It would be wrong to infer that children with less severe symptoms do not have autism, says Professor Andrew Whitehouse from the Telethon Kids Institute.
The so-called 'autism epidemic' is due to an increase in the diagnosis of children with less severe behavioural symptoms, not because there are more children being born with the disorder, a new Australian study shows.
There has been a 20-fold increase over the past 30 years in the prevalence of autism, which is now thought to affect at least one per cent of the Australian population.
Ten years ago, many children with milder autism symptoms would have been told they had a language impairment or not received any diagnosis at all.
But a greater understanding of the life-long developmental disorder over time means more and more children who fall on the mild to moderate side of the spectrum are being recognised, says Professor Andrew Whitehouse, Head of Autism Research at the Telethon Kids Institute.
Wanting to explain the rise in Autism Spectrum Disorder rates, Prof Whitehouse and a team of researchers examined the diagnostic information of more than 1200 West Australian children diagnosed between 2000 and 2006.
They looked at differences in both the percentage of newly diagnosed cases that met each of the 12 diagnostic criteria for autism, set by the Diagnostic and Statistical Manual of Mental Disorders, as well as severity ratings of the behaviours observed.
The study, published in international journal Autism Research, shows the increase in autism prevalence over this period was due to an increase in diagnosing children with less severe, or mild, behaviours.
Across the study period (2000 to 2006), there was in fact a reduction in the proportion of new cases rated as having "extreme" severity for six of the 12 diagnostic criteria.
Significantly, the percentage of new cases with no extreme rating on any criteria increased from 58.5 per cent to 86.6 per cent between 2000 to 2006.
Autism, for which there is no known cause, affects the way an individual relates to his or her environment and their interaction with other people.
Prof Whitehouse warned it would be wrong to infer that children with less severe symptoms do not have autism and do not face significant challenges.
Vicki Gibbs from Autism Spectrum Australia (ASPECT) agrees, and says this is a positive study because research has shown there is a lot of benefit to identifying people with these milder forms of autism.
"If they've got milder forms of autism they've actually got a lot of potential to contribute to society but they typically can't do that without some level of support," Ms Gibbs said.