title: Heritability estimates of intelligence in twins: effect of chorion type.
abstract: This study investigates the basic assumption of homogeneity of monozygotic (MZ) twins: are there differences according to the timing of the zygotic splitting, early in dichorionic (DC) and later in monochorionic (MC) pairs? We assessed the IQ of 451 same-sexed twin pairs of known zygosity and chorion type with the Wechsler Intelligence Scale for Children-Revised (WISC-R). The 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 chorion type was found: the MC twins resembled each other more than the DC-MZ twins on the subscales Arithmetic and Vocabulary. The effect accounts for respectively 14% and 10% of the total variance.
authors.affiliation: Center for Human Genetics, Katholieke Universiteit Leuven, Belgium.
authors.forename: N
authors.initials: N
authors.lastname: Jacobs
authors.forename: S
authors.initials: S
authors.lastname: Van Gestel
authors.forename: C
authors.initials: C
authors.lastname: Derom
authors.forename: E
authors.initials: E
authors.lastname: Thiery
authors.forename: P
authors.initials: P
authors.lastname: Vernon
authors.forename: R
authors.initials: R
authors.lastname: Derom
authors.forename: R
authors.initials: R
authors.lastname: Vlietinck
mesh_terms: D000328:Adult; D001530:Belgium; D002823:Chorion; D005260:Female; D006801:Humans; D007360:Intelligence / Q000235:genetics*; D008297:Male; D014429:Twins, Dizygotic / Q000235:genetics*; D014430:Twins, Monozygotic / Q000235:genetics*; D014888:Wechsler Scales