A study of factors affecting children's problem behaviors using two unique sibships, monozygotic twins adopted together from China (MZ-CTT twins) and virtual twins (same-age unrelated siblings reared together
VTs), is presented. MZ-CTT twins share genes and environments, whereas VTs share environments only, allowing assessment of how these factors influence behavioral difficulties. Participants (MZ-CTT: n = 43 pairs
VT: n = 41 pairs) were in ongoing studies of behavioral development at the Twin Studies Center at California State University, Fullerton. Mean ages were 7.00 years (SD = 2.59) for MZ-CTT pairs and 6.24 years (SD = 2.26) for VT pairs
MZ-CTT pairs were significantly (albeit only slightly) older. Parents completed demographic surveys and the Child Behavior Checklist for each child. The greater resemblance of age- and sex-corrected t scores of MZ-CTT pairs across Internalizing, Externalizing, and Total problem behavior scales (r