A series of multivariate twin models positing different combinations of additive genetic (A), shared (or common) environmental (C), and nonshared (or unique) environmental (E) components were fit to the individual criterion-level twin data on the 1073 complete twin pairs and the 623 single twins. The various independent pathway models were fit to 2-group (monozygotic/dizygotic, male/male same-sex twin pair) raw data using full-information maximum likelihood estimation as implemented in Mx.22 Each observed ordinal CD criterion was modeled as a set of estimated ordered thresholds on a normally distributed, continuous latent liability/response variable. Parameter estimation was carried out by integrating across these latent variable continua. Different threshold estimates were allowed for monozygotic and dizygotic male twin pairs. We began by fitting a “111_111” baseline model in which twin resemblance among the 10 CD criteria was posited to be adequately accounted for by single common additive genetic, shared environmental, and nonshared environmental components along with criterion-specific additive genetic, shared environmental, and nonshared environmental effects. Models omitting the common and criterion-specific C components, and then including additional A and E common components, were tested
