Greg Mankiw nails it To Obama-administration economists, as well as to many others, the recession that followed the financial crisis of 2008 seemed like a classic case of decline in aggregate demand. Because of the credit crisis, people were not able to obtain loans — for homes, cars, business equipment, or any of the countless other transactions that rely on credit in today's economy. And because people were unable to obtain loans, these sales and purchases couldn't take place, resulting in a significant drop in demand across the economy. So, inspired by the view that fiscal policy can prop up aggregate demand, Obama's advisors (and their congressional allies) began to design a stimulus plan heavy on direct government spending. A few days before President Obama's inauguration, his economic advisors released a document titled "The Job Impact of the American Recovery and Reinvestment Plan," in which they detailed some of their economic assumptions. They determined that the "government-purchases multiplier" — that is, the multiplier for direct spending — would be 1.57, while the tax-cut multiplier would be 0.99. In other words, every dollar spent by the government would yield $1.57 in aggregate demand, while every dollar in reduced taxes would yield only 99 cents in increased demand. And because 1.57 is larger than 0.99, the Obama team concluded it was better to increase spending than to cut taxes.
Obama and his advisors arrived at these numbers through a standard macroeconometric model of the sort economists have been using for years. Such models take various past relationships among economic variables (inflation and unemployment, for instance) and extrapolate them into the future. In essence, the economy is modeled as a system of equations, each describing how one variable responds to many others. University of Chicago economist (and Nobel laureate) Robert Lucas famously criticized these models for lacking an appreciation of people's changing expectations; many economists, however, still find such models valuable, and have continued to employ them for forecasting and policy analysis.
The question for economists now is whether the administration's assumptions, and the model based on them, were correct. After all, if we could be sure their model was right, we would know what to conclude when their stimulus plan was followed by 10% unemployment: The patient was sicker than they thought, and unemployment would surely have been higher still if not for the stimulus. (Indeed, since Obama's advisors do believe their model was right, this is the conclusion they have reached.)
The trouble is, we have no way of knowing for sure if the model was in fact correct.
