By common standards, a person is considered to be intellectually disabled if s/he has an IQ below 70 (the average is 90-110) and manifests serious deficits in adaptive functioning in comparison to his or her age-mates.

In Hong Kong, people with intellectual disabilities are divided into four grades according to their IQ:
Below 20

  • chromosomal abnormality (the X chromosome has a weak spot or an extra chromosome is present in pair 21)
  • problems in the adhesion of genes
  • metabolic disturbances due to genetic disorder
  • bacterial infection (e.g. rubella), incorrect medication, drugs, malnutrition, smoking, excessive drinking, substance abuse and radiation attack during conception can all lead to the birth of a mentally handicapped baby
  • premature birth, difficult labour and low weight can hinder the development of the baby's brain or do damage to it, thus causing mental handicap
  • bacterial infection (e.g. meningitis), malnutrition and brain injury during infancy and early childhood can cause mental handicap
  • postnatal environment the child is subjected to, such as the kind of parental care, the things it gets in touch with and the learning opportunities it is given, also has a bearing on its intellectual development

Characteristics of people with intellectual disabilities
Although people with intellectual disabilities are deficient in intelligence, they have thoughts and feelings just like ordinary people, and they need concern and acceptance from others. They also desire chances of demonstrating their abilities as a way to affirm their own worth. Due to their relatively low cognitive ability, they encounter greater difficulty in learning language, knowledge and skills. Special teaching and training methods have to be used. In respect of personality, they are on the whole simple and straightforward; however, some of them are emotional and tend to settle their negative emotions by some violent behaviour, such as yelling or hitting objects.

Ways to facilitate rehabilitation of people with intellectual disabilities
(1) Normalization
For peope with intellectual disabilities, a true-to-sense rehabilitation is to lead the life of a normal person, able to perform the tasks of daily living, have a job, friends and own interests, and get in touch with the real world. Good assessment and training are needed to achieve this. Of course, the level of intellectual disability has a relation with the degree of normalization.

(2) Behaviour modification
In helping people with intellectual disabilities improve their emotional and behavioural problems, behaviour modification has proved to be an effective method. It is based on the principle that the result of a behaviour largely determines the chances of its reoccurrence: a good result increases the chances and a bad result decreases them. The former includes the gain of something good (i.e. reward in common language) and the avoidance of something bad. The latter includes the imposition of something bad (i.e. punishment in common language) and the loss of something good. The effective use of the method requires an application schedule tailored to the conditions of the recipient. It also requires systematic observation and recording of the target behavior, which provide the necessary data for evaluation and adaptation of the schedule.

Attitude and skills to deal with people with intellectual disabilities
  • discard prejudices and learn about them in an objective manner
  • accept and respect them
  • exercise more patience and understanding towards them
  • provide opportunities for them to learn and try so as to make them more independent
  • encourage more often and criticize less
  • communicate with them in simple clear language with the help of gesture
  • keep reward and punishment clear and distinct - praise and reward be given for desirable behavior, immediate punishment be given for undesirable behavior