Level of response is considered by some to be an endophenotype. Defined by Gottesman and Gould118 as measurable indices of liability to a phenotype, these measures have gained popularity in genomic studies as they are heritable and assumed to be more proximal to the biological underpinnings of the behavior being studied.119 Although they may co-segregate with disease, they are more closely related to the causes than the consequence of disorder.120 There have been multiple putative endophenotypes proposed for addiction, including alcohol and drug-related attentional bias, frequently assessed using a modified version of the traditional Stroop task,121 and electro-encephalogram activity (for example, beta wave patterns, P300 amplitude).122, 123, 124 A number of these endophenotypes have been used in candidate gene efforts—for instance, SNPs in GABRA2 (Table 1) have been found to associate with resting electro-encephalogram beta waves.36, 125
