Parents of children diagnosed with autism were roughly twice as likely to have been hospitalized for a psychiatric disorder such as schizophrenia, depression, or neurotic disorders as a parent of a child without autism.
The researchers note an association between children with autism and mothers and fathers with schizophrenia. For other psychiatric disorders the increase in risk was seen only in children born to mothers with psychiatric disorders.
It has long been suspected that both genetic and environmental factors play a role in array of developmental syndromes known as autism spectrum disorder.
The new findings suggest that for some children with autism, genes plays a bigger role than for others, says University of North Carolina assistant professor of epidemiology Julie L. Daniels, PhD, MPH.
The study is published in the May issue of the journal Pediatrics.
"This study identifies a potential subtype of autism linked to a family predisposition for psychiatric disorders," Daniels tells WebMD.
Schizophrenia and Autism
Using comprehensive birth and hospitalization registries from Sweden, Daniels and colleagues searched for parents of children with and without autism who had been hospitalized for mental-health-related illnesses.
The study involved 1,237 Swedish children with autism born between 1977 and 2003 and 30,925 children without autism matched for age, sex, and place of birth. Roughly three out of four children in the study were boys, and half of those with autism were between the ages of 4 and 6 at diagnosis.
Overall, a parent with an autistic child was 70% more likely to have been hospitalized for a psychiatric disorder than a parent with a non-autistic child.
The timing of the child's diagnosis relative to the parent's hospitalization did not have a major influence on the findings.
Although the association was statistically significant, Daniels says the risk that the child of a parent with mental illness will develop autism is still quite small.
"We can't say anything about mild disorders that don't require hospitalization, because we didn't study those," she says.
Autism Research: The Road Ahead
The study is not the first to suggest a link between autism and having a parent with a mental illness, but it is one of the largest.
"Identifying families with a propensity for rare psychiatric conditions may help uncover rare genes that contribute to the susceptibility of both disorders," Daniels and colleagues write.
But even if these genes are identified, it is likely that they will help explain only a small fraction of autism cases, says William W. Eaton, PhD, of the Johns Hopkins Bloomberg School of Public Health.
In a study published in 2005, Eaton and colleagues also showed a strong link between parents' mental illness and autism in their offspring, with a more than threefold increase in autism risk seen among children born to a parent with a diagnosis of schizophrenia.
"What we know about both of these disorders is that there will be no easy answers," Eaton tells WebMD. "If there were one single gene or even half a dozen involved we would have found them by now."
A better understanding of the link between mental illness in parents and autism in children would be a step forward, he says. But there will still be plenty of steps to take. "It is a grain of sand in what will eventually be the mountain that explains the causes schizophrenia and autism."