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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Bailey, J.M., Horn, J.M. A source of variance in IQ unique to the lower-scoring monozygotic (MZ) cotwin. Behav Genet 16, 509–516 (1986). https://doi.org/10.1007/BF01066337
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DOI: https://doi.org/10.1007/BF01066337