PREVALENCE OF AUTISM
The 2009 SDAC showed an estimated 64,600 Australians had autism. This is an increase of 34,200 from the 2003 SDAC, or more than double the prevalence identified in 2003.
Autism spectrum disorders are more commonly found in males than females. In SDAC 2009, females made up only 18% of the reported cases. This is consistent with overseas estimates. For example, the Centre for Disease Control and Prevention in the US placed the female prevalence rate at somewhere between 17% and 20%2.
There was considerable variation in the prevalence of autism across age groups, with a marked drop off in prevalence in the late teens (Graph 1). Data is not presented for people aged over 30 years because the identified prevalence rate is too low for reliable estimates to be produced. There are several possible reasons for this variation in prevalence across age groups (diagnostic issues, survey scope and methodology) which are discussed below.
CHANGES IN DIAGNOSIS
The criteria used to diagnose autism have been subject to some variation over time. One of the key tools used as the basis for diagnosing autism is the Diagnostics and Statistics Manual of Mental Disorders (DSM). The DSM suggests diagnostic criteria clinicians can use in determining which mental and behavioural disorders people might have. The first two versions of the DSM (DSM (I) and DSM (II)) indicate children with autism should be classified as having ‘childhood schizophrenia’.
Diagnostic criteria explicitly for autism were added for the DSM (III), released in 1980. It was then amended for the DSM III (Revised), released in 1987 and revised again in the DSM IV, released in 1994. The changes from Version III and Version III (Revised) increased the amount of specific detail that should be considered in the diagnosis. These conditions were revised and reduced for the DSM IV, but at the same time, the number of criteria that had to be met to receive a diagnosis was also reduced. These changes in diagnostic criteria in the DSM appear to correlate with changes in the age specific prevalence rates shown in Graph 1.
It is unlikely however, that people are being diagnosed with other conditions instead of autism as there is no correlating increase in other conditions in the SDAC data that are likely to form alternative diagnoses (e.g. other developmental disorders, mental retardation/intellectual disability) in these late teenage years.
THE SCOPE OF THE SURVEY
The SDAC interviewed people living in private homes and a range of different accommodation options. It did not include people who are in gaol or were homeless. There is some evidence to suggest that people with autism3 may have a higher rate of homelessness, but details of the situation in Australian gaols is unknown. The proportion of people experiencing homelessness or incarceration, however, is low and this is not likely to explain the majority of the drop in prevalence observed in the late teens.
THE SURVEY QUESTIONS
The questions in the SDAC identify disability through asking about fifteen areas of limitation. Social interaction, a common (and diagnostic) deficit associated with autism spectrum disorders, is not one of these fifteen areas. The majority of people with autism (85%) are identified in the survey because they reported having difficulty learning or understanding. It may be that, as they move out of the education system and into an adult life, which they may arrange to be very structured and routine, they have less to learn or prove they understand on a daily basis and therefore no longer feel as challenged in this area.
Also, the first stage of the SDAC interview - the stage at which it is identified whether there is a person with a disability living in the household is asked of a household reference person. This includes the questions about the fifteen areas of limitations and the main condition causing each of these limitations. In instances where a child is living at home with their parents, it is more common for a parent to provide the responses to these questions. However, in the late teens when people start moving out of home, they will be more likely to answer these questions for themselves and it may be that there are differences in what the parents report with regard to child's limitations and what the child themselves reports.
There are other factors that may also be impacting on the change in prevalence with age. People may be learning coping strategies as they mature and no longer feel they are restricted in any of the areas of limitation considered in the SDAC. Similarly, for some people, intervention therapies effectively remediate the challenges of autism and they are able to function without experiencing limitations in their everyday activities.
It is also possible that people are less likely to identify a child as having autism as they get older because there is some sensitivity around the issue, either for the person or for the other family members.
Whilst none of these points explains the drop in prevalence with age in itself, it is possible they all contribute in some way.