Healthy pediatric MZ and DZ twins ages 4–24 years are recruited nationwide as part of a larger longitudinal study of normal brain development (Giedd et al., 1999). Siblings of twin pairs are included in the study as they are able to add to the power to detect shared environmental or dominance effects (Posthuma & Boomsma, 2000). Following a telephone screen, families are brought to NIH where participants receive physical examination and interview, participate in a neurocognitive testing, provide blood to confirm zygosity and for further genetic analysis, and undergo an MRI. Participants return for follow-up cognitive testing and MRI at approximately 2-year intervals.
