Principal investigator Dr. John N. Constantino of Washington University School of Medicine in St. Louis said half siblings share about 25 percent of their genes with their half-brothers and -sisters.
"We found that autism risk for half siblings is about half of what it is for full siblings," Constantino said in a statement. "Most of the half-siblings we studied had the same mothers. Given that half of the risk of transmission was lost and half was preserved among those maternal half siblings, mothers and fathers appear to be transmitting risk equally in families in which autism recurs."
The study, published online in the journal Molecular Psychiatry, involved 5,000 families with one child who had autism and at least one additional sibling. Among those families, 619 included at least one maternal half-sibling.
The researchers focused on maternal half-siblings rather than paternal half siblings because these children were more likely to live full-time with their biological mothers and to share the same environmental influences between the time they were born and the age of 2, the time at which the onset of autistic syndromes occur. The researchers compared autism recurrence among the 619 maternal half siblings to the rate among 4,832 full siblings.
After analyzing both sets of families, the researchers found 10 percent to 11 percent of full siblings had been diagnosed with autism, compared to 5 percent to 7 percent of half siblings, the study said.