Developmental disabilities is a general term that describes a wide range of disabilities that occur prenatal or during childhood. This heterogeneous category includes global mental retardation, distinct syndromes (e.g., Down syndrome, Fragile X syndrome), autism and related communication disorders, motor impairment such as cerebral palsy, and developmental delays of unknown etiology. Indeed, the term is rarely defined but generally refers to those individuals who are not exhibiting typical developmental patterns.

Definitional quagmires persist, as even the term mentally retarded is difficult to define. From its earliest conception until the present day, debate has existed about the criterion and terminology used in the definition of mental retardation. Predominant classification schemes from 1973 to 1992 defined individuals as mentally retarded if they scored 70 or less on standard intelligence tests.

This cutoff point reflects scores that lie 2 standard deviations below the mean of the population. The Diagnostic and Statistical Manual of Mental Disorder also included levels of severity based on IQ (e.g., borderline, mild, moderate, severe, and profound mental retardation).

Many individuals raised concerns about definitional criteria based solely on intelligence tests. Some argued that standardized intelligence tests are culturally and linguistically biased and discriminate against members of minority groups, resulting in an overrepresentation of minorities labeled with mental retardation.

Others noted that children with mental retardation often display markedly different performance within and between testing sessions, demonstrating avoidance behaviors and inconsistent motivation.

These behaviors tend to make scores on any one assessment unreliable. Current definitions of mental retardation have moved away from singular reliance on intelligence testing and instead incorporate knowledge of the individual's adaptive functioning. The latest definition of mental retardation endorsed by the American Association on Mental Retardation (AAMR) and by the American Psychological Association is substantial limitations in present functioning . . . characterized by significantly sub average intellectual functioning, existing concurrently with related limitations in two or more of the following applicable adaptive skill areas: communication, self care, home living, social skills, community use, self-direction, health and safety, functional academics, leisure and work. Mental retardation manifests before age 18. This definition also replaces earlier classification schemes that emphasized the severity of retardation (i.e., mild, moderate, severe, profound) with an approach that describes the amount of support needed by the individual in the various adaptive skill areas.

Four levels of support are delineated: intermittent, limited, extensive, and pervasive. Although this definition represents a move away from determining mental retardation based on cognitive deficit alone, it raises questions about appropriate methods of assessment. For example, many of the skill areas (e.g., community use, work) are not applicable at all ages, and none of the skill areas are useful in assessing newborns. Indeed, some claim that mental retardation (and by extension, developmental disabilities, more broadly) is, in fact, a socially constructed term , as the delineation of normality and abnormality are based on culturally constructed ideals of age-appropriate behaviors and skills.