In MOTWIN, inherited liability was parameterized by presence or absence of clinical-level externalizing behavior problems in the co-twin, according to the parent-report Child Behavior Checklist (CBCL). This yielded four levels of liability that were dummy coded in the following descending order of severity: Monozygotic (MZ) co-twin affected, dizygotic (DZ) co-twin affected, DZ co-twin unaffected, and MZ co-twin unaffected. Note that all twin pairs were reared together, so distinctions between MZ and DZ twin outcomes for a given co-twin status can be interpreted unequivocally as effects of gradations in inherited liability, [8] given the equal environment assumption that underlies all classic twin designs.
