With all this, it comes as no surprise that a recent study out of Iceland would report a prevalence greater than 1%. In Prevalence of autism spectrum disorders in an Icelandic birth cohort , the authors report a prevalence of 1.2% of clinically confirmed autism. The data, discussed below, shows a clear indication that better identification has played a major role in this prevalence.
For ASD, many had intellectual disability, but the majority did not ( click to enlarge ). For childhood autism (autistic disorder) the opposite is true. The majority (72%) had ID.
Other health conditions were reported, with epilepsy being the most prevalent co-occuring condition (other than ID). Epilepsy was more common among those with autism+epilepsy compared to autism without epilepsy, as found in other studies.
The authors note that 1.2% is much higher than previously reported, and that the prevalence just for childhood autism is also higher:
In the present study, the diagnostic category of CA represented a relatively small proportion (28%) of the total number of cases, even if the prevalence is high (0.34%). This prevalence is 7.7 times higher than that reported in the first study on autism published in Iceland.  and almost four times that reported in a more recent study.
citation  is: Magnússon GT. Athugun á geðveikum börnum á Íslandi: Börn fædd 1964–1973. (An investigation of psychotic children in Iceland: children born 1964–1973). Laeknabladid 1977;63:237–43.
The prevalence numbers are climbing with time even within the cohort in this study. The authors note:
In a 4-year period, from the end of 2005 to the end of 2009, the prevalence of ASD in the cohort studied doubled, moving from 0.6% to 1.2%. This increase cannot be explained by immigration to Iceland, confirmed by the National Registry,22 and migration of people from one part of the country to another is irrelevant since the area studied and the whole country are the same. As expected, children diagnosed earlier (by 2005) were more likely to have CA than AS and were generally more impaired than those diagnosed later (2006–2009), although the groups did not differ regarding the frequency of ID and medical conditions. In order to examine symptom severity from another angle than diagnostic classification, we compared the earlier and later diagnosed groups on ADI-R total score. This comparison did not reveal differences between groups. High scores on ADI-R for those diagnosed later indicate serious autistic symptoms, possibly in association with co-occurring developmental and psychiatric disorders. Another point of interest is that the number of boys did not increase, contrary to what is suggested by some investigators.33 One interpretation of these results is simply that as the cohort studied grows older, more girls are identified with ASD,34 and because girls with ASD are more likely to be cognitively impaired, it would counteract the predicted trend for fewer children with co-occurring ID as the prevalence of ASD increases. Comparing the distribution of boys and girls in the group of children with ID (n=91) diagnosed earlier or later with ASD revealed some support for this hypothesis, as the gender ratio was 2.8 and 1.2, respectively, although this difference fell short of statistical significance.
The prevalence doubled from 2005 to 2009. Doubled. This for kids who were born between 1994 and 1998. In 2005, the kids in this study were 7-11 years old, and over the next few years the fraction of those kids identified as autistic doubled. For critics of the idea that better identification is a major factor in prevalence increases, I await your explanation of this. Actually, I don’t await your explanation as this is not that surprising a result. Better identification, worldwide, has (and still is) a driving force behind increases of autism prevalence.
Here is the abstract (the full paper is online as well):
OBJECTIVES: A steady increase in the prevalence of autism spectrum disorders (ASD) has been reported in studies based on different methods, requiring adjustment for participation and missing data. Recent studies with high ASD prevalence rates rarely report on co-occurring medical conditions. The aim of the study was to describe the prevalence of clinically confirmed cases of ASD in Iceland and medical conditions.
DESIGN: The cohort is based on a nationwide database on ASD among children born during 1994-1998.
PARTICIPANTS: A total of 267 children were diagnosed with ASD, 197 boys and 70 girls. Only clinically confirmed cases were included. All received physical and neurological examination, standardised diagnostic workup for ASD, as well as cognitive testing. ASD diagnosis was established by interdisciplinary teams. Information on medical conditions and chromosomal testing was obtained by record linkage with hospital registers.
SETTING: Two tertiary institutions in Iceland. The population registry recorded 22 229 children in the birth cohort.
RESULTS: Prevalence of all ASD was 120.1/10 000 (95% CI 106.6 to 135.3), for boys 172.4/10 000 (95% CI 150.1 to 198.0) and for girls 64.8/10 000 (95% CI 51.3 to 81.8). Prevalence of all medical conditions was 17.2% (95% CI 13.2 to 22.2), including epilepsy of 7.1% (95% CI 4.6 to 10.8). The proportion of ASD cases with cognitive impairment (intellectual quotient <70) was 45.3%, but only 34.1% were diagnosed with intellectual disability (ID). Children diagnosed earlier or later did not differ on mean total score on a standardised interview for autism.
CONCLUSIONS: The number of clinically verified cases is larger than in previous studies, yielding a prevalence of ASD on a similar level as found in recent non-clinical studies. The prevalence of co-occurring medical conditions was high, considering the low proportion of ASD cases that also had ID. Earlier detection is clearly desirable in order to provide counselling and treatment.