In DSM-IV-TR (1), mental retardation (MR) is defined on the basis of three essential features: subnormal intellectual functioning, commensurate deficits in adaptive functioning, and onset before 18 years. Of these three criteria, the first two are most often discussed. Subnormal intellectual functioning is characterized by an intelligence quotient (IQ) lower than 70, based in most cases on the administration of an appropriate standardized assessment of intelligence. Deficits in adaptive skills, which involve one’s social and personal sufficiency and independence, are generally measured on instruments such as the recently re-revised Vineland Adaptive Behavior Scales (2) or a similar scale. DSM-IV-TR criteria for mental retardation are summarized in Table 5.1.2.1.

Various levels of MR are specified in the DSM-IV-TR: mild (IQ 50 to 70), moderate (IQ 35 to 49), severe (IQ 20 to 34), and profound (IQ <20). Borderline MR can be noted as a V code. Some flexibility is allowed for clinical judgment. Most persons with MR in childhood are those with mild MR (about 85% of cases); the remainder of cases comprise those with moderate (about 10%), severe (about 4%), and profound (1% to 2%) MR (Figure 5.1.2.1). In the past, the distinction was made between educable (IQ 50 to 70) and trainable (IQ <50). Although no longer commonly used, this distinction is important. Persons with mild MR often have psychiatric difficulties that are fundamentally similar (if generally more frequent) to those seen in the general population; this is not true for more severely impaired persons. Similarly, specific medical conditions associated with MR are more likely in the group with an IQ lower than 50, whereas lower socioeconomic status is more frequent in the group with mild MR (3). The proportion of persons with severe and profound MR is higher than would be expected given the normal curve, reflecting the impact of genetic disorders and severe medical problems on development (4).

The tests chosen for assessment of intellectual functioning should be appropriate to the patient, have reasonable reliability and validity, and be administered in a standardized way by appropriately trained examiners (see Chapter 4.2.4 for a discussion of psychological assessment.) Unfortunately, in some situations, the selection of an appropriate test can be difficult, such as for a very low-functioning person. Other aspects of assessment can also be problematic, such as when some modification must be made in terms of administration given the specific circumstance. Such modifications may limit the validity of the results obtained. The examiner must then make an informed decision depending on the nature of the issue at hand, for example, determination of eligibility for services versus information on levels of functioning that can guide remediation. Particularly in terms of eligibility for services, it is critical that the examiner administer the test in exactly the standardized fashion. Measures of adaptive skills are generally based on parent or caregiver report, although, in some cases, the person may be interviewed directly. In essence, the conceptual notion is that the term adaptive skills refers to the performance of day-to-day activities required for personal or social self-sufficiency.

The inclusion of adaptive skills in the definition of MR rests on the observation that some persons with IQ scores below 70 may, as adolescents or adults, have learned sufficient adaptive skills that they are able to function totally or largely independently. Technically, then, such individuals would not meet criteria for MR. This situation is more typical of persons who, as children, score in the mildly retarded range (5).

The approach to the definition of MR is fundamentally the same in the tenth edition of the International Classification of Diseases (ICD-10) (6). However, the definition of MR promulgated by the American Association of Mental Retardation (AAMR)— first in its 1992 manual (7) and later (in revised form) in its 2002 manual (8)— discards the use of IQ levels in favor of a “needs-based” nosology that identifies the intensity of supports that persons require to function best within multiple adaptive domains. This definition also gives the clinician leeway to extend the upper IQ bound to 75; this seemingly small increase would actually considerably broaden the diagnostic concept of MR, potentially doubling the total number of cases (9). The AAMR definition has been much criticized and has had very little empirical support. Partly as a result, the AAMR definition (particularly its 1992 version) has not been widely used either in research (10) or in state guidelines (11).

Interest in MR can be traced to antiquity (12,13). Modern interest in MR began at the time of the Enlightenment and increased greatly during the nineteenth century; this emphasis occurred at the time of great social upheaval and as infant and child mortality began to decline. There was increased interest in children, in education, and in the role of experience (nurture) versus endowment (nature). The interest in the “nature–nurture problem” is exemplified in Itard’s work with Victor, a child who was thought to be wild or “feral” but who may have had autism (14,15).

Subsequently, educators such as Seguin began to develop specific educational methods for stimulating children’s development. By the latter half of the nineteenth century, many facilities had been developed for the care of persons with mental retardation. Although the initial goal of such facilities was to provide a period of treatment before the child was returned to the family, these institutions gradually became places for custodial care (12). This problem has led to a strong counterreaction in recent years and to a renewed emphasis of caring for persons with MR in their homes and communities (16).

During the nineteenth century, various distinctions were made between levels of MR by using what now would be seen as rather pejorative terms (imbecile, cretin, idiot). Originally, the etiologic basis of any such distinctions was quite limited. On one hand, there was little systematic information on intellectual functioning that could be used for purposes of
categorization. On the other, there were few known etiologic causes of MR.

Toward the end of the nineteenth and the beginning of the twentieth centuries, both these limitations began to be addressed. Binet developed the first test of intelligence, which was translated into English and adapted in the United States by Terman (27,18). As a model psychometric assessment instrument for many years, the Stanford-Binet test allowed much more precise characterization of levels of intellectual disability. In addition, Terman had the brilliant notion of taking the mental age, dividing it by the child’s chronological age, and multiplying this quotient by 100. The resulting IQ score allowed for comparisons across children of different ages. Although Binet had originally developed his scale to identify children who were delayed in order to help them, the IQ score quickly became the object of much study.

Faith in the IQ as a predictor variable led to several extensions. First, developmental testing began to be performed on infants and young children (19). Second, proponents of the new tests believed that, when the test was properly administered, the resulting score from an IQ test was fixed and reflected a person’s genetic endowment. This proved incorrect. In a classic study, Skeels and Dye (20

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