Research is picking up on what I would call the homogenization theory of systemic risk. Instead of resorting to financial contagion or another propagation mechanism, this theory comes from the realization that bank failures are clustered not so much because failure spills over from banks to banks, but because banks’ balance sheets are relatively homogeneous. If banks invest in similar assets, then it is no wonder that they fail simultaneously, or to a lesser degree, it is no wonder that failure at a first bank triggers a reassessment crisis.

In a Journal of Financial Intermediation article (2010), Wolf Wagner makes the bold claim that it is in fact diversification that makes banks more homogeneous. To understand this claim, take Haldane & May’s example in Nature (2011);

Suppose you have N banks and N distinct, uncorrelated asset classes, each of which has some very small probability, ε, of having its value decline to the extent that a bank holding solely that asset would fail. At the inhomogeneous extreme, assume each bank holds the entirety of one of the N assets: the probability for any one bank to fail is now ε, whereas that for the system is a vastly smaller ε^N. At the opposite, homogeneous extreme, assume all banks are identical, each holding 1/N of every one of the N assets: the probability for any one bank to fail can now be calculated as N^N (ε^N)/N!, and this is obviously also the probability for all N of these banks to fail.

Now, Wagner takes his own version of this example at face value and claims that “diversification also makes the banks more similar to each other by exposing them to the same risks”. The optimal level of diversification, according to Wagner, is actually much lower than the “natural” level of diversification. This makes diversification driven homogenization an interesting market failure.

However, this is obviously only true if you’re completely oblivious to the world’s open-endedness. There is not “N” class of assets that banks can invest in, and the number of classes of assets is ever expanding. The only limit to the classes of assets you’re going to run into is human ingenuity and entrepreneurial spirit. While Wagner formulates no policy recommendation (but others do), this is still a good example of the kind of misplaced concreteness economic modeling is riddled with.

So, if diversification is not responsible for banks’ homogeneity, what is? Jeffrey Friedman and Wladimir Kraus offer in Engineering the Financial Crisis: Systemic Risk and the Failure of Regulation a convincing explanation of why that would be the case. Basel 2 capital rules’ risk weighting of assets created arbitrage possibilities that made mortgage backed securities particularly interesting for banks. Interestingly enough, in Haldane & May’s own words: “Tentative evidence comes from the fact that the world’s five largest banks have shown increasing concentrations of assets over the last ten years, in contrast to the top five hedge funds, whose less concentrated systems can give greater scope for diversity.” This would tend to support Friedman & Kraus; Basel 2/3 regulated banks are more homogeneous  while much more loosely regulated hedge funds are less.