Children who have an older sibling with autism have a 7% chance of developing this disease, while the risk to the general population is 1%. This is the conclusion reached by a study conducted by researchers at the University of Aarhus in Denmark.

Therese Gronborg, lead author of the research, whose results have been published in 'JAMA Pediatrics', has explained that the causes of autism are diverse and complex, and that if the disease had a single genetic component, the incidence among siblings would be much higher which they have found. In fact, other previous studies had determined the risk rate of having autism when the older brother suffers from the disorder by 18%.

If the autism had a single genetic component, the incidence among siblings would be much higher, explains the principal investigator of the study.

The researchers used data from births and civil and psychiatric registries to assess the 1.5 million children born in Denmark between 1980 and 2004. They found that children whose older sibling had autism had between 4.5 and a 10.5 percent chance -7% on average- of suffering from this pathology.

They also observed what happened when the children studied were stepbrothers; thus, if they had the same father and one of them was autistic, the risk of the other suffering the disorder was 1.5%, while if they were siblings on the mother's side this risk increased in the healthy child to 2, 4%.

According to Groborg, the lifestyle of the mother during pregnancy or some factor that intervenes in intrauterine development could influence the risk of their children suffering from autism, although it could also be associated with socio-cultural factors and education.

CDC Grand Rounds: Autism Spectrum Disorder (November 2019).