Greg Mankiw writes:
Fama on Fiscal Stimulus: Eugene Fama is a stimulus skeptic: In fact, he is even more skeptical than I am. I am willing to concede that many Keynesian effects work in the short run, although I prefer monetary policy to fiscal policy and, within fiscal policy, I prefer the use of tax instruments to government spending as a tool for short-run demand management. By contrast, I read Fama's article as a largely wholesale endorsement of the classical model with complete crowding out.
No, Greg. It's not an endorsement of any model. It's just a mistake. Fama mistakes the NIPA savings-investment accounting identity for a behavioral relationship that constrains the behavior of investment: when the government deficit goes up, Fama says, private investment must go down by the same amount.
When the government deficit goes up, private savings could go up by more--and private investment could increase. Private savings could go up by less--and private investment would fall by less than the rise in the government deficit. Private savings could remain unchanged. Or private savings could fall. Determining which of these is most likely to happen would require a model of the economy of some sort--and Fama does not have one: all he has is an accounting identity that he does not understand.