Younger siblings of those with autism are over a dozen times more likely than other kids to have the developmental disorder too, a new study suggests.

The risk did not appear to be affected by a child’s race or whether they were born early, at term or late. However, gender did seem to make a difference, according to findings published online this month in the Journal of Developmental & Behavioral Pediatrics.

For the study, researchers looked at medical records for Kaiser Permanente members in Southern California in cases where at least two siblings were born to the same mother between 2001 and 2010. Out of 53,336 children in the study, 592 were diagnosed with autism.

Among kids with an older sibling on the spectrum, 11.3 percent were ultimately diagnosed with autism as well. By comparison, just 0.92 percent of other children were found to have the developmental disorder.

The risk was higher for boys with older brothers with autism — 15 percent — compared to girls with affected older sisters who were diagnosed 7 percent of the time, the study found.

“Our study provides additional insights into how autism affects siblings,” said Darios Getahun of the Kaiser Permanente Southern California Department of Research & Evaluation who worked on the study. “These findings also contribute to a better understanding of the influence of factors such as gender on autism risk.”

Getahun said the findings could be influenced by outside factors including the increased likelihood that boys versus girls would be evaluated for autism and a greater chance that families with one child on the spectrum would have their other children tested.

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