Société Générale Lost Nearly Five Billion Euros Unwinding One Trader's Positions
In January 2008 the bank discovered enormous unauthorised equity index positions. The loss came substantially from closing them into a falling market.
The Discovery
In January 2008 Société Générale announced a loss of roughly 4.9 billion euros arising from unauthorised trading by Jérôme Kerviel, who worked on a desk conducting arbitrage between equity index products.
His authorised activity involved taking offsetting positions to capture small pricing differences. Instead he had accumulated very large one directional positions in European equity index futures.
The Concealment
The method resembles other cases with one specific feature. Kerviel had previously worked in the middle office, the function that verifies and processes trades, so he understood how the controls operated and what would trigger a query.
He entered fictitious offsetting trades to make his real positions appear hedged. Because the fake trades made the book look balanced, risk reports showed modest exposure. He reportedly timed entries and cancellations around confirmation cycles so the fictitious trades were removed before verification.
Experience in the control function is exactly the knowledge required to defeat it, which is why moving staff from operations into trading requires more oversight rather than less.
Why the Loss Was So Large
An important nuance is that the positions were not deeply underwater when discovered. The bank has stated they were close to flat at the point of detection.
The loss arose substantially from liquidation. The bank chose to close the positions immediately, over roughly three days, in a market that was falling sharply during a period of considerable turbulence.
Selling that quantity of index futures into a declining market moved prices against the seller. The exit generated the loss.
The Dilemma
The bank faced a genuine choice with no good option. Holding the positions meant retaining enormous unauthorised market exposure and hoping conditions improved, which is speculation with shareholders' money and would have been indefensible if it worsened.
Exiting immediately meant accepting the market impact of unwinding a very large position quickly.
They chose speed, and the decision has been debated since. The general principle is that liquidation cost scales with position size relative to market depth, which means the true risk of an oversized position includes what it costs to get out.
What It Changed
The response emphasised stronger separation between trading and control functions, closer scrutiny of cancelled and amended trades as a specific red flag, mandatory leave requirements, and monitoring of gross rather than only net exposure.
That last point is the substantive one. Risk systems that net offsetting positions can be defeated by fictitious offsets. Monitoring gross notional exposure reveals the actual size of what is being traded regardless of what it appears to be hedged against.
The Bottom Line
The positions were near flat when found and the exit created the loss. Position size determines liquidation cost, and monitoring net exposure alone lets fictitious hedges hide the real number.