New research suggests a majority of autism risk is genetic.

Autism spectrum disorder (ASD) tends to cluster in families, so a genetic link has been suspected. Now, researchers reanalysing a longitudinal study suggest genetics could explain 83 per cent of the likelihood for the disorder.

In a previous landmark study, ASD heritability was estimated to be 50 per cent.

To define presence or absence of ASD, the 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, Ph.D., of the Icahn School of Medicine at Mount Sinai, New York, and colleagues used an alternate method (used by other studies in the same field) to calculate the heritability of ASD.

The study used a dataset of children born in Sweden from 1982 to 2006, with follow-up for ASD in 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 ASD.

Various models were tested and using the best-fitting model, the ASD heritability was estimated as 83 per cent and the nonshared environmental influence was estimated as 17 per cent.

“This estimate [83 per cent] 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. Like earlier twin studies, shared environmental factors contributed minimally to the risk of ASD,” the authors write.

The researchers note that twin and family methods for calculating heritability require several, often untestable assumptions.

Because ASD is rare, estimates of heritability rely on few families with more than one affected child, and, coupled with the time trends in ASD prevalence, the heritability estimates are sensitive to the choice of methods.

“The method initially chosen in the previous study led to a lower estimate of heritability of ASD. The current estimate, using traditional methods for defining ASD discordance and concordance, more accurately captures the role of the genetic factors in ASD. However, in both analyses, the heritability of ASD was high and the risk of ASD increased with increasing genetic relatedness,” the researchers write.

The study is accessible here.