This work reports on a study to evaluate the relative contributions of genetic and environmental factors to both competence scales and behavioral/emotional syndromes as assessed by the Child Behavior Checklist (CBCL). A total of 279 pairs of twins and same-sex sib-pairs aged 12-16 years were recruited from 51 junior high schools in Taipei City, Taiwan. Twins' zygosity was determined by a combination of DNA typing and physical similarity. The Mx program was used to estimate parameters for a full model that contains effects from sex-specific additive genes, shared environment, and nonshared environment for the majority of the scales. The shared environment in the full model was replaced with nonadditive genetic factors for some scales when indicated. All girls' competence and behavioral/emotional syndromes exhibited a substantial heritability (h2 > 0.4), except for Social Competence and Withdrawn. For boys, though the heritability was also >0.4 for some scales (Social and School Competence, Thought Problems, Attention Problems, Delinquent Behavior, and Total Behavior Problems), environmental influences, especially shared environment, were predominant for most of the scales (10 out of 15 scales). Genetic factors are important for explaining adolescent behavioral problems, especially for girls, while shared environmental influences cannot be ignored for boys. Gender differences in heritability exist for various CBCL-based competence and behavioral/emotional problems.