A THEORETICAL AND METHODOLOGICAL CRITIQUE OF THE PRINCIPLE OF RELATIVE CONSTANCY

This article revisits the theoretical assumptions of the principle of relative constancy (PRC). This principle, or hypothesis, holds that co nsumers spend a constant fraction of their income on mass media over t ime (constancy assumption), although they are expected to alter their spending patterns within mass media categories in response to the intr oduction of new mass media products or services (functional equivalenc e assumption). But application of demand theory to the PRC reveals tha t whereas the functional equivalence assumption can be phrased in econ omic terms, there is no such validation for the constancy assumption. That assumption was found to be inconsistent with the Engel law/curve in that this traditional microeconomic model of consumer choice does n ot posit, as does the PRC, a proportional relationship between expendi tures on mass media and income. Furthermore, the empirical literature reviewed here suggests mixed support for the PRC, casting more doubts on the validity of the PRC as a theoretically grounded, empirically de termined hypothesis. The article identifies and discusses five methodo logical factors that may explain why PRC studies offer conflicting evi dence. Finally, it proposes a series of theoretical and methodological recommendations for conducting future research on consumer mass media expenditures.