Sex and the Single Sperm

FAU's wild Darwinians are at it again. And this time, they want to measure your nutsack.

Social scientists counter that evolutionary psychologists are too eager to explain every human behavior by Darwinian means. Three years after Randy Thornhill and his co-author, Craig Palmer, published A Natural History of Rape, critics responded with a book of their own, Evolution, Gender, and Rape, outlining the flaws in Thornhill and Palmer's work. They argued that the two misrepresented data to make their case.

"By cloaking themselves in science talk, Thornhill and Palmer aim to camouflage their unstated ideological agenda, deflect attention for the obvious flaws in their logic and supporting evidence, and inflate the importance of their own work," wrote Mary P. Koss, an expert on rape from the University of Arizona. "Rape long ago proved itself too complex to yield to such simplistic thinking."

When New Times sent Koss a copy of Shackelford and Goetz's 2004 "mate retention" paper, she responded with disgust.

"The average person can see for themselves that the language and concepts used in this paper obscure rather than clarify," she wrote. "Careful reading of the manuscript shows that the term 'mate retention tactics' actually describes what is defined by the US Centers for Disease Control and Prevention as intimate partner violence and the public generally recognize as battering and acquaintance rape. Are the authors suggesting that men consciously rape, batter, or psychologically control women as a seduction strategy to keep their partners from seeing other men?

"I could go on, but I have trouble staying on focus because this paper reads like Jon Stewart wrote it for the Daily Show," she added.

Patricia Adair Gowaty, an ecologist and geneticist at the University of Georgia, is troubled by Shackelford and Goetz's methodology — specifically, their reliance on surveys and questionnaires about people's sex lives.

"They did no explicit control for lies," she says. "They make universal statements from very, very limited samples. And I think their inferences about sexual behavior are potentially silly because of the operation of deceit and self-perception in human communication."

Gowaty, who has studied infidelity in birds, also says that Shackelford and Goetz incorrectly describe how rape works in animals. "They misrepresent the bird data rather dramatically," she says. Female ducks and geese are able to eject sperm from their bodies if they don't want to be impregnated, and at least one study has shown that zebra finches are also able to avoid getting pregnant when they are raped. No one has studied whether human females have similar adaptations, Gowaty says.

Furthermore, she adds, the idea that rape is about domination and control isn't necessarily at odds with evolution. Describing a 1998 review she wrote about how birds resist copulation, Gowaty says, "What's going on is that the so-called 'forced copulation' is not about immediate fertilization but about conditioning female behavior for the advantage of males." In other words, despite Shackelford's skepticism, evolution might well have designed men to use rape to dominate and control women rather than to create more children.

Another biologist, Alan Dixon, who studies sexual selection in primates in New Zealand, is also a critic. "My own view is that the evidence for human sperm competition has been exaggerated, and some of the work, especially that of Baker and Bellis, is deeply flawed."

Even Randy Thornhill, who put rape-as-an-adaptation on the map, as it were, says that Shackelford and Goetz's approach to human sexuality is "cutting edge" but also a little extreme.

"I would say it's a bit fringe," he says.

Shackelford, meanwhile, finds it ridiculous that anybody would attack his work as biased or badly designed. "I'm attacked for being a sexist and for trying to construct the world so that women would have a difficult time. And here I am with sisters and a mother and daughters... but no, my goal is to make life difficult for women. It's just bizarre."


In his FAU office, Shackelford opens a slim black case to reveal one of the instruments he plans to use in future research: a pair of calipers, capped by a small digital screen, designed to measure the exact size of a man's testicles.

The instrument will be the heart of an experiment that will test another of sperm competition's predictions: Men with larger testicles are programmed by evolution to be promiscuous.

This theory, which Shackelford calls the "bad boys have big balls" hypothesis, seems to work across species. Scientists have long been puzzled by the fact that testicles come in a wide range of sizes among different kinds of primates. Gorillas, for example, which can weigh 500 pounds, have surprisingly small testicles in relation to their body size — smaller than a typical man's. At the other extreme, chimpanzees, which are much smaller than humans, have enormous balls, weighing in at more than double a man's sack.

But gorillas and chimpanzees choose mates in very different ways. A dominant male gorilla maintains a harem by aggressively fending off other males. He doesn't need to produce much sperm to overwhelm the competition, because there isn't any. He makes sure of that.

Chimpanzees, on the other hand, have sex like maniacs multiple times a day with many different partners. A male chimpanzee's sperm, therefore, is constantly in competition with the sperm of other males, and the more volume he can produce, the better chance he will succeed.

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