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Because of this, our ancient male ancestor faced a quandary: If, while he was out hunting and gathering, the woman he thought he had impregnated stole away for sex in a nearby cave, he might end up putting all his time and energy into raising a child who didn't carry his genes.
In 1988, Shackelford's mentor, David Buss, predicted that an elaborate system of "mate retention tactics" would have evolved in response to this. Behaviors that range from giving lavish gifts to guilt trips, threats of violence, and outright rape. In 2003, Gordon Gallup, an evolutionary psychologist at SUNY Albany, succeeded in using dildos, artificial vaginas, and fake sperm made from one of Shackelford's recipes to show what Shackelford could not at FAU: that a human penis did indeed appear designed for "sperm displacement" to shovel out any rival sperm hanging around the vagina. The displacement only worked, Gallup found, during deep-thrusting sex: when the penis was inserted at least 75 percent of its length into a vagina.
To find whether these sperm competition behaviors show up in modern humans, Shackelford and Goetz have surveyed men and women in monogamous relationships, asking very specific and often invasive questions about how they behave and feel in their relationships.
One study, published in 2004, quizzed 305 men about two different sides of sperm competition: whether they practiced any of the mate retention tactics (giving gifts, etc.) and how long and how deeply they thrust during sex. Men catalogued how many times they refused to introduce their partner to a male friend, peeked at their partner's personal mail, or bought a small gift. They were also asked to rate their thrust depth on a scale of 1 to 9. Meanwhile, Shackelford and Goetz correlated this information with the men's description of the attractiveness and personality traits of their girlfriends or wives and how likely they thought their partners were to cheat.
After crunching the numbers, Shackelford and Goetz got what they expected: The hotter and more outgoing the girlfriend, the deeper men thrusted and displayed "mate retention tactics."
A man, they concluded, would go out of his way to prevent a good-looking mate from looking for other men but, to cover his bets, would thrust deeply during sex in case the woman had been unfaithful.
In 2006, the two surveyed 246 men and 276 women who were in at least a year-long relationship (but not with each other), this time with a new set of questions about how often they had performed or experienced "sexually coercive acts" in the past month.
Shackelford and Goetz again found what they expected: Men in relationships with women they suspected were unfaithful forced their partners to have sex more often. (The fact that acts of coercive sex reported in this study may have constituted actual sex crimes, however, didn't seem to prompt reports to law enforcement authorities.)
Of course, subjects sometimes lie to survey takers, which is the motivation behind Shackelford's largest project. In 1999, he received a grant from the National Institute of Mental Health, the nation's preeminent underwriter of scientific research, to launch the most comprehensive study of human sperm competition ever conducted. Already two years past its original due date, the study's data is so voluminous that Shackelford has three of his graduate students processing it full-time.
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."