The MTFS, which began in 1990, includes a representative population-based sample of 1382 pairs of like-sex twins and their parents. Participating MTFS families were originally ascertained from Minnesota state birth records for the years between 1972 and 1984. Families were eligible to participate if they lived within a day's drive of the University of Minnesota and if the twins had no physical or psychological disability that would interfere with their completing an assessment. Using publicly available data bases (e.g., phone books, Internet directories), we located addresses for over 90% of those twins still alive, and invited them to participate in a study of the maturation and health of twins.
