The term “developmental disability” first came into widespread usage in the late 1960s and had its first uniform definition around 1970 in U.S. Federal Legislation. It was commonly used to refer to three separate, but frequently coexisting conditions: intellectual disability, epilepsy, and cerebral palsy. Cerebral palsy is about 60 times more common among children with intellectual disabilities than among other children. Epilepsy is about 20-30 times more common among children with intellectual disabilities than among other children.

In Western Canada, the typical targets of eugenic sterilization were people believed to be unsuitable for parenthood due to undesirable physical or mental conditions feared to be heritable. People with various disabilities, but especially those with developmental disabilities, were targeted by such eugenic practices.

Today, developmental disabilities are frequently conceptualized as significant intellectual or physical impairments that are present from birth or identified during childhood and that result in functional limitations creating a lifelong need for supports and services. Major differences in definitions and the concepts that underlie them, however, result in significant limitations in the value of this term and result in serious issues in its practical application for the delivery of services.

What are developmental disabilities?
The term, developmental disability, has been used in different contexts with somewhat different meanings. In common use, “developmental disability” is sometimes used as a synonym for “intellectual disability” or euphemism for “mental retardation.” However, government agencies tend to use more specific definitions. According to the United States Department of Health and Human Services, Administration for Community Living and Administration on Intellectual and Developmental Disabilities:

The term “developmental disability” means a severe, chronic disability of an individual 5 years of age or older that- (A) is attributable to a mental or physical impairment or combination of mental and physical impairments; (B) is manifested before the individual attains age 22; (C) is likely to continue indefinitely; (D) results in substantial functional limitations in three or more of the following areas of major life activity- (i) self-care; ii) receptive and expressive language; (iii) learning; (iv) mobility; (v) self-direction; (vi) capacity for independent living; and (vii) economic self-sufficiency; and (E) reflects the individual's need for a combination and sequence of special, interdisciplinary, or generic services, supports, or other assistance that is of lifelong or extended duration and is individually planned and coordinated.

According to this definition, if developmental deficits are identified before the age of 5 they are generally termed as “developmental delays,” particularly when they are not associated with a more specific diagnosis with a known trajectory.

Other governments have used a much narrower definition. For example, in the Ontario Developmental Services Act, “developmental disability means a condition of mental impairment, present or occurring during a person’s formative years, that is associated with limitations in adaptive behaviour” (Ch. D.11, Section 1). Similarly, under Alberta’s Persons with Developmental Disabilities Services Act, “developmental disability means a state of functioning that (i) began in childhood, and (ii) is characterized by a significant limitation, described in the regulations, in both intellectual capacity and adaptive skills” (Ch. P-9.5, Section 1).

Another approach to defining developmental disability has been simply to list a number of more specific diagnostic categories that make up the broad category. For example, a major study conducted under the auspices of the US Centers for Disease Control and Prevention included all children over the age of 3 with attention deficit hyperactive disorder, autism, blindness, cerebral palsy, moderate to profound hearing loss, specific learning disability, intellectual disability, any history of stammering, stuttering or seizure in the last year, or any other developmental delay.

Historical context
During the late 19th and early 20th century, fueled primarily by the eugenics movement, large numbers of individuals with disabilities were labeled as intellectually defective and institutionalized for “care and control” in residential facilities for “the mentally retarded.” Families were discouraged from caring for their children at home while community-based educational, health, and social services, that might have supported these individuals in the community, were not supported.

Institutional inmates included many who actually had some degree of intellectual disability, but also others who had no intellectual impairment. For example, many individuals with epilepsy, autism, deaf-blindness, or severe cerebral palsy were mislabeled as having intellectual disabilities and were subsequently institutionalized. Many other institutional inmates had a combination of mild intellectual disabilities along with severe physical disabilities that resulted in a need for care.

After World War II, the eugenics movement was strongly rejected and there was an increased awareness of human rights. A growing family advocacy movement on behalf of individuals with intellectual disabilities followed this. Advocates exerted pressure for institutional reform and for community based services. The highly stigmatized term mental retardation began to be gradually replaced by alternatives and the term developmental disability began to be used to refer to the broader category of individuals who may or may not have intellectual disabilities, but have significant functional limitations that would require significant supports and services for a lifetime in the community.

Slowly, older terminology gradually gave way to newer. The US Mental Retardation Facilities and Community Mental Health Centers Construction Act of 1963 was renamed in the Developmental Disabilities Services and Facilities Construction Amendments of 1970. Many residential institutions were rechristened as Developmental Disabilities Centres. In 2007, the American Association on Mental Retardation renamed itself The American Association on Intellectual and Developmental Disabilities, renaming its journals to match.

Practical and administrative issues
While the definition of developmental disabilities may seem like an abstract philosophical issue, it has practical implications for the equitable distribution of services and resources. For example, since developmental disabilities are only a subset of the broader category of disability we would expect it to only include a fraction of the total number of individuals with disabilities. Nevertheless, according to the US Centers for Disease Control and Prevention, 15.04% of American children have developmental disabilities, while Statistics Canada reports that only 3.7% of Canadian children have a disability of any kind. Obviously, definitional differences make it impossible to meaningfully compare these percentages.

Even more strikingly, while some definitions only include children with some degree of intellectual disabilities in their concept of developmental disabilities, only 5.4% of the children in the CDC had intellectual disabilities. Furthermore, the CDC reports that since 1997 the percentage of children with intellectual disabilities is decreasing, while the numbers of children in other categories has been increasing substantially since 1987 (e.g., blindness +18.2%, Attention Deficit Hyperactive Disorder +33.0%, Autism +289.5%). As a result, the definition and conceptual focus of developmental disability is changing over time. Overall the percentage of American children considered to have developmental disabilities increased by 17%, but the percentage of children with intellectual disabilities dropped 1.5%.

The concept of developmental disability that was once almost synonymous with intellectual disability has now shifted its focus largely to other conditions. This shift inevitably results in winners and losers in the allocation of resources, services, and personnel preparation initiatives. The challenge in the allocation is not one of determining which categories of disability to include or exclude under the heading of developmental disability. The challenge is rather to establish real criteria for what we mean by substantial, lifelong disabilities that require substantial supports and services. Some individuals in each of the categories and most of the individuals included in some of the CDC disability categories will be able to become independent, self-sufficient adults with or without special intervention as children. Those with severe, lifelong disabilities that are extremely unlikely to resolve themselves in adult life make up only a small minority of those included in the CDC study and have radically different needs.

The inclusion of increasing numbers of new categories of individuals with “milder” disabilities in the concept of developmental disability helps more people obtain beneficial services, but it also tends to reallocate services from those with the greatest needs to those with less substantial needs. Due to the extreme variability in intended definitions, the term Developmental Disability requires an appended definition to have any precise meaning.

-Dick Sobsey

• Boyle, C. A., Boulet, S., Schieve, L. A., Cohen, R. A., Blumberg, S. J., Yeargin-Allsopp, M., et al. (2011). Trends in the prevalence of developmental disabilities in US children, 1997-2008. Pediatrics, 127(6), 1034-1042.

• Brown, I. (2007). What is meant by intellectual and developmental disabilities. In I. Brown & M. Percy (Eds.), A comprehensive guide to intellectual & developmental disabilities (pp. 5-16). Baltimore: Paul H. Brookes.

• The Developmental Disabilities Assistance and Bill of Rights Act of 2000, 42 USC 6001(5), Ontario Developmental Services Act, R.S.O. 1990, Chapter D.11