[Significance of twin birth for the appearance of cyclic psychoses].

The motility psychosis, one of the cycloid psychoses, is frequently encountered in twins. As this is also true of two-egg twins, similarity of character does not seem to be essential. Therefore the influence must probably become active during early childhood, perhaps even the first year of life. Somatic lesions before and after birth must be taken into consideration, as the weaker twin is at higher risk. On the other hand, a mutual influence between the twins has been suggested by the observation that confusion psychoses, which are closely related to motility psychoses, hardly occurred in twins. Investigations in siblings with motility psychosis and confusion psychosis confirmed the mutual interrelationship. Concerning the former, considerably more siblings were found. Many siblings, as well as twins, seem susceptible to the development of a motility psychosis, however they seem less susceptible to the development of a confusion psychosis. The investigation of siblings could lead to verification of early infancy as a period of influence.