Mental retardation is congenital or acquired in the early postnatal time interval underdevelopment of the psyche with manifestations of severe intellectual deficiency, difficulty, or total impossibility of an individual's social functioning. The term "mental retardation" in global psychiatry became firmly established for the last two decades, it entered the international classifications having replaced the previously used term "oligophrenia".
E. Kraepelin (1915) introduced the concept of "oligophrenia" and the term itself as a synonym for the concept of "general retardation of mental development" into the scientific vocabulary.
In ICD-10 (F70-F79), mental retardation is defined as follows: "A condition of arrested or incomplete development of the mind, which is especially characterised by impairment of skills manifested during the developmental period, skills which contribute to the overall level of intelligence, i.e., cognitive, language, motor, and social abilities." Obligatory indicators of mental retardation, including the early (up to 3 years) onset of intellectual disability and impaired adaptation to the social environment.
The main manifestation of oligophrenia is mental underdevelopment, depending on the particular form of the disease; it can be combined with various physical, neurological, mental, biochemical, endocrine, and vegetative disorders. The lack of progression is also its obligate indicator, in contradistinction to different variants of dementing processes.
The prevalence of oligophrenia has not been fully understood. Its maximum rates are at the age of 10-19. It is at this age that, special demands are made on a level of cognitive abilities (school education, entering an institution of higher education, calling to the colours). In official medical reports, the scatter of indicators is very wide - from 1.4 to 24.6 per 1,000 teenagers. Regional figures of the prevalence of oligophrenia also range from 12 to 78.5 and from 17.3 to 83.9 per 1,000 teenagers among representatives of the same ethnic group. This can be connected with the presence of isolates, the difference in environmental and socio-economic living conditions, and quality of medical care.