CATSS9w - The Child and Adolescent Twin Study in Sweden (9 year, web questionnaire)

The Child and Adolescent Twin Study in Sweden (CATSS) is an ongoing longitudinal twin study targeting all twins born in Sweden. Between 2004 and 2019, parents of 9-year-old twins were contacted and invited to participate in CATSS9. Data was collected through telephone interviews. In 2020, the data collection method transitioned to web-based questionnaires, which involved some revisions to the interview questions. To mark this transition and the revised questions, the survey is now referred to as CATSS9w. The focus remains on various physical and mental health issues as well as behaviors such as physical activity and social interaction. The twins in CATSS9w are born from 2011 onwards. Both identical (monozygotic) and fraternal (dizygotic) twins of the same or opposite sex within the twin pair are included. Currently, 8,500 twins (as of December 2025) are participating out of 17,400 invited, resulting in a response rate of 48%. In conjunction with the CATSS9w survey, parents are also asked to provide DNA samples via saliva from the twins. These sample are used for zygosity determination and genotyping. The number of genotyped twins who have participated in CATSS9w is 2,500 (as of December 2025). Parents are also asked to participate in a study on physical activity, where the twins wear an accelerometer for one week. The question areas included in the CATSS9w survey are listed below. Detailed information about the variables can be found at strdata.se. Administrative Information Weight and height Demography Physical activity Diseases Asthma, Eczema and Hayfever SDQ - Strength and Difficulties Questionnaire ADHD A-TAC - Autism - Tics, ADHD and other Comorbidities inventory (only Autism-questions) Language development LSC-R - Life Stressor Checklist- Revised Covid19