The GENDAAR study aims to investigate the transition through adolescence and into young adulthood, specifically identifying sex differences in individuals with autism spectrum disorders (ASD). The cohort includes three groups: individuals with ASD, siblings of children with ASD, and children with no family history of ASD. Participants were recruited from four sites across the United States of America (Yale University, Harvard University/Boston Children’s Hospital, University of California, Los Angeles, and University of Washington/Seattle Children’s Research Institute). From 2012, over 200 participants aged 5 to 17 years completed baseline data collection and were followed up a few years later when they were aged 10 to 26 years.