Summary This study is an outcome of the 1947 Scottish Mental Survey. In a Group Intelligence Test done by 70,805 eleven-year-old Scottish children, the average score of the twins was significantly lower than that of the non-twins. The difference is greater than that recorded by previous investigators. An examination of the relationship between test score and size of family, shows that this relationship is virtually the same for both categories, but that there is a tendency for twin births to arrest further enlargement of the family. There is no evidence of twinning being associated with fertility. The relationship between twinning and the socio-economic status of the family has still to be investigated.
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Mehrotra et al. (1949) studied this question.