The co-occurrence between internalizing and externalizing behaviors. A general population twin study.

Although Internalized and Externalized problem behaviors are described as separate phenomena at the psychometric and clinical levels, they frequently co-occur. Only few studies, however, have investigated the causes of such covariation. In a sample of 398 twin pairs aged 8-17 drawn from the general population-based Italian Twin Registry, we applied bivariate genetic analyses to parent-rated CBCL/6-18 Internalization and Externalization scores. Covariation of Internalizing and Externalizing problem behaviors was best explained by genetic and common environmental factors, while the influence of unique environmental factors upon covariance appeared negligible. Odds ratio values showed that a borderline/clinical level of Externalization is a robust predictor of co-existing Internalizing problems in the same child, or within a sibship. Our findings help to approximate individual risks (e.g., in clinical practice, predicting the presence of Internalization in an externalizing child, and vice-versa), and to recognize that several shared environmental and genetic factors can simultaneously affect a child's proneness to suffer from both types of problem behaviors.