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Pornography, sperm competition, and behavioural ecology

By Michael N. Pham, William F. McKibbin, and Todd K. Shackelford


Over millions of years, evolution by natural selection has produced adaptations in humans: biological and psychological traits that improved human survival and reproduction in ancestral environments. For example, ripe fruit was an infrequent but calorically rich part of the human ancestral diet. We therefore have a sweet tooth that rewards us when we eat ripe fruit.

But evolution works slowly and gradually, over many generations. Sometimes, the environment changes so quickly that our adaptations can’t evolve quickly enough in response to these changes. This is called an “adaptive mismatch.” Today, modern society presents us with many sweet-tasting goodies, like candy, that aren’t healthy for us. And yet, we continue to crave these unhealthy treats because they “parasitize” our sweet preference—an adaption that was designed to reward ripe fruit-eating.

But, what do adaptive mismatches have to do with pornography? A lot.

Heterosexual men become sexually aroused from seeing naked, fertile women. This sexual arousal is an adaptation that motivates men to prepare for the possibility of sex.

Like candy, pornography creates an adaptive mismatch. For a moment, try to see the world not from “human eyes” but from the eyes of an animal biologist. You might think that men’s enjoyment of pornography is bizarre: men are sexually aroused by the sight of ink that’s splattered on magazine pages, or computer pixels that display light. Nobody would argue that men evolved to have sex with magazines or computers. Adaptive mismatch? Quite.

Pornography is a formidable industry, with men as the primary consumers. And because pornography exploits slow-to-change adaptations, investigating men’s preferences in pornography can inform us about those adaptations.

A previous study documented that pornography depicting two men having sex with one woman (MMF) was more prevalent than pornography depicting two women having sex with one man (FFM). However, a different study documented that men report viewing FFM pornography preferentially over MMF pornography. To reconcile these contradictory findings, we recently published in Behavioral Ecology a paper documenting that adult DVDs containing more depictions of MMF on the DVD cover achieve better sales rankings than DVDs containing more depictions of FFM. Our results indicate two important things about men’s sexual psychology: (1) The type of pornography men say they view may differ from what they actually view, and (2) men’s greater sexual arousal from viewing MMF pornography may be a consequence of another adaptive mismatch: adaptations to sperm competition.

Sperm competition occurs when a woman has sex with two or more men within a sufficiently brief period of time, and the different men’s sperm compete to fertilize the ova. Men have evolved adaptations to increase their chances of success in sperm competition. Some adaptations to sperm competition involve increasing sexual arousal. For example, when men estimate a greater likelihood that their romantic partner recently had sex with another man, they ejaculate more sperm the next time they have sex with her, report greater interest in having sex with her, and sometimes, sexually coerce her.

To tie this all together, men’s preference for MMF pornography is evidence of adaptations to sperm competition. Men who see MMF scenes are “witnessing” sperm competition unfold between the two men in that scene. And as sperm competition theory predicts, men have adaptations that cause them to become sexually aroused by the risk of sperm competition, motivating them to purchase adult DVDs that contain depictions of it.

Sperm competition theory may help solve other puzzles about male sexuality. Notably, it may inform the question of why men become jealous—yet simultaneously, sexually aroused—by the thought of their romantic partner having sex with another man.

Michael N. Pham is a graduate student in evolutionary psychology at Oakland University. William F. McKibbin is an assistant professor of psychology at the University of Michigan—Flint. Todd K. Shackelford is chair and professor of psychology at Oakland University. They are the co-authors of the paper ‘Human sperm competition in postindustrial ecologies: sperm competition cues predict adult DVD sales’, published in the journal Behavioural Ecology.

Bringing together significant work on all aspects of the subject, Behavioral Ecology is broad-based and covers both empirical and theoretical approaches. Studies on the whole range of behaving organisms, including plants, invertebrates, vertebrates, and humans, are welcomed.

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Image credit: Spermatozoons, floating to ovule. By frentusha, via iStockphoto.

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