The genetic and environmental architecture at Wave 1 was also comparable to findings from the Minnesota Twin-Family Study (MTFS), which demonstrated a highly heritable “externalizing spectrum” construct (Hicks, Krueger, Iacono, McGue, & Patrick, 2004; Krueger et al., 2002). However, our Wave 2 results showed more modest genetic influences as well as modest shared environmental influences. Given the similar age at time of assessment (~17 years) in the LTS and MTFS samples, we attribute these differences in heritability to the fact that the MTFS twins were assessed with measures of more severe pathology, including symptoms of Diagnostic and Statistical Manual of Mental Disorders (3rd ed., rev.; American Psychiatric Association, 1987) defined antisocial personality disorder, as well as symptoms of alcohol and drug dependence, rather than experimentation. Deliberately targeting these more deviant behaviors likely defines a more genetically influenced syndrome (e.g., Rhee & Waldman, 2002) but one that would not be meaningful in twins as young as 12 years.
