(RxWiki News) Researchers have searched for possible causes of autism in just about every area imaginable. But it appears that at least part of the cause may be within a child's genes.
A recent study found that about half the risk of a child having autism appeared linked to the number of relatives with autism that the child had.
That means that environmental influences may still play a part in autism risk, but genetics and heredity also played a major role.
This study looked at the risk of autism among 2 million Swedish children, plus all the children who made up their siblings, half-siblings and cousins.
"Ask your pediatrician about early signs of autism."
This study, led by Sven Sandin, MSc, of the Department of Medical Epidemiology and Biostatistics at Karolinska Institutet in Sweden, aimed to learn how much autism may be related to genetics.
The researchers investigated the occurrence of autism spectrum disorders among just over 2 million children in Sweden, born between 1982 and 2006.
Within this group, there were 37,570 pairs of twins. Then the researchers also looked at the brothers, sisters and cousins of all the children in the group to expand the overall number they studied.
The researchers ended up with 2.6 million pairs of siblings, just over 432,000 pairs of half-siblings on the mother's side, about 445,531 pairs of half-siblings on the father's side and about 5.8 million pairs of children matched to cousins.
Among the children in the original birth group (the 2 million), 14,516 children had an autism spectrum disorder, including 5,689 children with autistic disorder.
The researchers then calculated something called the "relative recurrence risk," or RRR, of autism in these children's families.
The RRR refers to a number derived by comparing the risk of autism in one of the study participants who has a relative with autism (sibling or cousin, in this study) to the risk of autism in a study participants with no siblings or cousins with autism.
In calculating these RRR numbers, the researchers took into account differences among the children in terms of sex, birth year, age and their parents' age and psychiatric history.
Among identical twins, the rate of autism among those who had relatives with autism was 6,274 children per 100,000, compared to 27 children with autism out of 100,000 without a relative with autism.
Among fraternal twins, the rate of autism among those who had relatives with autism was 805 children per 100,000, compared to 55 children with autism out of 100,000 without a relative with autism.
Among full siblings that were not twins, the rate of autism among those who had relatives with autism was 829 children per 100,000, compared to 49 children with autism out of 100,000 without a relative with autism.
Among half-siblings on their mothers' side, the rate of autism among those who had relatives with autism was 492 children per 100,000, compared to 94 children with autism out of 100,000 without a relative with autism.
Among half-siblings on their fathers' side, the rate of autism among those who had relatives with autism was 371 children per 100,000, compared to 85 children with autism out of 100,000 without a relative with autism.
And finally, among cousins, the rate of autism among those who had relatives with autism was 155 children per 100,000, compared to 49 children with autism out of 100,000 without a relative with autism.
When the researchers made these same sorts of calculations for only autistic disorder (instead of all autism spectrum disorders), they found similar numbers, though with even greater likelihood of autistic disorder among those who had relatives with autism.
The researchers concluded that among these Swedish-born children, risk of autism spectrum disorders in children increased significantly as the presence of other individuals with autism in that child's family increased.
"Heritability of autism spectrum disorders and autistic disorder were estimated to be approximately 50 percent," the authors wrote. "These findings may inform the counseling of families with affected children."
Yet the findings are not surprising since they are consistent with previous research, according to Glen Elliott, MD, PhD, a clinical professor at the Stanford University Department of Psychiatry and Behavioral Sciences.
“But they do help more precisely characterize the genetic contribution of autism and autism spectrum disorders, with numbers large enough to look a more distant contributions such as cousins as an added bonus," Dr. Elliott said.
"As the authors conclude, the data help families who have a child with autism spectrum disorders weigh the risks of considering having another child," he said. "The data also again emphasize the importance of genetic contributions to autism spectrum disorders as well as yet-to-be identified environmental effects.”
This study was published May 3 in JAMA. The authors reported no conflicts of interest.
The research was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development, the National Institute of Environmental Health Sciences, the National Institute of Neurological Disorders and Stroke and the Beatrice and Samuel A. Seaver Foundation.