Mental retardation does not have a single cause, evolution, mechanism or prognosis and may be accompanied by any other somatic or mental disorder. The prevalence of mental retardation in the general population is 1%. Mild mental retardation has a prevalence of 0.4 - 0.6% in the general population, and moderate, severe and profound mental retardation, cumulatively, has a prevalence of 0.4%. The prevalence of other mental disorders is 3-4 times higher in individuals with mental retardation in comparison with the healthy population.