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Since 1900 the science of Genetics has been arriving at an ever more profound understanding of the problems of inheritance. The success achieved in the sphere of physiology soon prompted the investigation of the psychological problem. The practical aim behind the theoretical work was to determine the possibilities of therapy and of education, in other words to determine the influence of heredity on the one hand and of environment on the other on the development of the individual. In clinical genetic research the investigation of twins steadily grew in significance.
With regard to the inheritance of psychic characteristics one general consideration must be taken into account. Genes transmit certain dynamic possibilities to the progeny. No doubt there exist important causal connexions between these potentialities of inheritance and phenotypic characteristics such as mental disease, talents, character traits and so on. These connexions are, however, greatly affected by events in the external world. Psycho-analytic investigation has thrown light on the determining traumatic importance of such events in childhood and has discovered hitherto unsuspected connexions. I may refer here, for example, to the work of L. Hajdu who has lately been investigating the connexions which exist between schizophrenia and hunger in the suckling child.
In these circumstances I have felt myself justified in reporting the psycho-analysis of a pair of identical twins.
In November 1937 'Peter' X who was then twenty-nine years old, came for treatment on account of certain neurotic difficulties. He continued his treatment uninterruptedly till the summer of 1940. He was then called up for labour service and it was not possible afterwards to carry on continuous treatment though he had periods of several months' leave when he could take up his treatment again. When the war was over he went home and from time to time reported to me how he got on.
His twin brother 'Paul' came for treatment in t