There is at least one contradiction in Superfreakonomics.

In the epilogue of the book entitled “Monkeys are people too,” Levitt and Dubner discuss the research by M. Keith Chen and Laurie R. Santos with capuchin monkeys. Chen and Santos introduced money in a small group of capuchin monkeys and taught them how to use it. Eventually, the capuchins learned that coins had value and they could exchange them for valuable commodities like food.

One of the things that Chen and Santos discovered in their research is just how humanlike the capuchins are. As soon as they learned that coins had value, one of the male capuchins gave a coin to a female in exchange for . Yes, capuchins engage in prostitution. The observation that nonhuman species engage in prostitution is not new, however. Frans de Waal and other primatologists have long observed that bonobos also engaged in prostitution, by exchanging food for sex.

If monkeys and apes routinely engage in prostitution, then it means that the evolutionary origin of prostitution probably dates back before we were human. It means that prostitution is indeed the world’s oldest profession.

So far, so good. However, Levitt and Dubner contradict themselves in Superfreakonomics. In the Introduction, they describe an encounter where a venture capitalist, who attended one of Levitt’s lectures one day, had a “date” with a $300-an-hour prostitute later in the same evening. The venture capitalist notices a copy of Freakonomics in the prostitute’s apartment, and mentions that he had just attended a lecture by one of the authors of the book, in an attempt to impress the prostitute. Levitt and Dubner note: “The male instinct to impress the female is apparently strong even when the sex is already bought and paid for.”

This does not make sense. If monkeys and nonhuman apes routinely engage in prostitution, as the research by de Waal, Chen and Santos, and others seems to indicate, and if the evolutionary origin of prostitution thus dates back long before we were human, then it means that prostitution is evolutionarily familiar. If prostitution is evolutionarily familiar, then men’s brain should be able to recognize prostitutes and to treat them differently from “ordinary” women, whom they do have to impress if they want to have sex with them. In other words, there should be an evolved “hooker module” in the brain.

The deep evolutionary origin of prostitution and prostitutes and thus their evolutionary familiarity suggest that men would not try to impress prostitutes, because they know it is not necessary, contrary to what Levitt and Dubner suggest (perhaps in jest) in the Introduction. I don’t suppose there are any systematic and high-quality data on how men treat prostitutes, whether they indeed try to impress them, even when sex with them is a sure thing. If it turns out that men routinely attempt to impress prostitutes before having sex, then it means that prostitution is evolutionarily novel and it is not the world’s oldest profession.

There’s something else. The data from the General Social Surveys show that, even controlling for , income, age, race, religion, political attitude, , and survey year, more intelligent men are significantly more likely ever to have paid for sex. Intelligent men are more likely to have had sex with a prostitute than less intelligent men. According to the logic of the Hypothesis, this suggests that prostitution is evolutionarily novel and there were no prostitutes in the ancestral environment. That is why more intelligent men are more likely to recognize and comprehend prostitution, and to have sex with prostitutes.

Is prostitution evolutionarily familiar or evolutionarily novel? This is just one of the questions that keep me up at nights....