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 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.
Jacobs
N
N
Center for Human Genetics, Katholieke Universiteit Leuven, Belgium.
Van Gestel
S
S
Derom
C
C
Thiery
E
E
Vernon
P
P
Derom
R
R
Vlietinck
R
R
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