Freakonomics: odd word, two roots that don't belong to the same tree. The –onomics part speaks of the august, dour study of economics. But freak speaks of - freaks and the freaky.

The word says it all: Look closely at the reasons people do things, and what you find can be really - freaky.

Coined by economist Steven D. Levitt and journalist Stephen J. Dubner, freakonomics is on many people's lips. Dubner appears at 12:30 p.m. today at the Free Library to talk about their new book, Superfreakonomics.

The two met in 2003, when Dubner profiled Levitt for the New York Times Magazine. The article attracted such interest - they were besieged with questions from everyone from the New York Yankees to prison inmates - that the two realized they should combine forces and write a book.

It asked freaky questions: Why do real estate agents push to sell a house quickly and cheaply? Why do teachers help students cheat so much on standardized tests? How come so many drug dealers live with their parents? (Answers elsewhere on this page.)

They titled the 2005 book Freakonomics: A Rogue Economist Explores the Hidden Side of Everything. Powered in part by wild popularity in the blogosphere, it has sold more than four million copies.

This is serious econ, though, based on rigorous research. It's called "behavioral" economics. Other kinds, including "classical" economics, see human beings as making choices based on rational analysis. It's one of the best-known assumptions in all economics.

"Funny thing: Almost all economists know this is wrong," says Levitt, calling as he walks near the University of Chicago campus. "It assumes enormous, prodigious brainpower that allows people to calculate incredibly complex math problems and always get them right. No one can do such things - but talking as if they did is the easiest way to go in economics. Yet anything that relies heavily on the assumption of rationality risks being very wrong."

What makes behavioral economics different? Dubner, calling from his New York office, says it starts with realizing that what guides human behavior is not some inerrant rationality, but rather self-interest.

"It's important not to confuse self-interest with selfishness, which operates at the exclusion of other people," Dubner says. "Self-interest means only that you are concerned primarily with getting what you, or your family, or your team need."

So not reason, or not reason alone, but what we think best serves us and our needs. That pulls in the passions, the drives, and the situation. So while behavioral economics is data- and number-driven, much is also story-driven, attentive to the point of view of people as they choose.

Not everyone loves freakonomics. Some economists find it too "soft." Blogger Alex Pareene of the Web site Gawker calls it a "pop-econ trend that is the most annoying application of make-believe science since pop psychology." Others dive-bomb math errors or object to the public-policy implications of Levitt's conclusions. But the authors have found an audience and are challenging old ideas.

Whether the issue is prostitution, the fight against terrorism, or organ donation, Levitt and Dubner mix data with smart, humane storytelling.

"I think all of economics is about storytelling," Levitt says, "but that doesn't make this work any less rigorous."

"What turned me on to economics about 12, 13 years ago," says Dubner, "was the work of [psychologists] Amos Tversky and Daniel Kahneman. What made it so compelling was that it's often in story form. It's all in the framing. . . . Knowing the setting of a decision, and the mind-set of the decision-makers, is crucial to understanding the decision."

Superfreakonomics is written for noneconomists. Using wry humor, the authors explore unexpected areas - and often, a huge economic change turns on a noneconomic hinge. One memorable chapter focuses on prostitutes in Chicago. The average charge for one particular service has fallen greatly in recent decades. Why?

Turns out it's more than dollars and cents. As Levitt and Dubner write, a prostitute's harshest competitor is the sexual partner who is free. Attitudes and practices have changed, making free alternatives more common. Thus the price of a once-expensive service has slumped.

"All economics is like this, really," Levitt says. "Economists take an existing state of affairs, and then a shock happens: Say, health care costs so much at one point, and then it changes as people start living longer due to medical advances. Almost always, something like this is hidden, a social change, a technological change, that affects prices and quantity."

Levitt and Dubner say that the bigger and more life-changing a decision gets - should I marry or not? What profession should I have? - the harder it is to make. Such decisions are rare, so we lack experience making them. Worse, we can't be certain which choice is best.

"The decisions we fret most about," Levitt says, "are the ones we aren't really equipped to decide correctly. People agonize over them the most, because you're trying to figure out the direction of your life. Ironically, you may as well flip a coin."

Decisions such as what, if anything, to do about climate change are even cloudier. When they looked at climate change, Dubner says, "we were floored by the uncertainty, the power of external factors, how they create problems and make these issues even harder to solve."

The duo find that our models are crude; few agree on what must be done: It's not certain any policy will have measurable effect, and it would be very hard to create incentives for people to change their behaviors.

(The Union of Concerned Scientists, among others, is on record as saying Superfreakonomics misrepresents the science behind climate change. As one might expect.)

Should terrorists buy life insurance? If you're drunk after a party, is it better to walk home or drive? Has legalized abortion affected the crime rate? Freakonomics presents answers that surprise, cheer, and disconcert. Levitt and Dubner's books present a view of the world both fun and profound, in which human choice itself emerges as - superfreaky.

Freaky Economics 101

Freakonomics often comes up with unexpected questions and even less expected answers. A freakonomics Q&A:

Why do real estate agents push to sell a house quickly and cheaply? Real estate agents do get a commission, but as the sales price of a house rises, that commission doesn't rise enough to motivate them to wait and press for a higher price. So, it turns out, agents often advise buyers to underbid and houses end up selling more cheaply.

Why do teachers cheat so much on behalf of their students on standardized tests? Because these tests are supposed to evaluate the performance of both students and teachers, after all - which motivates teachers to pump up poor students' performance, and thus their own.

How come so many drug dealers live with their parents? Some drug kingpins may make a lot of money - but their underlings don't make much at all, so they tend to live with their moms.

If you're drunk after a party, is it a better idea to walk home or drive? If you walk home, statistics show, you're somewhat more likely to be killed (drunks tend to lurch, walk into the street, etc.). So drive, right? Wrong. If you drive, you're much more likely to kill others. Walk, and save lives.

Should suicide bombers buy life insurance? At first glance, no: Insurance companies don't pay out for suicides. But think again: The feds looking for terrorists see adult male with family but no life insurance as a red flag. That argues that you should buy it, since the feds are looking for men who don't.

Source: "Freakonomics" and "Superfreakonomics"

Contact staff writer John Timpane at 215-854-4406, jt@phillynews.com, or twitter.com/jtimpane.