As scientists and researchers continue to search for causes of it, simply defining autism has been a challenge for decades.
A new book, "In a Different Key: The Story of Autism" by John Donvan and Caren Zucker, explores the history of the diagnosis.
Autism, Donvan told MPR News host Tom Weber, is not a condition or syndrome that "has a biological marker. There's no DNA test for it. There's no blood test or cheek swab that says this person has autism and that person doesn't."
"Autism, from the time it was really first recognized, which was only about 75 years ago, is recognized by spotting certain behaviors in a person and matching them to the predominant — at the time — list of criteria for autism. There's room for all sorts of subjectivity and squishiness in that."
Today's definition of the disorder may still change, Zucker added. "The definition has become so broad now that you define somebody who is totally unable to care for themselves as the same diagnosis as someone who has a Ph.D. and is maybe bagging groceries and has some social issues; that's all one spectrum today."
At the moment, Donvan said the definition of autism "in the most broad sense ... is a collection of challenges with communication and social interaction."