Down Syndrome vs. Intellectual Disability: Distinct but Overlapping Concepts

Down syndrome (Trisomy 21) is a specific genetic condition. Intellectual disability (ID) is a neurodevelopmental diagnosis defined by deficits in intellectual functioning (IQ <70) and adaptive behavior.

Relationship

• Most (but not all) with Down syndrome have mild-to-moderate ID
• Many causes of ID exist: genetic (Fragile X, Rett), metabolic, infectious, traumatic, environmental
• ID severity in Down syndrome varies widely (IQ 30–70 typical)

Terminology Update

"Mental retardation" is obsolete and offensive. Use intellectual disability (DSM-5, ICD-11, Rosa's Law 2010).

Key Distinctions

Feature	Down Syndrome	Intellectual Disability (General)
Etiology	Trisomy 21 (95%), translocation, mosaic	Hundreds of known causes
Physical features	Characteristic facies, hypotonia, single palmar crease	Variable; syndrome-specific if genetic
Health comorbidities	CHD, thyroid, hearing, vision, atlantoaxial instability	Depends on etiology
Life expectancy	Now ~60 yrs	Varies by cause/severity

Person-first language: "person with Down syndrome," "person with intellectual disability."