Mothers of 155 sets of twins completed a questionnaire about the degree of physical similarity between the children and if, and by whom, they were confused. The twins were blood typed to determine zygosity. Parental perceptions of identical and fraternal twins were extremely different. A discriminant function analysis produced clear differentiation between the fraternal and identical groups with 98% agreement between zygosity assigned by the responses to the questions and zygosity determined by blood typing.
This approach to assigning zygosity to twin pairs may be of value in epidemiological and other studies of twins. The results of this study raise questions about an assumption that underlies many behavioral genetic investigations comparing identical and fraternal twins: that the life experiences of both types of twins are comparable. This assumption, basic to calculation of heritability coefficients and relevant to comparison of intrapair correlations, is questionable. There probably are psychological implications of being confused by one's parents, family, and strangers.