An underappreciated element of an endophenotype is its ability to identify youth without the manifest clinical phenotype who are nevertheless at high risk for developing it. This is a difficult criterion to establish because it requires longitudinal investigations in which a healthy sample of those with the endophenotype are followed for many years to determine their increased odds of developing psychopathology. Yoon et al. (Yoon, Malone, & Iacono, 2015) provided just such an example using reduced P300 event-related potential (ERP) amplitude, an endophenotype for externalizing disorders. This research team reported that healthy individuals assessed at age 17, who were subsequently diagnosed with a substance use disorder or antisocial personality disorder by age 29, had smaller P300 ERP amplitudes at age 17 than those who remained healthy. For each 1-microvolt decrease in age-17 P300 amplitude, the odds of an age-29 externalizing diagnosis were increased by 5%. Findings such as this point to the potential value of using endophenotypes to identify those at genetic high risk well before they become symptomatic. A longitudinal research strategy built from a sample enriched in this
