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The present investigation is part of a follow‐up study of twins from grade 4 through 8 (age 11 through 15). Intra‐pair similarities are studied, intra‐class correlations being used as a criterion of similarity. Scores on achievement tests administered to practically all the twins in grade 4 in Sweden were collected and analyzed. The intra‐class correlations obtained for an age group of twins in grade 6 are throughout higher than those obtained in grade 4. The findings refute the additive model of hereditary and environmental influences. Two alternative hypotheses are advanced to explain the results. Previous findings indicating an average superiority of single‐borns over twins in cognitive tests are confirmed.
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Torsten Hüén (1963) studied this question.
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