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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.