In addition, researchers may conduct bivariate quantitative genetic analyses of twin data to partition the covariation between two phenotypes into genetic, shared environmental, and nonshared environmental influences. As in the univariate case, interpretations of bivariate analyses including a parenting variable must be modified to account for passive gene-environment correlation (Table 1). Covariation due to common genetic influences evinces a nonpassive gene-environment correlation. Common shared “environmental” influences may include both: (1) true common shared environmental influences between maltreatment and conduct problems, and/or (2) passive gene-environment correlation. Common nonshared environmental influences reflect: (1) differences in levels of maltreatment due to nonshared environmental influences that are correlated with differences in levels of conduct problems due to environmental influences, and/or (2) error in measurement of conduct problems that is correlated with error in measurement of maltreatment.
