Source: Abnormal Psychology, University of Florida
Tags: intellectual disability, intellectual development disorder, mental retardation, IQ, adaptive functioning, Down syndrome, fragile X, PKU, Tay-Sachs, fetal alcohol syndrome, DSM-5, severity levels, neurodevelopmental disorder
Difficulty: Intermediate | Prerequisites: Chapter 10 overview of neurodevelopmental disorders, basic genetics (chromosomes, dominant/recessive inheritance).
Big picture: Intellectual disability (formerly called mental retardation) involves significant deficits across conceptual, social, and practical domains of life. The DSM-5 shifted the emphasis away from IQ scores and towards adaptive functioning, which is a key conceptual change to understand. Causes range from genetic and chromosomal abnormalities to prenatal damage, environmental toxins, and sociocultural deprivation. Treatment is long-term and multimodal, spanning drug therapy, behavioural approaches, and social integration efforts.
Intellectual disability is defined by significant deficits in intellectual and adaptive functioning across conceptual, social, and practical domains, with onset during childhood. The DSM-5 de-emphasises IQ scores in favour of adaptive functioning levels. Causes include genetic metabolic disorders, chromosomal abnormalities, prenatal damage, and sociocultural factors.
Intellectual disability
Significant deficits in three broad domains of life: conceptual (language, reading, reasoning, memory), social (awareness of others, interpersonal communication, social judgement), and practical (personal care, finances, job skills). Formerly referred to as mental retardation. Think of it as a condition where both intellectual capacity and everyday adaptive skills are substantially below typical levels.
Adaptive functioning
The ability to meet the demands of daily life across conceptual, social, and practical domains. The DSM-5 now prioritises this over raw IQ scores when determining severity.
Phenylketonuria (PKU)
A genetic metabolic disorder (1 in 20,000 births) in which the body cannot metabolise phenylalanine. The amino acid and its derivative build up and cause brain damage. Treatable with a phenylalanine-free diet from early age; untreated, it results in IQ below 50 and severe/profound intellectual disability.
Tay-Sachs disease
A genetic disorder carried by a recessive gene, found primarily in Jewish populations. Causes progressive degeneration of the nervous system beginning at 3–6 months of age, leading to mental and physical deterioration. Usually fatal before age 6, with no effective treatment.
Down syndrome (trisomy 21)
A chromosomal disorder in which chromosome 21 is present in triplicate rather than duplicate. Occurs in approximately 1 in 800 births. Almost all children have intellectual disability ranging from mild to profound. Associated with distinctive physical features, congenital heart defects, shortened life expectancy, and high rates of Alzheimer's disease after age 40.
Fragile X syndrome
A chromosomal disorder in which the tip of the X chromosome breaks off. Primarily affects males (who lack a second X to compensate). Results in severe to profound intellectual disability, speech defects, and severe interpersonal deficits. Males typically show large ears, long face, and enlarged testes.
Fetal alcohol syndrome (FAS)
A condition caused by maternal alcohol consumption during pregnancy. Children typically have a below-average IQ of around 68, poor judgement, distractibility, difficulty understanding social cues, and academic functioning at a 2nd–4th grade level. Occurs in 2–15 per 10,000 children.
Shaken baby syndrome
Brain injury caused by shaking an infant, leading to intracranial injury and retinal haemorrhage. Infants' heads are disproportionately large and their neck muscles too weak to stabilise the head, so shaking causes the brain to be bruised inside the skull. Can result in seizures, blindness, paralysis, intellectual disability, or death.
Conceptual domain: Language, reading, writing, maths, reasoning, knowledge, memory, problem solving.
Social domain: Awareness and understanding of others' experiences, interpersonal communication, making and keeping friends, social judgement, regulating one's own reactions in social situations.
Practical domain: Personal care, managing finances, recreation, transportation, organising oneself to hold a job or attend school.
People with intellectual disability also commonly have motor skill deficits (hand-eye coordination, balance).
Mild
Some limitations in acquiring typical academic and job-related skills.
May seem immature in social interaction; overly concrete in communication.
Limited social judgement and understanding of risk.
Able to care for themselves reasonably well, including in complex situations (legal/health decisions).
