From Slate:

The complex derivatives behind the current financial havoc aren’t literally martingales, but what’s wrong with the martingale is one of the things that’s wrong with the derivatives. There’s no question that you can reduce risk drastically by combining different investments in a single portfolio; that’s what plain-Jane instruments like index funds do. What sounds an alarm is the claim that you can get low risk and high returns in the same happy package. “Once the limits of diversification have been reached,” John Quiggin, an economist at the University of Queensland, told me, “rearranging the set of claims involved isn’t going to reduce risk any further, so if all parties appear to be making risk-free profits, the risk must have been shifted to some low-probability, high-consequence event.” In other words, if it sounds too good to be true, it’s probably heading toward some outcome too bad to be borne. Or, as financial skeptic Nassim Nicholas Taleb wrote last week, “It appears that financial institutions earn money on transactions (say fees on your mother-in-law’s checking account) and lose everything taking risks they don’t understand.”