MR is defined by the American association on metal deficiency (AAMD) in 1992 as a developmental period in which general intellectual functioning is significantly lower than that of the same age and is accompanied by deficits in adaptive behavior [23]. ICD I10), which was developed by the WHO in 1992, defines MR as a reduced level of intellectual functioning that occurs during the developmental period and is accompanied by a reduced ability to adapt to the demands of daily life in a normal social environment as a result of the reduced level of intellectual functioning [24]. The American Psychiatric Association’s Diagnostic and Statistical Manual for Mental Disorders (DSM I IV), published in May 1994, considers that the diagnosis of MR should meet the following three criteria: 1. Significantly lower than average intelligence: an intelligence quotient (IQ) score of <70. 2. Deficits or impairments in current adaptive functioning (the patient does not meet the level expected of a person of the same age from his or her cultural background), mainly in the following 10 areas: verbal communication, self-care family life, social or interpersonal skills, use of community facilities, self-discipline, study skills, work, leisure time leisure, and the ability to ensure health and safety. Deficits in 2 of the 10 adaptive behaviors listed above can be considered as deficits in adaptive behavior.3 The onset of the disease is before the age of 18 years [25]. 2001, the Chinese Medical Association Psychiatric Branch developed the Chinese Classification Scheme and Diagnostic Criteria for Mental Disorders, 3rd edition (CCMD I3), based on ICD I10 and DSM IIV in conjunction with China, which defines MR as a group of syndromes with incomplete or impaired mental development It is characterized by mental retardation and social adjustment difficulties, and starts before maturity (before 18 years of age) [26]. As can be seen above, with the development of medical science, although the diagnostic criteria for MR are currently inconsistent internationally, all need to meet the following three diagnostic criteria namely:1. the onset of the disease during the developmental period, i.e. before the age of 18 years; 2. a significantly lower than average intelligence; and 3. varying degrees of adaptive behavior deficits, i.e. the inability to meet social and personal requirements criteria depending on their age and cultural background. Intelligence is usually defined as a combination of perception, memory, language, thinking and practical activities. The IQ scale is commonly used to measure intelligence levels. Low intelligence generally refers to significantly lower than average intelligence, i.e., IQ is below 2 standard deviations from the normal mean, e.g., according to the Wechsler Intelligence Scale for Children (WISC), IQ < 70 points (the mean IQ score of the scale is 100 points and the standard deviation is 15 points, if IQ is below 2 standard deviations from the normal mean, i.e., 70 points) can be considered low intelligence. The severity of mental retardation can be measured according to the IQ level, and the ICD-10 classifies mental retardation into 4 levels according to the IQ results of the WISC test [24], mild: IQ 50-69, moderate: IQ 35-49, severe: IQ 20-34, and very severe IQ <20. Adaptive behavior, also known as social life skills, refers to an individual's ability to independently manage daily life and assume social responsibilities to the extent expected by his or her age and the sociocultural conditions in which he or she lives, that is, the individual's ability to cope with the natural environment and social needs. The adaptive behavior scale (ABS) is commonly used to measure adaptive behavior. Because the measure of social adaptive ability is complex and often related to age, occupational requirements, and socio-cultural background, the degree of mental retardation and social adaptive ability are not always the same in clinical work, so MR classification based on IQ alone often does not reflect the actual level of intellectual development. Based on IQ and adaptive behavior, the CCMD-3 classifies MR as mild, moderate, severe, and very severe [26]: 1. mild mental retardation (1) IQ score of 50-69, mental age of 9-12 years; (2) poor academic performance (often failing or repeating grades in general school) or poor work ability (only performing simple manual tasks); (3) able to take care of themselves; (4) no (4) No obvious speech impairment, but the ability to understand and use language is delayed to varying degrees. (1) IQ 35-49, mental age 6-9 years old; (2) cannot adapt to ordinary school learning, can do single-digit addition and subtraction; can do simple work, but with low quality and poor efficiency; (3) can learn to take care of simple life, but need supervision and assistance; (4) can master simple life language, but poor vocabulary. (3) Severe mental retardation (1) IQ score of 20-34, mental age of 3 years; (2) Significant motor impairment or other related deficits, unable to learn and work; (3) Inability to take care of themselves; (4) Severely impaired speech function, unable to communicate effectively in language. (4) Very severe mental retardation (1) IQ score below 20 and mental age below 3 years; (2) complete loss of social function and inability to escape from danger; (3) complete inability to take care of oneself and incontinence; (4) loss of speech function. In recent years, some scholars also believe that the 4-grade classification of MR is complex to diagnose, and it is often difficult to classify moderate, especially severe and very severe, and suggest that the above 4-grade classification should be changed to 2 grades of mild and severe MR by referring to the 1992 American Association on Mental Retardation (AAMR) criteria, which means that the 3 grades of moderate, severe and very severe are put together and collectively referred to as severe type [27], but this suggestion has not been widely adopted.