State-independence refers to the phenotype being stable and not simply a reflection of the disease process. As discussed above, the work of Schuckit and colleagues found that subjective responses to alcohol measured in the laboratory before the development of alcohol-related problems predict the development of alcohol use disorders at 8-year follow-up (Schuckit & Smith, 1996). That is true of even in the case of individuals who were relatively alcohol-naïve at the time of the alcohol challenge. Significantly less is known about the state-independence of alcohol’s reinforcing effects and its association to alcohol use disorders. Heritability is an important criterion for evaluating endophenotypes and represents the degree to which the phenotype is influenced by genetic variance. Ideal endophenotypes are heritable and one would wish for a highly heritable endophenotype very much in the same way we expect our behavioral measures to be reliable, so as to reduce “noise.” Subjective responses to alcohol, measured in an experimental twin study, had a heritability estimate of 60% (Viken et al., 2003). This study used a 22-item measure called Sensation Scale (Maisto et al., 1980),
