In the winter of 2001 at Florida Atlantic University, Shackelford and his graduate student Aaron Goetz were studying "sperm competition" in human beings the notion that, just as other male animals try to spread their seed (and genes) widely in the evolutionary game of life, men and their sperm have developed ways over the past few million years to impregnate as many women as possible.
Although their research usually involved asking college students to fill out surveys about their sex lives, Shackelford and Goetz were also interested in a theory called "sperm displacement," and investigating it called for something a little more hands-on.
They wanted to know if men use their penises in a particular, thrusting way to displace potential rival sperm lurking in a woman's vagina. But studies with actual people having actual sex would be difficult to get permission for at a public university, so Shackelford's small staff in the psychology department quietly spent the first part of 2002 preparing an experiment that would model thrusting movements with an artificial penis and vagina and lots of simulated semen.
"Fake, you know I made it with baking soda and some other nice things," Goetz says.
"There are recipes on the Internet, as you would imagine," Shackelford explains. The team planned to determine the best recipe via a ministudy, which they submitted to FAU's Institutional Review Board. "We would rate it for viscosity and if it felt real. It would be a blind situation. We had a couple of female graduate students and me and Aaron."
While waiting for the IRB's approval and thinking that a foreskin might make an especially effective sperm displacement tool, they also struggled to build an uncircumcised penis, which they planned to compare to penises without foreskins. "We know that penises in many cases appear designed to scoop out or grab sperm," Shackelford says. "In species of cats, for example, the penis has barbs and appears to be designed to scoop out semen placed there by a previous male. So it's not unreasonable. In human semen displacement, the foreskin might reach up and just grab hold of rival semen." But before they could construct a working foreskin, the team got a nasty surprise. FAU's IRB did not approve the proposal to test fake sperm.
Nathan Dean, then dean of FAU's College of Science, wrote Shackelford in February 2003 to deny him permission to test his fake semen. He had consulted two other faculty scientists who called Shackelford's work "highly speculative" and "poorly substantiated." In a memo to Shackelford, Dean wrote: "I would encourage you to explore more carefully the physiological basis of the work, in order to determine whether sperm competition in humans is a valid concept."
Shackelford says he felt shot straight through the heart. "What we got back was simply, 'This is pseudoscience,'" he says. "They were so offended by the notion that we were concocting simulated semen and had the wherewithal to say that we were going to rate it for how real it felt, they absolutely freaked out."
For 18 months, his work was on hold as he fought for the right to conduct his research. He produced a 600-page memo, packed with studies and articles, arguing that his life's work wasn't pseudoscience. Eventually, he abandoned the artificial semen and the functional foreskin entirely.
Five years later, Dean Dean has been replaced and Shackelford is tenured and is being considered for a full professorship. He is considered a rising star in the field of evolutionary psychology, and FAU, eager to get national attention, has apparently forgotten that it had any previous doubts about the legitimacy of his work.
Now Shackelford is steadily building his case, using state and federal money, to prove that jealousy, deep thrusts during sex, and even rape are all male behaviors that are biologically programmed by evolution so that men can pass on their genes to as many offspring as possible. And though few outside the growing and controversial field of evolutionary psychology seem aware of it, the professor is poised to make FAU a center of some of the most contentious science in the world.
As Aaron Goetz explains sperm competition to a room full of fellow scientists and students at FAU, the 27-year-old looks more like an actor playing a graduate student than the real deal. A dusting of stubble and slightly unruly hair softens his aquiline face as he talks casually about ejaculation and sperm motility while covering a blackboard with chalk drawings of sperm cells until there's no room for anything else.
In the front row, his mentor, 35-year-old Shackelford, fidgets as he watches his protégé through small, round-framed glasses and over a clipped beard.
"Some people are very resistant to this research," Goetz says, anticipating criticism, as he pulls up a PowerPoint slide illustrating an evolutionary tree of life representing all of the species on Earth. He points to a small label that reads "You are here" to make the point that evolution has shaped people just as much as it has shaped every other living creature. "If you still have doubts, I suggest that you read the literature."
Shackelford later makes the same point, saying that critics of his ideas simply don't like to admit that humans are subject to the same kind of instinctual, biological determinism that shapes the lives of other animals. Evolutionary psychologists believe that millions of years of evolution not only gave us big brains and an upright walk but also behaviors that adapted us to life in the wild. Today, living in societies and no longer on an African savanna, we are still influenced by those instinctual behaviors. Or so goes the theory.
For example, the pressures of natural selection may have favored men who rape their potentially straying mates. This has become the centerpiece of their work together: Shackelford and Goetz believe that a man is hard-wired in his brain to fuck his wife harder or even force her to have sex against her will if there's a chance that another man's sperm is inside her, on its way to creating a child.
"It is what it is," Shackelford says. "The data are the data. We propose an hypothesis, we collect the data, and we make tentative conclusions. That's good science, that's good practice. We've got big-time publishers publishing our books now. It's not just me and Aaron, off in never-never world. We've got peer-reviewed publications, dozens of them, in first-class journals."
But there is at least one critic in the room today. A man wonders if they're misreading their data. Could women resist their partners simply because they've already been satisfied by a third party?
When Goetz answers with a string of jargon and something called "resistance/persistence theory," a female psychology professor, Erika Hoff, chimes in with a wry laugh.
A woman refusing to have sex with her mate? "It couldn't possibly have anything to do with him," she chuckles.
Both Shackelford and Goetz were aware that applying evolutionary theory to modern human psychology was bound to ruffle feathers. But they aren't the only ones doing so at FAU. The Boca Raton-based state university has become a hothouse for the field, and earlier this year, New Times profiled one of Shackelford's colleagues, David Bjorklund, who is pursuing his own experiments in an effort to prove that the concept of God is a figment of the imagination and a byproduct of evolution ("The God Fossil," March 9).
