A further consideration is that although there is relatively extensive evidence that impulsive discounting predates addictive behavior, as reviewed above, there is also evidence that persistent drug exposure can also result in long-term increases in impulsive delay discounting (Dallery & Locey, 2005; Roesch, Takahashi, Gugsa, Bissonette, & Schoenbaum, 2007; Simon, Mendez, & Setlow, 2007) and probability discounting (Nasrallah, Yang, & Bernstein, 2009). Not all studies have found this to be the case and important methodological considerations apply (for a review, see Stein & Madden, in press), but, nonetheless, the aggregated findings suggest that impulsive delay discounting represents a recursive etiological process. In other words, impulsive discounting may well serve as both a risk factor and a characteristic that can be exacerbated by addictive behavior. Impulsive discounting from this perspective operates as a ‘Matthew Effect’ (i.e., advantage potentiating further advantage, disadvantage potentiating further disadvantage), with individuals with innate propensities to discount the future being at greatest risk of developing addictive disorders and in turn become progressively more impulsive, and vice versa. This recursive perspective further complicates the phenotypic picture, as a
