New research could finally put an end to the much contested debate as to how much of autism is genetic.

Autism is an incredibly complex condition, and although it is fairly certain that both genes and environment play a vital role, the conclusion as to how much of each has been the subject of differing opinions.

In a recent study published in JAMA, researchers have revealed that they have developed the most accurate figure to date for the role that genes play in autism. Led by Sven Sandin, an assistant professor of psychiatry at the Ichahn School of Medicine at Mount Sinai, the scientists re-analysed existing data from all children born in Sweden between 1982 and 2006. The team had looked at the same data previously, focusing on pairs of siblings, both of whom were diagnosed with autism. But this time, they applied a different method for tracking the diagnosis.

 Rather than looking at just one time point when both members of a sibling pair had been diagnosed, they incorporated the fact that not all siblings would be diagnosed at the same time. For example, they may start as being undiagnosed, then one would get diagnosed and, later, another might be determined to have autism.

Furthermore, Sandin also studied the different rates of autism among siblings with different shared genetic background, from half siblings who shared only one biological parent to biological siblings to both identical twins and fraternal twins. Sandin believes that not taking the change in autism status over time into account might have led to a skewed calculation of what role genetics plays in the disease. Due to it only looking at sibling pairs after both had been diagnosed, there is a chance that it may have over-estimated the influence of genes.

After tracking autism diagnoses over time among the sibling pairs, he found that genetics likely accounts for around 83% of the disorder. That compares to nearly90% reported in previous studies of twins only. Using the model, environmental factors probably contribute to approximately around 17% to the risk of developing autism.

Sandin explained, “This is why it is important to have different study designs. We have a family-based approach, and there are other approaches for twin studies and genetics studies. All of them seem to be converging on the same number of 80% to 90%.”

It is worth noting that the findings are based on the risk in a population and underscores the fact that the large majority of that risk for the condition is genetic. Therefore, the results should not be applied to individual sibling pairs to calculate the risk of developing autism.