contexts. Considerable conceptual controversy exists, growing out of different traditions that have generated several major disability frameworks found in contemporary literature. Processes of social change—including, for example, civil rights movements and development of assistive technology—have contributed to the emergence of varied, even competing, frameworks. Nonetheless, across the several different schools of thought, one can identify scholarly consensus on what constitutes the phenomenon of disability, which is the first step to achieving a common language.

How a society defines and treats persons with a limitation in ability or disability has roots in many different cultures. Contemporary Western thought can be traced to functionalism as expressed in Talcott Parsons’ definition of illness as “a state of disturbance in the normal functioning of the total human individual including both the state of the organism as a biological system, and of his personal and social adjustments” (Parsons, 1951, p. 431). This sociological perspective was the basis for definitions of disability focusing on functional status for many decades, resulting in an emphasis on the individual and individual adjustment with less attention to the extrinsic or environmental factors that influence a person’s ability to work or engage in meaningful activity. The Americans with Disabilities Act (ADA), for example, defines the term disability “with respect to an individual—(A) a physical or mental impairment that substantially limits one or more of the major life activities of such individual; (B) a record of such an impairment; or (C) being regarded as having such an impairment” (ADA, 2001).

Researchers have attempted to define disability by designing models that document the process of becoming disabled. Some of these models include: the Functional Limitation Paradigm (Nagi, 1965, 1979); the International Classification of Impairments, Disabilities, and Handicaps (ICIDH) (WHO, 1980), recently revised and renamed the International Classification of Functioning, Disability and Health (ICF); the Institute of Medicine (IOM) models (IOM, 1991, 1997b); and variations from other authors in many different contexts (Patrick and Peach, 1989; Verbrugge and Jette, 1994). While each of these models suggests a theoretical definition of disability, none offers a detailed operational definition, although the extensive classification system of the ICF permits multiple coding for individuals.

Scholarly definitions differ among themselves, but they share one thing in common. As long as they are confined to the abstract, theoretical level, they are free to add to, subtract from, or modify any of their terms and conditions, subject only to canons of logic and the scholar’s considered judgment. However, once the definitions are applied under real-world conditions, they necessarily operate under constraints of one sort or another, leading to numerous definitions used in public laws and pri-