By Randy Dotinga
TUESDAY, Sept. 26, 2017 (HealthDay News) -- Heredity contributes to about 83 percent of the risk of autism in children with the disorder, a new study suggests.
The estimate, from a re-analysis of a previous study, adds a new wrinkle to the ongoing debate over how much autism is inherited from parents. Essentially, the findings suggest that rare genetic traits combine in parents and explain about eight in 10 cases of the neurodevelopmental disorder in children.
However, study author Sven Sandin cautioned that "our results do not give any information about specific genes or other direct causes. It only informs us that genes are important."
Sandin, an assistant professor of psychiatry at the Icahn School of Medicine at Mount Sinai in New York City, noted that the findings also don't reflect anything about the reported increases in autism rates in recent years. The higher rates must have something to do with increased awareness or environmental factors, "and our study cannot shed any light on this," he said.
Previous research had estimated the heritability of autism as anywhere from more than 50 percent to as much as 90 percent, said Dr. Dan Geschwind, a geneticist who's familiar with the findings.
"We already know that autism has very substantial genetic contributions," said Geschwind, chair in human genetics at the University of California, Los Angeles School of Medicine. "The question is how much is genetic and how much is environmental?"
For the new study, researchers re-analyzed statistics from a previous study that tracked children born in Sweden between 1982 and 2006. The children were followed through 2009 to see if they developed autism spectrum disorders. The goal was to determine how common the disorders are in various types of siblings (such as twins), which would indicate the importance of genetics.
In total, the study looked at 37,570 pairs of twins, 2.6 million pairs of siblings, and nearly 888,000 pairs of half-siblings. Of all these, just over 14,500 children were diagnosed with autism spectrum disorders. The new study did not report the races of the children.
While the researchers estimated that inherited factors contribute to 83 percent of the risk, "even in couples who already have a child with autism, the likelihood that their next child will also develop autism is increased, but still not very high," Sandin said.
Still, Sandin noted, the heritability of autism appears to be higher than some psychiatric conditions. For example, "the heritability of schizophrenia has been estimated to be 80 percent, and for attention-deficit/hyperactivity disorder it has been estimated at 76 percent," he said.
"For cancer, it is very different for different types and for when they occur in life. For skin melanoma and prostate cancer, respectively, the heritability was recently estimated to 57 percent and 58 percent," Sandin said.
Geschwind noted that the study is large, which supports the validity of the findings. "At some level, it is important to show that it's heritable," he said. "But this finding won't really change the kind of work that most geneticists do."
In the United States, an estimated one in 68 school-aged children has an autism spectrum disorder, according to an estimate from the U.S. Centers for Disease Control and Prevention. Symptoms include difficulty communicating and interacting with others, and a tendency toward repetitive behaviors and obsessions.
The new study was published Sept. 26 in the Journal of the American Medical Association.