642 PA R T V I I Monetary Theory policy when institutions change, and provides more confidence in the direction of causation between M and Y If the structure of the model is not correctly specified because it leaves out important transmission mechanisms of monetary policy, it could be very misleading The reduced-form approach does not restrict the way monetary policy affects the economy and may be more likely to spot the full effect of changes in M on Y However, reduced-form evidence cannot rule out reverse causation, whereby changes in output cause changes in money, or the possibility that an outside factor drives changes in both output and money A high correlation of money and output might then be misleading because controlling the money supply would not help control the level of output Armed with the framework to evaluate empirical evidence we have outlined here, we can now use it to evaluate the empirical debate on the importance of monetary policy to economic fluctuations APP LI CAT IO N The Debate on the Importance of Monetary Policy to Economic Fluctuations We can apply our understanding of the advantages and disadvantages of structural model versus reduced-form evidence to a debate that has been ongoing for over seventy years: how important is monetary policy to economic fluctuations? The followers of Milton Friedman, known as monetarists, tended to focus on reducedform evidence and found that changes in the money supply are very important to economic fluctuations Early followers of John Maynard Keynes, known as Keynesians, focused on structural model evidence based on the components approach to determination of aggregate demand, which was less likely to find that monetary policy is important We evaluate the evidence that monetarists and Keynesians brought to bear on the importance of monetary policy using the analysis we developed in the previous section Early Keynesian Evidence on the Importance of Money Although Keynes proposed his theory for analyzing aggregate economic activity in 1936, his views reached their peak of popularity among economists in the 1950s and early 1960s, when the majority of economists had accepted his framework Although most Keynesians currently believe that monetary policy has important effects on economic activity, the early Keynesians of the 1950s and early 1960s characteristically held the view that monetary policy does not matter at all to movements in aggregate output and hence to the business cycle Their belief in the ineffectiveness of monetary policy stemmed from three pieces of structural model evidence: During the Great Depression, interest rates on Canadian and U.S government securities fell to extremely low levels Early Keynesians believed monetary policy affected aggregate demand solely through its effect on nominal interest rates, which in turn affected investment spending; they believed that low interest rates during the Depression indicated that monetary policy was easy because it encouraged investment spending and so could not have played a contractionary role during this period Since monetary policy was not capable of explaining why the worst economic contraction in history had taken place,