Hi dear parents thanks for the query, I am sorry for your situation, I understand your concern about your child illness. Yes it is really difficult to say what exactly could be the reason for mental retardation. It can be genetic/hereditary, problem during pregnancy / prenatal development, or environmental or toxin related issues or problem during the delivery etc.. Rather finding out how it occurred, please focus on what exactly the child is suffering how we can help the child to cope with it. There are various things for illness but few things we need to understand, 1. With the experts help Try to know what exactly the child is suffering 2. You can follow the guidance  from the psychiatrist about next step to be done.. 3. Follow up is really important in this area. 4. Support from the parents and educating the parents with regard to problem is required. 5. Parents attending counseling sessions might help to handle the situation in better way. Take care.