Much of what happens in the brain is not evident to the brain itself, and thus people are better at playing these sorts of tricks on themselves than at catching themselves in the act. People realize that humans deceive themselves, of course, but they don't seem to realize that they too are human.

A Princeton University research team asked people to estimate how susceptible they and "the average person" were to a long list of judgmental biases; the majority of people claimed to be less biased than the majority of people. A 2001 study of medical residents found that 84 percent thought that their colleagues were influenced by gifts from pharmaceutical companies, but only 16 percent thought that they were similarly influenced. Dozens of studies have shown that when people try to overcome their judgmental biases -- for example, when they are given information and told not to let it influence their judgment -- they simply can't comply, even when money is at stake.

And yet, if decision-makers are more biased than they realize, they are less biased than the rest of us suspect. Research shows that while people underestimate the influence of self-interest on their own judgments and decisions, they overestimate its influence on others.

For instance, two psychologists, Dale Miller and Rebecca Ratner, asked people to predict how many others would agree to give blood for free or for $15, and people predicted that the monetary incentive would double the rate of blood donation. But when the researchers actually asked people to give blood, they found they were just as willing to do it for nothing as they were for a $15 reward.

The same researchers measured people's attitudes toward smoking bans and asked them to guess the attitudes of others. They found that smokers vastly overestimated the support of nonsmokers for the bans, as did nonsmokers the opposition of smokers to the bans -- in other words, neither group was quite as self-interested as the other group believed.

Behavioral economics bolsters psychology's case. When subjects play laboratory games that allow them to walk away with cash, self-interest dictates that they should get all the cash they can carry. But scores of experiments show that subjects are willing to forgo cash in order to play nice.

For instance, when subjects are given a sum of money and told that they can split it with an unseen stranger in any proportion they like, they typically give the stranger a third or more, even though they could just as easily have given him nothing. When subjects play the opposite role and are made the recipients of such splits, they typically refuse any split they consider grossly unfair, preferring to walk away with nothing than to accept an unjust distribution.