This summer two American senators have had their secret lives revealed in humiliating circumstances. Senator David Vitter of Louisiana admitted to a “very serious sin”, after his name appeared on the phone records of a Washington escort agency. Senator Larry Craig was given a 10-day suspended jail sentence after apparently cruising for gay sex in a public lavatory.

This weekend, Mr Craig resigned from the Senate. Mr Vitter, however, is hanging on. So what does it take for a sex scandal to be truly fatal? Why do some politicians survive this sort of thing and others perish?