Often hold competitive jobs that do not emphasise conceptual skills.
Moderate
Significant delays in language development (4–10 words by age 3).
Physically clumsy; trouble dressing and feeding themselves.
Academic achievement typically does not exceed second-grade level, though simple vocational skills can be acquired with special education.
Can learn personal care with extensive training.
Social interactions impaired by communication difficulties and poor social judgement.
Severe
Very limited vocabulary; speech in 2–3 word sentences.
Significant motor development deficits; plays with toys inappropriately.
Can feed themselves with a spoon and dress themselves in simple clothing; require support for all aspects of daily living.
Cannot travel alone, shop, or cook independently.
Generally lack awareness of risk; easily led by others.
Profound
Do not develop conceptual skills beyond simple matching of concrete physical features of objects.
Co-occurring sensory and motor impairments may prevent functional use of objects.
Understand only simple, concrete instructions and gestures.
Maladaptive behaviour present in a significant minority.
Individually administered intelligence tests measure verbal comprehension, working memory, perceptual reasoning, quantitative reasoning, abstract thought, and processing speed.
The traditional threshold is two standard deviations below the mean: an IQ score of approximately 70.
The DSM-5 de-emphasises IQ scores and focuses on the individual's level of adaptive functioning across the conceptual, social, and practical domains.
Prevalence: 1–3% of the population.
Genetic metabolic disorders
PKU (1 in 20,000): inability to metabolise phenylalanine causes brain damage. A phenylalanine-free diet from early age prevents intellectual disability. Untreated, IQ falls below 50.
Tay-Sachs: recessive gene, primarily Jewish populations. Progressive nervous system degeneration from 3–6 months of age. Fatal before age 6; no effective treatment exists.
Chromosomal disorders
Down syndrome (trisomy 21): 1 in 800 births. Distinctive physical features (round, flat face, almond-shaped eyes, short stature). Nearly all develop Alzheimer's after age 40.
Fragile X syndrome: primarily affects males. Severe to profound ID with speech defects.
Trisomy 13 and Trisomy 18 also cause intellectual disability.
Risk of chromosomal abnormality increases with parental age (chromosomes more likely to have degenerated or been damaged by toxins).
Brain damage during gestation and early life
Maternal infections during pregnancy (rubella, herpes, syphilis) can damage the foetus.
Chronic maternal disorders (high blood pressure, diabetes) increase risk, though effective treatment during pregnancy lowers that risk.
Fetal alcohol syndrome: even low-to-moderate alcohol consumption during pregnancy can lead to miscarriage, low birth weight, congenital abnormalities, and impaired social/cognitive development.
Shaken baby syndrome: intracranial injury from shaking.
Exposure to toxic substances (lead, arsenic, mercury).
Motor vehicle accidents where a child is not properly restrained.
Low socioeconomic backgrounds are associated with higher rates of intellectual disability.
Mechanisms include: parents with ID unable to secure well-paying jobs, poor prenatal care, increased exposure to lead (old buildings with lead paint), poorly funded schools, less parental involvement in schooling.
These sociocultural factors exacerbate underlying biological vulnerabilities.
Drug therapy
Aims to reduce seizures, control self-injurious/aggressive behaviour, and improve mood.
Neuroleptics reduce aggressive and antisocial behaviour, but neurological side effects make them controversial.
Antipsychotics (e.g. risperidone) reduce aggression and self-injury without serious neurological side effects.
Antidepressants can reduce depressive symptoms, improve sleep, and help control self-injury.
Behavioural therapy
Aims to enhance positive behaviours and reduce negative ones.
Social and communication skills can be taught (initiating conversations, clear articulation).
Desired behaviour is modelled in incremental steps with rewards as the individual gets closer to the target skill.
Focus is on maximising functioning in society, not isolated skills.
Social interventions
Integration of children into mainstream settings wherever possible.
Early intervention programmes (e.g. Infant Health and Development Program) include home visits by trained counsellors, child development centres, and parent support groups. After 36 months, children in these programmes are less likely to have IQ scores in the low range.
Mainstream classroom placement has disadvantages: children may be viewed negatively by peers and may not receive the specialised training they need.
Adults with intellectual disability often live in group homes with assistance and training, including workshops for unskilled/semiskilled labour.
