The prevalence of autism spectrum disorders among British adults in a population-based survey was about 1%, researchers said -- closely matching rates seen recently among British and U.S. children, but contradicting suggestions that the incidence has risen dramatically over time.
Based on in-person clinical assessments performed on 618 adults of all ages, researchers from several U.K. universities calculated a community-wide prevalence of 9.8 cases of autism spectrum disorder per 1,000 population, according to a paper in the May issue of Archives of General Psychiatry.
Two recent prevalence studies in U.S. children also came up with figures near 10 per 1,000.
However, rates in the new British study did not differ markedly by age, "suggesting that the causes of autism are temporally constant," wrote Traolach S. Brugha, MD, of the University of Leicester in England, and colleagues.
The study is the first to attempt a population-based prospective assessment of autism-related disorders in adults, the researchers indicated.
Other studies, especially in the U.S., have suggested rising rates of autism among children over time, which ought to translate to substantially lower rates among older adults.
Although Brugha and colleagues did find a small decrease in prevalence with age -- about 1% per year -- the 95% confidence interval was a 4% increase to a 6% decrease per year of age.
"Based on [previous published studies], we would expect substantially lower rates in earlier birth cohorts (older respondents) if rates of autism spectrum disorders have been rising considerably during recent decades, and we would also expect lower rates in younger adults than in recent childhood surveys using the ADOS-4 [Autism Diagnostic Observation Schedule Module 4]. We did not find this," the researchers wrote.
"Overall, our findings suggest that prevalence is neither rising nor falling significantly," they added. "This favors the interpretation that methods of ascertainment have changed in more recent surveys of children compared with the earliest surveys in which the rates reported were considerably lower."
On the other hand, Brugha and colleagues did find several factors significantly associated with adult autism and related disorders in multivariate analyses. These included male sex, low educational attainment, and living in public housing
"It was surprising to all of us," said Dr. Traolach Brugha, a psychiatrist at the University of Leicester, who worked on the study. "If this study is correct, it does put a big question mark over the autism epidemic."
In fact, more and more research hints that some if not all of the increase in autism may be due to changes in how, and how often, the disorder is diagnosed. Kids who used to be classified as mentally retarded or just plain eccentric, for instance, might now get an autism-spectrum label instead.
"That simply means more people are coming forth and being recognized," Brugha told Reuters Health.
Brugha said he was confident in the results, but that they should still be confirmed in other studies given the small number of people with autism found in this study.
Ha added that he had been disappointed to discover that none of those who got the diagnosis based on the study's clinical assessment were aware of their condition.
"None of them had been diagnosed (previously) with autism," he said. "I think for me the issue is that people have been ignoring autism in adulthood and only focusing on children."