Reliably separating identical from fraternal twins.

Blood typing is the most reliable method for assigning zygosity to twinships in psychological research. Cost, ethical considerations, and practical difficulties in obtaining blood specimens from a large group of children suggested the need for a questionnaire method used with young children and completed by parents. One was designed to assess zygosity based on the extent to which the children were rated as looking alike and being confused by family and strangers. Validity was determined with a sample of twins whose zygosity was demonstrated by blood typing. To determine test-retest reliability, and to explore parental beliefs about zygosity, mothers of same-sex twinships completed the questionnaire on two separate occasions, showing very high agreement. The major difference in parental perceptions of monozygotic and dizygotic twinships is convenient for epidemiological research. This difference, however, questions the assumption, made in estimates of heritability using twin data, that both twinships have identical environmental experiences.