Rape and Evolution IV: A response to a different history professor

Thank you for your thoughtful note.

Here is the Thornhill and Palmer assertion. “The most convincing study of pregnancy and rape in peacetime settings (Holmes et al., 1996) involved a three-year longitudinal study of a representative sample of several thousand American women. Among victims of reproductive age (12-45), the rape-related pregnancy rate was 5% per rape, or 6 percent per victim. … [T]he figures reported by Holmes et al. probably should be corrected to about 2 percent. At this time it is not known whether false rape allegations influence this percentage.”

Thornhill and Palmer (2000), 100 citing M.H. Holmes et al., “Rape-related pregnancy: Estimates and descriptive characteristics from a national sample of women, American Journal of Obstetrics and Gynecology 175 (1996): 320-325. Note Holmes et al. (1996) apparently state that the probability of conception following rape is 5.3% for women 12-17 and 4.7% for those ages 18-45.

You asked about the conception rates for consensual sex. It is 3.1%. A. J. Wilcox et al., “Likelihood of conception with a single act of intercourse: providing benchmark rapes for assessment of post-coital contraceptives,” Contraception 63 (2001): 211-215 cited in Fessler. Note that age differences make the comparison to rape-related frequency tricky.

You asked about the comparative rate of conception. “Moreover, analysis of conception rates reveals that the probability of conception following rape does not differ from that following consensual coitus.” Daniel Fessler, “Rape is not less frequent during the ovulatory phase of the menstrual cycle,” Sexualities, Evolution, & Gender 5.3 (2003): 127-147.

Note that the percentages need not be that high for natural selection to operate. “Natural selection can operate effectively with small reproductive advantages, as little as 1 percent.” Steven Pinker, The Blank Slate (2002): 368.

You ask where Thornhill and Palmer (2000) got their data. They went to the Holmes et al. 1996 study. Where did Holmes et al. go to get their data? I don’t know the answer to this. I am assuming that the American Journal of Obstetrics and Gynecology is a peer-reviewed journal that would have been sensitive to this issue, but this is just an assumption.

You point out that conception is not the same as the production of offspring who will themselves reproduce. This is correct, but all the conception-data was used is to show that there is evidence that some of the known effects of rape are consistent with evolutionary theory. Compare this to predictions made by the feminist theory (rape-is-not-about-sex) and the anti-evolution theory (rape is not an evolution-based adaptation or effect of such an adaptation or adaptations). Is there any evidence for these theories?

I’m curious as to whether you found Johnston-Robledo and McVicker’s arguments #1 through #7 (my numbering) to be weak.

1. If so, I’m not sure why a curiosity about one piece of data would have led you to sign a letter containing such arguments.

2. If not, I’m wondering why neither you nor anyone else has presented a plausible defense of any of these arguments.

Thanks again for the note. I hope your semester is finishing well,
Steve K