 3. During the past generation, there has been a radical transformation in the attitudes of economists toward empirical causal inquiry, especially in the form of field experiments and natural experiments, often employing instrumental variables. For example, about two-thirds of the articles in the February, 2018 American Economic Review are based on empirical studies. The titles of the first four entries in the table of contents are: “The Effects of Pretrial Detention on Conviction, Future Crime, and Employment: Evidence from Randomly Assigned Judges,” “Implications of US Tax Policy for House Prices, Rents, and Homeownership,” “The Welfare Cost of Perceived Policy Uncertainty: Evidence from Social Security,” “The Economic Consequences of Hospital Admissions.” If one goes back twenty-five years, only about one-eighth of the first issue of the 1993 American Economic Review appear to rely on any empirical studies. The first four entries are: “Today’s Task for Economists,” “Trigger Points and Budget Cuts: Explaining the Effects of Fiscal Austerity,” “Economic Policy, Economic Performance, and Elections,” “The Macroeconomics of Dr. Strangelove.” A Rip Van Winkle who had gone to sleep in 1983 reading the principal economics journals would be staggered when he awoke in 2018.