Abstract
A number of experiments with SDR methodology have yielded inter-species performance differences that are consistent with taxonomic rankings. However, one troublesome problem that plagues such comparisons is the question of whether or not such differences might be due to uncontrolled variation in either drive ( D) or incentive ( K) levels across species. As an initial step in the evaluation of the problem a series of experiments designed to study the effect of variation of both variables, within and across species, upon SDR measures are reviewed. While it is clear that both D and K do effect SDR measures, previously obtained inter-species performance differences are recovered when species are equated on these variables.