An Experimental Study of Mutual Behaviour Imitation and Person Perception in MZ and DZ Twins — Implications for an experimental-psychometric analysis of heritability coefficients

Abstract

In an earlier study (Wilde, 1964) on 88 MZ and 42 DZ twin pairs in Holland, DZ pairs, who had been separated for a minimum of five years, were compared with DZ twins living together on measures of neuroticism, extraversion, test-taking attitude and masculinity/femininity. The striking observation was made that separated DZ twins were systematically more similar than DZ twins living together across all personality traits concerned. No such difference was found between MZ pairs living together and MZ pairs living separately. In order to account for these findings, the hypothesis was put forward that the persons in the direct environment of a twin pair have certain behavioural expectancies with regard to the twin members, which depend upon the observation whether the twins are MZ or DZ. As soon as the twin pairs leave the parental home these behavioural expectancies come to a halt and the “true personalities” of the twins will emerge. This might explain why DZ pairs were found to be more similar after separation, and this reasoning also shows that it is inadequate to say that “the environment is the same” or “the environment is different”, without specifying which environmental variable is the same and which one is not. In some respects the environments of separated DZ pairs may have been more similar than the environments of DZ pairs living together.