Sunday, June 24, 2007

Sperm competition and pornography   posted by agnostic @ 6/24/2007 09:08:00 PM

Recently Razib posted on a review of sperm competition (PDF), part of which claims:

Kilgallon & Simmons [(2005)] documented that men produce a higher percentage of motile sperm in their ejaculates after viewing sexually explicit images of two men and one woman (sperm competition images) than after viewing sexually explicit images of three women.

Motile sperm are the kind that are capable of moving themselves by swimming. The idea is that if a guy watches a video with two guys and one girl, he'll try to leave more swimming sperm since they now have to compete with those of the other man (so his mind thinks). Now, the proper reaction is to think that two guys going at one girl simultaneously was probably as rare, or even more rare, in our evolutionary past than it is today, so how would we show an adaptation to it? Fortunately the study in question is available online for free -- here -- and it's only two pages long, so read it. Let's review the key findings.

In the Results, we read:

Subjects viewing images of sperm competition had a greater proportion of motile sperm in their ejaculates than those viewing images of females (52.1 +/- 7.3% versus 49.3 +/- 8.0%; F1,23=5.08, p=0.034).

That 3% difference in means is swamped by the standard-error bars of 7% - 8%, and the p-value, while under the 0.05 threshold, is uncomfortably close to it, considering the margin of error and the smallish sample size. So this result could easily be a fluke -- or not, but it warrants no confident statement that the study "documented" a pattern. "Somewhat suggested," perhaps, but gimme a break.

This equivocal data aside, it gets worse when the authors looked at the concentration of sperm per volume:

Men viewing images depicting sperm competition had fewer sperm in their ejaculate than those viewing images of females (61.35 +/- 1.27 versus 76.64 +/- 1.26 * 10^6 sperm ml^-1; F1,36=8.48, p=0.0061). [my emphasis]

Now the effect is clear, but it contradicts the hypothesis that men's bodies will make more of an effort to defeat the other male's sperm when watching "sperm competition" images. In fact, given these two findings, maybe "two guys on one girl" ought to be called "sperm anti-competition" images.

The strongest finding (read the paper for the data) was that men who thought the pornography was "more explicit" than what they'd seen previously had a much higher percentage of motile sperm and much higher concentration of sperm per volume. The authors suggest that men for whom this sort of pornography was old news had become habituated to it. That rings true anecdotally: I'm sure most guys recall how loudly their heart was pounding when they watched their first adult film.

This raises an interesting possibility: if it's largely the novelty factor that's causing men to produce more sperm, and more motile sperm, would this carry over into the case where novelty was based on the ethnicity of the girl? The study design would be pretty simple: recruit a bunch of Latino-American and Anglo-American men, and randomly assign them to two groups, one that watches a scene featuring one man and one woman of the same ethnicity as the viewer, and another that watches a scene featuring one man of the same ethnicity as the viewer and one woman of the opposite ethnicity (Latin or Anglo). Then see if "jungle fever" played any role in how much sperm the men produce. I'm assuming the actresses in all scenes would be rated beforehand to ensure that any guy would find them very attractive. That's why it has to be Latinos and Anglos -- Blacks and Anglos might not work simply because too many White men don't find Black women as attractive as women of other groups, and that could be a problem.

The other reason you might expect guys to produce more sperm when viewing a different-ethnicity girl is that throughout most of human history, including today in most parts of the world, it was incredibly rare to see someone from a noticeably different ethnic group. Even in the few cases where it happened, one of them would have been "just passing through" the other's region. Thus, the male would not know anything about her mating habits, and would have to assume the worst -- that she had a mate already. And if such encounters were fleeting, "strangers in the night" situations, then he would stand to gain everything by impregnating her. He'd never see her again, after all, so why not go the extra mile sperm-wise to make sure? Then he'd have another man raising his child: all benefit and little cost.

You could also look at sperm content from real-life interracial couples, but there could be confounding factors. Maybe if you conducted the research where there were hordes of youngsters who were more-or-less open to mating with anyone -- say, Cancun during Spring Break -- you might be able to collect enough subjects to randomly assign them to the real-life versions of the video study suggested above. "Y'know, as long as you're mating with any old person, why not take part in our study?"

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