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Business cycle theory emerged as a core subfield of macroeconomics, dedicated to explaining the recurrent fluctuations in aggregate output, employment, and prices. Early pre-Keynesian thought, including classical and Austrian approaches, often treated cycles as minor deviations or self-correcting episodes driven by real factors like technology or credit, but lacked a unified framework. The Keynesian revolution of the 1930s provided the first comprehensive theory, attributing cycles to volatile aggregate demand amplified by sticky wages and prices, with investment playing a central role. This paradigm dominated postwar analysis through the Neoclassical Synthesis, which combined Keynesian demand management with classical long-run equilibrium, though it maintained an ad hoc treatment of expectations.
The monetarist school, led by Milton Friedman, challenged this consensus by emphasizing monetary shocks as the primary cause of cycles and advocating for policy rules over discretion. Monetarism's empirical critiques paved the way for the New Classical Macroeconomics revolution in the 1970s, which introduced rational expectations and continuous market clearing. This school rejected Keynesian models for lacking microfoundations and argued that systematic policy was ineffective, viewing cycles as efficient responses to real disturbances. Its most influential offspring, Real Business Cycle Theory (RBC), developed in the 1980s, attributed fluctuations almost entirely to technology shocks within dynamic stochastic general equilibrium models, sidelining monetary factors and reinforcing a supply-side narrative.
New Keynesian economics arose in the 1980s and 1990s as a direct response, incorporating rigorous microfoundations while restoring a role for demand shocks and policy. By embedding nominal rigidities, imperfect competition, and coordination failures into dynamic models, New Keynesian theory explained how market imperfections could propagate shocks and justify stabilization policy. This synthesis with New Classical methodology became the backbone of modern policy analysis, particularly during the Great Moderation, and integrated into dynamic stochastic general equilibrium frameworks with active monetary rules.
Contemporary developments have expanded these core paradigms. The Financial Crisis of 2008 spurred renewed interest in financial frictions and heterogeneous agent models, blending New Keynesian insights with Minskyian financial instability. Post-Keynesian and other heterodox schools, emphasizing fundamental uncertainty and income distribution, continue to offer alternative narratives. Despite evolving tools, the field remains structured around the rival legacies of Keynesian, New Classical, and New Keynesian schools, each refining how shocks and propagation mechanisms drive economic oscillations.