It was that openness to radical ideas, Shackelford and Goetz both say, that lured them to the school.
"I definitely appreciate pushing the envelope and making people, including myself, uncomfortable," Shackelford says. "I thrive on it. And I enjoy the controversy."
Shackelford found his controversial calling when he was a teenager in the Bible Belt. When he opened his textbook in biology class at his very Catholic high school in Tulsa, Oklahoma, he knew he was meant for Darwin.
"I swear to you, the stuff on evolution was blacked out," he remembers, sitting in his small, windowless office in the FAU psychology department. "And I was like: That's what I want to study. I loved the ideas, because they were sort of 'wrong. '"
As an undergraduate, he attended the University of New Mexico, the academic home of several evolutionary psychology luminaries. "I didn't know it," Shackelford says, "but I was at the center of evolutionary studies."
While reading The Origin of Species in an honors psychology class, Shackelford asked unwelcome questions about how natural selection applied to humans. The professor, an avowed creationist, referred Shackelford to the evolutionary psychologists. "He didn't seem happy about applying Darwinian thinking to humans," Shackelford says. "But I loved thinking about things that I wasn't supposed to be thinking about."
By the time he'd finished his doctorate at the University of Texas in 1997, Shackelford was bucking against the very edges of the discipline itself. After blanketing universities across the country with applications, he found himself interviewing at only three an absurdly small number for a qualified young scientist. Two of the interviews were unpleasant. The third was at FAU.
"Perhaps we are a little more open-minded," Bjorklund says about Shackelford's almost universal rejection from prestigious American universities. "He was selling himself as a social psychologist from an evolutionary perspective. It reflects the state of the field ten years ago, when EP was still a novelty and there were very few people who would call themselves evolutionary psychologists getting a foothold in academia."
By the time Aaron Goetz was in college, times had changed. As a child growing up in San Antonio, Texas, he thought that he might someday want to be a shrink. But on the first day taking a class at the University of Texas at Austin from David Buss, one of Shackelford's own mentors, he knew that evolutionary psychology was for him. "I called my mom and said, 'Mom, I know what I want to be when I grow up,'" he says. "And that was an evolutionary psychologist. It was the integration of evolutionary thinking into human behavior, which made complete sense to me. I mean, how else could it be?"
It made so much sense that he used an EP perspective in his thesis, "Alternative Female Mating Tactics: The Role of the Satellite Female," to study the tactics unattractive women use to pick up men while out with more attractive female friends. He concluded that ugly girls use humor in place of looks. "Perhaps it is no coincidence that most female comedians have unattractive bodies," he suggested.
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.
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."
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.
When the testicles of primates are averaged against body size, gorillas as a group come in at the low end and chimpanzees at the top. Humans, however, have middling sized testicles, which appears to tell us little about how much our genitals evolved to deal with sperm competition. Did we develop medium-sized jewels because ancestral men guarded a small harem or happily traded lots of female partners with other men or maintained monogamous relationships? There's just no way to know for sure.
But Shackelford, undaunted, wants to measure individual men while surveying them for how promiscuous they are.
"We'd say, 'The final part of this study is to please take these calipers to the restroom,'" he says. "We'd show them how to turn it on, and then we give them a diagram and show them 'Here's where you're going to put the width of your testes' and 'Here's where you put the length.' They'd measure, write it down, and then you would have them put it into an envelope and bring it back, so you don't have them worry if we're trying to see anything."
Shackelford expects that the men with larger balls would report more sleeping around than those with smaller balls. He'd also ask the men about their attitudes toward children and child-rearing in order to test another prediction: that big-balled men are more likely to invest their time and energy in impregnating as many women as possible, while the small-balled would focus on being good fathers. Shackelford calls it a "cads versus dads" scenario.
And it turns out, Shackelford knows something about both categories. Along with the calipers, another thing that catches your eye in his office are the photographs of all of his own genetic successes five children, the youngest of whom is named after Darwin.
Shackelford met his current wife, Viviana Shackelford, when she was an undergrad and he a grad student at FAU in 1997. (Now Viviana works alongside him as an evolutionary psychologist at FAU.) At that point still married to his first wife, Shackelford performed an evolutionarily advantageous behavior: He cheated. It took him three years to divorce and remarry. Accustomed to questions about his personal life, Shackelford doesn't bristle when asked if he's using his research to justify his own infidelity.
"To justify it? No. Maybe to explain it. But it was a nightmare, for everybody involved," he says. "The pain is very real."
Shackelford knows that to theorize about such behavior may look like an attempt to vindicate it. But he insists that we're better off understanding the root causes of jealous and violent impulses.
"I'm trying to find the truth," he says. "Sometimes, the truth is upsetting. But if we're not trying to identify what the truth is, then we're in the wrong. That's in principle what we're doing trying to get a little closer to what reality is.
"It's just like parental love," he adds. "I can appreciate that children, in some very real sense, are parasites. But at the same time, I can also truly say that I love them. I can recognize that those feelings are there by design. I don't think that they're conflicting ways of thinking about things. They dovetail beautifully."
And being aware of the evolutionary forces pulsing through their veins, Shackelford and Goetz say, can even be good for their relationships.
"You can really get a handle on jealousy," Goetz says. "So we just have these natural feelings, like when your partner's talking to some guy just a little too much and you feel these jealousy pangs. Then you can just think, 'Oh, it's all right that's just a perceived threat to my relationship.' And you know that you're not acting immature or irrational. You can just blow it off because you understand exactly what the function of jealousy is."
"Unless you determine that the threat is real...," Shackelford teases, and both men laugh.