These studies using animal models reveal both aggregate genetic influences on impulsive choice and, in several cases, the connection between these influences and drug motivation. However, there are limits to generalizability, such as whether the same genetic variants are associated with greater discounting across strains or whether variation in discounting in humans is similarly genetically-influenced and, if so, attributable to homologous genes. Toward addressing the question of heritability in humans, the gold standard is the twin methodology, in which a given phenotype is examined in both monozygotic and dizygotic twins and statistical modeling is used to infer the amount of variation attributable to genetic factors based on known differences in the amount of shared genetic variation (i.e., effectively 100% in monozygotic twins versus 50% on average in dizygotic twins). More specifically, this approach permits parsing the relative overlap in the characteristic or condition to additive genetic factors (the sum of individual genetic effects); non-additive genetic factors, such as gene×gene interactions; shared environmental experiences (events or experience that pertain to both twins), and non-shared environmental factors (events or experiences that are
