严重智力障碍约占小儿总数的0.3～0.4％。据医院资料统计:20％与环境因素有关,14％系染色体异常(大多为先天性愚型),8％为单基因遗传疾患,约60％病因不明。公共卫生部门资料更具代表性,唯诊断欠精确。据146例分析:4％为环境因素致病, Serious mental retardation accounted for about 0.3 to 0.4% of the total number of children. According to hospital statistics: 20% are related to environmental factors, 14% are chromosomal abnormalities (mostly congenital stupid), 8% are single-gene genetic disorders and about 60% are unknown. Public health department information more representative, only the diagnosis is less accurate. According to 146 cases analysis: 4% is caused by environmental factors,