Participants (115 monozygotic pairs of twins, 32 dizygotic pairs, and 881 non-twin students) took the Tanaka B Intelligence Test (New Edition) when in junior high school and high school. The data were analyzed by a type of structural equation model, a genetic ACE (Additive genetic, Common environment, non-shared Environment) model, combined with a model for analyzing longitudinal data. Intelligence scores (IS) and scores on 7 sub-tests were examined. Non-shared environment accounted for a considerable portion of the variance in each score, at both the junior high and high school levels.
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