What have economists learned about monetary policy over the past fifty years?

What have economists learned about monetary policy over the past fifty years? In economic research, fifty years is a long time. I will actually start in 1967, with Milton Friedman’s presidential address at the meeting of the American Economic Association, so I will cover only about forty years. I will give a very personal view of what economists have learned since Friedman’s address that is most relevant for practical monetary policy. In order to be brief, I will have to leave out many important research contributions. I will hence be very selective, eclectic, and possibly controversial.1