Ken Arrow says that we are not done yet in working out how to understand financial crises.

The current financial crisis, the loss of asset values, the refusal to extend normally-given credit and the great increase in defaults on obligations ranging from individual mortgages to the debts of great investment banks presents, of course, a pressing challenge to the fiscal authorities and central banks to take measures to minimise the consequences. But they also present a challenge to standard economic theory, a challenge all the more important since the development of policies to prevent future financial crises will depend on a deeper understanding of the processes at work.

The problem is that diversity of asset holdings is good for spreading risk but that very complexity makes it more likely that the costly consequences of asymmetric information will rear its ugly head.