992 Apr. But that’s not what they cite.

In any case, both of these sets of numbers are estimates. Both are likely based on decent rationale, but I had guessed that the answer lay somewhere between these two numbers.

It gets worse

I decided to crunch some numbers myself. Using US Census Bureau data for July 1996 (arbitrary, but I wanted to choose the same year as publication of the Holmes et al), there were 60,704,500 women between 15-44 years of age. If 13% of them were raped, then the range of possible pregnancies is between 244,639 (3.1% pregnancy rate from Wilcox et al (2001)) and 394,579 (5% pregnancy rate). Semen is found in about half of rapes (Riggs et al., 2000), about half of women use hormonal contraception. So let’s quarter those numbers.

That still works out to 61,160 to 98,645 pregnancies from rape.

Ok, let’s factor in the use of emergency contraception (EC). I’m getting desperate now. In one survey, 85.9% of emergency departments provide EC counseling, and 87.7% of those will administer the first dose right there (Azikiwe et al., 2005). If all of the women who were raped actually went to emergency departments, if all of them who were offered EC took it, and if it worked for ALL of them, then…

…it’s only 15,082 to 24,129 pregnancies from rape. If rape survivors always went to emergency departments for treatment, which they rarely do.

An estimate is an estimate. None of these numbers captures the reality of the situation, except that they are all unacceptably big. Rather than accept any one of these values as the right one, perhaps we need to just decide that any number greater than zero is too many.

AN UPDATE (8/22/2012, 9:28am CST):

Based on some feedback I’ve received that the 13% was likely the incidence over 3 years according to the Holmes et al study, we could cut the yearly incidence in third again. So that would be 5,027 to 8,043 pregnancies from rape each year, if everything goes perfectly under the rather unrealistic conditions I’ve set above (regarding access to EC).

References

Azikiwe N, Wright J, Cheng T, D’Angelo LJ. 2005. Management of rape victims (regarding STD treatment and pregnancy prevention): Do academic emergency departments practice what they preach? Journal of Adolescent Health 36(5):446-448.

Holmes MM, Resnick HS, Kilpatrick DG, Best CL. 1996. Rape-related pregnancy: Estimates and descriptive characteristics from a national sample of women. American Journal of Obstetrics and Gynecology 175(2):320-325.

Riggs N, Houry D, Long G, Markovchick V, Feldhaus K. 2000. Analysis of 1,076 cases of sexual assault. Ann Emerg Med 35:358-362.

Wilcox AJ, Dunson DB, Weinberg CR, Trussell J, Baird DD. 2001. Likelihood of conception with a single act of intercourse: providing benchmark rates for assessment of post-coital contraceptives. Contraception 63(4):211-215.

Kate Clancy About the Author: Dr. Kate Clancy is an Assistant Professor of Anthropology at the University of Illinois. She studies the evolutionary medicine of women’s reproductive physiology, and blogs about her field, the evolution of human behavior and issues for women in science. Find her comment policy here. Follow on Twitter @KateClancy.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. leerwilson 5:43 pm 08/21/2012

    at 13% that’s 7.8m rapes per year, in 1996. as under reported as rape is, I don’t think it was under reported by a factor of 80 – when using the FBI crime stats as a comparison. http://www2.fbi.gov/ucr/cius2009/data/table_01.html

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  2. 2. sonoran 6:54 pm 08/21/2012

    There’s another aspect to the question of rape-induced pregnancies and being forced, as the National Republic platform on abortion would demand, to bring the child to term. Is there a genetic basis to the violent assaultive behavior of rapists?

    While I’d imagine there’d be some variation in this, it certainly could be hypothesized that forcible sex is a reproductive strategy that’s represented in a population somewhat in proportion to its success in breeding.

    Do children born of rapists go on to commit rape themselves at a higher rate than others?

