Within families with multiple kids on the spectrum, varying genetic roots are often at play, according to a new study that’s highlighting just how complicated the developmental disorder is.

Roughly 11 percent of families affected by autism have more than one child on the spectrum. While it seems logical that shared genetic variations would be responsible, researchers found otherwise.

The study involved an analysis of genetic data collected from 1,532 families of those with autism. In 80 percent of families in the study, multiple members had the developmental disorder.

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Not surprisingly, in these families known as “multiplex,” inherited copy-number variations were more likely to be involved. However, it was rare that multiple siblings on the spectrum shared the same autism-linked inherited variation.

“It’s actually the exception rather than the rule,” said Daniel Geschwind, a professor of human genetics at the University of California, Los Angeles who led the study published this month in the American Journal of Human Genetics. “So for example, there are families in which only two of three kids have the rare CNV inherited from the parent, yet they all have been diagnosed with an autism disorder.”

It’s unclear why siblings would so infrequently possess the same autism-linked genetic variation, the researchers said. It could simply be luck or there could be other factors that have not yet been identified.

Geschwind said the findings highlight the importance of studying multiplex families.

“This is the largest study of its kind in families with multiple autism spectrum disorder children, and it shows that looking at such families can provide significant new insights,” he said.