Twin pairs were recruited from FinnTwin16-25 (FT16-25), a longitudinal study of 5 consecutive birth cohorts of Finnish twins. FT16-25 was initiated during 1991 when twins from the first cohort, born in 1975, were sequentially enrolled over 12 months time as twins reached age 16. After 60 months of baseline data-collection from all 5 cohorts was completed, pair-wise response rates exceeded 88%, yielding baseline data on 2,733 twin pairs. Subsequent follow-up assessments were made at age 17, 18½, and 25. From all participating twins in these questionnaire studies, 448 twin pairs were selected for laboratory studies conducted in Helsinki. One or both co-twins were never reached in 16 of the invited pairs; an additional 32 pairs were ineligible for participation, because 1 or both co-twins were living outside Finland (N = 13); because 1 or both co-twins was ill, in late stages of pregnancy, or handicapped (N = 18); or deceased (N = 1). Of the 436 eligible pairs successfully contacted and invited to participate, 300 (68.8%) did so. The first 177 of these normally hearing twin pairs (plus individual twins
