Given the exploratory nature of the present report, it necessarily has some notable limitations. The brain electrophysiological measures were recorded from a limited number of electrodes: 3 parietal sites for P3 and its related TF components, and Cz and bilateral parietal sites for beta power. The implications are that we have limited topographic information and that we may not be capturing all measures that have been associated with EXT in the literature. For instance, a frontal theta TF component has been associated with alcoholism (Jones et al., 2006). It will be important to examine a denser array of electrodes in followup studies. Also, the sample consisted solely of twins in late adolescence and was not racially or ethnically diverse. This age (~17-years-old), however, is that at which the most is known regarding the endophenotypic relationship between the measures in the current study and EXT (e.g. Gilmore et al., in press; Iacono et al., 2002; Rangaswamy et al., 2007). Thus, we initially focused our efforts on this age. Further research needs to be done, however, regarding the generalizability of the findings to other age, racial, and ethnic groups.
