A child in Australia is more likely to have autism if he or she is the first-born, is born to a woman who is older than 40 years, or belongs to a family of higher socio-economic status, according to a study published in March in PLoS One.
By contrast, children born to women younger than 20 years old, of later birth order or from families with fewer social advantages are more likely to be diagnosed with intellectual disability.
The prevalence of autism varies considerably among countries, a fact that may in part be the result of complicated cultural variables that affect diagnosis. Risk factors such as paternal or maternal age can also affect the prevalence of the disorder.
In the new study, researchers looked at these variables for nearly 400,000 children born in Australia between 1984 and 1999. They found that the 1,179 children diagnosed with autism tend to belong to wealthier families compared with the 4,576 children diagnosed with intellectual disability.
This is especially true for those who have autism without intellectual disability compared with those who have mild to moderate intellectual disability. The risk factors for those who have both autism and intellectual disability lie somewhere in between.
The researchers found that parents who are married, taller-than-average, and living in urban areas have higher odds of having a child with autism, and lesser odds of one with intellectual disability. Children of mothers of aboriginal descent are more likely to have intellectual disability than autism.
The results of the study suggest that factors that increase social standing may result in a child being diagnosed with autism instead of intellectual disability, a trend also seen in the United States.