Although P3AR appears to index genetic risk for externalizing psychopathology broadly conceived, it is not the case that those at risk develop all of the externalizing disorders, and specific genetic influences are likely to guide the development of each disorder. These specific influences may have their own endophenotypes, and combinations of endophenotypes may be profitably used to identify risk for specific disorders. Iacono et al. (2000) showed that when P300 amplitude and an autonomic measure derived from a stressful coping task were used in conjunction, those scoring in the deviant direction on both variables had especially heightened risk for developing SUDs, and the two endophenotypes were not significantly correlated (r=.15). Subsequent work with the autonomic measure has shown that rather than being associated with externalizing disorders generally, it appears to be related only to SUDs (Taylor, 2004), thus raising the possibility that using the two measures in combination may aid the identification of genes specific to SUDs.
