Mental retardation is not a disease or specific disability. It is an administrative cover-all blanket term for a variety of different genetic, social and specific medical conditions sharing the one common feature that individuals affected test (reliably) below IQ 70 (or 67–73) on specific IQ tests (see below). In certain countries, a diagnosis of mental retardation will not be made unless the affected individual is in need of the support of society for his daily life.

Mental retardation is subgrouped according to the level of tested IQ. Profound mental retardation is the term often used for cases with IQ under 20. If this category is used at all, then severe mental retardation is the term used for cases with IQ of 20–34/39 and moderate mental retardation for cases with IQ in the 35/40–49/54 range. Mild mental retardation is the category applied to cases with IQ in the 50/55–69/74 range.

It is becoming increasingly common to include only two levels, i.e. severe mental retardation (SMR) for cases with IQ < 50 and mild mental retardation (MMR) for IQ-levels in the 50–69 range. Borderline intellectual functioning is the term applied when IQ is in the 70–84 range. This subgrouping system is the one which will be used in this chapter.

The DSM-III-R (the Diagnostic and Statistical Manual of Mental Disorders. Third Edition Revised, 1987) includes four levels of severity of mental retardation. Mild mental retardation corresponds to IQ-levels of 50–55 to approximately 70.