Virtual twins and intelligence: Updated and new analyses of within-family environmental influences

Virtual twins (VTs) are same age, unrelated siblings reared together from early infancy who replay twinship, but without genetic relatedness. A previous report from an ongoing study of behavioral similarities and differences between VTs, based on 90 pairs (Segal, 2000b), indicated a modest within-family contribution to general intelligence and a negligible within-family contribution to the cognitive ability profile. New data, based on a larger VT sample (mean age = 8.10 years; SD = 8.56), supported these findings, yielding a within-pair IQ correlation of .26 (n = 113 pairs) and a profile correlation of .07 (n = 111 pairs). This special class of adopted siblings also enabled updated analyses of the IQ similarity between adopted-adopted and adopted-biological pairs. In addition, a first assessment of sibling interaction found positive and significant effects for each IQ measure. These findings are interpreted with reference to available twin and adoption data.