What Caused the Recession of 2008?

Casey Mulligan, contrarian blogger, just put out a working paper on the role of productivity in our current recession. The abstract (NBER subscription):

A labor market tautology says that any change in labor usage can be decomposed into a movement along a marginal productivity schedule and a shift of the schedule. I calculate this decomposition for the recession of 2008, assuming an aggregate Cobb-Douglas marginal productivity schedule, and find that all of the decline in employment and hours since December 2007 is a movement along the schedule. This finding suggests that a reduction in labor supply and/or an increase in labor market distortions are major factors in the 2008 recession. The decline in aggregate consumption suggests that the reduction in labor supply (if any) is neither a wealth nor an intertemporal substitution effect. "Sticky real wages" or the emergence of significant work disincentives are possible explanations for these findings.