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  3. 3. Kate Clancy in reply to Kate Clancy 7:27 pm 08/21/2012

    Hi Lee, thanks for your comment. The 13% value comes from the Holmes et al. And I hope you didn’t miss the broader point of the post — we can estimate values all we want, and arrive at different ones. They are all unacceptably high.

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  4. 4. Kate Clancy in reply to Kate Clancy 7:29 pm 08/21/2012

    Sonoran, I don’t know the answer to this, but my best guess is no. From what I understand, most rapists rape because of narcissism and cultural conditioning that makes them think it is appropriate to force their way into whatever they want, and/or they want to punish or control women. I don’t mean to be a die-hard nurture person here, but my guess is that family and peer environments would be strong predictors of this type of behavior.

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  5. 5. Mr. Natural 7:30 pm 08/21/2012

    Would we accept such sloppy statistics for other violent crimes? Any number above zero is unacceptable for murder, as well, but we know the murder rate in the nation.

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  6. 6. Kate Clancy in reply to Kate Clancy 7:32 pm 08/21/2012

    Mr. Natural, when you change our culture so that it is easy for women to report when they are raped free from shame, we will have an amazing dataset from which to calculate rape and rape-related pregnancies. We’ll probably also have a lot fewer rapes.

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  7. 7. leerwilson 7:51 pm 08/21/2012

    kate, i agree we must not miss the larger point. was just trying to drill down why there were two different numbers.

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  8. 8. sonoran 7:56 pm 08/21/2012

    Kate, perhaps you’re right, but the interaction between nature and nurture in behavior can be pretty complicated. What the rapist and perhaps his psychologist might perceive to be the nature of his underlying motives has little to do with whether there’s some underlying genetic component at work. A particular constellation of brain-mediated behavioral traits pertaining to dominance, impulse control, sexual drive etc. might predispose someone to this kind of behavior.

    This is an action that has a direct effect on reproduction, not something tangential to it. Seems like a trait that might have a reasonable probability of being highly conserved.

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  9. 9. cystal3719 11:37 pm 08/21/2012

    And then one has to consider how many cases of venereal diseases and/or AIDs these misfortunate women and children contract. Has anyone made an estimate of that?

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  10. 10. blmccormick 2:39 am 08/22/2012

    Lee and Dr. Clancy,
    13% of women are estimated survivors of sexual assault. So, if there were 60,704,500 women in the U.S. in 1996, and 13% of them are survivors, then there are 7.8 survivors in the United States in that year, approximately. The assault could have happened a year, 7 yrs, or 20 yrs ago. So Dr. Clancy’s estimated rate of pregnancies resulting from rape are for women over the course of the years throughout the middle of the lifespan and Holmes et al. seem to be for one year. They measure different things.

    Regardless, I long for the day that reporting a rape is not shameful and that responsibility is placed on the criminal and not the victim. If we can end victim-blaming and rape myth perpetuation, reports would go up, people would realize what a true problem it is, and interventions would be put in place to reduce the gigantic number.

    Love the last line of the article. Spread that idea far and wide!

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  11. 11. zstansfi 3:25 am 08/22/2012

    leerwilson, that’s 13% of their sample over a period of 3 years, although many of those women were raped more than once.

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  12. 12. jbairddo 7:30 am 08/22/2012

    While it may be a genetic advantage in countries who have greater tolerance for this behavior, certainly in the last 20 years there has been a significant change in how rape is viewed. Also the availability of legal abortion (and morning after pills) means that rape victims have the ability to terminate a pregnancy. The questions become A) is there a gene or set of genes B) is the advantage of the gene outweighing its disadvantage-parenting and helping the women raise the child C) with the above conditions (abortion, morning after pills, jail for rapists) can the gene be prevented being passed on to the point that it won’t be viable population wise (at least until the GOP sends women’s rights back to the stone age and makes MAP’s and abortion illegal again).

