The reason a trader goes to work at an investment bank is the same as a punter's in visiting a casino. Traders, like gamblers, intend to make money in spite of their certain knowledge that it is very easy to lose. This creates a heady cocktail in the mind composed of fear and greed. Everyone who works in financial markets knows it is so. Banks construct complex systems of risk control, but there are always traders evade them.

Kweku Adoboli, who lost $2bn operating a trading system known as Delta One, is the latest trader to do so; Jerome Kerviel of the French bank Société Générale is the biggest loser (€4.9bn). Nick Leeson single-handedly bust Barings, a banking legend. Irish and Japanese banks have confessed to embarrassing losses. Rogue traders are always with us.

Greed needs to be carefully defined, because the rogue traders we know about have not made anything like the money they have lost. Their gains have been in higher bonuses when the going was good. Their satisfaction often comes from the awe in which their alleged profits are held by their colleagues, whose bonuses reflect their success. Nick Leeson's apparent success allowed him to believe he was just as good at making money as generations of Barings, many of whom, unlike him, had inherited toffee noses. Kerviel confessed that making a sudden surge of profit was like having an orgasm.

Rogue traders share a contempt for the managers who ought to know what they are up to, but fail fully to understand their statistical models. As the profits build up, they may start to think they are invincible. But fear comes from the knowledge that as time goes by the odds move against them. They deal with the fear by trying to mask it. Leeson drank heavily; Kerviel persuaded himself that his managers approved of what he was doing. The inevitable jail sentence does not deter them. Indeed, Leeson and Kerviel have become heroes to people who see banks and bankers as the real enemy.

Stephen Fay is a former Deputy Editor of The Independent on Sunday and the author of 'The Collapse of Barings' (Norton)