The Child and Adolescent Twin Study in Sweden (CATSS) is a nation-wide cohort that focuses on all Swedish twins turning 9 or 12 years since July 2004. CATSS has an 80% response rate, making it a highly representative population sample. Data were available on 12 446 children: N= 5944 for 9-year olds and 6496 for 12-year olds (the two samples were independent). The Autism—Tics, AD/HD, and other Comorbidities inventory (A-TAC) (55) was used for phenotypic assessment in the CATSS sample. This study used genetic material from the first DNA collection from CATSS, including information from 887 girls. DNA was extracted from saliva samples that were collected using OraGeneR DNA self-collection kit. Zygosity was determined primarily by genotyping.
