1. The original objective of these studies was the study of the roles of heredity and environment in determining mental capacities of various sorts. 2. The first essential was to learn how to diagnose with certainty the two types of twins, monozygotic and dizygotic. 3. Only about 25 per cents of all twins are monozygotic. Collections that depart widely from this figure have probably been incorrectly diagnosed. 4. Only about 42 per cent. of same-sexed twins are monozygotic. 5. The method of diagnosis used in this study combines the best features of the methods of Dahlberg, Siemens, and Komai. Certain refinements of technique are added, the details of which are explained in the text. 6. Out of a collection of one hundred two pairs of same-sexed twins, only six pairs caused any difficulty, three of which are now classified as monozygotic and three as dizygotic. 7. The details concerning the three "similar fraternals " are presented and the reasons for their diagnosis as dizygotic twins are given. 8. The result is that we have now a collection of fifty pairs of monozygotic and fifty-two pairs of dizygotic same-sexed twins accurately diagnosed. These are to be used for further biological and psychological study.