A source of variance in IQ unique to the lower-scoring monozygotic (MZ) cotwin

J. Michael Bailey; Joseph M. Horn

doi:10.1007/BF01066337

A source of variance in IQ unique to the lower-scoring monozygotic (MZ) cotwin

J. Michael Bailey^*, Joseph M. Horn

^*Corresponding author for this work

Psychology

Research output: Contribution to journal › Article › peer-review

7 Scopus citations

Abstract

Environmental factors more common in twins than singletons appear to be disadvantageous with respect to intelligence. If these factors contribute to within-pair differences in IQ, the effect should be found more in the cotwin lower in IQ (the low twin) than in the cotwin higher in IQ (the high twin). In this case, the variance of the low twins should exceed that of the high twins. This hypothesis was tested for five samples of monozygotic (MZ) twins and three samples of dizygotic (DZ) twins but was confirmed only for the MZ samples. These results suggest that there are disadvantageous environmental factors unique to MZ twins which contribute to within-pair variability. Such factors will cause heritability coefficients obtained using MZ twins to be underestimated.

Original language	English (US)
Pages (from-to)	509-516
Number of pages	8
Journal	Behavior Genetics
Volume	16
Issue number	5
DOIs	https://doi.org/10.1007/BF01066337
State	Published - Sep 1 1986

Keywords

environment
heritability
intelligence
twins

ASJC Scopus subject areas

Ecology, Evolution, Behavior and Systematics
Genetics
Genetics(clinical)

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10.1007/BF01066337

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abstract = "Environmental factors more common in twins than singletons appear to be disadvantageous with respect to intelligence. If these factors contribute to within-pair differences in IQ, the effect should be found more in the cotwin lower in IQ (the low twin) than in the cotwin higher in IQ (the high twin). In this case, the variance of the low twins should exceed that of the high twins. This hypothesis was tested for five samples of monozygotic (MZ) twins and three samples of dizygotic (DZ) twins but was confirmed only for the MZ samples. These results suggest that there are disadvantageous environmental factors unique to MZ twins which contribute to within-pair variability. Such factors will cause heritability coefficients obtained using MZ twins to be underestimated.",

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N2 - Environmental factors more common in twins than singletons appear to be disadvantageous with respect to intelligence. If these factors contribute to within-pair differences in IQ, the effect should be found more in the cotwin lower in IQ (the low twin) than in the cotwin higher in IQ (the high twin). In this case, the variance of the low twins should exceed that of the high twins. This hypothesis was tested for five samples of monozygotic (MZ) twins and three samples of dizygotic (DZ) twins but was confirmed only for the MZ samples. These results suggest that there are disadvantageous environmental factors unique to MZ twins which contribute to within-pair variability. Such factors will cause heritability coefficients obtained using MZ twins to be underestimated.

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A source of variance in IQ unique to the lower-scoring monozygotic (MZ) cotwin

Abstract

Keywords

ASJC Scopus subject areas

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