Professor David Skuse explains that symptoms of autism are not precisely distinct from 'normal' behavior.
One of the questions that I think is still unanswered is, in respect of what is autism, is how is autism different from normal development? How is autism different from the way that, say, you and I behave? The answer to that is we don’t actually know. Autism is not a distinct separate condition that is completely different from normal behavior. It is the extreme of certain behaviors that we might see in many typical children or adults that don’t have autism at all.
One of the questions that is as yet unanswered is, where is the boundary between what we recognize at typically autistic behavior and normal development, normal behavior? We can only answer that by looking for autistic traits, autistic symptoms, if you will, in a very large number of typical people, let’s say a very large sample of normally developing children.