The investigation concerns two main problems: (1) the distribution of ability test scores among twins in comparison with single‐borns; and (2) intra‐pair similarity among the various twin categories. More than 1000 individual twins were compared with 2700 single‐borns on 21 test variables covering Swedish, Mathematics and English. The twin group consistently showed a lower average than the single‐borns due to a displacement of the test score distribution as a whole among twins. The intra‐pair similarities, expressed as intra‐class correlations, run between 0.85 and 0.90 for identical pairs, whereas like‐sex fraternals showed coefficients about 0.60. The unlike‐sex fraternals had coefficients ten to twenty hundredths lower than those of the like‐sex pairs.
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Torsten Husén (1960) studied this question.