Developmental trends in intelligence revisited with novel kinships: Monozygotic twins reared apart v. same-age unrelated siblings reared together

Prior research has reported convergence of monozygotic (MZ) cotwins' intelligence scores over time, with divergence observed between dizygotic (DZ) cotwins. These patterns have been variously explained with reference to the increased influence of genetic factors (MZ and DZ twins) and nonshared environmental factors (DZ twins), and the reduced effects of shared environmental events (DZ twins). Studies of unrelated siblings have found modest within-pair resemblance in intelligence, with increasing divergence over time. This subject is revisited in a study using three novel kinships: young reared-apart MZ twins from China (CTA-MZ), adult reared-apart twins from Denmark (D-MZA) and findings from a previous study of same-age unrelated siblings (virtual twins or VTs). Despite modest sample sizes, the anticipated trends described above were observed, replicating extant findings. Intraclass correlations for overall IQ score at Time 1 and Time 2, respectively, were ris = 0.51, 0.81 (CTA-MZs) and ris = 0.64, 0.74 (D-MZAs). The hypothesis that VTs would show score divergence at Time 2 was confirmed. Increased genetic influence (CTA-MZ, D-MZA and VT), reduced impact of shared environments (VT), and increased effects of non-shared environments (VT) appear to best explain the findings. These results also inform parent and educator expectations regarding twins' and siblings' academic performance.