Beyond proximal environmental influences, research on the etiology of externalizing problems has focused on the role of individual differences. Twin studies show evidence for genetic influences on general externalizing behavior (Hicks et al., 2004; Krueger et al., 2002). More recently, specific genes have been identified as potential candidates that may confer differential risk for behavioral outcomes. For example, CHRM2, a gene encoding the muscarinic acetylcholine receptor M2, has been associated with a number of cognitive and biological markers known to correlate with externalizing problems. Specifically, CHRM2 has been implicated as playing an important role in neurocognitive processes such as disinhibition (Casey et al., 2007) and novelty seeking (Dick et al., 2007a). In addition, CHRM2 has been associated with P300 event-related potentials, a biomarker shared across externalizing disorders (Jones et al., 2006; Jones et al., 2004), as well as with alcohol spectrum disorders directly (Dick et al., 2006; Rangaswamy & Porjesz, 2008). In one study, CHRM2 showed the strongest association with a composite factor encompassing symptoms from a variety of specific externalizing problems and disorders (Dick, 2007; Dick et al., 2008).
