The CATSS is an ongoing longitudinal study targeting all twins born in Sweden since July 1, 1992.35 Our sample consisted of participants (born January 1, 1993, to December 31, 2001) who provided information on OCD, anxiety, depression, and substance misuse symptoms at 18 years of age and who endorsed using alcohol or drugs (n = 9230). Participants with missing values were excluded (n = 3473). A subset of the sample had follow-up data at 24 years of age. Those who endorsed using alcohol or drugs were included (n = 3414). Participants who had not used alcohol or drugs at 18 years of age but reported use at 24 years of age (n = 426) were also included in the age 24 sample.
