Mental retardation can be caused by any condition which impairs development of the brain before birth, during birth or in the childhood years. Several hundred causes have been discovered, but in about one-third of the people affected, the cause remains unknown. The three major known causes of mental retardation are Down syndrome, fetal alcohol syndrome and fragile X.

The causes can be categorized as follows:
· Genetic conditions - These result from abnormality of genes inherited from parents, errors when genes combine, or from other disorders of the genes caused during pregnancy by infections, overexposure to x-rays and other factors. Inborn errors of metabolism which may produce mental retardation, such as PKU (phenylketonuria), fall in this category. Chromosomal abnormalities have likewise been related to some forms of mental retardation, such as Down syndrome and fragile X syndrome.
· Problems during pregnancy - Use of alcohol or drugs by the pregnant mother can cause mental retardation. Malnutrition, rubella, glanular disorders and diabetes, cytomegalovirus, and many other illneses of the mother during pregnancy may result in a child being born with mental retardation. Physical malformations of the brain and HIV infection originating in prenatal life may also result in mental retardation.
· Problems at birth - Although any birth condition of unusual stress may injure the infant’s brain, prematurity and low birth weight predict serious problems more often than any other conditions.
· Problems after birth - Childhood diseases such as whooping cough, chicken pox, measles, and Hib disease which may lead to meningitis and encephalitis can damage the brain, as can accidents such as a blow to the head or near drowning. Substances such as lead and mercury can cause irreparable damage to the brain and nervous system.
· Poverty and cultural deprivation - Children in poor families may become mentally retarded because of malnutrition, disease-producing conditions, inadequate medical care and environmental health hazards. Also, children in disadvantaged areas may be deprived of many common cultural and day- to-day experiences provided to other youngsters. Research suggests that such under-stimulation can result in irreversible damage and can serve as a cause of mental retardation.