Heritability estimates of intelligence in twins: Effect of chorion type

This study investigates the basic assumption of homogeneity of monozygotic (MZ) twins: are there differences according to the timing of the zygotic sp litting, early in dichorionic (DC) and later in monochorionic (MC) pairs? W e assessed the IQ of 451 same-sexed twin pairs of known zygosity and chorio n type with the Wechsler Intelligence Scale for Children-Revised (WISCR). T he variances of within-pair differences were compared for monochorionic (MC ), dichorionic monozygotic (DC-MZ) and dizygotic same-sexed (DZ) twins and structural equation modeling was applied. High heritability estimates were found for almost all subscales and IQ-scores. A significant effect of chori on type was found: the MC twins resembled each other more than the DC-MZ tw ins on the subscales Arithmetic and Vocabulary. The effect accounts for res pectively 14% and 10% of the total variance.