What is it about politics that generates corrupt behaviour? Silvio Berlusconi, the Italian Prime Minister, is due to stand trial on 16 November to answer allegations of tax fraud and false accounting and then, just 11 days later, to explain the alleged payment of a £370,000 bribe to his British tax accountant, David Mills.

Signor Berlusconi would have to stand down if convicted, wouldn't he? Of course not. This is politics. He states: "If there is a conviction at trial, we would be confronted with such a subversion of the truth that I would all the more feel the duty to resist and stay at my post to defend democracy and the rule of law."

Then there is former French president, Jacques Chirac, who has recently been ordered by an investigating judge to appear in court to answer charges of misuse of public funds dating back to his time as Mayor of Paris in the 1990s. On Wednesday Mr Chirac declared he had "nothing to be ashamed of" and was ready to face trial for corruption.

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Alain Juppé, the former prime minister and colleague of Mr Chirac, was tried some years ago on more or less the same grounds. He was convicted and sentenced. The court commented that it was regrettable that Mr Juppé, whose intellectual qualities were unanimously recognised, did not judge it appropriate to assume before the court his entire criminal responsibility and that he kept on denying established facts.

The fiddling of expenses by members of the House of Commons and the Lords is on a much smaller scale. Nonetheless, some members of both Houses will probably be charged with criminal offences. And the public's broad view is that many MPs have in effect been stealing from the public purse.

Indeed, on a close reading of the facts, that is often the inescapable conclusion. Or as Sir Christopher Kelly puts it in his report released earlier this week: "MPs have been able to misuse for personal gain an expenses regime which was intended simply to reimburse them for the additional costs necessarily incurred in performing their jobs."

The explanation for the stink of dishonesty that pervades politics comes in three parts. There is, first, the old adage that "power corrupts; absolute power corrupts absolutely". This is because, once given responsibility for the conduct of the business of the state, it becomes easy to convince oneself that the needs of the state override all other considerations. Illegal or unethical actions that might conceivably be excusable for short periods when the country is in great danger become a habit. Indeed, politicians often persuade themselves that their own survival in office is itself in the interests of the state. Gordon Brown, for instance, fills his speeches with boastful untruths, and in this he is no worse than his predecessors.

Second, politicians come to feel a powerful sense of entitlement. This has been a major factor in the scandal of parliamentary expenses. As one member of the public told Sir Christopher: "The root of the problem lies in one simple principle: the rules applied to MPs' expenses are quite different to those applied to the taxpaying public."

Many MPs came to believe that they were in some fashion "owed" a lifestyle more luxurious or convenient than their bare parliamentary pay would provide. MPs should be able to clean their moats, build themselves duck houses or create little property empires.

A coded expression of this sentiment is found in the argument that the strict rules proposed by Sir Christopher could have the effect of discouraging less wealthy people from becoming MPs. To which Sir Christopher rightly said that this was a matter that had to be addressed through MP's pay, not allowances.

Third, closed societies, such as the two Houses of Parliament themselves or, say, individual police forces, or members of certain specialised financial markets, can develop a form of institutional rot. Once one or two members start to take advantages for themselves at the expense of those whom they are meant to serve, the malpractice quickly spreads and becomes viewed as normal – "everyone does it!"

Then, when challenged from the outside, the members of the closed circle are incredulous and "don't get it". This is why, as Sir Christopher notes in his report, "many MPs had failed to understand the urgent need for reform".

Sir Christopher's recommendations will undoubtedly do the job, but they are not the end of the matter, for the electorate has yet to give its judgment. Many errant MPs will be booted out. Only when this has happened can Parliament begin to regain the trust it has lost.

a.whittamsmith@independent.co.uk