For each phenotype, alternative univariate models containing A, E, and either C, D, or S were compared. Because C, D, and S are estimated using the same information, they cannot be included in the same model. The best fitting model for each phenotype was chosen on the basis of BIC (Table 1). For CD, inattention, and hyperactivity-impulsivity, AE+S models had the lowest BICs, although estimates of S were small (CD = -.08, hyperactivity-impulsivity = -.11, and inattention= -.15). Negative estimates of S are interpreted as reflecting biases due to raters contrasting twin behavior or competition between twins.34