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  13. 13. Heteromeles 10:56 am 08/22/2012

    As for the problematic data-set, I agree, and it’s far from the only one. For example, there are rumors that more men are raped in American prison than women are raped outside prison, but I have no idea how one could get accurate numbers on that either.

    As for the Republicans, they seem so fond of illegal industries in their red states–meth labs, marijuana grows, moonshining, among others–that I’m somehow not surprised they’d want to make abortion illegal too. Yes, that’s an unfair stereotype, and I do apologize to the many decent people who live in the red states. Still, I don’t want to live in some Republican mock-up of a 1950s American that never actually existed, and I honestly resent their attempts to suck me into their fantasy.

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  14. 14. libelula 12:37 pm 08/22/2012

    Kate,

    Indignation fuelled fingers typing evidence-based arguments to stop the nonsense in its tracks.

    Thank you.
    Alejandra Nuñez-de la Mora

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  15. 15. Kate Clancy in reply to Kate Clancy 1:23 pm 08/22/2012

    Thank you, Alejandra, and I hope I get to see you at the AAPAs!

    Link to this
  16. 16. leerwilson 2:33 pm 08/22/2012

    ah, I misremembered the 13% being an overall lifetime risk factor for women, (actually 17.6%, 14.8 completed and 2.8 attempted, according to RAINN) so, D’oh on that.
    the last things Ill say about the Holmes study are that rape has declined 60% since 1993 (according to RAINN) so using Holmes, et al’s 32,101 number and lopping off 60% we get 12,845 today, admittedly a rough calculation.
    Second, if 5% = 32,101 then 100% = 642,020. Or 6.6 times more than the FBI crime survey reports.
    In short, I don’t think the Holmes et al study is all that great and Todd Akin is an ass.

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  17. 17. kegrant 2:39 pm 08/22/2012

    Two good, thorough, and thought-provoking posts on a difficult subject. Forgive me for sticking to the statistics.

    If I look at a rape rate of 13% over 3 years, that corresponds to an 87% probability of not being raped in that period. If I take the (1/3) power of that, I get a 95.5% probability of not being raped in one year and a 24.8% likelihood of not being raped over 30 years (30th power of 0.955). A lifetime likelihood of 75.2% of being raped seems high. RAINN gives 14.8% for lifetime completed (versus attempted) rapes.

    If I use the RAINN figure, the lifetime probability of not being raped is 85.2%. Taking the (1/30th) power, gives a yearly likelihood of 99.47% of not being raped and, correspondingly, a 0.53% of being raped. Applying that to 60,704,500 gives an estimate of 322,000 rapes per year. At 5%, roughly 16,000 pregnancies would result per year.

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  18. 18. brynnscott 3:02 pm 08/22/2012

    As a long time pro-choice advocate, I recall attending an Ohio legislative Health Committee hearing several years ago for a bill to require Hospital Emergency rooms to carry (and provide accurate counseling regarding) EC for sexual assault victims. The question was posed by the anti-crowd as to well how many women does this really affect (ie let’s not force the [Catholic] hospitals for some insignificant number of women–such delicate consciences these hospital administrators have). If I remember correctly – the Cleveland AIDS Task Force representative testified that from their own statistics, roughly 4.5% of women they had worked with who were raped became pregnant. Now if the statistic I have oft heard quoted that roughly 5% of women who are in their fertile years are ovulating at any time is correct – that seems a fairly frightening statistic.

    As to outcome, whether or not there are genetic components to be concerned about in the off-spring of rape, I would not be surprised if there were epi-genetic components. Certainly it is known that depression during pregnancy can lead to poor outcomes for the fetus. I can’t imagine how a women who is raped would not be depressed… something to think about.

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  19. 19. Kate Clancy in reply to Kate Clancy 3:05 pm 08/22/2012

    Kegrant, I certainly forgive you :) . I appreciate all of you helping me sort through the stats here, and am just frustrated that all of them are so unsatisfying/difficult to parse.

