In all the uproar over AIG, the most important lesson has been ignored. AIG failed because it sold large amounts of credit default swaps (CDS) without properly offsetting or covering their positions. What we must take away from this is that CDS are toxic instruments whose use ought to be strictly regulated: Only those who own the underlying bonds ought to be allowed to buy them. Instituting this rule would tame a destructive force and cut the price of the swaps. It would also save the U.S. Treasury a lot of money by reducing the loss on AIG’s outstanding positions without abrogating any contracts.

CDS came into existence as a way of providing insurance on bonds against default. Since they are tradable instruments, they became bear-market warrants for speculating on deteriorating conditions in a company or country. What makes them toxic is that such speculation can be self-validating.

Up until the crash of 2008, the prevailing view — called the efficient market hypothesis — was that the prices of financial instruments accurately reflect all the available information (i.e. the underlying reality). But this is not true. Financial markets don’t deal with the current reality, but with the future — a matter of anticipation, not knowledge. Thus, we must understand financial markets through a new paradigm which recognizes that they always provide a biased view of the future, and that the distortion of prices in financial markets may affect the underlying reality that those prices are supposed to reflect. (I call this feedback mechanism “reflexivity.”)

With the help of this new paradigm, the poisonous nature of CDS can be demonstrated in a three-step argument. The first step is to acknowledge that being long and selling short in the stock market has an asymmetric risk/reward profile. 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. This asymmetry discourages short-selling.

The second step is to recognize that the CDS market offers a convenient way of shorting bonds, but the risk/reward asymmetry works in the opposite way. Going short on bonds by buying a CDS contract carries limited risk but almost unlimited profit potential. By contrast, selling CDS offers limited profits but practically unlimited risks. This asymmetry encourages speculating on the short side, which in turn exerts a downward pressure on the underlying bonds. The negative effect is reinforced by the fact that CDS are tradable and therefore tend to be priced as warrants, which can be sold at anytime, not as options, which would require an actual default to be cashed in. People buy them not because they expect an eventual default, but because they expect the CDS to appreciate in response to adverse developments.
AIG thought it was selling insurance on bonds, and as such, they considered CDS outrageously overpriced. In fact, it was selling bear-market warrants and it severely underestimated the risk.

The third step is to recognize reflexivity, which means that the mispricing of financial instruments can affect the fundamentals that market prices are supposed to reflect. Nowhere is this phenomenon more pronounced than in the case of financial institutions, whose ability to do business is so dependent on trust. A decline in their share and bond prices can increase their financing costs. That means that bear raids on financial institutions can be self-validating.

Taking these three considerations together, it’s clear that AIG, Bear Stearns, Lehman Brothers and others were destroyed by bear raids in which the shorting of stocks and buying CDS mutually amplified and reinforced each other. The unlimited shorting of stocks was made possible by the abolition of the uptick rule, which would have hindered bear raids by allowing short selling only when prices were rising. The unlimited shorting of bonds was facilitated by the CDS market. The two made a lethal combination. And AIG failed to understand this.

Many argue now that CDS ought to be traded on regulated exchanges. I believe that they are toxic and should only be allowed to be used by those who own the bonds, not by others who want to speculate against countries or companies. Under this rule — which would require international agreement and federal legislation — the buying pressure on CDS would greatly diminish, and all outstanding CDS would drop in price. As a collateral benefit, the U.S. Treasury would save a great deal of money on its exposure to AIG.

Mr. Soros is chair of Soros Fund Management and author of “The Crash of 2008” (PublicAffairs, 2009).