Finally, it is worth noting that the issue at hand is a hypothesis, not a truism. The prospect of impulsive discounting as an endophenotype should be systematically investigated and the results considered critically, with the recognition that it may not be supported. It is notable that because endophenotypes are putatively more proximal to genetic variation, the associations between endophenotypes and specific genotypes are theorized to be larger in magnitudes and more reliable. In a meta-analysis of several of the most widely studied endophenotypes, however, Flint and Munafò (2007) did not find evidence of larger effect sizes. They recommended caution against presuming that endophenotypes necessarily have a simple genetic architecture, and this may well be the case for discounting also. Furthermore, in thinking critically about genetic influences on discounting, there are potentially influences beyond individual genetic factors. For example, impulsive discounting may in fact be a function of one or more gene × environment interactions, meaning that genetic influences may only be evident in individuals with certain environmental exposures, such as early life stress (e.g., Enoch, 2012). Alternatively, if level of
