In another sign pointing to an inherited component to autism, a study released on Monday found that having a schizophrenic parent or a mother with psychiatric problems roughly doubled a child's risk of being autistic.
"Our research shows that mothers and fathers diagnosed with schizophrenia were about twice as likely to have a child diagnosed with autism," said Julie Daniels of the University of North Carolina, Chapel Hill, who worked on the study.
"We also saw higher rates of depression and personality disorders among mothers, but not fathers," she said in a statement.
The study of families in Sweden with children born between 1977 and 2003 involved 1,227 children diagnosed with autism. They were compared with families of nearly 31,000 children who did not have autism. Sweden's detailed health registry provides a wealth of data for such studies.
Autism, which is marked by impaired social interaction and communication, or a related disorder like Asperger's syndrome, affects an estimated one out of every 150 U.S. children, the U.S. Centers for Disease Control and Prevention estimates. Asperger's is marked by mild social awkwardness.
No one knows what causes autism, but researchers think it is likely that several genes and possibly environmental factors contribute. Some autism advocates believe childhood vaccinations play a role, although most medical experts say it is extremely unlikely.
Which genes lie behind various mental illnesses are also poorly understood, according to the researchers, whose study appeared in the journal Pediatrics, published by the American Academy of Pediatrics.
"Earlier studies have shown a higher rate of psychiatric disorders in families of autistic children than in the general population," Daniels said.
The association between a child's autism and mental illness in the parent was strongest with schizophrenia, and was less powerful when the mother suffered from depression or personality disorders. There was little association between autism and parental addiction to alcohol or drugs or some other types of mental illness
It was not clear if it was significant that having a mother, but not a father, with certain mental illnesses, raised the risk of autism.
"Establishing an association between autism and other psychiatric disorders might enable future investigators to better focus on genetic and environmental factors that might be shared among these disorders," Daniels said.