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Young children who have an older, autistic sibling might be at a greater risk for autism, a new study reported. Autism is a behavioral disorder that is characterized by a lack of social interaction and communication. Even though autism can range from mild to severe, children with this condition still need early behavioral treatment interventions. In order to offer these treatments as soon as possible, researchers have been focused on the risk factors. In this new study, researchers found that the risk of autism for younger siblings with an older brother or sister with an autism spectrum disorder is seven-fold.

"I think a lot of autism researchers agree that the causes of autism are many and it's very complex," the lead investigator of the study, Therese Gronborg said according to Reuters. "If it was only genetics we would see a much higher recurrence rate."

For this study conducted at Aarhus University, the researchers examined data provided from birth, civil and psychiatric registries. The registries provided information on 1.5 million children who were born between 1980 and 2004 in Denmark. By 2010, the researchers found that a little over 13,000 of these children were diagnosed with autism. Of this rate, there were 276 children who had an older sibling diagnosed with autism as well.

The researchers calculated that a younger child's risk of being diagnosed with autism if they have an autistic older sibling was anywhere from 4.5 to 10.5 percent. The average risk was seven percent. The researchers then calculated the risk for siblings who were only related through their fathers. They found that younger half-siblings had a 1.5 times greater risk of being diagnosed with autism if they had an older sibling with autism. For half-siblings that shared a mother, the risk of autism jumped to a 2.4 greater chance.

Although the researchers are not sure what causes this trend, they provided several plausible explanations. First, the risk factor could be due to genetics as well as the mother's lifestyle and environment during pregnancy.  Second, the increased risk could be due to upbringing. Third, the researchers reasoned that parents who already have an autistic child might see the symptoms of autism better, which would lead to a faster diagnosis where as parents who are not aware of the symptoms might accidentally leave their autistic child untreated. The researchers hope that this finding could help with diagnosing and treating children with autism.

The findings were published in JAMA Pediatrics.