More importantly, there is also evidence from studies with both animals and human twins that delay discounting is heritable. In the former case, a number of studies have investigated differences in discounting between inbred rodent strains. As these strains are isogenetic within each strain (Beck et al., 2000) and receive identical rearing environments, systematic differences in discounting across strains can be attributed to genetic differences. The first study revealing strain differences did so in the context of investigating clomipramine as a pharmacotherapy for impulsivity in Lewis and Fischer 344 rats (Anderson & Woolverton, 2005), finding no effect of the medication, but significantly higher delay discounting in the Lewis rats. This finding has subsequently been replicated twice using a steady-state paradigm to minimize within-session carryover (Madden, Smith, Brewer, Pinkston, & Johnson, 2008; Stein, Pinkston, Brewer, Francisco, & Madden, 2012). Using six strains, Wilhelm and Mitchell (2009) found significantly greater discounting in Fischer rats compared to Copenhagen and Noble rats, but not Lewis rats, although a recent study suggests that this failure to replicate may be a function of procedure rather than
