Already shouldering a hefty financial burden, new research indicates that parents of children with autism tend to earn nearly one-third less than those with typically developing children.

After adjusting for educational attainment, location, age and other factors, researchers presenting this week at the International Meeting for Autism Research in San Diego said they found that autism is associated with an average drop in household income of 27 percent, or $17,640 per year.

Even as compared to families with children who had other functional limitations, having a child with autism still resulted in a 20 percent hit to the wallet, the study found.

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The reason: mothers of children with autism are less likely to work and when they are employed these moms tend to earn less than others, leaving many families with only one breadwinner.

Those behind the study say they don’t believe that characteristics of autism are to blame for fewer mothers working, but instead cite a lack of supports for families dealing with the developmental disorder.

“Families of children with autism don’t have a care system in place the way that other families do,” said David Mandell of the University of Pennsylvania who worked on the study.

While there are established treatment protocols and support systems in place for children with other types of developmental disabilities, Mandell said parents of children with autism are often left to hobble together an appropriate mix on their own.

“What happens is that the mother drops out of the labor market to become the case manager for the child,” he says.

For the study, Mandell and his colleagues looked at survey data collected between 2002 and 2007 on 47,942 children, 147 of whom had an autism diagnosis. Although a significant number of mothers were affected by whether or not their child had autism, they did not find much difference in employment rate or earnings for fathers.

While the findings are preliminary, the study is among the first to look at the impact of lost income on families who have a child with autism. Previously, most studies looking at the economic implications of the developmental disorder have focused on the direct cost of treatment and other care.