Families with a history of schizophrenia or bipolar disorder are also more likely to have a child with autism, new research from Sweden and Israel suggests.
Researchers found that kids whose parents or siblings had been diagnosed with schizophrenia were almost three times more likely to have an autism spectrum disorder, including autism and Asperger syndrome.
The link was weaker for bipolar disorder, but still consistent, according to findings published this week in the Archives of General Psychiatry.
"Most people with a family history of one of these disorders actually get nothing - the vast majority in fact," said Dr. Patrick Sullivan, the study's lead author from the University of North Carolina at Chapel Hill.
Still, he told Reuters Health, "Maybe there is something that is more fundamental but common to both," such as certain changes in gene patterns that are passed from parents to children.
The number of autism diagnoses in the United States has been rising, and the Centers for Disease Control and Prevention now estimates one in 88 kids has an autism spectrum disorder. That's up from one in 150 a decade ago.
One of the questions in recent autism research has been how much genetics is involved in who gets the condition, versus the prenatal or early childhood environment.
Sullivan and his colleagues note that autism used to be considered the childhood version of schizophrenia, before researchers learned more about the two disorders. And some research has continued to suggest certain genetic changes are common in both conditions, as well as in bipolar disorder.
So for the new study, the researchers analyzed information from three separate databases, including two that tracked kids and families in Sweden and their medical diagnoses and a third that recorded data on all Israeli citizens entering the army draft, including sibling pairs.
All in all, Sullivan's team had data on more than 30,000 young people with autism.
The researchers found that in the Swedish studies, kids whose parent or sibling had been diagnosed with schizophrenia, according to family medical records, were 2.6 to 2.9 times more likely to have autism. The trend was the same for siblings of teens with schizophrenia in Israel.
When a parent had bipolar disorder, on the other hand, kids were 1.6 to 1.9 times more likely to have autism or another autism spectrum disorder.
"It's been questioned for many years, whether there's a relationship" between schizophrenia and autism, said Dr. Andrew Zimmerman, a neurologist from the Lurie Center for Autism at MassGeneral Hospital for Children in Lexington, Massachusetts.
"In fact, I think those of us that see a lot of patients with autism have seen this relationship in relatives for a long time," he added.
Sullivan said there are certain rare genetic mutations that seem to predispose people to both autism and schizophrenia.
And there could be other gene variations, he said, where differences in a person's environment in the womb or growing up could influence whether the same mutation ends up leading to schizophrenia, autism or nothing.
"Just because two things share a risk factor doesn't mean that they're the same thing," Sullivan added. "The needs of people with autism and schizophrenia, and the treatments that we know work for one or the other, don't overlap very well."
Zimmerman, who wasn't involved in the current research, told Reuters Health there are still many questions about how the conditions may be related.
The new study "is important because it makes us think," he said. "It doesn't tell us whether (autism and schizophrenia) are genetic or environmental - certainly both are still possible - but it makes us think there might be relationships between the two."