The researchers analyzed 69 studies that collectively include 6,679 children with an autistic sibling and 21,263 controls; 30 of the studies are unpublished dissertations, theses, abstracts and research posters. Including unpublished work helps to combat publication bias — the fact that journals favor findings that support an association over those that show none, Shivers says. Most of the studies are based on either self-report or parent questionnaires.