Suddenly, everything old is New Deal again. Reagan is out; F.D.R. is in. Still, how much guidance does the Roosevelt era really offer for today’s world? The answer is, a lot. But Barack Obama should learn from F.D.R.’s failures as well as from his achievements: the truth is that the New Deal wasn’t as successful in the short run as it was in the long run. And the reason for F.D.R.’s limited short-run success, which almost undid his whole program, was the fact that his economic policies were too cautious.

In our macroeconomics class today we’ll be looking at FDR actions in response to the Great Depression. The president earned wide spread admiration for taking bold action to reverse the effects of the depression, yet the country stumbled in and out of recovery for 6-8 years afterwards. Krugman argues that FDR didn’t spend enough. This is a worthwhile read but not the full story.

But when Olivier Blanchard, the I.M.F.’s chief economist, was asked about the possibility of the world sinking into another Great Depression, he reassuringly replied that the chance was “nearly nil.” He added, “We’ve learned a few things in 80 years.”

Yes, we have. But have we learned what caused the Depression of the 1930s? Most important, have we learned enough to avoid doing the same thing again?

In a week or so we’ll look at the Great Depression in my Principles of Macroeconomics class. Just as geologists study earthquakes, economists and policy makers need to understand the Depression.

Mankiw places more emphasis on Roosevelt’s regulation of markets as a force that turned an ordinary recession into a depression. Bernanke points to the collapse of the banking system and the sharp reduction in money as the main culprit. Either way, these discussions are instructive.