Environmental factors may be more important than genes in determining whether a child develops autism, according to a controversial new analysis of the disorder in twins.
That finding runs counter to decades of prior research, which has generally found that genetic inheritance is the biggest determinant of a child’s risk of autism. The authors of the new study, published online Monday, 4 July, by the journal Archives of General Psychiatry, came to their conclusion after studying 192 pairs of identical and fraternal twins in which at least one twin met clinical criteria for the neurodevelopment disorder.
But the authors’ conclusion that environmental influences — perhaps chemical exposures, infections, diet or stress levels — could be so influential was roundly criticised by other autism experts.
“I think they’re really on shaky ground to say that,” said Dr. Paul Law, director of the Interactive Autism Network at the Kennedy Krieger Institute in Baltimore.
“It’s a massive claim,” said Angelica Ronald, a behaviour geneticist at Birkbeck University of London. “It flies in the face of the previous data. I don’t see why the results have come out the way they have.”
The study authors acknowledged that their calculations were subject to a wide margin of error and thus could be incorrect. Still, they said that the analysis highlights the need for more research into environmental factors that may contribute to autism.
“Genetics don’t explain it,” said co-author Neil Risch, a genetic epidemiologist at UC San Francisco. “They’re part of the story, but only part of the story.”
Scientific thinking on the causes of autism has undergone enormous shifts over the decades. For many years it was blamed on detached, unemotional “refrigerator mothers.”
But starting in the 1970s, a series of studies looking at twins recast autism as a genetic disorder. They found that if one twin had autism, the odds that the other had it too — a figure known as the concordance rate — depended primarily on how much DNA they shared.
If the twins were identical, the odds were 80 per cent or higher. But if the twins were fraternal — and shared only half their DNA, on average — the odds were 10 per cent or lower.
Those studies led scientists to zero in on genetics as the main cause of the disorder, which has been one of the world’s fastest-growing diagnoses over the last two decades. Researchers and activists have collected DNA samples from thousands of families, resulting in the discovery of a few dozen genes that appear to be associated with the disorder.
Since the 1990s, the definition of autism has expanded to include milder cases. Using the new definition, two recent studies have found that when one member of a fraternal twin pair had some form of the disorder, the odds that the other twin had it too were 31 per cent.
In the latest study, researchers used records from the California Department of Developmental Services to identify children labelled with autism. They conducted their own exams on as many of those children as they could, ultimately building a database of 192 twin pairs in which at least one twin has the disorder.
“Genetics don’t explain it.” Neil Risch, report co-author