Genetic and Environmental Factors in the Development of ‘Primary Reaction Patterns’*

In the course of a longitudinal study of 128 children, individual styles of behaviour have been delineated and followed through successive age‐periods. The contribution of genetic forces to the formation of such reactive patterns has been examined by the study of 56 sibs and twins who form part of the study population. Various non‐genetic influences were examined through a longitudinal analysis of stability and instability over time of the characteristics of reactivity for the whole sample of sibs and twins. The relevance of the findings for a theory of individuality in psychological development is considered.