And there is perhaps no student more attentive to this fact, and glad for it, than a call girl named "Allie" who asked to be included in Dubner and Levitt's research.
"She is a very bright woman. She understands the economics pretty well," said Dubner. "She said, 'Thank God prostitution is illegal,' 'cause if it weren't, I wouldn't be making $500 an hour; I'd be offered maybe $100 an hour, and I wouldn't be a prostitute. I would probably be back doing what I was doing -- which was working as a computer technician for a Fortune 50 company.'"
But Allie had ambitions of working for a different Fortune-listed company -- her own. Adapting business concepts of low overhead, market value and the technological expansiveness of the Internet, she used her bedroom as an office and took a high-heeled first step into the world of high-end escorts.
"I originally thought it would be thrilling to try once," Allie told ABC News in an interview. And it wasn't. It wasn't as glamorous as I thought. But it wasn't awful. And the guy was very nice. I walked away from it thinking, "That wasn't so bad. I was probably better than a lot of my online dating situations and now I have a car payment paid."
For Allie, it was a case of simple mathematics.
"It just became really hard to sit at a computer and program for 40, 50, 60 hours a week for the exact same amount of money I would get going to the Four Seasons for three hours and have a bottle of champagne and, you know, in the company of a gentleman," said Allie.
Allie is a direct beneficiary of all those women who marched for equal rights, as Dubner and Levitt see it. Access to a military education made her a savvy businesswoman. Liberation gave her the freedom to use her body as she sees fit.
But it was Allie's natural business acumen that impressed Dubner and Levitt.
"I had been talking a lot with businesses, trying to help businesses to be more profitable. And I was asking Allie the same sorts of questions. And she was giving great answers -- better than many of the CEOs that I interact with," said Levitt.
Allie's approach to safeguarding her protection during client meetings was especially memorable to Levitt.
"She came up with this brilliant idea," said Levitt. "She calls customers at their work, before she'll ever meet them. And, ostensibly, she calls them just to tell them how much she is looking forward to their date that night. But really what she is telling them is, 'I know where you work. And if anything goes wrong, I am gonna call you there.'
"And it turns out to be an incredibly simple fix, a way of letting the customer know that if anything goes wrong, there is gonna be big trouble, uh, but doing it in such a nice way that no one could possibly be offended."
It seems that even a call girl can teach an economist a new trick.
"When I think about well, how economics works in the world and how you use information to your own advantage -- that is one of the most brilliant fixes I have ever heard," said Levitt.
But vetting clients in advance isn't always fail-proof.
"There were several times I, you know, had to ask someone to leave," said Allie. "You're, of course, in a vulnerable situation. I'd never been hurt, thank goodness, but you know, someone shows up at your door and they're not kind."