Within-session changes in rats' food-demand elasticity

A common finding in behavioral economics is that demand assessed under an open economy is more elastic than that obtained under a closed economy. Although elasticity traditionally has been conceptualized in terms of price sensitivity, a potential confounding variable is session duration because open economies typically arrange markedly briefer sessions than closed economies. To assess the role of session duration on demand elasticity, we arranged an open economy in which 6 male Long-Evans rats lever-pressed for food and water reinforcers in 1-and 6-hr sessions. The fixed ratio for food reinforcers increased across sessions. Both reinforcer magnitude (1 or 2 pellets) and session duration were manipulated across conditions. Cumulative hourly food consumption data were fit to both the linear and exponential demand models such that the elasticity parameters yielded could be compared. For all rats, as time within the session elapsed, cumulative exponential demand for food became progressively less elastic, as did initial linear demand elasticity; however cumulative linear demand for food remained relatively unchanged for 5 of 6 rats. These results suggest that session duration modulates only initial demand elasticity with small price increases, and that these changes do not reflect changes in price sensitivity. Thus, session duration does not appear to account for the differences in elasticity previously obtained under open and closed economies.