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In view of the widespread interest aroused when the subject of identical twins is raised, the literature is surprisingly scant. Almost everyone can recount a personal experience with identical twins, and in all cultures, from antiquity onward, identical twins have had special significance. It may not be a coincidence that the founders of both the Jewish group (Jacob and Esau) and Rome (Romulus and Remus) were identical twins (Niederland, 1961). Apollo and Artemis, Castor and Pollux, Heracles and Iphicles were all twins of Greek mythology (Larousse, 1959). A present-day Nigerian tribe has customs in which twins—identical or fraternal—are honored and glorified (Report of the Delacorte Gallery, 1960).
Scientific interest in identical twins dates from the work of Galton about one hundred years ago. More recent investigators, such as Kallman (1948), Newman et al. (1937), and Slater (1953), have tended to use identical twin pairs as experiments in nature useful to determine the relative roles of heredity and environment. Fraternal or binovular twins have been ignored in such studies because they are regarded as distinct individuals whose development would not throw light on the relative role of heredity and environment in psychologic development. As Arlow (1960) points out, these are not studies of twins but studies on twins. Psychiatric studies along these lines have been appearing with increasing frequency (Jacobs and Mesnikoff, 1960); (Lowinger, 1960).