Age & Sexual Coercion

On Thursday of last week, I gave what I find to me one of the most difficult lectures to give in my Evolutionary Psychology class, focusing on sexual coercion and rape. The difficulty isn’t only because of the controversy that surrounds evolutionary approaches to the topic, but for me I find it very difficult to talk about because of the nature of the topic itself. It is, I think, important to discuss, but there is horror behind the numbers that makes the presentation a particular challenge.

This year, as I often have in the past, I left time at the end of the class for students to ask questions and comments; also as in the past, several students came up to the front to speak to me individually after the lecture. I found the students’ remarks and questions insightful and illuminating. I thought I would share parts of the conversation.

Figure from Rothman et al.

Figure from Rothman et al.

I didn’t systematically poll the students, but at minimum a minority of students came to class with the assumption that rape is motivated by the desire to control or exert power over women. I discuss this idea, most famously associated with Susan Brownmiller, briefly in my lecture, mostly in the context of asking the students to think about what predictions such a view makes. Because the topic is sensitive, while I try to present some relevant data, I try not to push them too strongly toward one view or another. My goal is to get them to think about the relationship between different explanations (both ultimate and proximate) and the existing data.

Other students seemed to come to the discussion with a different view. Perhaps not surprisingly, primed perhaps by my earlier lectures in which I discuss sexual coercion among non-human animals, some students thought that rape might be a short term sexual strategy. Certainly such a view has been entertained in various forms by people working in the area. The proposal is that one way male humans might have increased reproductive success in ancestral environments was through sexual coercion, gaining a single sexual encounter through force instead of through being chosen as a mate. Obviously, much has been written about this and related ideas, with Thornhill and Palmer’s A Natural History of Rape being one early and well-known example.

In class, I asked the students to think about what predictions a short term sexual strategy view makes, in particular with respect to the issue of, if this were correct, which individuals you would expect to be targeted most frequently, holding everything else equal (which is, obviously, an important caveat). The answer, it seems to me – and I’m happy to be corrected – is that the most targeted individuals ought to be those individuals with properties that correlate with the maximum probability of conception given a single act of intercourse (i.e., high fecundity).

This idea intersects with a new paper that crossed my path, a study of “fecundability” in a large sample in Denmark. Fecundability refers to how likely conception is during one menstrual cycle if a woman is having unprotected sex during the course of the cycle. It’s not a perfect measure for the present purpose, but here I take it as a good proxy for the probability of conception during a single sexual act. I’ve included Figure 1 from the paper here. The data show, as one observes in substantial numbers of other similar datasets, that this value peaks in the late 20’s. Here is the authors’ conclusion:

In our study, peak fecundability was approximately 29–30 years among parous women and 27–28 years among nulliparous women. Among parous women, age was associated with increasing fecundability until age 30 years, after which it decreased.

I presented some older data in class, showing largely the same pattern, and I asked the students to consider these findings in the context of statistics regarding the age of victims of rape. The most recent Bureau of Justice statistics I could find were quite old, dating from the late nineties, but I have no reason to believe that the patterns have changed greatly. (If someone has more recent data, please let me know where to find it.) In the BoJ data, roughly 37% of victims are 17 or younger. 62% are age 24 and younger.

Source: Criminal Victimization in the U.S., 2005. Dept. of Justice.

Source: Criminal Victimization in the U.S., 2005. Dept. of Justice.

If one thinks that rape is a short term strategy, then one might predict that victims should be deferentially  likely to be those individuals most likely to conceive given one act of sex. The Bureau of Justice data make it appear as if targets of rape are considerably younger than this. The median age of a rape victim is 22.

There are, of course, good explanations for why younger, rather than older, women might be targeted. Insofar as age and wealth are correlated, older women are in a better position to be able to afford protections that reduce exposure to the risk of rape. Similarly, as women age, they might learn strategies that make them better able to defend themselves. Other explanations are possible as well. These different explanations make predictions which some might already have tested. For example, if the issue is that one gets safer, in general, with age, then similar patterns should emerge when one looks at other violent crimes. For assault, armed robbery, and murder, does the pattern look similar? Above, I’m showing some data from the Bureau of Justice from 2005, which allows a comparison of robbery and sexual assault broken out by age and sex. To my eye, the robbery data look relatively uniform across age for female victims, but the sexual assault data seem to have a spike at 24 years old and below. Again, I would be pleased if readers directed my attention to appropriate sources which might provide better evidence.

Age is, of course, not the only property of victims that might merit scrutiny. However, because the proposal that rape is a short term sexual strategy seems to point to age as an important parameter, it seemed to me a good place to direct my students’ attention as they considered this issue.

My reading of these data is that victims of rape tend to be younger than one would predict under the proposal that rape is a short term sexual strategy designed to maximize the chance of conception in a one-time sexual encounter. This is not necessarily fatal to the proposal insofar as perpetrators might prefer such victims but choose younger victims for some of the reasons indicated above, or for any of a number of other reasons.

I also alluded to one other set of findings. In a short paper published in 1982, Wislon and Durrenberger report some surprising findings regarding the chances that a victim will date their attacker:

…39% of 52 rape victims as contrasted to 12% of 58 attempted rape victims dated their attackers again, after the assault…

Similar proportions have been reported in more recent data (Ellis et al., 2009), a finding that, I believe, also came as a surprise to my students, as they did to me the first time that I encountered them.

Before concluding, a few caveats. First, by talking about rape, broadly, above, I don’t mean to imply that it’s an undifferentiated, homogenous set of acts. Different motives might very well be at play in different cases, and indeed I find that to be very likely. Second, I’m not trying to speak here directly to the adaptation/byproduct discussion that surrounds this issue. Like so many observers of this debate, it seems to me that this issue is still to be established one way or the other, and I don’t find myself convinced in either direction.

Lastly, it ought to go without saying, but to be clear, an explanation for rape is in no way condoning the behavior. Rape is a horrible crime, and the perpetrators are responsible for their actions; understanding their motives does not excuse the behavior in the least. At the end of class, I ended with some slides that deviate from my usual practice of staying away from non-scientific issues, and I showed some statistics regarding how likely a rapist is to be caught, arrested, convicted, and jailed. (These statistics are aggregated in sites like this one, for instance.) These statistics are sobering.


Ellis, L., Widmayer, A., & Palmer, C. T. (2009). Perpetrators of sexual assault continuing to have sex with their victims following the initial assault. International Journal of Offender Therapy and Comparative Criminology, 53, 454-463.

Rothman, K. J., Wise, L. A., Sørensen, H. T., Riis, A. H., Mikkelsen, E. M., & Hatch, E. E. (2013). Volitional determinants and age-related decline in fecundability: a general population prospective cohort study in Denmark.Fertility and Sterility.

Wilson, W., & Durrenberger, R. (1982). Comparison of rape and attempted rape victims. Psychological Reports, 50, 198.

02. April 2013 by kurzbanepblog
Categories: Blog | 21 comments

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