The comparative study of identical and fraternal twins is a powerful method for identifying the relative contributions of genes and environments to individual differences in behavioral, physical, and health-related characteristics. Studies of reared-apart twins provide even more compelling ways to approach this same class of questions, given that the co-twins were raised in different homes, communities and/or countries, so were unable to influence one another.

This talk first reviews the different types of twins and the logic behind twin designs before presenting findings from past, recent and current studies of twins who grew up separately. Here, the focus will be on the Minnesota Study of Twins Reared Apart and the Fullerton Study of Chinese Twins; however, research using reared together twins, virtual twins (same-age-unrelated individuals reared together who replicated twinship but without a genetic link) and unrelated look-alikes will also be described with reference to family relations, tacit coordination, personality similarity, and bereavement.

The vast body of evidence indicates that genetic influence is pervasive, affecting virtually every measured human trait, but environmental factors also play a role. The occasional abuse of twin studies must be recalled to prevent their future occurrence.

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Prof. Nancy L. Segal, PhD is Professor of Psychology and Director of the Twins Studies Center at California State University, Fullerton. She has authored over 250 articles and six books on twins; her seventh book, Deliberately Divided: Inside the Controversial Study of Twins and Triplets Adopted Apart was released in November 2021. Dr. Segal’s 2012 book, Born Together-Reared Apart: The Landmark Minnesota Twin Study, won the 2013 William James Book Award from the American Psychological Association. She was a speaker at a TedX event in Manhattan Beach, California (2017), and delivered invited addresses at the Human Behavior and Evolution Society in Amsterdam, the Netherlands (2018) and the International Society for the Study of Individual Differences, in Florence, Italy (2019). She was also an invited participant in a debate on parenting organized by Intelligence Squared in New York City (2019). Dr. Segal’s work has been featured in the New York Times and the Wall Street Journal. She has appeared on national and international television and radio programs, including The Today Show, 20/20, Long Lost Lives, AirTalk (NPR) and Hidden Brain (NPR). Dr Segal can be contacted via email at: [email protected], and you can learn more about her work on: drnancysegaltwins.org

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