Introduction

Intellectual disability, which until recently was referred to as “mental retardation”, is characterized by intellectual functioning falling well below population norms, in tandem with difficulties with everyday functioning. Historically, both the label and the definition of this disability have undergone changes. The first body to attempt to clearly define the characteristics of this disorder was the American Association on Mental Deficiency, in 1908. This most recent reworking of the definition, described in the 10th edition of Mental Retardation: Definition, Classification, and Systems of Supports [1], is as follows: “Mental retardation is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills. This disability originates before the age of 18.” Relevant to this discussion, in 2007 the American Association on Mental Retardation (AAMR) spearheaded a change in the diagnostic label for mental retardation, due largely to concerns about the stigma this term holds [2]. The membership approved adoption of the term “intellectual disability”, and an accompanying change to the organization name, to the American Association on Intellectual and Developmental Disabilities (AAIDD). As a result, the common term now is “intellectual disability”, which will be used throughout this chapter, even though DSM-IV-TR [3] has not yet updated its terminology.

In this chapter, the history of definitions of intellectual disability will first be briefly reviewed, with an emphasis on current controversies regarding diagnostic conceptualization and criteria. Next, various etiologies of intellectual disability will be described.