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This finding took a darker turn when he applied it to expectations about culture and race. Stereotypes hold that Asian-Americans excel at math but women do not. So Ariely divided a group of Asian-American women into two. One group answered questions related to gender; the other, race and gender. Then he gave them all a math test. The second group did better than the first.

"The results," Ariely writes, "show that even our behavior can be influenced by our stereotypes and that activation of stereotypes can depend on our current state of mind and how we view ourselves at the moment."

Those findings dovetail with his research on the power of suggestion.

For instance, people like to believe that their moral compass always points to true north. Through a series of experiments, Ariely showed that ethics are often situational -- in general, people will cheat on a test if they are sure they can get away with it. Unless, that is, they are asked about the Ten Commandments or told to sign an honor pledge beforehand. Then they stop cheating completely.

Now consider drugs. A truly rational person will judge a drug's effectiveness by how well it works. Ariely let one group of test subjects believe a pain-relief pill cost $2.50 and another that it cost 10 cents. He zapped both with excruciating charges. Guess who thought the medication was more effective?

Ariely's research also debunks the adage about people knowing the price of everything but the value of nothing. In fact, our understanding of a fair price is highly relative. Williams-Sonoma had few takers when it introduced its $275 bread maker. "People were unfamiliar with the product," he said, "so they didn't know if that was a fair price."

Sales rose when the company introduced a far more expensive model. "Nothing had changed," Ariely observed, "except now that looked like a bargain."

"Predictably Irrational" bursts with such provocative insights: why it is always best to be the first one at your table to order in a restaurant, why we overvalue the things we own, why free will is not all it is cracked up to be.

"The bad news is that we are at the mercy of powerful forces," Ariely said. "The good news is that we can identify them and, perhaps, do something about them."

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