The Tennessee Twin Study (TTS)3 is representative of 6-17-year-old twins born in Tennessee and living in one of the state's five metropolitan statistical areas (MSAs) in 2000-2001. These MSAs include the 28 urban, suburban, and rural counties surrounding the cities of Nashville, Memphis, Knoxville, Chattanooga, and Bristol. The Tennessee Department of Health identified 7,794 birth records representing all twin pairs born in Tennessee in the eligible age range and used external databases to locate families (2,431 twin pairs were eliminated because they lived outside the five MSAs). A random sample was selected from the remaining families, stratified by age and 35 geographic subareas, proportional to the number of listed families in each subarea. Of 4,012 selected households, 3,592 (89.5%) were located and screened, with 2,646 of the screened families being eligible (neither twin was autistic or psychotic, both twins co-resided with the adult caretaker ≥50% time during the past 6 months, the twins and caretaker spoke English, and the twins were at least 6 years old at the time of the interview and no more than 17 years old at
