In the past, whenever the financial system came close to a breakdown, the authorities rode to the rescue and prevented it from going over the brink. That is what I expected in 2008 but that is not what happened. On Monday September 15, Lehman Brothers, the US investment bank, was allowed to go into bankruptcy without proper preparation. It was a game-changing event with catastrophic consequences.

For a start, the price of credit default swaps, a form of insurance against companies defaulting on debt, went through the roof as investors took cover. AIG, the insurance giant, was carrying a large short position in CDS and faced imminent default. By the next day Hank Paulson, then US Treasury secretary, had to reverse himself and come to the rescue of AIG.

But worse was to come. Lehman was one of the main market-makers in commercial paper and a large issuer of these short-term obligations to boot. Reserve Primary, an independent money market fund, held Lehman paper and, since it had no deep pocket to turn to, it had to “break the buck” – stop redeeming its shares at par. That caused panic among depositors: by Thursday a run on money market funds was in full swing.

The panic then spread to the stock market. The financial system suffered cardiac arrest and had to be put on artificial life support.

How could Lehman have been left to go under? The responsibility lies squarely with the financial authorities, notably the Treasury and the Federal Reserve. The claim that they lacked the necessary legal powers is a lame excuse. In an emergency they could and should have done whatever was necessary to prevent the system from collapsing. That is what they have done on other occasions. The fact is, they allowed it to happen.

On a deeper level, too, credit default swaps played a critical role in Lehman's demise. My explanation is controversial and all three steps of my argument will take the reader to unfamiliar ground.

First, there is an asymmetry in the risk/reward ratio between being long or short in the stock market. (Being long means owning a stock, being short means selling a stock one does not own.) Being long has unlimited potential on the upside but limited exposure on the downside. Being short is the reverse. The asymmetry manifests itself in the following way: losing on a long position reduces one's risk exposure while losing on a short position increases it. As a result, one can be more patient being long and wrong than being short and wrong. The asymmetry serves to discourage the short-selling of stocks.

The second step is to understand credit default swaps and to recognise that the CDS market offers a convenient way of shorting bonds. In that market the asymmetry in risk/reward works in the opposite way to stocks. Going short on bonds by buying a CDS contract carries limited risk but unlimited profit potential; by contrast, selling credit default swaps offers limited profits but practically unlimited risks.

The asymmetry encourages speculating on the short side, which in turn exerts a downward pressure on the underlying bonds. When an adverse development is expected, the negative effect can become overwhelming because CDS tend to be priced as warrants, not as options: people buy them not because they expect an eventual default but because they expect the CDS to appreciate during the lifetime of the contract.

No arbitrage can correct the mispricing. That can be clearly seen in US and UK government bonds, whose actual price is much higher than that implied by CDS. These asymmetries are difficult to reconcile with the efficient market hypothesis, the notion that securities prices accurately reflect all known information.