It is primarily driven by the expansion of the diagnosis itself. DSM-3 Autism (1980) is quite different from DSM-5 Autism (today). Today's autism includes things that used to have totally different names. It also "allows" for a diagnosis much later in life. Autism used to be something very specific and discreet. From another perspective, the diagnosis rate may not be soaring. That is, many more people in the 1980s and 1990s would be autistic by today's new standard.