Why do so many politicians throw it all away for a moment of madness? This week Anthony Weiner, a candidate for mayor of New York and former Democratic Congressman, publicly admitted he's been sending lewd messages to strange women online.
Again. Weiner resigned from Congress in 2011 after confessing to "sexting" several females via Twitter. He sent a photo of his boxer shorts, beneath which he was visibly aroused, to a 21-year-old student.
When the conservative blogger Andrew Breitbart got hold of the picture and went public with it, Weiner denied everything - saying he did not recognise the offending bulge and that he was the victim of a right-wing conspiracy. Only when yet more photos came out did he quit the day job. But you can't keep a bad man down, and in May a "reformed" Weiner declared his candidacy for the New York mayoralty.
New Yorkers might have been prepared to give him a second chance, but history repeated itself. On Thursday, Weiner gave a shamefaced press conference in which he admitted that he had continued to engage in online sex chat after he left Congress. Calling himself Carlos Danger, he'd started a relationship with a 22-year-old woman identifying herself as "Sydney Leathers". Displaying a self-confidence that is heroic bordering on the deluded, Weiner says he's staying in the mayoral race.
What compelled the otherwise talented Weiner to go on sinning? Shortly before Breitbart's tragic death from a heart attack, I had lunch with Weiner's nemesis and he spun me an interesting theory. The Congressman was close friends with actor Ben Affleck, and they had a habit of burnishing each other's egos: Affleck told Weiner he would make a great mayor and Weiner encouraged Affleck to run for office.
Breitbart thought that Weiner came to see himself as a star rather than a politician, and had adopted Hollywood's nastier habits of vanity and sexual incontinence.
It's a neat idea, but it doesn't explain the antics of politicians who don't live in the shadow of Hollywood. In Britain, plenty of less well-connected men have engaged in risky sexual adventures that imperilled otherwise promising careers. Recall Ron Davies, Tony Blair's first Secretary of State for Wales, who resigned after a mysterious "moment of madness" on Clapham Common. Or the formerly uninteresting Lib Dem MP Mark Oaten, who quit his job when a newspaper alleged he had engaged in a menage a trois with rent boys. Neither man had spent much time in Tinseltown.
Perhaps the real answer is that a certain kind of personality is drawn to politics: a business that rewards machismo and risk-taking. Politicians need to force their personalities on others, and that quality is reinforced by sycophantic supporters.
So it's not surprising that office-holders come to believe that they are special, that the rules don't apply to them and that they can get away with almost anything.
But politics can also breed fragility. There's the politician's need to be loved, to have their likeability affirmed by others. They lead strange lives, separated for long periods from their families, in which they exist more as a public figure than as a private person. The search for risky sex can be about reclaiming a private world, creating a space within which they can either be who they really are or else live out some fantasy that can't be articulated publicly - they can become "Carlos Danger".
Weiner's choice of the name "Danger" probably says more than he intended. Some politicians simply get off on the risk involved in extra-marital shenanigans; others are inviting exposure, an excuse to end a career that is driving them mad.
Scandal can be motivated by a very human desire to escape. Speaking of his own moments of madness, Mark Oaten described his visits to the rent boys thus: "When I was there I did not feel guilt or panic, and the guy made me feel as though this was the most normal thing in the world."
When your life is as abnormal as a politician's, a quiet afternoon spent with a person paid to fake love and understanding is a visit to planet normal. Continental voters are far more sympathetic towards this desire to conduct two separate lives; few blushed when the mistress of the former French President Francois Mitterrand showed up at his funeral.
In the case of Weiner, there is evidence of both an addictive personality and enormous arrogance. But there is also vulnerability. In one message to his "fantasy" woman, he asked her how he looked and added: "I am deeply flawed." As are we all. Sometimes, when we see stressed men under extraordinary pressure doing daft things, we might remember: "There but for the grace of God go I."