Similar to these preceding factors, another source of potential confounding is a failure to address comorbidities in sampling. The association between impulsive discounting and addiction is arguably the most well established, but there is also evidence that it is associated with several related conditions, such as ADHD (e.g., Scheres et al., 2006; Hurst, Kepley, McCalla, & Livermore, 2011), antisocial personality disorder (e.g., Acheson et al., 2011; Petry, 2002), and bipolar disorder (e.g., Ahn et al., 2011). As such, case-control studies using clinical cases compared to a matched control group will need to be sure that both groups are fully characterized in terms of present and past psychiatric conditions to avoid potential confounding. More generally, collateral assessment of relevant variables on continua, such as attentional control and antisociality, would be optimal in order to characterize the relationship to the discounting phenotype simultaneously in genetic studies. This issue is essentially the same as the potential demographic and cognitive confounds; a well-designed study should be able to specifically attribute genetic influences to the discounting phenotype and in turn to addictive behavior, but not
