The quick answer:
I am frequently asked, “why do we think the prevalence of autism has increased?”
That question can be taken two ways. First, do we really believe that it has increased? And, second, if we do believe that it’s increased, why it is increased?
Do we really believe that it has increased?
We are very confident about the answer to the first question.
We understand that some people disagree, including the authors of this paper. But, as we have explained in some detail, the idea that the prevalence of ASD has not dramatically changed over time cannot be reconciled with several observations.
It’s a hypothesis that we can eliminate since it contradicts observations. My view is that the only thing still supporting that hypothesis is the fact that most people don’t know of a reasonable alternative. They are unaware of the monumental body of evidence pointing at acetaminophen exposure in susceptible babies and children as a cause of autism.
If we do believe that it’s increased, why it is increased?
The answer to the second question is also evident. Why did the increase actually happen? What caused it?
First, drugs were introduced in the late 1800’s that resulted in exposure of some babies and children to acetaminophen.
Then, starting in the 1980s, several factors drove up acetaminophen use in the pediatric population. Those factors include a switch from aspirin to acetaminophen in the early 1980s due to a connection between aspirin and Reye syndrome. Direct-to-consumer advertising by the pharmaceutical industry also played a very important role in driving up use of drugs in the pediatric population. We now know that the drug was never proven safe and that it would not pass current drug testing standards, but it was nevertheless effectively marked as safe.
Indeed, we found literally thousands (not hundreds) of papers in the medical literature which state that acetaminophen is safe for use in babies and/or children when used as directed, without the science to back it up. The sales strategy and marketing campaign effectively influenced healthcare workers as well as parents and guardians. If the sales strategy wasn’t working, a company producing the drug would not have spent almost half a billion dollars (inflation adjusted) per year marketing the drug.
What’s the math behind 70 times?
The question is, how did we decide that the prevalence of autism spectrum disorder has increased 70-fold since acetaminophen became popular? We are being conservative. We suspect it’s much more than that, and I’ll explain. But first, the 70-fold change was easy to calculate. In the 1960s and 1970s, the prevalence of autism was estimated to be about 4 in 10000 (the same as 1 in 2500) at most. You can find those number here. The most recent measured prevalence is 1 in 36. The ratio is 2500/36, or 69.4444, or about 70-times different.
We doubt that autism spectrum disorder as we know it existed at all prior to the 1880s, when nobody was exposed to the drug. So, yes, the increase is real, we know why it increased, and a very conservative guess is that the increase is 70-fold. It’s probably more.