Younger siblings of individuals with autism are nearly seven times more likely than those with only typically-developing brothers and sisters to have the developmental disorder too, researchers say.

In the first population-based study examining what’s known as the recurrence risk for autism, Danish researchers found that when parents had one child on the spectrum, the chance of having another child with the disorder was about 7.5 percent.

For half siblings, however, the recurrence risk decreased to 2.4 percent in cases where the children had the same mother and was not significant if the children only shared a father.

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That’s “substantially lower” than the risk for siblings noted in previous studies, the researchers said in their findings published Monday in the journal JAMA Pediatrics. Accordingly, they indicated that the roughly 7 percent recurrence risk should be “reassuring” to parents of children with autism who are considering having another child.

The current study looked at the roughly 1.5 million children born in Denmark between 1980 and 2004.

Researchers behind the report said that the findings offer some clues as to what might be responsible for the developmental disorder.

“The difference in the recurrence risk between full and half siblings supports the role of genetics in ASDs, while the significant recurrence risk in maternal half-siblings may support the role of factors associated with pregnancy and the maternal intrauterine environment in ASDs,” wrote Therese Gronborg of Aarhus University in Denmark and her colleagues in the study.