Modèles d'équilibre et de déséquilibre dans les théories contemporaines des cycles

[fre] revanche ambigu sur le travail de Robinson : l'augmentation de richesse lui permet de consommer plus aujourd'hui en travaillant moins, mais il a aussi intérêt à travailler plus aujourd'hui, puisque la cueillette est aisée, et moins demain où elle sera plus difficile. Si ce second effet l'emporte, l'investissement, la consommation et l'emploi fluctueront en phase avec la production, dans l'île, et donc dans l'économie réelle. En particulier, le chômage (loisir de Robinson) diminuera dans les phases d'expansion et augmentera dans les phases de récession, pour le plus grand bonheur de Robinson. Si ces modèles arrivent à reproduire les fluctuations des principales grandeurs macroéconomiques, c'est en grande partie parce que la variable qui sert généralement à mesurer les chocs technologiques, le « résidu de Solow », reflète très largement les fluctuations de la demande. La conclusion plaide pour une approche non mécaniste des fluctuations économiques qui peuvent résulter aussi bien de chocs de demande (dans les années soixante) que de chocs d'offre (dans les années soixante-dix et quatre-vingt) et conduire de ce fait à des dynamiques contrastées. [eng] Equilibrium and Disequilibrium Models in the Theory of Business Cycles Pierre-Alain Muet This article examines the contributions of equilibrium and disequilibrium models to the understanding of economic fluctuations. In disequilibrium models, short-run price and wage rigidities lead to quantity adjustments which cause economic fluctuations. Capital accumulation plays, via multiplier-accelerator effects, a fundamental role in business cycles, as illustrated by studies of dynamic econometric models and the adequacy of this simple model to the cyclical pattern of macroeconomic variables. The dynamics of price and quantity, that is, the response of prices, wages and interest rates to market disequilibria and their impact on demand, constitute the other elements of disequilibrium dynamics. In the mid-sixties, a theory of fluctuations was developed based on the assumption that markets were in a permanent state of equilibrium. In the early models of the new classical theory, departures from stationary equilibria were caused by unanticipated nominal shocks which thus had real temporary economic effects. But the most original developments of the application of rational expectations to business cycle theory were bubbles and the fact that extra-economic speculative phenomena, such as sunspots, can influence economic fluctuations. The most recent reincarnation of equilibrium models, the theory of real business cycles, interprets economic fluctuations as the optimal reaction of economic agents to technological shocks. Its main contribution only appears to be the new methodology which it fostered. We illustrate the economic mechanisms by the story of Robinson Crusoe. What happens if there is an abundance of coconuts today? If the shock is temporary, Robinson should invest (use the nuts to plant coconut trees) to be able to consume more tomorrow. Its impact is, however, ambiguous on the work of Robinson: the increase in wealth allows him to consume more today and work less, but, on the other hand, he should work more today as it is easy and less tomorrow as it will be harder. If the second effect dominates, investment, consumption and employment are procyclical on the island and thus in the real economy. In particular, unemployment (Mr. Crusoe's leisure) diminishes in the phases of economic expansion and increases during recessions, to Robinson Crusoe's joy. If these models are able to reproduce the fluctuations of the main macroeconomic variables, it is because the variable which serves to measure technological shocks — the Solow residual — largely reflects demand fluctuations. The conclusion calls for a non-mechanical approach to economic fluctuations which could be the result of demand (as in the sixties) or supply (as in the seventies) shocks and thus lead to different dynamics.