The Diagnosis of Twin Zygosity

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Abstract
Objective rules for diagnosing zygosity of twins from questionnaire items concerning physical similarity were developed on the basis of the responses of 123 sets of twins diagnosed by blood typing. Cross-validation of a preliminary set of rules and intraclass correlations of intelligence for 1,239 sets of twins diagnosed by various methods indicates that about 95% of all cases can be diagnosed by the rules with greater than 90% accuracy.