The distribution of the four blood groups was determined in a control series of 1444 individuals and the distribution of the M-N types in a similar series of 1905 individuals. With these results were compared the percentile distributions of the blood groups and M-N types in a series of 1849 patients with mental disease. With regard to mental disease in general, no significant deviation from the control-series percentages was found in either the blood groups or M-N types. In addition, the following groups of psychoses were analyzed individually: the catatonic and paranoid types of dementia præcox, the manic depressive, with cerebral arteriosclerosis, general paresis, with epilepsy, involutional, alcoholic, and miscellaneous. In none of these formal groupings could any significant variation from the normal percentages be found. In one instance a difference three times the probable error and in another instance a difference of five times the probable error was found, but as was pointed out even differences of these magnitudes can occur by chance alone, especially when a long series of comparisons is made.

Hence, the present study indicates very definitely that the distribution of the blood groups and the M-N types in mental disease in general and also in the various mental diseases considered individually is identical with that occurring in normal individuals. The contradictory reports published in the literature are due chiefly to the failure to take into account the probable error of the observed frequencies, and in occasional instances to errors in technic.

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