Dual-Deficit Requirement
Definition
DSM-5 requirement for intellectual disability diagnosis: deficits in both measured intellectual functioning and adaptive behaviour, both present before age 18.
- Source
- DSM-5, Intellectual Disability criteria
- Domain 1
- Intellectual functioning (IQ testing)
- Domain 2
- Adaptive behaviour (conceptual, social, practical)
- Onset window
- Before age 18
Common questions
Why does DSM-5 require both deficits instead of just a low IQ score?+
A low IQ score alone does not show a person cannot function independently in daily life. Adaptive behaviour testing checks whether the person can actually manage tasks like communication, self-care, and social judgement, so both measures together guard against diagnosing someone purely on a test number.
What happens if adaptive functioning is impaired but the onset was after age 18?+
The case would not meet the DSM-5 definition of intellectual disability, since the onset criterion excludes deficits that first appear in adulthood, for example after a traumatic brain injury. Such cases are classified under other diagnostic categories instead.
Related terms
- Atkins V. Virginia
- US Supreme Court 2002 decision holding that executing intellectually disabled defendants violates the Eighth Amendment; requires DSM-standard intellectual disability determination including Flynn-corrected...
- BNSS § 367
- Provision of India's Bharatiya Nagarik Suraksha Sanhita 2023 governing court procedure when an accused person appears to be of unsound mind; cognitive...
- Flynn Effect
- Documented approximately 3-point-per-decade rise in population mean IQ scores; requires correction when IQ scores obtained with older norms are used in legal...
- FSIQ
- Full-Scale IQ; derived from 10 core WAIS-IV subtests; primary score used in intellectual disability determinations and Atkins proceedings.
- Hall V. Florida
- US Supreme Court 2014 decision requiring courts to account for IQ measurement error and the Flynn effect before enforcing rigid IQ cutoffs...
- MacCAT-CA
- MacArthur Competence Assessment Tool for Criminal Adjudication; a 22-item standardised instrument measuring Understanding, Reasoning, and Appreciation of the trial process, grounded in...
- NIMHANS Neuropsychological Battery
- Normative neuropsychological battery published by NIMHANS Bangalore with Indian-population norms; primary reference for forensic cognitive assessment in India.
- Performance Validity Test (PVT)
- Any test designed to detect inadequate effort or deliberate underperformance in cognitive assessment; TOMM is the leading example for memory; embedded PVTs...
- Vineland-3
- Vineland Adaptive Behavior Scales, Third Edition; standardised semi-structured interview measuring adaptive functioning; required for DSM-5 intellectual disability determination alongside IQ testing.
- WAIS-IV
- Wechsler Adult Intelligence Scale, Fourth Edition; 15-subtest, 4-index IQ battery; gold standard for forensic intellectual functioning assessment.
- WMS-IV
- Wechsler Memory Scale, Fourth Edition; standardised memory battery providing five Memory Index scores; core of forensic neuropsychological memory assessment.