He seems nice and friendly, but you can't stop wondering why he acts the way he does. He may have a condition called mental retardation. People with mental retardation are usually born with it, or it develops early in their life. They may also have some difficulty with daily living skills such as learning to read and write and caring for themselves. Doctors and other professionals determine that a person has mental retardation based on their intelligence and how well they can do everyday activities.
Intelligence is the ability to learn and understand.
Levels of intelligence are measured by special tests called intelligence tests. The score a person gets on one of these tests gives a numerical measure of a person's intelligence. An average score on an IQ test is about 90 to 110. A person with mental retardation will usually score below 75 on an IQ test. The IQ test alone does not determine whether someone is mentally retarded.