There are a lot of reasons there are more cases of what we call autism, but, and I’m speaking as someone who worked extensively with autistic people, here are two phenomena that, IMO, have contributed greatly to the increase in people being labeled as autistic.
- Until 1974, there were few places for children with autism other than institutions. A few states had private clinics, but not all of them were residential, and all of them were private. Parents had to be able to pay for them, or get grants, and either send their children there, sometimes out-of-state, or drive them long distances every day. And the programs had long wait-lists, and were selective of the students they accepted.
It is very likely that many autistic children were deliberately misdiagnosed as mentally retarded, because just about every state had a residential school for the mentally retarded, which was free or required only a minimal fee that could be waived for poor families. Meanwhile, family doctors who may have suspected mild autism in a high functioning child, especially a bright one, probably kept it to themselves. Even if the child was unhappy in public school as “the weird kid,” it was way better than an institution, or being expelled, which the school could do if the child acquired a label like “autistic.”
After 1974, Public law 94-142 went into effect, and public schools had to provide education for all handicapped children, which meant that autistic children of any level of functioning were legally entitled to a free public school education. From this year forward, diagnoses became more honest.
- There used to be a lot of other things a child with the same symptoms as autism could be labeled with. One was “Emotionally Disturbed,” and another was “Childhood schizophrenia.” Because people believed that autism was a sort of PTSD, caused by bad mothering, and a retreat into self, a child who exhibited symptoms of it, but either had a very mild case, or else clearly was not the subject of bad mothering, sometimes got labeled “Emotionally Disturbed.”
Childhood schizophrenia was a label that children who talked a lot, but often had speech did not seem to make sense, and resembled the “word salads” of adult schizophrenics, got. People did not know the cause of either schizophrenia or autism, and there were some theories that postulated a similar cause. Schizophrenics were known to sometimes experience hallucinations, and there was a theory that some clearly atypical children did too, and preferred their hallucinations to the real world (stimming was thought to be interacting with their hallucinations). It is easy now to see that these individuals are autistic, but people were flying by the seats of their pants when these theories were making the rounds. It was before brain imaging, and before computers allowed different researchers in different places to easily compare their work, so two people with similar children never realized the children they were working with clearly had a syndrome and there were more children, and more children. All they knew about was the one child before them.
Anyway, in the 1980s, a lot of adults who still carried “schizophrenia” labels from childhood had them revised to autism, so many of the new cases that spiked in the 80s were actually adult cases.
Those are probably the two major factors. There are minor factors. One is two Rubella epidemics, in the late 1960s and early 1970s, that did actually cause a small bump in cases of autism; another is the baby boomlet in the 1980s, which caused a rise in total number of cases as a factor of more children in general; yet another is the sad fact that in the days when parenting meant turning kids loose in the morning, and expecting them back for supper, beginning about the time they were three, a lot of autistic kids were probably lost to accidents when they were small, and never had a chance to be diagnosed with anything. Awareness is another: many more children are referred for diagnosis these days. It used to be that in the 1970s, for every child presented for diagnoses, probably two slipped through the cracks. Now, not every child presented for diagnosis even gets one. Maybe two in three, or two in four, and since the 1990s, it’s been pretty difficult for a child to get missed. Some parents refuse the referral, and children who are homeschooled may get missed, especially if they aren’t taken to the doctor regularly, but it’s rare for a child to fall through the cracks anymore. However bear in mind that better identification does not mean there are more cases, just more diagnoses.
TL;DR: No, autism is not suddenly increasing exponentially.