Data from monozygotic (MZ) and dizygotic (DZ) twins can be utilized to estimate the relative contribution of additive genetic (A), non-shared environmental (E), and either shared environmental (C) or non-additive genetic (including dominance, and thus denoted as D) influences on population variation in a given behavior. C and D cannot be jointly estimated when data from twins alone are used, so we fitted models incorporating C and D separately (i.e., ACE and ADE models) to the data.
