Strong and widely accepted evidence shows that the natural rate of unemployment varies over time with substantial amplitude. The frictions in the labor market that account for positive normal levels of unemployment are not simple and mechanical. Instead, as a rich modern body of theory demonstrates, the natural rate of unemployment is an equilibrium in which the volumes of job-seeking by workers and worker-seeking by employers reach a balance controlled by fundamental determinants of the relative prices of the two activities. In recessions, unemployment rises, and job vacancies fall. The natural explanation is an economywide fall in labor demand. But a compelling model that generates a fall in labor demand without a counterfactual fall in productivity has eluded theorists to date. Nonetheless, policymakers have appropriately adopted the view that the natural rate varies over time and is not a simple benchmark for setting monetary instruments.
Hall, Robert E..
2003."Modern Theory of Unemployment Fluctuations: Empirics and Policy Applications ."American Economic Review,
93(2): 145-150.DOI: 10.1257/000282803321946958