Researchers analysed about 5,205 whole genomes from families affected by autism.
"It's noteworthy that we're still finding new autism genes, let alone 18 of them, after a decade of intense focus," said Mathew Pletcher, vice president at Autism Speaks, an advocacy organisation in the US.
"With each new gene discovery, we're able to explain more cases of autism, each with its own set of behavioural effects and many with associated medical concerns," he added.
Researchers found at least two of the autism-associated gene changes were associated with increased risk for seizures.
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The findings from the Autism Speaks MSSNG project - the world's largest autism genome sequencing programme - illustrate how whole genome sequencing for autism can provide additional medical guidance to individuals, families and their physicians, researchers said.
They also determined that many of the 18 newly identified autism genes affect the operation of a small subset of biological pathways in the brain.
All of these pathways affect how brain cells develop and communicate with each other.
"In all, 80 per cent of the 61 gene variations discovered through MSSNG affect biochemical pathways that have clear potential as targets for future medicines," Pletcher said.
"We believe that this, too, is an important part of the neurodiversity story," said Stephen Scherer.
The study was published the journal Nature Neuroscience.
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