"The reality is, there were real issues with folate and iron deficiency 40 years ago, and that's not when we had autism," Insel said. "This paper adds one more edge piece to the very large jigsaw puzzle."
Autism is a complex set of disorders with overlapping genetic and environmental -- both social and biological -- contributors, making it difficult for researchers to understand its underlying causes and develop appropriate treatments.
"I think we've made tremendous progress over the last couple years in areas of genetics and what's happening in the brains of children with autism," Insel said. "Now we need to focus on environmental factors."
This study supports the idea that whatever environmental factors are leading to the increase in autism, they're probably active during pregnancy, Insel said. Research linking autism and early childhood vaccination published by Andrew Wakefield and colleagues a decade ago was deemed fraudulent in a Jan. 5 editorial published in the British Medical Journal.
But until researchers uncover specific risk factors, parents who are expecting or planning to get pregnant shouldn't worry about autism risk, Insel said.
"This is still an odd and rare occurrence," Insel said.
Although 1,188 children conceived within 12 months after a sibling were diagnosed with autism, 154,846 children were not.
"Even with a closely spaced pregnancy, 99 times out of 100 you're not going to have a child with autism," Insel said.