The Seoul Twin Family Study seeks to investigate genetic and environmental influences on the development of cognitive abilities and other psychological traits among children and adolescents in Seoul, South Korea. In November, 2001 with the assistance of the Seoul Metropolitan Office of Education, we began to solicit all private and public schools in Seoul to obtain names, addresses, and telephone numbers of the twins enrolled in schools in Seoul. The vast majority of the recruited twins were between 6 and 18 years old. The recruited twins and their non-twin siblings and parents will undergo comprehensive assessment of cognitive abilities, interests, daily activities, personality, and familial and non-familial environments. We plan to reassess the twins and their families every two years as the twins progress through major changes that characterize childhood, adolescence, and adulthood. In addition, medical records of the twins during the prenatal period will be examined to determine the effects of intrauterine environment on individual differences in cognitive abilities and other psychological traits. We also plan to undertake molecular genetic studies to identify genes involved in specific cognitive abilities.