In this program, Dr. Mercy will discuss studies performed with identical and fraternal twins as participants. If you have been a participant in a twin study perhaps through one of the large registries that exist through the United States and the world, we would love to hear your experiences. Twin studies can be cross-sectional or longitudinal; they often focus on the interrelationship among genetics, shared environment, and non-shared environment in order to determine the relative effects on personality, aptitudes, life experiences, and health. Some studies have been done with MRI technology to measure the size of fetal brains. Dr. Mercy will cover these resources and will also discuss the recent findings that identical twin DNA is not necessarily identical; she welcomes callers to discuss the implications of such results for society and the twins themselves. She will also report on various twin registries across the world, their purposes, and some results of the longitudinal studies that these registries are used to conduct.