The project, called the "Daily Study of Sexual Behavior and Conflict in Marriage," tracks the sexual behavior of 45 married couples over 30 days. Each night, subjects filled out 15-page questionnaires with excruciatingly specific sexual and personal details, from how attractive they found their partner to whether they noticed their partner's smell and taste during oral sex.
Shackelford is excited about the results but refuses to give away any statistics before he finally publishes his findings. And he warns that even this is still a preliminary experiment in a new field.
"There's not a shred of direct evidence that these things increase conception," he says. "Ultimately, we'll never be able to go back in time and pull out the videotape of our ancestors and see what they're doing. But you build an argument."
And in that statement, Shackelford has hit on the number-one criticism of his field: That there's no way to go back in time to see what was actually happening on the African savanna when human beings arose as a species.
Biologists study animals in the wild to understand how natural selection might have shaped their forms and behavior. But who knows how human beings lived "in the wild"? Even the most "primitive" of human societies today are steeped in culture beliefs, customs, and rituals handed down generation to generation that may or may not counteract the instinctual urges of our genes.
For decades, for example, rape has been studied by social scientists not as a genetic birthright but as a learned behavior men are affected in their youth by what they see adults do and then later exert domination and power over women in violent ways. The act has little to do with sex or procreation.
But Shackelford and Goetz dismiss such power-and-domination ideas and argue that, from an evolutionary perspective, they make no sense.
"I think it needs to be explained, using this domination theory, why men consistently put their penises in women's vaginas and ejaculate," Shackelford says. "Why not just club her over the head to demonstrate your dominance, or why not have anal sex with her? But that's not what men do! Men put their penis inside the woman's vagina and then ejaculate, which isn't required to dominate a woman."
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.