Twins and virtual twins: Comparative analysis of problem behaviors

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_is = .72, .71, and .77, respectively) compared with VT pair scores (r_is = .25, .40, and .63, respectively) demonstrates genetic influence on these measures. VT pairs with a biological child and an adoptee showed greater resemblance than those with two adoptees. This probably reflects the correlation of biological children's genotype with the adoptees' environment. Regression analyses revealed that for MZ-CTT pairs less variance in problem behaviors was explained by parental variables, such as maternal education, relative to VT pairs. This result may speak to the greater heterogeneity of the backgrounds of VT participants. In summary, this study of two unique kinships shows that both genetic and environmental factors influence child behavioral problems. Future studies using novel and informative kinships are warranted.