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Intellectual Disability


“Intellectual Disability” (ID) is a disorder characterized by significant limitations in both intellectual and adaptive behaviour. These limitations may lead to restrictions in daily life functioning such as learning, communication, self-care, social and practical adaptive skills. ID is manifested during the developmental period of childhood or adolescence. The severity of ID ranges from mild to profound levels.

The term “developmental delay” is used to describe pre-school children whose developmental levels are substantially behind the average expectations of children of the same age in two or more developmental domains. Due to rapid growth and development during infancy and preschool years, clinical presentations and outcomes can be highly variable. Very often, developmental delay represents an early warning sign of later developmental problems, including ID.

Children with ID may present with different clinical profiles depending on the degree of disability and the pattern of intellectual impairment. They are slower than other children of comparable age in the areas of motor, language, intellectual, self-care and social skills development.

Preschool children:

Preschool children with mild developmental delay can develop varying degrees of social and communication skills, with milder impairment in motor and sensory skills. However, children with severe developmental delay may show very limited communicative speech and significant impairment in sensorimotor functioning.

School age children:

School age children with ID may acquire different levels of functional academic and self-care skills. Children with mild ID can learn some degree of academic skills and receive prevocational training, while children with moderate ID may be able to learn functional skills of daily living to become independent in familiar surroundings. Those with severe to profound ID may acquire limited basic communication and self-care skills.

According to international statistics, ID occurs in approximately about 1 in 100 persons in the general population, with a male to female ratio of 1.5 to 1. Amongst them, about 85% fall within mild grade, 10% within moderate grade, and the remaining within the severe (around 4%) or profound (around 1%) range. In Hong Kong, the prevalence rate was reported by the Census and Statistical Department (2021) as 1.0 to 1.2%. Enrollment statistics for students with ID in 2015/16 from the Education Bureau indicated that ID occurred in 0.8% of the school age population.

In clinical settings, clear organic or biological causes can be identified in approximately 25 to 50% of cases, with most of these individuals showing moderate to profound ID. Chromosomal abnormalities, hereditary conditions, early alterations of embryonic development and acquired brain injury (such as during birth or from infections or trauma) may be the contributing factors. However, for 30 to 40% of all cases no clear identifiable causes can be demonstrated. Most individuals with mild ID do not have neurological complications. The role of adverse environmental influence as a causative factor has not been substantiated.

There are conditions that may be confused with ID. These include specific learning disorders, autism spectrum disorder and language disorder. On the other hand, ID can co-exist with some of these disorders.

Children with ID have increased incidences of co-existing medical problems such as epilepsy, cerebral palsy, hearing impairment and visual impairment. Commonly associated developmental problems in children with ID include attention deficit hyperactivity disorder, autism spectrum disorder and mood disorders.

Early treatment programs providing developmentally appropriate training activities are important for infants and young children with developmental delay. Fostering positive parent-child interaction is essential. For parents, support services may include parent programmes on play and language stimulation and training in positive parenting skills. Access to relevant community service and parent support groups are important. These can help them better understand, accept and cope with their children’s conditions, as well as enable them to understand their children’s rights and the means to obtain appropriate services.

Children with ID should receive educational experiences that are appropriate for their learning needs. These involve availability and provision of necessary support services and supplementary aids for both the children as well as their teachers, within mainstream or special schools depending on the degree of adaptation required. Some special schools also provide residential placement. Some children with ID may need to employ assistive devices to support their communication, environmental control, mobility and activities of daily living. These devices also make it easier for the children to learn new concepts and to acquire information. Children with complex communication disorders may benefit from augmentative and alternative communication systems such as picture communication devices and electronic communication appliances.

The Social Welfare Department provides special preschool training to children with special needs. These are provided through early education and training centres, integrated programmes in Kingergarten-cum-Child Care Centre, On-site Preschool Rehabilitation Services and special child care centres.

For primary school placement, those functioning at the higher end of mild ID may consider inclusive education in mainstream schools with intensive remedial support. Other children with ID will benefit from attending special schools where there are tailored and individualized programmes and facilities to meet their developmental and educational needs. School placement and selection will depend on severity levels and other developmental and psychosocial factors. Some special schools also provide residential care to meet the training and social needs of students and their families.

Although ID is not curable through medical measures, improvement in environmental conditions and the presence of needed support can improve an individual’s ability to meet the routine demands of life. Through these accommodations, life functioning, adaptation and participation of these individuals can be improved and their personal potential may be fully achieved.

The Social Welfare Department offers a number of services to adults with ID, aiming at facilitating maximal community participation. Individuals with less severe disability can live and work independently in the community with appropriate support. Some may be able to perform semi-skilled work through vocational training, or “supported employment” placement in a work setting with the aid of an on-site supervisor. Others can work under supervision in sheltered workshops and perform structured activities such assembly lines or packaging operations. Some may be placed in work-activity programs where they are engaged in limited remunerative work along with schedules of educational, social and recreational programmes. Those with profound disability may need continual care and attention in daily activities.

  • American Association on Intellectual and Developmental Disabilities, AAIDD (2010). Intellectual Disability: Definition, Classification, and Systems of Supports. (11th Ed.). Washington D.C.: American Association on Intellectual and Developmental Disabilities.
  • American Psychiatric Association, APA (2013). Diagnostic and Statistical Manual of Mental Disorders: Fifth Edition. Washington D.C.: American Psychiatric Association.
  • Census and Statistic Department. Social data collected via the General Household Survey: Special topics report no. 63: Persons with disabilities and chronic illness. Hong Kong: Hong Kong Government Printer; 2021. https://www.censtatd.gov.hk/en/data/stat_report/product/C0000055/att/B11301632021XXXXB0100.pdf
  • Shevell, M. I. (2008). Global developmental delay and mental retardation or intellectual disability: conceptualization, evaluation, and etiology. Pediatric Clinic of North America, 55, 1071–1084.
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