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 to which they treat their nontwin children similarly. Failing to measure T may lead to inflated estimates of C. Like C, T may capture passive gene-environment correlation. For example, a parent’s genetic vulnerability to antisocial behavior may not be triggered until the arrival of a major stressor, by which time an older sibling may not be in the period of risk for maltreatment, though two younger twin children are. Neiderhiser demonstrated that the magnitude of passive gene-environment correlation can be estimated in a study using a parent-based design (i.e., examining the parenting of MZ and DZ parents).
