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 respondents (1981–1990) (Colorado Department of Public Health and Environment, State birth rates from 1979 to 2004). The sample was predominantly made up of complete twin pairs (818 pairs; 97 single twins) and an equivalent distribution of participants of each gender across all age categories. A comparison of subjects without data available at Wave 2 and those being examined in this study revealed similar rates of substance use and disorders in Wave 1.
