We have enrolled 5,600 twins and their families ascertained from nation-wide birth cohorts born 1983–1987. In the first phase the twins, their parents and teachers filled out questionnaires when the twins were 11–12 years old, with two follow-ups at ages 14 & 17½ years (Kaprio, Pulkkinen & Rose, 2002, Kaprio 2006). Parents and twins completed questionnaire assessments of their use of alcohol, smoking, lifestyle and health status; parents and teachers assessed aspects of the twins’ behavior. We have achieved response rates of 85 – 90 % across all aspects of the study. Parents also reported on pre-pregnancy (such as infertility treatments) and pregnancy events (such as elevated blood pressure), use of alcohol and tobacco during pregnancy and early childhood conditions and child development. From the five consecutive birth cohorts a subset of 1,035 families of twins was formed for the intensive study. All intensive subset parents were interviewed (N=1,860) with a semi-structured psychiatric assessment interview, and at age 14, their twins have been interviewed (N=1,852) with the adolescent version of the same instrument to assess substance use, dependence and psychiatric
