Non-normative mental development is analysed as a special type of ontogenesis. Using an activity approach, the author places non-normative mental development between mental anomaly and the mental norm and argues that it is determined by biological, social and psychological factors. Non-normative mental development manifests itself on an organismic level as underdevelopment (or hyperdevelopment) of cognitive, affective and instrumental processes due to the immaturity of brain structures (or early maturity); on the level of individual - as non-normative personality traits due to incorrect, partial and especially inconsistent up-bringing; and on the level of the personality - as self-realization by actions of an egotist using other people as a means to an end. Psychological correction of non-normative mental development can be rendered as a social optimization of child's developmental situation.