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  20. 20. santaidm 9:05 pm 08/22/2012

    I am not an expert in statistics, genetics or rape but I think that a significant number of rapes are committed by the victim’s father and/or brother or other near family. That explains why some women get raped more than once, defying probabilities. That issue alone is enough to foul the data so that regular statistics rules don’t apply very well. It would also infer on genetics, the perpetrator and the victim being of the same blood. On top of it all if a pregnancy occurred in such a case, then both nature and nurture would combine their effects.

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  21. 21. Kate Clancy in reply to Kate Clancy 10:28 am 08/23/2012

    If anyone reading here has realized that their comment has not made it through moderation and it’s been more than a few hours (not including comments overnight), you may want to consider re-reading this post and the comments policy: http://blogs.scientificamerican.com/context-and-variation/2012/01/24/blogging-while-female/.

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  22. 22. seabee123 5:07 pm 08/23/2012

    Kate, these past two articles have been fantastic, but I feel like you’re ignoring a sort of obvious tactic here. The problem is that pointing to a study that states a 5% pregnancy rate among victims can elicit the reaction of “Well clearly those women were lying about being raped.” I think that it would be really interesting to point out that if the pregnancy rate among raped women is the same as the pregnancy rate among the average population at large, mathematically there can be only two conclusions: A) rape does not occur or B) rape does indeed result in pregnancy.

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  23. 23. Kate Clancy in reply to Kate Clancy 5:26 pm 08/23/2012

    seabee123, I don’t think I understand your point, or your problem.

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  24. 24. seabee123 6:12 pm 08/23/2012

    Kate, sorry if I’m not being very clear. Let me see if I can clarify things.

    Todd Akin says, “Raped women don’t get pregnant.”

    Your response to him is, “But look at these statistics! Look at the incidence of rape among rape survivors!”

    The problem is that people like Todd Akin believe that if they are pregnant, they are not rape survivors, but that the rape was, for whatever reason, not legitimate. Thus your argument, from their point of view, is begging the question–you can’t prove rape pregnancy incidence by to someone who believes you can’t get pregnant from rape by citing rape pregnancy statistics.

    However, there is a mathematical argument to be made here. Given the statistics, only two conclusions can be made logically:

    1) If we are to assume that Todd Akin was right, the logical conclusion is that there are no “legitimate” rapes. Why? Let us say that X is the number of “legitimate” rapes and Y is the number of “false” rapes (ugh). X + Y = Z, the total number of women who reported being rape survivors in the study. Let’s also, for simplicity’s sake, bring the pregnancy rate in the Holmes et al study in line with the Wilcox study–this should be fine since the pregnancy rate is actually HIGHER in the Holmes study. Then, this inequality would hold true whenever X > 0: X*0 + Y*0.031 < Z * 0.5. If, say, X = Z/2, then the expected pregnancy rate in a group would be 1/2 of what it is in the general population. This, of course, implies that fully half of these rape survivors are lying.

    2) Since the pregnancy rate is, at best, the same in both groups, the incidence of pregnancy must be the same both in cases of rape and consensual sex.

    Since no reasonable or sane person can assert that rape doesn't exist or that rape is so vastly overreported as to make #1 a viable option, mathematically, the conclusion HAS to be that #2 is true–that a woman's body does not (as is obvious to any normal person) prevent her from getting pregnant in the case of rape.

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  25. 25. seabee123 6:32 pm 08/23/2012

    Erm, that should have been “look at the incidence of pregnancy among rape survivors.” Wish I could edit my comment!

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  26. 26. kienhua68 7:16 pm 08/26/2012

    With the population increasing around the world, the last
    thing we need is forcing rape victims to carry an unwanted,
    genetically uncertain quantity into the world.
    Equally bad or even worse are diseases of the incurable type.
    Last but certainly not least is the mental trauma of the act.
    To burden someone with all that for political purpose is
    nothing short of criminal.

    Link to this

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