CTR is a population-based registry that holds records from twins born in Colorado in the United States of America, or who were living in Colorado at the time, as ascertained through school records. The study is lead by the Institute for Behavioral Genetics (IBG), University of Colorado Boulder. Recruitment of families began in 1984 with mailings from the Colorado Department of Health (CDH) to the parents of twins born in 1982. For twins born between 1982 and 1998, families that expressed interest in twin research were enrolled in the CTR. A similar procedure was used to contact and enroll twins born between 1968 and 1981 as young adults.

Recruitment is ongoing