It seems autism rates, which have been steadily increasing for decades, have hit a 3-year plateau. Why have the rates been increasing, and why has it hit a plateau? The answer is likely the same: a combination of changing definitions of autism over the years and improved diagnosis.
If diagnosis rates have flattened out, we have probably discovered the actual autism rate.
Autism is probably not a single thing, but is likely rather a variety of syndromes. This is possible because in complex systems like the developing brain, a variety of causes can have the same effects, and the same cause can result in different effects in its interactions with other causes.
Alterations in 16p11.2 have been identified with autism and most recently with specific neuroanatomic differences. Note, though, in their descriptions, that they identify low IQ and poor language skills. If this is typical of 16p11.2 variants, it can hardly explain those of us who are identified as being on the spectrum and yet having high IQs and even a degree of language mastery. The Intense World Theory version of autism probably does better with people such as my son and I, as well as many of the more gifted autistics. But that would imply at least two major divisions within autism that it may be a good idea to completely separate from each other.