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President Barack Obama isn't as great as he thinks he is. To be fair, neither were Presidents Bush or Clinton — or Washington or Lincoln, for that matter. The same can be said for every general who ever commanded an army or every boss who ever ran an office. The fact is, if there's one thing that defines people in powerful positions, it's that they overestimate what they can do with that power.

That, at least, is the conclusion of a study published in the current issue of the journal Psychological Science. And while you may have always suspected that the folks who run the world aren't all they're cracked up to be, don't take too much satisfaction from the fact. It's the rest of us who wind up paying for their overreaching. (See pictures of how Presidents age in office.)

Having a lot of faith in yourself can be a very good thing. Decades of psychological studies have found that people who believe they have some control of their lives and circumstances are more optimistic and proactive and have higher levels of self-esteem than others. People who believe events control them are likelier to be depressed and pessimistic and to avoid challenging situations. But what happens when your sense of control spins out of control? Try to cross the ocean with nothing but a rowboat and muscle, and you're not going to get very far. (See TIME's list of the 100 most influential people of 2008.)

To explore how we come by our illusory sense of power, a team led by business professors Nathanael Fast and Deborah Gruenfeld of Stanford University devised a series of experiments. In the first, they recruited 38 students and divided them into three groups. They asked one group to write about an experience in which they felt they had control over other people, and another group to write about a time they felt out of control. The third group wrote nothing. All of the students were then given dice and told that if they correctly guessed the number they rolled, they would win $5. They were also given the choice of rolling the dice themselves or having someone else roll for them.

The results were striking. One hundred percent of the students who had written about being in charge rolled the dice themselves, compared with only 58% of the students who had written about someone else having power over them. Sixty-nine percent of the control group chose to roll. There may be nothing quite as random as a roll of the dice, but the students on a power high appeared to believe they could do it better. (Read "Does Power Corrupt? Absolutely Not.")

"Choosing to roll the die represents an illusory sense of control," the researchers write. "[But] the outcomes were uncontrollable."

In a second test, 30 volunteers were paired off for a role-playing game in which one partner was assigned the part of the superior and the other the part of the subordinate. All of the volunteers were then asked to read a scenario about a fictional marketing agency and rate how likely they would be to improve the agency's profitability next year. The people who had been designated superiors in the role-playing game consistently believed they would do better at the helm of the agency than the people who had been designated subordinates. In the third study, 79 volunteers again wrote about a time they had either been in control or under another's control, and then completed a questionnaire designed to measure their self-esteem, initiative and perceived ability to influence their world. Across the board, the people who had written about being in charge scored higher. That's good — but not if it goes too far.

"By producing an illusion of personal control," the authors write, "power may cause people to lose touch with reality in ways that lead to overconfident decision-making." (Read "Gut Decisions May Not Be Smart.")

With stock markets collapsing around the world, nobody needs an illustration of where that kind of hubris can lead. Ordinary folks with bills to pay may smell something funny in fiscal instruments with names like "credit-default swaps," but when you work on Wall Street and people call you a "master of the universe," you think you can make the things pay off. Even nonpartisans would agree that George W. Bush waded into the Iraq mess with more certainty than strategy. And Bill and Hillary Clinton might actually have achieved health-care reform if they had tried negotiating with the opposition instead of steamrolling it. (See George W. Bush's biggest economic mistakes.)

"One way people in power can guard against this is to place themselves into a deliberative mind-set, focusing on the pros and cons," says Fast. "This takes a great deal of discipline, however, as the tendency after taking power is to move straight to action."

None of this is to say that all cockiness leads to ruin — sometimes it's what's needed for a leader seeking greatness. During his brief presidency, John F. Kennedy had the preposterous idea that the CIA could topple the government of Cuba and the equally foolish notion that the U.S. could put a man on the moon before 1970. One plan led to the Bay of Pigs, the other to the Sea of Tranquility.

"It is likely," says Fast, "that some of history's most tragic failures and inspiring successes were orchestrated by power holders who overestimated their abilities to control future outcomes." Here's hoping that the leaders we've got now are guessing right.

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• Jeffrey Kluger