- Nylander & Gillberg (2001) screened adult outpatients of a psychiatric hospital and found that 89.5% (17/19) of "definite autistics" did not have a prior autism diagnosis. The most common existing diagnosis in these adult autistics was found to be schizophrenia, which was considerably more common than the diagnosis of autism itself.
- Stahlberg et al. (2004) found that 30% of referred adult patients with ADHD had co-occurring ASD. The prevalence of ADHD in adults is estimated at 4.2% [source].
- Over 25 years ago, Shah et al. (1982) found that 38% of the adult patients at a mental handicap hospital had impairment in two-way social interaction, which was "very significantly associated with abnormalities of communication and imaginative activities." This confirmed an earlier study of mentally retarded children (Wing & Gould, 1979). The prevalence of mental retardation is roughly 1%.
- Baron-Cohen et al. (2001) assessed adults with the AQ test. About 2% of those in the group of randomly selected controls scored in the 32+ range. The researchers also interviewed 11 students who scored in the 32+ range and found that all of them met 3 or more DSM-IV criteria. About 60% of the 11 met threshold criteria. The general distribution of AQ test scores was replicated in Japan by Wakabayashi et al. (2004).
It's possible to propose alternative ways to interpret each piece of evidence, no doubt. But taken together, it's hard to argue with the inescapable no-nonsense conclusion: ASD is common in adults, albeit often undiagnosed.
When I first listed this evidence, one response was that perhaps ASD is common in adults, but that it would have to be a "mild" form of ASD. This is the "children are more severe these days" argument. There's absolutely no evidence that could lead us to believe this is the case, first of all. Second, the world doesn't work like that. In any group of people, there will be a wide range of variation of skills and characteristics. There has to be a distribution of "severity" in ASD adults that goes from very obviously autistic to nearly neurotypical. Sure, the shape of the distribution could have shifted, but is there any compelling reason to suspect this is the case? Furthermore, Shah et al. (1982) provides evidence of high prevalence of ASD in adults who are by no means "mild" anything.