Jérôme Kerviel didn’t just bet the bank, we now discover. He bet more than the bank. The face value of his secret wagers, as defined by the maximum he could have lost, was more than €50 billion or £37 billion, according to the French presidential aide Raymond Soubie. That’s roughly the GDP of a lesser European power such as Slovakia, and rather more than the entire market value of Société Générale, his employer.

It’s another example of the parallel with Barings and its nemesis – Nick Leeson, whose bets really did wipe out that once-august institution. Yet for all the hand-wringing that followed that collapse in 1995, banks and regulators seem to have forgotten the lessons. Lesson one is that the capacity to inflict colossal self-damage on an organisation has nothing to do with the seniority of the person involved. The lowliest private can destroy his own army if given the keys to the arsenal. Leeson was a nongraduate with an F in maths A level dispatched to the remote outpost of Singapore. Kerviel was a relative grunt, paid less than €100,000 (£50,000) a year.

Lesson two is that rogue traders are more likely to be people who know what’s under the bonnet than how to drive the car. Leeson spent years settling and processing trades before being allowed anywhere near the trading floor. He understood the nuts and bolts of the back office – and stayed in charge after he was promoted to trader. Kerviel spent his first three years at SocGen in risk management and compliance, where he had time to absorb the workings of the password-protected systems and trading controls. Rogue traders are as likely to be geeks as masters of the universe.

Lesson three is that the rogues tend to gravitate to areas where regulators and bank compliance officers aren’t looking. Leeson and Kerviel both traded exactly the same types of security – index futures. These are the most bog-standard of products where the financial police don’t expect trouble.

SocGen was explaining on Thursday that the securities Kerviel bought were “plain vanilla” – as if this somehow excused its failure to monitor him carefully.

Lesson four is that it’s always very tempting to bend the rules when you are betting with other people’s money. Leeson and Kerviel were both supposed to confine themselves to arbitrage – the business of exploiting minuscule differences in the prices of the same asset in different market places. It’s relatively low-risk, relatively dull and relatively poorly rewarded.

Leeson and Kerviel both strayed, taking big ballsy positions that would yield huge rewards if they were right and serious losses if they were wrong. They were wrong to break the house rules, but were perhaps seduced by the culture of the organisations in which they worked. For taking big bets has seeped into the bones of modern-day investment bankers. No longer do banks only trade securities on behalf of clients or to hedge themselves against operational risks. They bet their own capital on their own hunch-es. It’s called proprietary trading – prop trading for short – and until last summer’s credit crunch it was responsible for a huge chunk of bank profits.

Switzerland’s biggest bank, UBS, admitted last month to losing billions of dollars in US sub-prime, not because it had directly lent money to America’s struggling trailer-park homeowners, not even because its whizzy investment bankers were left stranded after packaging up these mortgages into fancy investment products that they then couldn’t sell, but because its prop traders fancied a punt and got it wrong.

Times Recommends

• B&B hit by £18m fraud charge
• Mortgage rescue fund at the ready
• Zoom airline collapses and halts all flights