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The unity of twins, the meaning they have for each other, their similarity, and the special ways they interrelate have long been subjects of psychological interest. Some researchers (Kallman, 1953; Newman, 1940; Slater, 1953) have stressed genetic identity as the major factor determining the behavioral and personality similarities of identical twins. Other more psychoanalytically oriented writers (Cronin, 1933; Joseph, 1961; Joseph and Tabor, 1961) have focused on the psychic relatedness of twins and have described the partial fusion of some identical twins with each other. Above and beyond genetic similarities, they see aspects of the life experience inherent in being a twin as crucial in what they call the “twinning reaction.” Thus, they suggest that such factors as the gratification twins offer each other, the way in which they are treated as a unit by others, their use of each other in acting out conflicts, and the fact that they pass through the various developmental stages simultaneously, determine the mutual interidentification and diffuse ego boundaries sometimes found in twins.
Identity fusion has more often been described in identical twins, but is at times also seen in fraternal twins. For example, Orr (1941) has described what he calls a “joint ego” in a fraternal twin, underlining the critical importance of the twin experience itself in the personality development of twins. Ortmeyer (1970), reporting on the psychoanalysis of a twin, has thrown further light on twin interaction. Proposing the concept of a “we-self,” he has suggested that the psychological unit of twins involves a “complementarity of mutually shared aspects of personality” (p. 126). He asserts that, rather than being identical in personality, twins may tend to develop “nonduplicative and nonidentical” traits, certain of these traits becoming more fully developed in one twin than in the other and then serving both partners. In this way each twin offers the other complementary personality at