The biggest study to date on the cause of autism spectrum disorders reveals the risk of autism jumps when a family already has one child suffering from ASD and states heritability as a 50 percent factor and environmental conditions as the second 50 percent factor.

"Among children born in Sweden, the individual risk of ASD and autistic disorder increased with increasing genetic relatedness. These findings may inform the counseling of families with affected children," notes the study, "The Familial Risk of Autism."

The King's College London research studied data on over two million children born in Sweden between the years of 1982 and 2006, of which 14,516 are diagnosed with diagnosis of autism spectrum disorder. The study focused on twins, siblings, half siblings and cousins.

Results indicate that heritage and environmental factors are the two prime causes of austism.

"Heritability is a population measure, so whilst it does not tell us much about risk at an individual level, it does tell us where to look for causes. We were surprised by our findings as we did not expect the importance of environmental factors in autism to be so strong," said Professor Avi Reichenberg, one of the study authors.

"Recent research efforts have tended to focus on genes, but it's now clear that we need much more research to focus on identifying what these environmental factors are. In the same way that there are multiple genetic factors to consider, there will likely be many different environmental factors contributing to the development of autism" he said.

The latest study is just one of several that have been released on autism in the past month. As Tech Times has reported one study focused on antidepressant use during pregnancy as a possible cause; another study focused on the age of parents and children diagnosed with autism. Yet another targeted air pollution as a factor.

The King's College study also shows children with a brother or sister with autism are 10 times more likely to develop autism; three times if they have a half-brother or sister; and twice as likely if they have a cousin with autism.

"The new study is the largest and most comprehensive to date and estimates heritability of autism to be 50%, with the other 50% explained by non-heritable or environmental factors," states a release on the study.

"Our study was prompted by a very basic question which parents often ask: 'if I have a child with autism, what is the risk my next child will too?' Our study shows that at an individual level, the risk of autism increases according to how close you are genetically to other relatives with autism. We can now provide accurate information about autism risk which can comfort and guide parents and clinicians in their decisions," states Sven Dr Sandin, one of the study authors.

The study was funded by the Swedish Research Council, the US National Institutes of Health and the Beatrice and Samuel A. Seaver Foundation, New York.

An editorial, in the same JAMA issue, focuses on the on genetic and environmental contributions to autism and notes that much more research needs to be done on familial risk in autism.