There is a mystery of autism that has been published this week. The fact is that parents of a child with autism face a risk of almost one in five that their next child will also develop the disorder. What does this mean? The largest study of siblings of children with autism attempts to explain.

The risk is higher than previous estimates, and goes even higher if the second child is a boy. In fact, the risk rises to over 26% if the second child is male, because the truth is that the disorder is mostly found among boys and over 32% for infants with more than one older sibling with autism.

Scientists are working hard to narrow down the regions on chromosomes 7 and 15 to identify the specific gene(s) related to autism on these chromosomes. They are also expanding their searches to investigate other chromosomes for which there is preliminary suggestive evidence.

Researchers in the U.S., Canada and Israel, included 664 infants whose average age was eight months at the start of the study, and who were tested for autism when they reached three years. A total of 132 infants met the criteria for ASD, with the said 26% of boys compared with 9% of girls diagnosed in the study as a whole.

The overall rate of autism among all study participants was 18.7%, and in families with one older child with autism, or simplex families, the rate of incidence was 20.1%. In families with more than one sibling with autism, the recurrence rate was 32.2% to be exact.

Sally Ozonoff, professor of psychiatry and behavioral sciences at the MIND Institute at the University of California-Davis, Sacramento, who led the study, said:

‘This is the largest study of the siblings of children with autism ever conducted. It’s important to recognize that these are estimates that are averaged across all of the families. So, for some families, the risk will be greater than 18%, and for other families it would be less than 18%. At the present time, unfortunately, we do not know how to estimate an individual family’s actual risk.”

Previous estimates put the risk at between 3 and 10% that an infant with an older sibling with autism would also develop the disorder, which affects the ability to interact socially and communicate. The new study shows a recorded jump up in risk by 8.7%.

Ozonoff adds:

“There is no previous study that identified a risk of recurrence that is this high.”

During the past decade, scientists have made significant breakthroughs in understanding the genetics of autism. Researchers are now focusing on specific chromosomal regions that may contain autism related genes. This has been accomplished by studying chromosomal abnormalities in individuals with autism, and by screening each chromosome for evidence of genes associated with autism.

Although suggestive evidence for the existence of autism-related genes has been found for several chromosomal regions, researchers are currently focusing on five different chromosomes: 2, 3, 7, 15 and X.

Written by Sy Kraft