Abstraction skillfulness in monozygotic and dizygotic twin pairs

The computerized version of the Wisconsin Card Sorting Test (WCST) was administered to a sample of 96 subjects (Ss), constituted in equal parts by monozygotic twins (MZ), dizygotic twins (DZ), unique children and couples of "almost contemporary" brothers. The statistic tests (Analysis of principal components, ANOVA) underline, as far as the rapidity to define a category is concerned, a statistically significant difference between DZ and singletons, independently from the fact that the latter may be unique children. A significant difference emerged neither between MZ and singletons, nor between MZ and DZ.