To be honest, I had assumed that those people who do not speak - but DO communicate - have physical issues with forming words. After all, I reasoned, if a person wants to communicate but doesn't speak, it must be because speech is just too difficult.
But it turns out that I may be wrong. In fact, according to a top researcher in autism and communication (Dr. Helen Tager-Flusberg, PhD of Boston University), "...the short answer is that we really don't know why some children with ASD don't learn to speak at all."