Twin psychology has been studied to a very limited extent. Only few remarkable peculiarities of the twins' development are known for sure, such as delayed intellectual development, language retardation with frequent cryptophasia, difficulties and fragility of self consciousness, reduced sociability. These and other peculiarities may result from a number of different factors: biological, peculiar parental attitude, and especially the twin situation itself, the psychology of the twin as an individual being a function of the psychology of the twin pair. Twin peculiarities have for a long time been ignored. On the one hand, psychology ignored the couple reality (it was a “one-body” psychology), and on the other, the twin method classically postulated that twins are not atypical and may therefore be used as a test for the general population. As a matter of fact, twin psychology opens a new way to science. The twin situation may serve as a paradigm for the general study of the couple effect, namely, for the objective analysis of personality as a consequence of the relations between self and others. For certain traits of personality, it is already known that MZ twins brought-up together are significantly less similar than MZ twins brought-up apart. In fact, couple effects may mask or considerably reduce genetic factors.