After reviewing the work of a number of authors on the specific problems of identification of twins, the author examines the difficulties met with in the establishment of good symbiotic relationship with a maternal object when a twin is always interposed between the mother and the child. It is very much up to the mother to ensure that the constant presence of a third individual is not experienced as parasitism by the other twin. The disastrous consequences of such a situation in an adult patient are illustrated by a clinical example.