On the Implications of Jointness in a Normative Model of Behavior Based on an Activity Hierarchy

T he widespread belief that it is often useful to "kill two birds with one stone" makes it clear that most consumers recognize that there exists considerable scope for using an input gainfully in the production of more than one consumption activity. However, economic theorists often forget this lay belief as they set up their formal models, and consequently they come to inappropriate conclusions. This recently occurred in Coursey's (1985) normative analysis of consumer behavior in this Journal where he models priority-based choice over activities.' This article indicates how the implications of his model may be modified if activities use a common input.2 In contrast to Coursey, we show that priority-based choice does not necessarily imply that (1) an increase in an individual's income will lead to a reduction in the percentage of a consumer's budget spent on inframarginal activities, or (2) a change in the price of a commodity producing a marginal activity will only lead to changes in an individual's consumption of marginal and extramarginal commodities. Put simply, it appears that Coursey's general results are valid only when one adopts his implicit assumption that there is no jointness in production.