New research has found that the risk of younger siblings developing an autism spectrum disorder (ASD) is 14 times higher if an older sibling has ASD.

Carried out by the Kaiser Permente group, the research also found that the risk level was consistent across gestational age at birth.

Darios Getahun, study senior author, Kaiser Permanente Southern California, Department of Research & Evaluation, said: “Our study provides additional insights into how autism affects siblings.

“These findings also contribute to a better understanding of the influence of factors such as gender on autism risk.”

Researchers had examined the records of 53,336 children, and found that children had an 11.3 per cent chance of being diagnosed with ASD if they had a diagnosed older sibling, and just 0.92 per cent if not.

Dr Getahun said: “It’s possible that parents who have an older child with an autism diagnosis are more likely to have their younger siblings tested, too, resulting in a higher rate of diagnoses among younger siblings, compared with parents who do not have children diagnosed with an autism spectrum disorder”.

Autism – most commonly associated with impairments in social interaction and communication – affects around 1 in 68 children, and its cause is still unknown.