eTable 1. Classification of mental retardation (MR) according to Zhang et al. (e5).

Level	Degree of disability(IQ)	Adults	Children
0	None	No impairment	Development appropriate for age
1	Borderline (<70)	Attendance of regular schools for several years; needs little support	Milestones reached at normal ages; slight developmental delay apparent in first years at school
2	Very mild (<65)	Attendance of regular schools for a few years; needs a lot of support; simple skills in reading, writing, and numeracy	Milestones reached at normal ages for the first few years; slight developmental delay apparent from age of 2 to 3 years
3	Mild (<50)	Good language comprehension, even long sentences are understood; limited skills in reading, writing, and numeracy	Achievement of milestones delayed by a few months; apparent from the age of 1 to 2 years
4	Moderate (<35)	Good language comprehension; speaks short sentences, makes a lot of gestures	Achievement of milestones delayed by several months; apparent from the 2nd year of life
5	Severe (<20)	Understands simple common sentences; speaks two-to three word sentences, makes a lot of gestures; can walk	Achievement of milestones delayed by several months to a year; apparent from the 1st year of life
6	Very severe (<10)	Understands single words; speaks single words or not at all; can walk unsteadily/with assistance	Developmental delay by several years; becomes apparent in first 6 months of life
7	Profound	No or only slight reaction; can sit and stand with assistance, walks rarely or not at all	No unassisted sitting at age of 5 years

IQ, Intelligence quotient