The results of the study of psychological defenses in adolescents with a clinical diagnosis (F70 «mild mental retardation») brought up in incomplete families are presented. The mechanisms of psychological defenses included in the maladaptive, neurotic and psychotic group are identified. The mechanisms of psychological defenses are presented in the form of: displacement of isolation, hypochondria, splitting, denial, and reactive formation, as well as the main reasons for the manifestation of these types of defense mechanisms. The influence of the social factor of an incomplete family on the formation of a group of maladaptive psychological defenses in boys and girls with mild mental retardation is clearly shown.