Abstract: The article considers the statistical studies of Petr Petrovich Kashchenko (1859 – 1920), a well-known psychiatrist and public person, in the field of psychiatry. The research subject is the statistical report of one of the first census of people with mental diseases in Russia. The author considers the peculiarities of this statistical study of mental diseases, points at the difficulties faced and the mistakes made, and analyzes the obtained results. The author notes the significant influence of land statistics in the late 19th century on the science and the society. The study is based on the principle of unity of logic and history in science. The author applies the categorical and conceptual analysis, problemological analysis and historiographic analysis. The scientific novelty of the work consists in the consideration of the results of the statistical study of mental diseases spreading among rural population of Nizhny Novgorod province in the late 19th century, and in the specification of the data, acquired by P.P. Kashchenko (compared with the previously published works in this field). The author demonstrates the quantity of mentally deranged people according to their diseases; the percentage ratio of particular diseases in the total number of diseases, the percentage ratio of deranged men and women. The author emphasizes the contribution of Kashchenko to the development of psychiatry in the late 19th century. 

Keywords: population census, Nizhny Novgorod, mental disease, land statistics, history of science, history of psychiatry, psychiatry, Kashchenko, XIX century, statistical data

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