There are many possible reasons for mental retardation. The causes can be sorted into a couple of classifications. Diseases like HIV, meningitis, listeriosis, encephalitis, congenital oxoplasmos is, congenital rubella and congenital CMV present previously or happening after birth can cause mental retardation.Toxins like alcohol, cocaine, amphetamines and different medications the mother polishes off while pregnant can likewise be the causes.