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Autism is mostly caused by genetic factors, with heritability of the condition 33 per cent higher than previously thought, new research suggests.
Reanalysis of data from a previous study on the familial risk of Autism Spectrum Disorder (ASD) estimates the heritability to be 83 per cent, according to a study published by JAMA.
Prior studies have found that autism aggregates in families, but heritability was previously estimated to be around 50 per cent.
To define presence or absence of autism, the previous study used a data set created to take into account time-to-event effects in the data, which may have reduced the heritability estimates.
Using the same underlying data from this study, Sven Sandin of the Icahn School of Medicine at Mount Sinai, New York, and colleagues used an alternative method to calculate the heritability.
The study included a group of children born in Sweden 1982 through 2006, with follow-up for autism through December 2009.
The analysis included 37,570 twin pairs, 2,642,064 full sibling pairs, and 432,281 maternal and 445,531 paternal half-sibling pairs.
Of these, 14,516 children were diagnosed with autism, and the heritability was estimated as 83 per cent, while the non-shared environmental influence was estimated as 17 per cent.
Authors of the study said: “This estimate is slightly lower than the approximately 90 per cent estimate reported in earlier twin studies and higher than the 38 per cent estimate reported in a California twin study, but was estimated with higher precision.
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“Like earlier twin studies, shared environmental factors contributed minimally to the risk of ASD.”
According to the website Autism Speaks , environmental factors which could be linked to autism include prenatal exposure to toxins, such as the chemicals thalidomide and valproic acid.
Other risk factors appear to include influences such as parental age at conception, maternal nutrition, infection during pregnancy and prematurity.
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