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20% of risk factor due to “as-yet unidentified environmental causes”. That’s a lot of “unidentified”.
Also, no mention of the vast increase in the rate of diagnosis - that’s the big question today, now up to 1 in 59 or better than 1.67% of all births. Apparently the rate was 1 in 5000 in 1975 and 1 in 2000 in the 80s.
Say 1 in 3000 and split the difference. That’s .03%.
From .03% to 1.67% in 30 years. That’s a 55x increase.
That 20% environmental slice could explain the entire increase, in theory.
The vast increase in diagnosis is due in large part to the widening of the definition, in combination with the lessening of the stigma associated with the diagnosis.
When I was a kid in the 1970s being diagnosed with autism was considered a massive blow to parents and teachers as they were seen as being the cause of the condition in the child.
As a result, people simply didn't go to a psychologist to have their child diagnosed if they suspected a mental health problem.
And if they did, the child was likely to be diagnosed with the general term "mental retardation" rather than autism or asperger's (which 2 conditions are now both called autism).
Add in the massive distrust of people towards mental health professionals that existed at the time because of the abuse against mental health patients in mental health hospitals (called insane asylums at the time still) and people were even less likely to seek professional help if mental health issues were suspected.
For me and many others, as a result of all that, the diagnosis of autism came 40 years later than it probably would today.
Don't forget the money factor. Millions and millions of taxpayer dollars are doled out to schools for autistic children. The numbers must be increased to get more funds.
In 1951 I met an autistic child, about 11 or so, the brother of one of my friends. My friend was mortified as his brother was supposed to be in his room. That is where he stayed most of the time. He didn't go to school and he could not be left alone. He was the definition of autism then. Today autistic kids play sports, get jobs, graduate from high school, even get married. When I was young they couldn't even wipe themselves after going to the bathroom. So they have definitely expanded the definition to include children that would have probably been considered normal 60 years ago.
Diagnostic changes strongly linked to funding, a potent fuel for the sudden fire. As several noted above, now there's a "spectrum" that includes old-fashioned autism at one end and Asperger's on the other. Even Asperger's is expanded to include behavior that once would have been merely eccentric. How many professionals are now willing to check a "spectrum" box that might deliver funds to a struggling family? In some hands, the diagnosis is about as specific, and useful, as "learning disorder."
I agree with Bird Dog that it's mostly diagnostic changes. We used to refer to "Idiot Savants," who were considered mentally retarded but had surprising cognitive skills. Many of those we would not consider autistic. The spectrum is now broken out diagnostically, creating a difference in statistics, but not reality.
The other 20% are not automatically unknown environmental facets just because they aren't genetic. As with IQ and other traits, the poorly-understood remainders may be mostly just random.
Assistant Village Idiot
Or they may be environmental.
I know too many horror stories from people I know personally that point to environmental factors to dismiss that as a piece of this puzzle.
Knowing parents of kids who suddenly change overnight tends to dampen your appetite for abstract theories based on definitional changes.
I believe the rate of autism was one in 10,000 in the early 80's. Today its more than one in a 100. That is a hundred-fold increase in less than two generations (closer to one generation). The fastest rate of genetic change in humans is the decline of sickle-cell anemia in African-Americans as compared to Africans, a decline of 10-fold over about 10 generations.
That autism is due mostly to genetics is simply not plausible.