Environmental bias in twin studies

Abstract

This study examines the responses of mothers of twin girls about similarities and differences of their monozygotic (MZ) and dizygotic (DZ) twins. After these measures had been completed and scored, the investigator obtained zygosity diagnoses of the twins made by extensive blood‐group analyses. Of the 61 pairs of twins, 11 were misclassified by their mothers. Despite these mothers’ erroneous beliefs about the zygosity of their twins, they described the twins as having similarities and differences appropriate to their true degree of genetic relatedness.