[Tyler Cowen is a professor of economics at George Mason University.]

MANY people are looking back to the Great Depression and the New Deal for answers to our problems. But while we can learn important lessons from this period, they’re not always the ones taught in school.

The traditional story is that President Franklin D. Roosevelt rescued capitalism by resorting to extensive government intervention; the truth is that Roosevelt changed course from year to year, trying a mix of policies, some good and some bad. It’s worth sorting through this grab bag now, to evaluate whether any of these policies might be helpful.

If I were preparing a “New Deal crib sheet,” I would start with the following lessons:

MONETARY POLICY IS KEY As Milton Friedman and Anna Jacobson Schwartz argued in a classic book, “A Monetary History of the United States,” the single biggest cause of the Great Depression was that the Federal Reserve let the money supply fall by one-third, causing deflation. Furthermore, banks were allowed to fail, causing a credit crisis. Roosevelt’s best policies were those designed to increase the money supply, get the banking system back on its feet and restore trust in financial institutions.

A study of the 1930s by Christina D. Romer, a professor at the University of California, Berkeley (“What Ended the Great Depression?,” Journal of Economic History, 1992), confirmed that expansionary monetary policy was the key to the partial recovery of the 1930s. The worst years of the New Deal were 1937 and 1938, right after the Fed increased reserve requirements for banks, thereby curbing lending and moving the economy back to dangerous deflationary pressures.

Today, expansionary monetary policy isn’t so easy to put into effect, as we are seeing a shrinkage of credit and a contraction of the “shadow banking sector,” as represented by forms of derivatives trading, hedge funds and other investments. So don’t expect the benefits of monetary expansion to kick in right now, or even six months from now.

Still, the Fed needs to stand ready to prevent a downward spiral and to stimulate the economy once it’s possible....