Mental Retardation

Definition

Psychologists, psychiatrists, and other professionals who provide services to individuals with mental retardation do not always agree on its definition. Nonetheless, all currently accepted definitions share three essential components: (1) significant deficits in intelligence; (2) substantial limitations in adaptive behavior; and (3) manifestation in the childhood period (American Association on Mental Retardation, 2002; American Psychiatric Association, 2000; Jacobson & Mulick, 1996).

Intellectual Deficits

All definitions consider that a significant deficit in intelligence is at least two standard deviations below the mean IQ of the population, or approximately 70. Most definitions specify levels of mental retardation based on IQ scores, with mild, moderate, severe, and profound levels delineating, respectively, IQ scores that are between 2 and 3, 3 and 4, 4 and 5, and 5 and below standard deviations from the mean. At least 75% of persons with mental retardation have IQ scores in the mild range.

Adaptive Behavior

The definition and assessment of adaptive behavior is rooted in the kinds of daily tasks that an individual of a given age and culture is expected to perform successfully. Most definitions use a criterion of significant limitations in at least two of the following domains: communication, self-care, self-direction, social and interpersonal skills, home living, academic tasks, health, safety, leisure, use of community resources, and work. It is relatively straightforward to determine who does and does not have some medical conditions, but mental retardation is at least in part a social construction, based on the complexity of the society in which the individual lives and the demands of the society for individual functioning. For example, if literacy is not expected of most adults in a society, then some persons who have trouble acquiring it might not receive the mental retardation diagnosis.

Manifestation in Childhood

All definitions agree that mental retardation is a developmental disorder and must manifest prior to adulthood or before the age of 18 or 22, depending on the definition. Individuals with more severe mental retardation, especially those with syndromes that have known physical characteristics, are likely to be diagnosed at a younger age than those who have mild mental retardation and no obvious accompanying physical markers.

Prevalence

Estimates of the prevalence of mental retardation in the United States vary by time period and age of the child. Historically, most estimates were in the 2% to 3% range, based largely on the theoretical normal distribution of IQ scores. However, current estimates are much lower. Even during the school years, when the incidence of mental retardation increases, the prevalence rarely goes beyond 1% of school-aged children. This peak during school is acknowledged by the phrase “six-hour retardation,” referring to the increase in identification and diagnosis during the school years when children with mild mental retardation who frequently have no recognizable medical conditions are identified for the first time as the result of failure at academic tasks (Graziano, 2002). Many of these children “lose” the label when they leave school and enter the world of work. Although they may live on the social and economic margins of society, they are less likely to receive services than are individuals with more severe mental retardation.

...