Next, we tested for any disorder specific transmission effects by allowing the residual variance of each parental symptom count to covary with residual variances of the corresponding offspring symptom count variable. Parent-child transmission effects were allowed to vary for biological and adoptive parent-child relationships. To maximize power to detect effects, we tested for one disorder at a time (2 df test). None of the father-child disorder specific effects resulted in a lower BIC value, the criterion by which we used to judge a significant improvement in model fit. For mother-child disorder specific effects, only the effect for nicotine dependence resulted in a lower BIC value. The effect was small, but statistically significant for biological mothers (r = 0.11, 95% Confidence Interval [CI] = 0.06-0.17), but not adoptive mothers (r = 0.03, 95% CI = -0.06-0.13).
