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The main object of this communication is to report a study of 142 pairs of twins with so-called schizophrenic psychoses in one or both of the twins in each pair. These twins are classified as follows: monozygotic 41 pairs; same-sex dizygotic 53 pairs; opposite-sex dizygotic 48 pairs. There is a marked contrast between monozygotic and dizygotic twins with respect to the proportion of cases of both twins of the pair affected. These proportions are, respectively, 68.3 per cent and 14.9 per cent. In the medical literature of twin pathology great stress has been laid on close similarity or even identicalness of manifestations, particularly in monozygotic twins. However, as regards the so-called schizophrenic psychoses, close similarity or identicalness of manifestations is the exception rather than the rule. Instances of similar psychoses, quantitative dissimilarity (i. e., dissimilarity of age of onset, particular symptomatology, course, outcome, etc.), qualitative dissimilarity (i. e., one twin having a schi...

Twelve sets of triplets, comprising 33 surviving and three stillborn individuals, were incidentally included, along with over a thousand pairs of twins, collected for the purpose of a study of the etiology of mental disorders. The twins and triplets were selected on the basis of at least one in each pair or set having a mental disorder. Neuropsychiatric conditions were found affecting 17 of the 33 subjects, consisting of subnormal intelligence with or without the following complications: child behavior difficulties, juvenile delinquency, adult criminality, psychotic disease and epilepsy. Comparisons of the monozygotic with the dizygotic pairs of subjects among the triplets have failed to reveal any significant contrast with respect to frequency of concordance. The findings yielded by this material, though limited in amount, serve, nevertheless, not only to confirm the conclusions reached by us from our larger-scale studies of twins, but also to further discount the factor of heredity in the etiology of th...

The main object of this communication is to present a study of 90 pairs of twins with manic-depressive syndromes. Among 23 pairs of monozygotic twins included in this material both twins were found to be affected in 16 instances, and only one of each pair in 7 instances. Among the remaining 67 pairs of twins, which are dizygotic, both twins were affected in 11 instances, and only one in 56 instances. Hereditary or germinal factors play an important part in the etiology of manic-depressive syndromes, as is shown by the contrast between monozygotic and dizygotic twins in the percentage of cases of both twins of the pair affected—69.6 per cent for the monozygotic and 16.4 per cent for the dizygotic twins in our material. Hereditary or germinal factors, in themselves, are often inadequate, i. e., they do not suffice to produce a manic-depressive syndrome, as is shown by the high proportion of cases among monozygotic twins in which only one of the pair is affected—30.4 per cent in our material. In other words,...