The agreement between zygosity diagnosis in twins by reference to the twins' own answers to written questions and diagnosis based on an objective assessment of zygosity by blood-typing is studied for 178 pairs of same sex twins. Seventy-two female and 106 male twin pairs answered two questions concerning their similarity in appearance and frequency of confusion in childhood, and were blood-grouped using up to 23 different antisera. Of those pairs concordant for all markers and diagnosed as MZ, 98% agreed that they were confused in childhood and that they are alike in physical appearance. Only 55% of those pairs discordant for at least one marker and classified as DZ agreed that they were never confused and are not alike in appearance. The replies of the remaining 45% are spread over all the other combinations of responses. If all twins who agree that they were confused in childhood and are alike in appearance were to be classified as MZ on the basis of the questionnaire alone, and all others as DZ, the diagnoses of only seven pairs out of the total 178 (3–9%) would disagree with those based on blood group data.
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