An additional body of research is concerned with measuring psychophysiological activity and responses to laboratory stimuli to assess the degree to which different psychophysiological measures may serve as endophenotypes for externalizing psychopathology (Iacono et al., 2008). Because they are thought to be more proximal to genetic influence than, for example, clinical diagnoses, endophenotypes may help researchers identify genes that contribute to the development of psychological disorders (Gottesman & Gould, 2003) as well as provide insight into the nature of genetic risk underlying those disorders (Iacono et al., 2008). Our work, as well as that of others, has indicated that one promising endophenotype, P300 amplitude reduction (P3AR), appears to serve as an endophenotype for the spectrum of externalizing disorders (Iacono et al., 2002b; Patrick et al., 2006), and that the association between P3AR and externalizing psychopathology is genetically mediated (Hicks et al., 2007). In addition, investigators have begun to explore other measures of electroencephalography (EEG) activity that appear to relate more directly to specific neural processes, which in turn may be linked to relevant genes. Indeed, some of this work has
