Stanley Fischer said,

To conclude, the U.S. financial system has changed a great deal over the past several decades. One of the most important changes has been the rapid growth of the nonbank sector. Many reforms have been adopted for both banks and nonbank financial institutions. But regulation is a cat and mouse game. Regulators need to respond to existing regulatory gaps and to keep pace with further changes. We hope we will succeed in doing so. But we know that we will never be able to identify in advance all the threats to stability that are out there, and that it is therefore all the more critical to maintain and strengthen the robustness of our financial institutions, and of the financial system as a whole.

Read the whole thing. Pointer from Mark Thoma.

Both theoretical and empirical work in macroeconomics tends to ignore structural changes of this kind. On the empirical side, think of econometrics. There are three classes of factors at work in macroeconomics data. One is short-term noise, such as a cash-for-clunkers program adding to auto sales one quarter and subtracting from them the next. Another is cyclical drivers–the sorts of things that your theory suggests as causal factors in macroeconomic fluctuations. Finally, there are structural changes, such as the changes in financial markets that Fischer is talking about.

I do not think that it is possible to sift through these three factors without using judgment. Just dumping the data into your econometrics software is an exercise in garbage-in, garbage-out.