Milton Friedman (Savage’s mentor and co-author) and Anna J. Schwartz, in their 1963 book A Monetary History of the United States , showed that monetary-policy anomalies – a prime example of an external shock – were a significant factor in the Great Depression of the 1930’s. Economists such as Barry Eichengreen, Jeffrey Sachs, and Ben Bernanke have helped us to understand that these anomalies were the result of individual central banks’ effort to stay on the gold standard, causing them to keep interest rates relatively high despite economic weakness.