ASL is a natural language as proved to the satisfaction of the linguistic community by William Stokoe. It is a manual language meaning that the information is expressed not with combinations of sounds but with combinations of handshapes, movements of the hands, arms and body, and facial expressions. It is used natively and predominantly by the deaf of the United States and Canada.
Although it often seems as though the signs are meaningful of themselves, in fact they are as arbitrary as words in spoken language. For example, hearing children often make the mistake of using "you" to refer to themselves, since others refer to them as "you." Children who acquire the sign YOU (pointing at one's interlocutor) make similar mistakes - they will point at others to mean themselves, indicating that even something as seemingly explicit as pointing is an arbitrary sign in ASL, like words in a spoken language.
In recent years, it has been shown that ASL has had a positive impact on the intellect of hearing children who are exposed to it. When infants are taught the language early, parents are able to respond accordingly to the infant at a developmental stage when verbal speech, which requires extremely fine control of many, interacting parts, is not yet able to be formed. The ability of the child to actively communicate and interact earlier than would otherwise be possible accelerates the cognitive development of the child.
ASL has been taught to chimpanzees, bonobos, and gorillas. Several have mastered more than one hundred signs. One deaf native speaker on the Washoe research team was asked to write a sign down whenever she saw them and although the hearing people on the team were turning in long lists of signs, what she saw were not signs at all, but simply gestures. The researchers in the studies of Koko and Washoe also refused to share their raw data with the scientific community.