The initial MTFS sample includes two cohorts: one initially assessed at age 11 (the younger cohort) and one initially assessed at age 17 (the older cohort). Intake and follow-up assessments are scheduled to coincide with major transitions in the lives of adolescents and young adults. Assessments were also completed by 99% of biological mothers, 91% of living biological fathers, and over 85% of step parents, thus providing comprehensive information on the biological and rearing backgrounds of the twins. Analysis of participation and non-participation at study intake and follow-up has indicated little evidence of participation bias (Iacono et al., 1999; Iacono & McGue, 2002a; Iacono et al., 2006).
