ABILITIES OF TWINS

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.