What is the theory behind it?

The direct reinforcement that comes from immediately getting what you want is the key to PECS. Without having to use spoken words, a child is able to turn an inner desire into an external reward. It is thought that tangible rewards are more reinforcing to children with autism than social rewards, at least during the first steps of communication learning. However, if these rewards are too difficult to receive – that is, if it is very difficult for the child to form words – then the point of communication may, at first, be lost on these children. This can result in tantrums and other undesirable behavior, because the child cannot clearly communicate what s/he wants. However, when children with autism are trained in PECS, problem behaviors often subside as the benefits of communication become more tangible ^{(1, 4, 5)}.

PECS may also help improve social interactions in children with autism. Because the child is in charge of approaching the communication partner, the child learns how to make the first move. For children with autism, approaching another person socially can be difficult. However, in this case, the child is not expected to speak, so the initial approach may be less intimidating ^{(1, 2)}.