IX.—The Intellectual Resemblance of Twins

Abstract
The characteristics of social behaviour in man are conditioned by previous experience. What is observed is the product on the one hand of a certain genetic constitution and on the other of an intricate, prolonged, and at present largely obscure, process of training and physical environment, including both the environment of the fœtus and family influences, social and physical. The experimental methods for detecting differences due to single gene substitutions cannot be applied directly. Indeed, we can see no immediate prospect of applying to social behaviour methods of genetic analysis such as have led to the mapping of the chromosomes in animals and in plants. With methods available at present, genetic inquiry can undertake to detect whether any gene differences are associated with observed differences, and whether such gene differences are recognisable throughout a comparatively wide or narrow range of social and physical environment.

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