Why do retarded people try to act smart?

I have known many mentally retarded people and not one of them try to "act" smart. There are degrees of retardation and many can understand derision directed at them, at that time someone may become defensive and say things that you may interpret trying to "act" smart. Those that have experienced a lot of negative treatment may develop a "public persona" in an effort to fly under the radar of abusers. And, believe it or not, many retarded people are also quite smart, they will sometimes amaze you what they figure out and how they perceive things.