What Is Intellectual Disability?

  • Christina WeirMSc, Biotechnology, Bioprocessing & Business Management, University of Warwick, UK


Intellectual disability, previously known as mental retardation, is characterised by low IQ and poor functional and adaptive skills.1 Intellectual disability significantly impacts an individual's ability to learn and function in daily life. It can also be associated with other mental health issues, neurodevelopmental disorders, neurological conditions, and medical conditions like meningitis.2 Intellectual disability refers to a significant impairment in cognitive abilities, affecting both intellectual and adaptive functioning. It usually manifests during childhood or adolescence, varying in severity from mild to severe. The condition hinders the individual's ability to learn, reason, problem-solve, and communicate effectively.1,3 The severity of intellectual disabilities can vary, with 85% of the cases reported being considered to be mild cases, followed by 10 % of the cases being moderate, 4% being severe (4%), and 2% being reported as profound cases.3 This article provides a comprehensive overview of intellectual disability, its causes, signs and symptoms, diagnosis, management, and treatment. Furthermore, it addresses frequently asked questions pertaining to intellectual disability.

Intellectual functioning is typically measured through standardised tests.1 Intellectual functioning comprises components such as verbal comprehension, working memory, perceptual reasoning (to analyze and solve problems that involve visual information like puzzles), and quantitative reasoning (to analyze and solve problems using numbers).4 Intellectual disability can lead to difficulties in social, conceptual, and practical skills which can affect various aspects of an individual's life.1

Causes of intellectual disability

Intellectual disability can result from various factors including:1 

  • Genetic conditions (such as Down syndrome and Fragile X syndrome) 
  • Prenatal factors (such as exposure to infections or toxins during pregnancy), birth complications, postnatal factors (such as infections, traumatic brain injury, or malnutrition) 
  • Environmental factors (such as neglect or abuse)

Genetic conditions 

Genetic abnormalities leading to intellectual disability can arise from inborn errors of metabolism, neurodevelopmental defects, and neurodegeneration.1 

  • Inborn errors of metabolism: Toxic accumulation of by-products can also contribute to intellectual disability and behavioural problems.1 For example, phenylketonuria (PKU) affects roughly 0.01% of newborns where the newborn’s body cannot break the amino acid called phenylalanine (to para-tyrosine), and the accumulation of this amino acid leads to brain damage1 
  • Neurodegeneration: As seen in Rett syndrome, can also lead to intellectual disability1
  • Neurodevelopmental defects: Down syndrome is an example of a chromosomal cause; people with Down syndrome are born with an extra chromosome, while Fragile X syndrome represents a genetic cause.It is caused by a gene mutation in the FMR1 gene (which makes a protein called FMRP that is needed for brain development)1  

Additional examples of genetic causes include Lesch-Nyhan syndrome, Hunter disease, Niemann-Pick disease, Hartnup disease, homocystinuria, and galactosemia.1,4 Genetic syndromes, such as DiGeorge syndrome, can result in alterations that contribute to intellectual disability.1

Prenatal and Postnatal Factors 

Intellectual disability can be acquired when there are developmental issues at a young age and can be prenatal (entire pregnancy period before childbirth), perinatal (time period right before and immediately after childbirth), postnatal (time period immediately after childbirth) that can be caused by congenital, developmental, and environmental factors.4 Congenital factors can be metabolic (neonatal hypothyroidism), toxic (lead poisoning and fetal alcohol poisoning), or infections (like rubella, syphilis, and simple herpes).4 Intellectual disability can occur postnatally in early childhood due to factors such as infection (meningitis and encephalitis), head injury, asphyxia, intracranial tumours (brain tumours), malnutrition, and exposure to toxic substances.1

Environmental Factors

Environmental exposure during pregnancy can also lead to intellectual disability.1 A common environmental cause of intellectual disability is fetal alcohol syndrome.1  For instance, fetal alcohol syndrome halts the production of retinoic acid (a metabolite of vitamin A), which plays an import role in sending messages in the nervous system, leading to intellectual disability.1 

Maternal exposure to toxins, infectious agents, and birth complications can play a role.1 Infections such as rubella and HIV can also cause intellectual disability, with maternal rubella infection in the first trimester carrying a 10-15% risk.1

Other causes of intellectual disability in newborns include syphilis, toxoplasmosis, and cytomegalovirus inclusion disease.1 Maternal obesity, urinary tract infections, pregnancy hypertension, asthma, and pre-gestational diabetes can also increase the risk.1

Signs and symptoms of intellectual disability

The signs and symptoms of intellectual disability can vary depending on its severity, with earlier recognition and diagnosis more likely in severe cases. In infants, signs may include delayed milestones such as walking, crawling, sitting up, and speech development. Poor memory, limited social cues, and difficulties with problem-solving are also common indicators. Intellectual disability typically peaks between the ages of 10 and 14 and is 1.5 times more prevalent in children assigned male at birth than in children assigned female at birth.1 Individuals with intellectual disability may experience difficulties in communication, personal care, social skills and community integration.3 

While an IQ score below 70 is often used as an indicator of intellectual disability, it's important to consider other factors, such as adaptive functioning, including communication, social participation, and independent living.1 Severity levels of intellectual disability are categorized based on IQ points. 

