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This is one of several reports concerning the analysis of a collection of 1,014 pairs of twins with some kind of mental disorder in one or both of the twins in each pair. Previously published studies in this series have dealt with the etiology of mongolism and with that of epilepsy. This report deals exclusively with mental deficiency. The arbitrary range for mental deficiency is placed at 79 points I. Q. and lower. In the whole collection of twins there were found to be 366 pairs one or both of whom were mentally deficient. Of these, 126 pairs were diagnosed by the Siemen resemblance method as monozygotic, 101 pairs as same-sexed dizygotic, and 139 pairs were opposited-sex twins and therefore dizygotic. Siemen's concordance-discordance method in a modified form was used in the analysis. The authors conclude that only a little more than half of the cases of mental deficiency are
The Etiology of Mental Deficiency with Special Reference to Its Occurrence in Twins: A Chapter in the Genetic History of Human Intelligence. JAMA. 1937;109(18):1478. doi:10.1001/jama.1937.02780440068036