Commodity specific rates of temporal discounting: does metabolic function underlie differences in rates of discounting?

Discounting rates vary as a function of commodity type. Previous studies suggest five potential characteristics of the commodity that could explain these differences: type of reinforcer (primary or secondary), if the commodity is perishable, if the commodity is satiable, if the commodity can be directly consumed, and immediacy of consumption. This paper suggests that these characteristics may best be viewed as related to a more fundamental characteristic: metabolic processing. In order to explore the possibility that metabolic processing underlies changes in discount rates, the difference in discounting between food, money, music CDs, DVDs, and books are compared. Music CDs, DVDs, and books share many characteristics in common with food, including gaining value through a physiological process, but are not directly metabolized. Results are consistent with previous findings of commodity specific discount rates and show that metabolic function plays a role in determining discount rates with those commodities that are metabolized being discounted at a higher rate. These results are interpreted as evidence that the discount rate for different commodities lies along a continuum with those that serve an exchange function rather than a direct function (money) anchoring the low end and those that serve a direct metabolic function capping the high end (food, alcohol, drugs).