in our general population sample is not comparable to its density in COGA, problematic alcohol use is quite prevalent in the MCTFR in general (Hicks, Schalet, Malone, Iacono, & McGue, 2011; McGue et al., 2013), including MTFS twins (Hamdi & Iacono, 2014). Ascertainment in COGA on such high levels of genetic susceptibility is likely to increase the relative importance of rare variants with large effects in genetic analyses. It also may amplify the genetic signal common to P3 amplitude and alcoholism risk more than the P3-specific signal, which is small in population-based samples such as ours (Hicks et al., 2007).
