The Twin study of CHild and Adolescent Development (TCHAD) is a longitudinal study on the health and behavior of 1480 Swedish twin pairs born between May 1985 and December 1986 (53). This study used self-report questionnaire data regarding behavior in romantic relationships from the Relationship Quality Survey (RQS) and parent-report questionnaire data regarding child behavior from the Child Behavior Checklist (CBCL) from two different times. In 1994, 1106 (75%) parents of twins 8–9 years old replied to the questionnaire and in the 2005 follow-up assessment the 19–20 year old twins’ self-reports had a response rate of 59% (n=1705). Zygosity was determined using a discriminant analysis that was previously developed using information from 385 twin pairs with known zygosity (54). DNA was extracted from mouthwash samples that were collected using DNA self-collection kit. In total, 1240 (528 boys and 712 girls) provided usable DNA samples. Of these 142 boys and 323 girls were in a romantic relationship at the age of 19–20.
