About 20 percent of younger siblings of those with autism are on the spectrum too and they often show symptoms of the developmental disorder as young as 18 months, researchers say.

Warning signs like poor eye contact and repetitive behaviors were apparent in 57 percent of siblings at just a year and a half old, according to findings published this month in the Journal of the American Academy of Child & Adolescent Psychiatry.

Among siblings without symptoms at 18 months, those that were later diagnosed with autism showed signs of the developmental disorder by 36 months, the study found.

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“While the majority of siblings of children with ASD will not develop the condition themselves, for those who do, one of the key priorities is finding more effective ways of identifying and treating them as early as possible,” said Katarzyna Chawarska of the Yale Child Study Center and the Yale School of Medicine who led the research. “Our study reinforces the need for repeated diagnostic screening in the first three years of life to identify individual cases of ASD as soon as behavioral symptoms are apparent.”

For the study, researchers looked at data on 719 infants who had older siblings on the spectrum. The children were assessed at 18 months and again at 36 months to identify social, communication and repetitive behaviors that could be predictive of autism.

Of those ultimately found to have autism, researchers said that when and how the kids began displaying symptoms varied. About half exhibited poor eye contact alongside limited gestures and imaginative play. Other children, however, had repetitive behaviors and lacked nonverbal communication skills, the study found.

“Not only do the behavioral symptoms appear at different ages, but different combinations of early symptoms may predict the diagnostic outcome,” Chawarska said.