In a study of adolescent and mother reports of maternal behavior, Neiderhiser et al. (2004) described how univariate twin studies may be used to test for the presence of passive and nonpassive gene-environment correlation (Table 1) in parenting variables (i.e. maternal control, warmth, positivity, and negativity). In child-based designs (i.e., examining the parenting of MZ and DZ children), the additive genetic factor (A) captures variance in parental behavior associated with genetically influenced characteristics of children (genetically influenced child effects/nonpassive gene-environment correlation). The “shared environment” (C) captures the extent to which parents treat their children similarly regardless of whether they are MZ or DZ twins or full siblings, including true environmental influences. This factor also captures the effects of any passive gene-environment correlation. The nonshared environmental factor (E) includes differences in the way parents treat their children due to environmental influences and measurement error. Though not described by Neiderhiser, twin studies that also include full sibling pairs may also estimate the “twin environment” (T), which captures the extent to which parents treat their twin children similarly over and above the extent
