Differences in Individual Development Within a Pair of Identical Twins1

Our interest in twins developed during a psycho-physiological study of the normal personality, carried out since 1954 with Henry M. Fox and Arthur F. Valenstein (1961), (1965). We are investigating possible correlations between ego structure and homeostatic patterns in healthy young college men, including individual subjects and identical and fraternal twins. We have also observed some discordant adult twins, one of whom had a medical or psychiatric illness and the other was apparently normal, often the healthy donor for kidney-transplanation. In their developmental histories we were impressed by the importance of physical and behavioral charateristics present at birth, such as intrapair differences in size, feeding and sleeping patterns, motor activity, and perceptual responsiveness, which could be related to individual differences in adult personality structure, including identifications, choice of defences, predilections to neurosis and psychosomatic illness, and the use of special skills and sublimations. Individual development within twin pairs represented the outcome of a complex interaction among three factors: (i) the contribution of innate constitutional differences to early ego-formation, (ii) parental attitudes toward these individual differences that selectively augment or diminish their importance, and (iii) the relationship between the twins themselves, tending toward equalization and similarity or toward differentiation and divergent development.

These impressions were in keeping with the current genetic theories of Weiss (1961) and Dobzhansky (1959), with Freud's (1912) concept of heredity and environment as 'a perpetual interchange of both', and with Benjamin's (1960a) formulation of development as 'a continuous interaction between the innate and the experiential.' In monozygotic twins, however intrapair differences present at birth are presumably the result of individual prenatal experiences, including differ