Are all politicians mad? As they turn up on the beaches of Cornwall or Corfu for their summer holidays, this is the time when they should seem most normal. But, in fact, it is the season when they look less than ever like the voters they represent.

They appear as uncomfortable in their Boden swimming trunks as their ill-fitting leisure wear and can’t stop tweeting or blogging from the beach. While most people aspire to nothing more taxing than a sunlounger by the pool and a relaxing novel, Gordon Brown is planning to spend a week doing voluntary work. I wonder how long it will be before a Tory frontbencher is caught creeping on to a yacht.

Of course politicians are not actually insane but my question is not entirely spurious. Privately, over lunch at Westminster, MPs frequently question the sanity of their colleagues. In an age when, as Drew Westen, the American writer, has put it, “the political brain is an emotional brain”, psychology is as powerful a force at Westminster as ideology.

Mr Brown was famously described as “psychologically flawed” by the Blairites — I remember one of Tony Blair’s aides predicting that he would be “carried out of No 10 by the men in white coats”. To the Brownites, Mr Blair was out of control, messianic and mad with power. Only last week a Cabinet minister described Harriet Harman as bonkers. When the Tories lost power in 1997, MPs said the experience was like having a “collective nervous breakdown”.

This may not just be rhetorical. According to a survey by the all-party parliamentary group on mental health, one in five MPs has had mental health problems. Just before the Commons rose for the recess, the government whips met to discuss their concerns about politicians’ psychological wellbeing after the expenses row — some MPs have been suicidal, others paranoid, convinced that everyone is muttering about them whenever they go into a shop.

Lord Mandelson believes that most politicians have psychological issues. “I’ve never met a perfect politician, including the one who’s talking to you,” he told me in an interview recently. “I’ve come through my catharsis and emerged a more relaxed, less spiky, less insular individual.”

History seems to indicate that he is right. Winston Churchill was plagued by the “black dog” of depression and Abraham Lincoln suffered from violent mood swings — “I may seem to enjoy life rapturously when I am in company. But when I am alone I am so often overcome by mental depression that I dare not carry a penknife,” he once wrote.

When David Owen became an MP in 1966, he was working as a junior doctor at St Thomas’ Hospital, specialising in neurology and psychiatry. He was shocked by the number of politicians he was treating.

Perhaps Westminster appeals to those who crave endorsement more than most. Margaret Hodge, the former Culture Minister, once told me that she thought many MPs are driven towards politics by traumas in their childhood — in her case it was the death of her mother when she was 13. “I’ve often thought that politicians choose such a mad life because we’ve come from some sort of traumatic background,” she said. “Part of what attracts you to the public life of politics is the feeling that you need to have yourself validated by the voters.”

Certainly, an unusually high proportion of politicians have had difficult experiences in their youth. When Mr Blair was 14, his father had a serious stroke — Leo Blair had to give up his own ambition to go into politics but transferred it to his son. “I felt I couldn’t let him down,” the former Prime Minister later said.

For Mr Brown a rugby accident that left him pinned to a hospital bed for months and caused him to lose the sight in one eye was the formative traumatic experience. Friends say that it created an enormous sense of determination and drive — but also left him with an underlying feeling that he was battling against a hostile world.

Alan Johnson was orphaned at the age of 12 and raised by his 15-year-old sister. In a different way David Cameron’s character was formed by the birth of his disabled son Ivan. Those who know him well think that the privileged public school boy would never have been able to lead his party without having suffered such an emotional shock.

Politics demands certain characteristics. It requires extraordinary levels of ambition and perseverance just to get a seat — let alone to reach Cabinet. MPs have to cope with pressure that would be unbearable for most people, juggling long hours and commuting between their constituencies and London.

They must also develop what is, in effect, a split personality, combining unstinting loyalty to their party with a ruthless ability to stab colleagues in the back. They must be as narcissistic as a Hollywood actor and as selfless as a parish priest, compulsive risk-takers and obsessively careful about staying “on message”. Mark Oaten, the Liberal Democrat MP, has written a book, Screwing Up, which describes the levels of stress and depression that led him to visit rent boys — a decision that he must have known would eventually emerge and destroy his career.

There is a case that being involved in politics can damage mental wellbeing. In his book In Sickness and in Power, Dr Owen argues that the sense of being “intoxicated” by power may actually change how the brain works. The former Foreign Secretary identifies what he thinks may be a medical condition, “hubris syndrome”, which was in his view behind Mr Blair’s decision to go to war in Iraq. “It has been observed for centuries that something happens to some people’s mental stability when in power ... Power is a heady drug which not every political leader has the necessary rooted character to counteract.”

So politicians are not mad, nor are they bad, but we — and they — should remain alert to their psychological flaws.

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