In technologically advanced countries, do most autistic adults not know they are autistic?

Question

I understand this is not an easy question to address. There are differences between the terms autism and Aspergers both in public perception and in the professional world, and it's a continuum or spectrum or constellation of characteristics (syndromal); there's no simple binary yes/no pass/fail, positive/negative test that I know of. There is of course the Autism-Spectrum Quotient or AQ test (examples: 1, 2) which provides a score, but the score must then be interpreted; there's still no fixed threshold.

There are however both genetic and neurological markers that correlate with aspects of autism, refer for example to the videos

• Autism: An evolutionary perspective, Professor Simon Baron-Cohen, 1st Symposium of EPSIG, 2016
• Simon Baron-Cohen: Autism and the male brain

Presumably Baron-Cohen put "the male brain" in the title to emphasize how much less is known about autism in females, perhaps due to some combination of lower frequency and different presentation especially in adults.

So while the question is simple to ask, there will not be a simple answer like "Yes" or "No" followed by a percentage of those that don't know.

But as this is an important question, I'm hoping that at least some work has been done to assess the fraction of the adult autistic population who are as yet unaware of it and remain uninformed of it by medical and mental health professionals who may not be looking for it.

Have any studies along this line been done? Is there at least some estimate?

---

Different but potentially related and/or helpful:

• is it possible to be severely autistic yet not being "visibly autistic"? (discusses diagnostic criteria)
• What is the relationship between mathematical ability and autism? (some interesting references)
• Are the incidents of autism spectrum disorder on the rise, or is there an increase due to better definition of diagnostic criteria?
• What is the evidence that mathematicians are more likely to have autistic traits than the rest of the population?

Answer 1

Short answer: Yes.

Modern estimates of global prevalence for autism are around 1-2% (for systematic reviews, see Baxter et al, 2015; Chiarotti & Venerosi, 2020). Prevalence rates appear stable across countries, age groups, and probably time, though there is a great deal of variability between individual studies. Virtually all data is from countries in the "technologically advanced" category, as data for lower-income countries is lacking.

To figure out the proportion of autistic adults who don't know that they are autistic, we just need the number who already have an autism diagnosis, known as the "administrative prevalence" rate. While population prevalence is thought to be stable, administrative prevalence has been on the rise for over 20 years. Determining the administrative prevalence for adults has turned out to be a significant challenge, as the number of different agencies (medical, support, assistance, insurance, education, etc) under which adults may be registered as autistic is both vast and uncoordinated, and there is a paucity of studies looking at adults at all.

One study that I was able to find that both managed the Herculean effort of coordinating these independent agencies, and also specifically looked at adults, is the Pennsylvania Autism Census Project (2009):

In 2005, we counted 1,421 adults with ASD who were 21 years of age or older, just 7.1% of the total ASD population in Pennsylvania. ... By 2020, the adult ASD population will be roughly equal to Pennsylvania’s entire ASD population, children and adults, in 2005.

That is, this study estimated that in 2005, only 7.1% of autistic adults knew that they had autism, based on current diagnosis rates for children expected to become autistic adults by 2020.

The 2011 Scotland census, reported in Rydzewska et al (2018), provides a more direct approach, asking respondents directly to self-report on their autism status. In this census, comprising 94% of Scotland's population, 0.2% of adults 25+ reported knowing that they were autistic. Compared to the estimated prevalence of 1-2%, this suggests that about 10-20% of autistic adults in 2011 knew of their condition.

Another study of Northern California by Croen et al (2015) found autism in 1507 out of 1,578,657 insured adults, an administrative prevalence rate of <0.1%. Given a population prevalence rate for the same region and time period, from Windham et al (2011), of 0.47%, this suggests that about 20% of adults with autism knew about it at the time (using a more recent prevalence of 1-2%, it would be more like 5-10%).

A primer on autism by Lord et al (2020) also says:

... in high-income countries with strong ASD public health policies, there is evidence that ASD in adults goes largely unrecognized ... most people in the world who are autistic are adults , and ... many of these individuals have not received a diagnosis of autism ...

Finally, population prevalence estimates generally source data from epidemiological "population studies" that sample directly from the general population (rather than a known patient community or population subset). A population study of adults in England by Brugha et al (2011) found a number of autistic individuals in its sample, and asked them if they knew that they were autistic:

None of the cases found in this study had previously been given a formal autism assessment or diagnosis.

An extension of this study found a significant number of autistic individuals living in communal care, or registered with learning disabilities. This brings up the final point that I'd like to make.

The above studies seem to suggest that the majority of autistic adults are unaware of their condition, and are therefore unsupported. However, this is unlikely the case, due to diagnostic substitution: Essentially, prior to increased awareness and broadening definitions of autism, most autistic children were instead diagnosed with other conditions, such as intellectual disability, language disorders, learning difficulties, and so on. The phenomenon of diagnostic substitution in autism is well illustrated by this graph (source):

So while most autistic adults are unaware of their autism, many of them are receiving support and care under different labels. And, as the increasing number of children being diagnosed with autism over the past 20 years become adults, the number of adults unaware of their condition will decline accordingly.