"Our study of discordant twins -- twin pairs in which only one twin was affected by autism spectrum disorder -- found birth weight to be a very strong predictor of autism spectrum disorder," lead author Molly Losh of Northwestern University said in a statement.
Prior studies showed when one identical twin had autism spectrum disorder, the other twin was much more likely to have autism spectrum disorder as well.
"Because identical twins share virtually 100 percent of their genes, this is strong evidence for the role of genetics in autism," said Losh. "Yet it is not 100 percent the case that autism spectrum disorder affects both identical twins in a twin pair."
When only one twin is affected by autism spectrum disorder in some identical twin pairs, this suggests environmental factors may play a role, either independently or in interaction with autism risk genes, Losh said.
"And because autism is a developmental disorder impacting brain development early on, it suggests that prenatal and perinatal environmental factors may be of particular importance," she said.
The findings are published in the journal Psychological Medicine.