New research from Washington University School of Medicine in St. Louis shows that the genetic reach of the disorder often extends to half siblings as well. On the surface, the finding may not be surprising — half siblings share about 25 percent of their genes. But the discovery is giving scientists new clues to how autism is inherited. According to principal investigator John N. Constantino, MD, the new research adds to recent evidence that even though autism is far more common in males, females still can inherit and pass along genetic risk for autism. The study found that autism risk for half siblings is about half of what it is for full siblings. Most of the half siblings 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.