Bank of Minneapolis and University of Minnesota. We thank the NSF for financial support. The views expressed herein are those of the authors and not necessarily those of the Federal Reserve Bank of Minneapolis or the Federal Reserve System. The Great Depression is not yet well understood. Economists have offered many theories for both the massive decline and the slow recovery of output during 1929—39, but no consensus has formed on the main forces behind this major economic event. Here we describe and demonstrate a simple methodology for determining which types of theories are the most promising. Several prominent theories blame the Great Depression on frictions in labor and capital markets. The sticky wage theory is that wage stickiness together with a monetary contraction produces a downturn in output. (See Michael Bordo, Christopher Erceg, and Charles Evans 2001.) The cartelization theory is that an increase in cartelization and unionization leads to a slow recovery. (See Harold Cole and Lee Ohanian 2001.) The investment friction theory is that monetary contractions increase frictions in capital markets that produce investment-driven downturns in output.