Mental retardation is a developmental disability that first appears in children under the age of 18. It is defined as an intellectual functioning level (as measured by standard tests for intelligence quotient) that is well below average and significant limitations in daily living skills (adaptive functioning)
Factors causing mental retardation are a heterogeneous group of disorders characterized by a significant impairment of cognitive and development due to abnormalities in the structure or function of brain. Mentally Retarded persons have IQ less than 70
trauma before birth, such as an infection or exposure to alcohol, drugs, or other toxins. trauma during birth, such as oxygen deprivation or premature delivery. Inherited disorders, such as phenyiketonuria
Or Tay-Sachs disease. Chromosome abnormalities such as Down syndrome .There are four levels of intellectual disability: mild, moderate. Server and profound.