Background: Dissociation--a pattern of general disruption in memory and consciousness--has been found to be an important cognitive component of children's and adults' coping with severe trauma. Dissociative experiences include amnesia, identity disturbance, age regression, difficulty with concentration, and trance states. Stable individual differences in dissociative behaviors may represent a dissociative tendency trait that varies in the population independent of the influence of trauma. Method: In the current study, we examined genetic and environmental sources of variance in some of these behaviors by comparing 86 pairs of adoptive siblings and 102 pairs of full siblings from the Colorado Adoption Project (parents' and teachers' ratings), and 218 pairs of identical and 173 pairs of same-sex fraternal twins from the British Register for Child Twins (parents' ratings). The study used a dissociation scale comprised of six CBCL items. Results: Developmentally, there was no change in mean dissociation scores across middle childhood and adolescence, and individual differences were moderately stable. Both parents' and teachers' ratings showed moderate to substantial amounts of genetic and nonshared environmental variance and negligible shared environmental variance, and most of the parent-teacher agreement in their ratings was accounted for by overlapping genetic variance. Conclusions: The results support further research into possible genetic and environmental factors that contribute to dissociative tendencies in children and adolescents.