Sperm competition and pornography

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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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  1. That rings true anecdotally: I’m sure most guys recall how loudly their heart was pounding when they watched their first adult film. 
    bill margold has also talked about people who’ve watched so much nasty porn that the only thing that will get a “rise” out of them is lactating-post-op-trannies-being-DPed and stuff. 
    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. 
    this is impressionistically correct (i’ve done surveys of “top” porn actresses on this weblog before, mostly blonde and eurasian). i’d like to see it quantitized more though….

  2. Gang rape usually follows conquest of another village or country. Men become incredibly oversexed for some reason (See A. Beevor’s Berlin: The Downfall 1945), could be some sperm competition going on.

  3. I prefer 2 women any day,any offers?.Its not surprising since I’m an alpha male.

  4. Men become incredibly oversexed for some reason  
    because they can. women of a conquered village are at their mercy, and it isn’t like there is a system or rules for who gets who beyond the level of the war leader (remember the iliad).

  5. Is it true that white men tend to find black women less attractive? I do know that myself (white) and my wife (black) find very few other couples of the same racial mix as ourselves, but I was thinking there were other explanations.  
    Recently I read (here?) that there was a STRONG tendency for women to prefer same-race partners, and that black women were the most race-preferring. In monetary terms, I recall $250K difference in annual income was what a white man would have to make to be preferable to a similarly attractive black man. 
    I suppose the Porn actress issue that razib brings up is good evidence in the pro- camp though.

  6. From what I’ve read of online-dating sites, asian women were the only group that disfavors their own race. I’ll post the link if I remember where it was, though if I recall correctly it was from Gene Expression or Dusk in Autumn.

  7. From what I’ve read of online-dating sites, asian women were the only group that disfavors their own race.  
    i thought it was that they didn’t strongly disfavor white males (other race that is).

  8. Is it possible they have things almost exactly backwards? Given that sperm really doesn’t cost that much and replenishes pretty quickly, maybe it’s not so much that multi-dude pron makes guys shoot bigger loads, but rather multi-chick porn that causes smaller ones. I figure the only strong reason not to shoot your whole wad is if you will likely be banging a different chick in the very near future. 
    I have some ideas as to how one might go about testing this theory.

  9. TGGP was thinking of this study, where white males could earn less than Asian males to get a response. 

  10. A survey of adult literature content and preferences has revealed that men prefer images depicting scenarios that would promote sperm competition (two males and a female) to images depicting situations that would not (two females and a male; Pound 2002). 
    I never would have suspected that, and jaspa seems to agree. I don’t know if you consider this too much info, but I find nude images (and videos) of women (whether engaged in “adult activities” or not) hotter the fewer the number of men simultaneously visible, with none being the best, given that watching naked men turns me off even if they’re with women. Do I disprove some theories about sperm competition?

  11. And to those who, almost inevitably, will answer my above post with comments along the lines of “you must be a real homophobe”–that’s not it. Several of my friends definitely seem more homophobic than me, and they aren’t bigots.

  12. I should look up that study as well — I took it for granted, since it was a preference survey, not just an observation that there exists more 2-guy-1-girl porn than the converse. That would be true just for financial reasons: an extra guy must be an order of magnitude cheaper to pay than for an extra girl. 
    I too found the result bizarre…