The interval between the two waves of assessment was approximately 5 years (mean = 5.06; standard deviation (S.D.) = 0.65). DuringWave 1 the twin sample had an age range of 11.5–18.49 years (mean age = 14.73 years; S.D. = 2.14). At the Wave 2 follow-up, the sample had an age range of 16.50–25.49 years (mean age = 19.79 years; S.D. = 2.35). We define adolescence as the age range 12–18 and young adulthood as ages 17–25 (with the overlapping ages 17 and 18 representing transitional years into young adulthood). With the exception of age category 23+, in our analyses and presentation of results the age of each respondent was rounded to the nearest year and placed into separate age categories (12–22); the age 23+ category was composed of respondents between the ages of 22.5 and 25.49 (mean age = 23.47, S.D. = 0.63). Table 1 provides the racial composition and age distribution by gender at each wave. The racial distribution of the twin sample is comparable to that of the state of Colorado between the years of birth of the
