There is also a very robust empirical connection between discounting and addictive behavior. Many cross-sectional categorical studies have been completed, comparing delay discounting in a criterion group of individuals exhibiting addictive behavior to a control group. More impulsive discounting has been found in criterion groups based on levels of alcohol misuse and dependence (e.g., Petry, 2001; Vuchinich & Simpson, 1998), nicotine dependence (e.g., Bickel, Odum, & Madden, 1999), stimulant dependence (e.g., Coffey, Gudleski, Saladin, & Brady, 2003), opiate dependence (e.g., Madden, Petry, Badger, & Bickel, 1997), and pathological gambling (e.g., MacKillop, Anderson, Castelda, Mattson, & Donovick, 2006). Moreover, a recent meta-analysis of 46 cross-sectional studies found consistent and substantial evidence of differences between groups in these categorical studies (MacKillop et al., 2011). Specifically, as illustrated in Figure 2, the meta-analysis revealed a statistically significant medium effect size difference between groups across studies, and also revealed statistically significant and medium effect size differences within addictive behavior categories, suggesting this was common across addictive drugs (albeit with the exception of marijuana use disorders). In addition, the meta-analysis revealed a significantly larger effect
