Why do powerful men risk everything for sex? It has to do with brain chemistry, evolution and, yes, testosterone.

We'll never know exactly what New York Gov. Eliot Spitzer was thinking when he allegedly arranged a dalliance with a high-priced prostitute, risking the collapse of both his career and his family. Even he may not fully understand his own actions. But all too many powerful men can at least identify with him, because they've been there. Spitzer is simply the latest married politician caught with his pants down, a group so large that "pretty soon there will be enough of them to do a scientific study," says Texas psychologist Brian Gladue. Why do men with so much to lose take the chance that they may in fact lose it? Psychologists say they fit a profile: the traits that help them succeed at high-powered jobs are often the same ones that cause them to fail in their personal lives. Newsweek's Mary Carmichael asked several analysts to put the typical philandering politician on the couch.

Gimme More: Many fallen politicians fit a personality type known as a "sensation seeker," defined in the early 1970s. Sensation seekers crave novel and intense experiences more than other people do, and, as part of that, they tend to have many sexual partners. "They get a bigger kick out of things," says Marvin Zuckerman, a pioneering psychologist and author of the 2006 book Sensation Seeking and Risky Behavior. There's chemical evidence: sensation seekers have lower levels of monoamine oxidase A, which regulates the brain's levels of dopamine, the "pleasure" neurotransmitter.

Of course, loving life isn't always a bad thing: sensation seekers are often high-energy, high-functioning people. The problem is that they never seem to get enough excitement. "Their experiences have to be either very new or very intense, or both, or else they get very restless," says Zuckerman. "When things get monotonous, they have to do something else to increase their arousal." That's the flipside of finding pleasure more pleasurable: for sensation seekers, boredom is also more boring.

Risk Rules! Sensation seekers don't just lust after things--they take them, often disregarding the risks that block their way. "When you're dealing with these high-level, in-your-face, go-for-everything guys, you're dealing with people who take a lot of risks. If that results in gains for them, they get on a roll, and pretty soon their risk management starts to fade a little," says Gladue, who is based at the University of North Texas Health Science Center. "At some point, they can't manage every aspect of their lives. They have to blow off some steam, so they say to themselves, 'this is something I'm going to do for thrills or chills or fun. It's kind of dangerous, and I'm not going to worry about it.' For politicians, that's often in their private life, where they don't have people managing them all the time. And that's where things get out of hand."

For these types, the risk itself is part of the reward. "Breaking rules is a thrill for them," says Frank Farley, a psychologist at Temple University. "Look at Spitzer: he's Mr. Rectitude, the terror of Wall Street, and he busts prostitution rings, and yet he allegedly goes into that very lions' den - the prostitution ring - and partakes. If that isn't risk-taking I don't know what other label to put on it."

He's Hormonal. Alpha males are high on testosterone, the hormone that underlies almost all the typical traits of the politico-sexual animal: high levels of testosterone make for a high sex drive, a love of risks, aggressiveness and competitiveness. "These people have a strong need to win at games, which is obviously important in power politics," says Zuckerman. Success sends their testosterone spiraling up, while a loss brings the levels down - a phenomenon that's been documented in the lab as well as in athletes and chess champions.

Women's testosterone levels also rise when there's competition on the line, but the actual act of winning - or, for that matter, losing - doesn't have any effect on the levels either way. It's the game, not the outcome, that makes the difference for women. Success, then, may not set them off-balance the same way. Evolutionary psychology also suggests that women leaders wouldn't be as likely as men to get caught in sex scandals. "Men and women play different roles in reproduction, so I don't think that you'd see the same kind of pattern where high-status women would be more likely to seek out lots and lots of men," says Daniel Kruger, a research scientist at the University of Michigan who has studied risk-taking behavior. "That's not going to really benefit them that much because they're limited in the number of children they could have." Men, on the other hand, have more of a biological imperative to spread their genes far and wide--the kind of privilege that often comes with being an alpha male.

Hungry for Power. Not everyone wants to be a high-profile politician. It takes, among other things, supreme confidence - the kind that may shade into egocentrism and lead to downfall. "For high-profile offices - we're not talking about the school board, but mayors, governors, senators, some members of Congress and the presidency - you have to have a kind of personality where you are very interested in yourself and your personal needs, as well as the needs of others," says John Gastil, a University of Washington political scientist. "When the gratification of your desire for social change becomes the justification for so much of what you do in your career, it's not a leap to then say, 'Well, my other desires and needs are equally justified.' You come up with elaborate justifications. 'Hey, 23 hours day I'm working hard for the people of New York. Time for a little me time!'"

Ironically, that kind of confidence is part of what appeals to voters. "We love charismatic people, the 'micro-messiahs'," says Gastil. "We favor the candidates who are already concerned with projecting certainty and power and strength - and we cultivate those characteristics in people. We want a little bit of that sense that these people are special and different. Does that go to their heads? Of course it does."

And then power has its own corrosive effects. A person who seeks out power may already be compromised. But once he's got that power, he may be tempted beyond anything he's experienced before. "We sometimes say, 'God, what do these people think, the rules don't apply to them?' Well, that's often true. They really do live in a different world from most of us," says Gastil. "Spitzer apparently had access to a service where you pay top dollar for exclusivity and discretion - one that most people don't have access to. Probably your average philanderer doesn't know such a company even exists." Remember the explanation Bill Clinton gave for his cheating: "I did something for the worst possible reason - just because I could."

As the saying goes, power is also an aphrodisiac - and that's been true, says Kruger, as long as humans have been around. "In our evolutionary history, men who had lots of resources and status and power were able to have more than one partner. Your body is basically saying if you have this power, you should use it, because that's what has worked before," he says. "Even in modern history, whether you're talking about medieval kings or sultans or rock stars, quite a few have multiple partners. So you're not so surprised to see this dynamic in politics today."

He Thinks He's Invincible. Bloggers and commenters have been floating the idea that Spitzer was subconsiously hoping to be caught. But that, at least, is one negative trait that psychologists hesitate to ascribe to him. "The idea of a death wish, that he was self-destructive - I don't think there's a shred of reason to believe that," says Farley.

Instead, the opposite may be true: not only was Spitzer hoping to get away with something, he honestly thought he'd be able to. "It does have an element of Greek tragedy to it. There's a certain amount of hubris that goes with getting to the top," says Gladue. "You think you're invincible. You just don't think it could happen to you." Until, of course, it does.