Most subjects were interviewed by telephone. After a complete explanation of the research protocol, verbal or signed informed consent was obtained before all interviews. This project was approved by the Committee for the Conduct of Human Research at Virginia Commonwealth University. Interviewers had a Master’s degree in a mental health-related field or a Bachelor’s degree in this area and at least 2 years of clinical experience. Each member of a twin pair was always interviewed by a different interviewer. Zygosity was determined by a combination of self-report measures, photographs and DNA polymorphisms (Kendler et al. 2000).
