A second challenge to the Keynesian school arose in the 1970s, when the American economist Robert E. Lucas, Jr., laid the foundations of what came to be known as the New Classical school of thought in economics. Lucas’s key introduced the rational-expectations hypothesis. As opposed to the ideas in earlier Keynesian and monetarist models that viewed the individual decision makers in the economy as shortsighted and backward-looking, Lucas argued that decision makers, insofar as they are rational, do not base their decisions solely on current and past data; they also form expectations about the future on the basis of a vast array of information available to them. That fact implies that a change in monetary policy, if it has been predicted by rational agents, will have no effect on real variables such as output and the unemployment rate, because the agents will have acted upon the implications of such a policy even before it is implemented. As a result, predictable changes in monetary policy will result in changes in nominal variables such as prices and wages but will not have any real effects.

Following Lucas’s pioneering work, economists including Finn E. Kydland and Edward C. Prescott developed rigorous macroeconomic models to explain the fluctuations of the business cycle, which came to be known in the macroeconomic literature as real-business-cycle (RBC) models. RBC models were based on strong mathematical foundations and utilized Lucas’s idea of rational expectations. An important outcome of the RBC models was that they were able to explain macroeconomic fluctuations as the product of a myriad of external and internal shocks (unpredictable events that hit the economy). Primarily, they argued that shocks that result from changes in technology can account for the majority of the fluctuations in the business cycle.

The tendency of RBC models to overemphasize technology-driven fluctuations as the primary cause of business cycles and to underemphasize the role of monetary and fiscal policy led to the development of a new Keynesian response in the 1980s. New Keynesians, including John B. Taylor and Stanley Fischer, adopted the rigorous modeling approach introduced by Kydland and Prescott in the RBC literature but expanded it by altering some key underlying assumptions. Previous models had relied on the fact that nominal variables such as prices and wages are flexible and respond very quickly to changes in supply and demand. However, in the real world, most wages and many prices are locked in by contractual agreements. That fact introduces “stickiness,” or resistance to change, in those economic variables. Because wages and prices tend to be sticky, economic decision makers may react to macroeconomic events by altering other variables. For example, if wages are sticky, businesses will find themselves laying off more workers than they would in an unrealistic environment in which every employee’s salary could be cut in half.

Introducing market imperfections such as wage and price stickiness helped Taylor and Fischer to build macroeconomic models that represented the business cycle more accurately. In particular, they were able to show that in a world of market imperfections such as stickiness, monetary policy will have a direct impact on output and on employment in the short run, until enough time has passed for wages and prices to adjust. Therefore, central banks that control the supply of money can very well influence the business cycle in the short run. In the long run, however, the imperfections become less binding, as contracts can be renegotiated, and monetary policy can influence only prices.

Following the new Keynesian revolution, macroeconomists seemed to reach a consensus that monetary policy is effective in the short run and can be used as a tool to tame business cycles. Many other macroeconomic models were developed to measure the extent to which monetary policy can influence output. More recently, the impact of the financial crisis of 2007–08 and the Great Recession that followed it, coupled with the fact that many governments adopted a very Keynesian response to those events, brought about a revival of interest in the new Keynesian approach to macroeconomics, which seemed likely to lead to improved theories and better macroeconomic models in the future.