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INTRODUCTION

History

A series of terms have been used over time to refer to people with varying degrees of intellectual impairment. New terms have been created to replace previous ones that were considered inappropriate or degrading, and often became viewed as derogatory themselves. For instance, the psychologist Henry Gaddard coined the term moron to replace the term feeble-minded. The term moron, however, became viewed as disparaging over time. In the late 19th century, the term retarded, which was not considered defamatory at the time, began to replace others such as idiot, moron, and imbecile, until it also became offensive to others by 1960.1 On October 5, 2010, then President Barak Obama signed Rosa’s law (Pub.L. 111–256), named after a girl with Down syndrome (DS), which recognized her initiatives along with those of her family in the state of Maryland, to replace mental retardation with intellectual disability and mentally retarded individual with individual with intellectual disability in federal law.2 In 2013, the American Psychiatric Association (APA) also adopted the new term intellectual disability in place of mental retardation for the latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).3

Terminology and Definition

Traditionally, patients younger than the age of 5 years who are not meeting their cognitive or developmental milestones, are referred to as having global developmental delay. This term is used until a comprehensive and accurate assessment of intellectual functioning can be obtained.3 The latest version of DSM (DSM-5) requires “onset during the developmental period that includes both intellectual and adaptive functioning deficits in conceptual, social, and practical domains” for the diagnosis of Intellectual Disability (ID).3 The DSM-5 also included the term Intellectual Developmental Disorder (IDD) in parentheses next to the term Intellectual Disability (ID) to acknowledge changes made in the terminology in the latest version of the International Classification of Diseases (ICD-11).3,4 Diagnostic criteria by both ICD-11 and the American Association on Intellectual and Developmental Disabilities (AAIDD) also describe ID/IDD as impairment in “intellectual functioning” and “adaptive behavior” with onset during the developmental period.4,5 Both DSM-5 and ICD-11 have classified ID/IDD under the parent category, Neurodevelopmental Disorders. The term Intellectual Developmental Disorder (IDD) has also been adopted by organizations including AAIDD and the U.S. Department of Education. Please see DSM-5, ICD-11, and AAIDD for full diagnostic criteria for ID/IDD.3–5 DSM-5 determines the severity of IDD as mild, moderate, severe, or profound based on the level of adaptive functioning,3 whereas ICD-11 employs a similar classification of mild, moderate, severe, or profound (in addition to provisional or unspecified) as defined by the degree of intellectual impairment and disruption of adaptive behavior.4

EPIDEMIOLOGY

Prevalence

Based on a meta-analysis of 52 studies involving nearly 30 ...

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