I. The concept of mental retardation Mental retardation, also known as mental deficiency, is generally referred to as a result of organic damage to the brain or incomplete brain development, resulting in persistent impairment of cognitive activity and the entire mental activity. Mental retardation is a condition in which the brain of a fetus or infant does not develop normally or is incompletely developed due to harmful factors such as genetic mutation, infection, poisoning, head injury, cranial malformation, or endocrine abnormalities, so that the development of intellectual activity remains at a relatively low stage. If the brain is damaged by physical, chemical or viral or germ damage, the original normal intelligence is damaged and defects are caused, it is called dementia. The diagnostic criteria of mental retardation are 1, intelligence is significantly lower than the level of the same age, that is, above 2 standard deviations below the mean, below 70; 2, there are also deficits in social skills, verbal interaction, daily life care, personal independence and other adaptive functions, the degree of which is much lower than the standard expected by the cultural environment of children of the same age; 3, the onset of the disease before the age of 18. The prevalence of intellectual disability is estimated at 2% to 3% globally. Mild (50