Risk of developing autism is mostly caused by genetics

Genetic factors may explain most of the risk of developing autism spectrum disorder (ASD), suggests a new analysis.

It calculated that ASD has 83 percent heritability - this is a measure of the extent to which a particular trait is inherited within a population.

The paper, published in JAMA, is a re-analysis of a previous study by the same researchers in twins and siblings that concluded that ASD was 50 percent heritable. Many studies have attempted to quantify the contributions of genetics and environment to the risk of ASD, and estimates vary widely.

'We already know that autism has very substantial genetic contributions,' Dr Daniel Geschwind, a geneticist at the University of California, Los Angeles School of Medicine, told HealthDay News. 'The question is how much is genetic and how much is environmental?'

The authors of the new analysis put the findings into context. 'This estimate [83 percent] is slightly lower than the approximately 90 percent estimate reported in earlier twin studies and higher than the 38 percent estimate reported in a California twin study, but was estimated with higher precision,' they wrote. 'Like earlier twin studies, shared environmental factors contributed minimally to the risk of ASD.'

Dr Sven Sandin at the Icahn School of Medicine at Mount Sinai, New York, and colleagues used an alternate method to re-analyse their data from a large study of children born in Sweden between 1986 and 2006, which followed them until December 2009. This included approximately 37,600 sets of twins, 2,642,000 full sibling pairs, and about 877,800 half-sibling pairs. Of these, about 14,500 children were diagnosed with ASD.

Although the two analyses produced hugely different results for heritability, the authors of the latest analysis note: 'In both analyses, the heritability of ASD was high and the risk of ASD increased with increasing genetic relatedness.'

But Dr Sandin added: 'Even in couples who already have a child with autism, the likelihood that their next child will also develop autism is increased, but still not very high.'

The large size of the study supports the validity of the findings, said Dr Geschwind.