Mental Retardation (MR) in Popular Psychology

Mental retardation (MR) is a disorder characterized primarily by intellectual functioning that is markedly below average, as well as a general deficit in social and self-care skills. Most people’s understanding of MR begins and ends with IQ scores, but there is substantially more to the disorder than an unusually low IQ. Poor intellectual functioning is only one of three major components of the standard definition produced by the American Association on Mental Retardation (now echoed in both the DSM-IV and federal law).

  • Significantly sub average intellectual functioning (indicated by an IQ of 70 or less on a standard, individually administered test).
  • A deficit in adaptive behavior. This means that in two or more of the following areas, the patient has more trouble functioning than would be expected for age and cultural group: communication, self-care, home living, social and interpersonal skills, using community resources, self-direction, academic ability, work, free time, health, and safety.
  • Origin in the developmental period. This means that the signs of mental retardation are evident in childhood, or before age eighteen.

The second requirement is in the definition to clarify MR’s status as a psychological disorder. Many reasons exist as to why a person might score poorly on an intelligence test, but that person has no disorder if that does not interfere with his or her functioning otherwise. The third requirement is there as a recognition that other circumstances, such as a serious head injury or disease affecting the brain, can lead to both a low IQ score and a loss of independent self-care skills. If such an incident occurs to an adult, mental retardation is an inappropriate label.

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U.S. federal law, again reflecting the standards of the AAMR and the DSM-IV, further recognizes four distinct levels of mental retardation, classified according to IQ level: mild (IQ range 55–70), moderate (IQ range 40–54), severe (IQ range 25–39), and profound (IQ below 25). The prevalence of mental retardation in the U.S. population has been estimated at between 1 and 3 percent.

The large majority of cases of mental retardation fall in the mild range and have no clear physiological cause. These cases are sometimes referred to as cultural-familial or sociocultural retardation, as the primary cause appears to be a severely deprived and understimulated childhood. In cases where an organic cause exists, the most common causes appear to be Down syndrome, Fragile X syndrome (a chromosomal anomaly involving damage to the X chromosome), and Fetal Alcohol Syndrome (FAS).

As the leading identified cause of mental retardation, FAS merits special attention, because unlike genetic anomalies, it is completely preventable. Negative effects of binge drinking on developing fetuses have been documented for much of human history, but the syndrome was only formally identified and named in 1973. In addition to mental retardation, individuals with FAS have a distinct pattern of facial abnormalities, growth deficiency and evidence of central nervous system dysfunction. They may also have other neurological deficits such as poor motor skills and hand-eye coordination, as well as behavioral and learning problems, including difficulties with memory, attention, and judgment. Although most cases are associated with binge drinking by the mother, there is no completely safe known level of alcohol consumption during pregnancy. Prevention of FAS is simple, and it is one of the great medical success stories of the late twentieth century: simply advising pregnant women to avoid drinking alcohol has resulted in a substantial drop in the number of FAS cases.

The terminology used to refer to persons with mental retardation has changed dramatically over the last 150 years, as our understanding of the disorder has improved and become more refined. As recently as the mid-nineteenth century, people who would today be considered mentally retarded received no treatment other than to be locked away in asylums, with the only diagnosis being to occasionally distinguish the “idiots” from the “lunatics.”

The first book devoted to treatment methods specifically intended for the “feeble-minded” (another popular nineteenth and early-twentieth century label), Seguín’s Idiocy and Its Treatment by Physiological Methods, only appeared in 1866. By the late-nineteenth century, recognition was growing that feeblemindedness could occur at differing levels of severity, and “idiot” was joined by the higher-functioning “imbecile.” In the United States, H. H. Goddard added an additional level, corresponding to those mildly retarded persons whose disability was not as obvious to the casual observer: he called them “morons.” Due to their widespread use as pejorative terms, all of these are obsolete, long since replaced by the four levels of mental retardation described above. In the early 1990s, the AAMR attempted to again change the diagnostic labels, partly in recognition of the extent to which “retarded” has joined the older terms as a stigma. They proposed replacing the four levels of retardation with terms describing the degree of services required: mild, moderate, severe, and profound became intermittent, limited, extensive, and pervasive. These terms have now been used in two different editions of the AAMR diagnostic manual, but they have failed to catch on among most professionals in the field or among textbook authors, who continue largely to use the older terms.


  1. American Association on Mental Retardation. Mental Retardation: Definition, Classification, and Systems of Supports. 9th ed. Washington, DC: AAMR, 1992;
  2. American Association on Mental Retardation. Mental Retardation: Definition, Classification, and Systems of Supports. 10th ed. Washington, DC: AAMR, 2002;
  3. Beirne-Smith, M., Patton, J. R., and Ittenbach, R. Mental Retardation. 4th ed. New York: Merrill, 1994.