The present study addresses the issue of differential heritability with increasing severity of parent-reported internalizing and externalizing behavior problems assessed by the Child Behavior Checklist. The sample includes 526 identical and 389 fraternal same-sexed twin pairs from five national birth cohorts, aged 5-6, 8-9, and 12-15 years. Heritability (h2), common environment (c2), and changes in these parameters as a function of proband score were analyzed by multiple regression models (Cherny et al., 1992). Internalizing and externalizing behavior showed significant heritability. A small increment in h2 and a reduction of c2 with increasing severity of externalizing behavior were independent of sex and age. For internalizing behavior h2 increased and c2 declined with increasing severity for the 5-6 and 8-9 year olds. Logarithmic transformation of scores lowered h2 and increased c2, particularly for externalizing behavior. The changes in heritability with severity were nonsignificant for the transformed variables.