PKU is one of the clearest examples in medicine of a genetic disorder that can be entirely managed through environmental intervention (diet). It illustrates the gene-environment interaction principle: a strong genetic predisposition does not guarantee the outcome if the environment is modified early enough. Newborn screening for PKU is now standard practice in most countries.
Students often think IQ is the primary determinant of intellectual disability severity in the DSM-5. The DSM-5 deliberately shifted emphasis to adaptive functioning across the three domains.
Students sometimes assume Down syndrome always produces profound disability. Severity varies from mild to profound.
Students may not realise that fetal alcohol syndrome is entirely preventable and that even moderate alcohol consumption during pregnancy carries risk.
Students occasionally confuse fragile X syndrome (chromosomal, primarily affects males) with Down syndrome (trisomy 21, affects both sexes equally).
⚠️ The DSM-5's shift from IQ-based to adaptive-functioning-based severity levels is a key conceptual change. Be able to explain why this matters.
⚠️ Know the four severity levels (mild, moderate, severe, profound) and be able to distinguish them by adaptive functioning, not just IQ.
⚠️ Be able to describe PKU, Down syndrome, fragile X, Tay-Sachs, and fetal alcohol syndrome, including their genetic/chromosomal mechanisms and outcomes.
⚠️ Sociocultural factors are exam-relevant: understand how poverty exacerbates biological risk.
⚠️ The Infant Health and Development Program outcome (reduced low-range IQ scores after 36 months) is a likely test point.
True or false: The DSM-5 primarily uses IQ scores to determine the severity of intellectual disability. (False, it focuses on adaptive functioning.)
Fill in the blank: Down syndrome involves a triplication of chromosome ___. (21.)
True or false: PKU causes intellectual disability that cannot be prevented. (False, a phenylalanine-free diet from early age prevents it.)
Fill in the blank: Fragile X syndrome primarily affects ________ because they lack a second X chromosome to compensate. (Males.)
True or false: Fetal alcohol syndrome occurs only with heavy alcohol consumption during pregnancy. (False, even low-to-moderate levels carry risk.)
Q: What are the three domains of adaptive functioning assessed in intellectual disability?
A: Conceptual (language, reasoning, memory, academics), social (interpersonal communication, social judgement, awareness of others), and practical (personal care, finances, job skills, daily living).
Q: How does the DSM-5 approach to intellectual disability differ from the DSM-IV?
A: The DSM-5 de-emphasises IQ scores and instead focuses on the individual's level of adaptive functioning across conceptual, social, and practical domains to determine severity.
Q: What is PKU, and why is it a key example in abnormal psychology?
A: PKU is a genetic metabolic disorder in which phenylalanine cannot be metabolised, causing brain damage. It demonstrates that a strong genetic predisposition can be entirely managed through early dietary intervention, making it a classic gene-environment interaction example.
Q: What are the main physical and cognitive features of Down syndrome?
A: Round, flat face, almond-shaped eyes, short stature, congenital heart defects, gastrointestinal difficulties, and intellectual disability varying from mild to profound. Nearly all individuals develop Alzheimer's disease after age 40.
Q: How do sociocultural factors contribute to intellectual disability?
A: Low socioeconomic status leads to poor prenatal care, exposure to toxins (e.g. lead paint in old buildings), poorly funded schools, less parental involvement, and overall exacerbation of biological risk factors.
Intellectual disability frequently co-occurs with ASD (covered in the autism notes) and is linked to chromosomal abnormalities that also appear in the genetics chapters. Fetal alcohol syndrome connects to substance use disorders. The DSM-5 emphasis on adaptive functioning over IQ ties into broader debates about the validity and utility of intelligence testing. Sociocultural factors link to discussions of health disparities and social determinants of mental health throughout the course.
Intellectual disability, intellectual development disorder, mental retardation, adaptive functioning, IQ, DSM-5 severity levels, mild moderate severe profound, Down syndrome, trisomy 21, fragile X syndrome, PKU, phenylketonuria, Tay-Sachs disease, fetal alcohol syndrome, FAS, shaken baby syndrome, chromosomal abnormality, early intervention, mainstreaming, group home, behavioural therapy for ID