Starting in the 1990s, US bank assets grew more concentrated among a few large institutions. We explore the changing role of idiosyncratic volatility as a shaping force of the bank asset power law distribution. Our results reveal that idiosyncratic asset volatilities for bank-holding companies declined since the 1990s. To the extent that firm-specific shocks can have significant macroeconomic consequences, this result implies that even as one obvious source of aggregate risk and contagion--bank asset concentration--has increased, another important source--idiosyncratic volatility--has diminished.

In the boom before the Great Depression, capital requirements for commercial banks were low and fixed. Bankers faced double liability. Failing banks were not bailed out. During the boom before the Great Recession, capital requirements were proportional to risk-weighted assets. Bankers faced limited liability. Banks deemed too big to fail received bailouts. During the 1920s, the largest banks increased capital levels as asset prices rose. During the boom from 2002 to 2007, the largest institutions kept capital levels near regulatory minimums. Our results suggest more market discipline would have induced the largest U.S. banks to hold greater capital buffers prior to the financial crisis of 2008.