Intellectual Disability CategoryIQ PointsCharacteristics
Mild intellectual disability50-70Primarily affects conceptual development, social skills, and daily living skills.4 Individuals at this level can learn life skills and function reasonably well in everyday life, typically requiring minimal support.4
Moderate intellectual disability35-50
Severe intellectual disability20-35Significant developmental delays, limited communication skills, and a need for supervision in social settings, often relying on family care and supervision.4
Profound intellectual disabilityBelow 20Have congenital syndromes, are unable to live independently, require close supervision assistance with self-care, have limited communication abilities, and experience physical limitations.4

Management and treatment for intellectual disability

While intellectual disability is a lifelong condition, early intervention and appropriate management can significantly enhance an individual's quality of life. Treatment plans typically involve a multidisciplinary approach that incorporates 

  • Specialised education 
  • Speech and occupational therapy 
  • Behaviour modification techniques 
  • Social skills training 

By implementing these interventions, individuals with intellectual disabilities can develop necessary life skills and positive behaviours, which contribute to their overall well-being.1 It is crucial to initiate intervention and support for intellectual disability as early as possible in order to prevent further deterioration and minimize symptoms.1 

Educational support is essential for teaching life skills and promoting functional abilities. Behavioural interventions, including cognitive therapy, can encourage positive behaviours and foster cognitive development.1 Additionally, vocational training programs are crucial for individuals with intellectual disabilities, as they help them integrate into society and find meaningful employment opportunities.1 

Through management and treatment, individuals with intellectual disability can enhance their overall functioning, independence, and participation in society.


The diagnosis of intellectual disability requires comprehensive assessments conducted by healthcare professionals specializing in the field. These evaluations typically include a range of assessments, such as cognitive testing, evaluations of adaptive behaviour, medical examinations, and a review of the patient's medical history.


How can I prevent intellectual disability?

Preventive measures primarily focus on addressing factors that contribute to intellectual disability during the prenatal and postnatal stages. These may include receiving proper prenatal care, avoiding exposure to harmful substances, ensuring a safe and nurturing environment for children, and timely identification and treatment of medical conditions that can impact cognitive development.

How common is intellectual disability?

Intellectual disability affects approximately 1-3% of the global population.1 The prevalence varies across regions and is influenced by factors such as socioeconomic status, access to healthcare, and the presence of genetic conditions within specific populations.

Who is at risk of intellectual disability?

Intellectual disability can affect anyone regardless of age, gender, or ethnicity. However, certain factors increase the risk, including genetic disorders, maternal substance abuse (using drugs, consuming alcohol, smoking, etc, during pregnancy), premature birth and exposure to infections or toxins during pregnancy.

What can I expect if I have an intellectual disability?

The impact of intellectual disability on an individual's life can vary widely based on the severity of the condition. However, with appropriate support, early intervention, and individualized accommodations, individuals with intellectual disability can lead fulfilling lives, achieve personal milestones, and contribute to their communities.

When should I see a doctor?

If you notice any developmental delays, difficulties with learning, or concerns regarding your child's intellectual abilities, it is crucial to consult a healthcare professional. Early intervention and timely diagnosis can facilitate access to appropriate support and resources. If you suspect your child of having an intellectual disability, you should speak to your general practitioner or see a developmental specialist.


In conclusion, intellectual disability is a complex condition characterized by low IQ and poor functional and adaptive skills. It can be caused by a combination of genetic abnormalities and environmental factors, with a range of prenatal, perinatal, and postnatal factors contributing to its development. The signs and symptoms of intellectual disability can vary in severity and may include delayed milestones, speech and language challenges, limited social skills, and difficulties with problem-solving.

Early intervention and appropriate management are crucial in improving the quality of life for individuals with intellectual disability. Treatment plans and supportive services are also essential in promoting independence and integration within society.

Accurate diagnosis, achieved through comprehensive assessments by healthcare professionals, is essential for guiding interventions and providing appropriate support. By implementing these comprehensive approaches, individuals with intellectual disability can enhance their overall functioning, independence, and participation in society. 


  1. K L, M C, R M. Intellectual Disability [Internet]. Europepmc.org. 2019. Available from: https://europepmc.org/article/NBK/NBK547654
  2. Maulik PK, Mascarenhas MN, Mathers CD, Dua T, Saxena S. Corrigendum to “Prevalence of intellectual disability: A meta-analysis of population-based studies” [Res. Dev. Disabil. 32 (2) (2011) 419–436]. Research in Developmental Disabilities. 2013 Feb;34(2):729.  Available from: https://www.sciencedirect.com/science/article/abs/pii/S0891422210003082?via%3Dihub
  3. Katz G, Lazcano-Ponce E. Intellectual disability: definition, etiological factors, classification, diagnosis, treatment and prognosis. Salud Publica de Mexico [Internet]. 2008;50(S2):132–41. Available from: https://www.medigraphic.com/cgi-bin/new/resumenI.cgi?IDREVISTA=79&IDARTICULO=19124&IDPUBLICACION=1954
  4. Committee to Evaluate the Supplemental Security Income Disability Program for Children with Mental Disorders, Board on the Health of Select Populations, Board on Children, Youth, and Families, Institute of Medicine, Division of Behavioral and Social Sciences and Education, The National Academies of Sciences, Engineering, and Medicine. Mental Disorders and Disabilities Among Low-Income Children [Internet]. Boat TF, Wu JT, editors. PubMed. Washington (DC): National Academies Press (US); 2015. Available from: https://www.ncbi.nlm.nih.gov/books/NBK332882/
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Christina Weir

Master of Science - MS, Biotechnology, Bioprocessing & Business Management, University of Warwick

Hey there, I'm Christina (Krysia), and I'm thrilled to be an article writer for Klarity! I recently completed my master's degree in Biotechnology from the University of Warwick, and currently, I work at The Francis Crick Institute in Science Operations. I love being involved with the institute's exciting biomedical research and have a passion for Science Communications. My goal is to simplify science so everyone can join in and learn something new!

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