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Goetz came to FAU specifically because of Shackelford. "I knew that sperm competition was the most exciting research going," he says. By 2002, Goetz was on the front lines of evolutionary psychology, researching fake sperm recipes.
Shackelford and Goetz, like all evolutionary psychologists, see sex as a heartless matter of genetic warfare. A winner-takes-all contest where the prize is passing on your genes, it's a battlefield where those with adaptations that help them get laid pass their genes on to future generations.
"We're essentially organic robots," Goetz says. Our behavior, he explains, has nothing to do with what we consciously want those emotions and desires are just a smoke screen for evolutionary programming. Especially when it comes to sex. "If you think of a penis as a tool," he says, "men might use that tool."
Charles Darwin himself first mapped out the theory of "sexual selection" and devoted the last years of his life to describing it. He came to believe that the competition over mates had as big an effect on evolution as the other environmental influences of natural selection.
It's sexual selection that explains a lion's mane or a peacock's tail anything, in short, that animals use to attract the opposite sex. And in the 1970s, scientists began to wonder if that kind of competition also took place on the level of sperm.
The term "sperm competition" was coined by biologist Geoffrey Parker, who noted that a male yellow dungfly will sometimes interrupt another male having sex with a female by lifting him off, tossing him away, then immediately taking his place. If both dungflies' sperm were in the female at the same time, Parker reasoned, the male with the faster-swimming sperm would turn out the evolutionary winner.
"That was the announcement of a new type of sexual selection," says Randy Thornhill, a prominent evolutionary psychologist and co-author of the highly controversial 2000 book A Natural History of Rape, which argues that evolution programmed men to rape women when they otherwise couldn't find willing mates.
"[Parker] extended Darwin's earlier work on sexual selection tremendously. He showed that sperm between different males compete... that this was a real evolutionary force in insects."
Once scientists started looking for evidence of sperm competition in animals, they found all sorts of examples. Some species have sperm that actively attack and subdue the sperm of rival males inside the female. Others sport penises with elaborate hooks and curlicues that grab competing sperm out of a vagina. Male birds of some species "guard" their mates to prevent being cuckolded, while others "poach" females to increase the chance of spreading their genes widely. There's even a type of duck that forcibly rapes its mate if it catches her "cheating" on him.
Evolutionary psychologists pounced on the growing body of evidence of sperm competition in animals as a reason to research it in humans. If evolution had programmed birds and insects to rape and guard, they reasoned, it might have programmed humans too.
"There's incredible evidence for sperm competition in birds, fish, and mice," Shackelford says. "There's no reason to believe that humans are the oddball at all."
But the first scientists to apply the theory of sperm competition to humans, a pair of researchers named Robin Baker and Mark Bellis, may have done more harm than good. In 1995, they published Human Sperm Competition, a book that outlined their ideas from several years of research. Disappointed by the lack of reaction to the book, Baker rewrote it for a general audience and called it Sperm Wars.
Baker and Bellis' work contained surprising ideas, such as their theory that some human sperm are "kamikaze" cells designed to sacrifice themselves to sabotage the sperm of competitors. They suggested that men adjust the number and quality of sperm they ejaculate depending on how likely it is their partner has had sex with someone else and that individual human sperm are designed to wrap around the sperm of rival males and strangle them. Some of their ideas especially kamikaze sperm have been shown to be thinly supported.
"Their work was pioneering but certainly not conclusive," Thornhill says. "They were reasonable hypotheses. Then Todd began to get interested in this stuff and has taken it very seriously. I would put Todd there as the person who has done most of the most important empirical work to look at human sperm competition."
Shackelford was inspired by Baker and Bellis' work, despite its methodological flaws. But he added his own approach along with looking for physical signs of sperm competition, such as "attack" sperm, Shackelford and Goetz looked for psychological effects as well: behaviors, like jealousy and rape, that may have evolved in response to the pressures of sperm competition.
When it comes to procreating, men and women face very different outcomes. A woman never has to worry whether the baby she's putting all her time and energy into is passing on her genes: She knows it is, because she saw it come out of her body. But a man can never be sure: All he knows is that his sperm went into a woman and a baby eventually came out.