Description of psychopathological symptoms in mentally retarded subjects is of great heuristic value because of the association with a known genetic disorder as well as the heterogenous and atypical phenomenology. The concept of behavioural phenotypes postulates a specific relationship between gene defect and behaviour, although considerable overlap between syndromes and differences within syndromes concerning behavioural manifestations are frequently observed. Since syndrome specific psychiatric disorders can be established too, the term psychopathological phenotype seems to be appropriate.

Classical psychiatric taxonomy appears to be increasingly inadequate in the diagnostic process of mentally retarded subjects. The reasons for this are the atypical presentation of classical psychiatric disorders, special diagnostic entities like cycloid psychosis and unstable mood disorder and the existence of so-called behavioural phenotypes related to syndromes with a known genetic etiology. In addition, disturbed behaviour may be the result of somatic or neurologic comorbidity and induced to adverse drug events.