According to University of Illinois, educable mentally retarded people are of below-average mental function with an adult mental age between eight and 12 years who can learn academic subjects up to a sixth grade level. This is the highest functional level among those considered to be mentally retarded. In order to be considered mental retardation, the below-average function must originate before the age of 16.
The University of Illinois mentions two other classifications of mental retardation: trainable mental retardation and profound mental retardation. Those with trainable mental retardation cannot learn academic information but are able to learn basic self-care. On the other hand, profound mental retardation often requires life-long care and supervision, usually within the confines of an institution designed for this purpose.
According to the University of Illinois, the causes of mental retardation are many, and not all have been discovered. Genetic causes include mutations from x-ray exposure, incompatibility of parental genes or Rh blood factor, and Down's syndrome. Complications during pregnancy, such as malnutrition, German measles, endocrine disorders, and infections, are also known causes. Problems during birth, particularly those that restrict blood flow to the brain, sometimes permanently damage mental function. Environmental factors after birth that are linked to mental retardation include severe emotional distress and lack of access to learning.