The paper considers the phenomenon of «mental atony» in children with developmental delay, intellectual disability and autism. The condition of mental atony is a congenital or early acquired even decrease in mental tone, manifested by the reduced ability to concentrate attention, a decline of the targeted activity, difficulties in contacting, weakening of unconditioned reflexes, lack of instinctive manifestations, «mobilization» of mental functions by persistent verbal and tactile stimulation. The article provides the history of the «mental atony» theory, as well as clinical features, epidemiology, etiology, pathogenesis and nosological specificity of the condition. Moreover, the criteria for the differential diagnosis between the atonic form of intellectual disability and autism are discussed in detail. Based on the analysis of literature and the results of own clinical observations, the authors conclude that a thorough differential analysis between childhood autism and atonic form of intellectual disability in children is necessary, otherwise, there is a risk of overdiagnosis of autistic disorders. The authors also point to the role of the brain lesion, particularly in the area of reticular formation, in the occurrence of phenomena of mental atony.