Science and Film logo

Photo of Nancy L. Segal

Nancy L. Segal

Dr. Nancy L. Segal is Professor of Psychology at California State University, Fullerton (CSUF) and Director of the Twin Studies Center which she founded in 1991. Her research focuses on the behavioral and physical development of twins, the nature of twins’ social relationships, twins separated at birth, the behavioral consequences of twin loss, and the personality similarity of unrelated look-alikes.

Dr. Segal has written several books about twins, including Twin Mythconceptions: False Beliefs, Fables, and Facts About Twins (San Diego: Elsevier, 2017) and Accidental Brothers: The Story of Twins Exchanged at Birth and the Power of Nature and Nurture (NY: St. Martin’s Press, 2018). She is also is the author of the award-winning Born Together-Reared Apart: The Landmark Minnesota Twin Study (Cambridge: Harvard University Press, 2012), as well as Someone Else’s Twin: The True Story of Babies Switched at Birth (Amherst, NY: Prometheus Books, 2011), Indivisible by Two: Lives of Extraordinary Twins (Cambridge: Harvard University Press, 2005), and Entwined Lives: Twins and What They Tell Us About Human Behavior (NY: Dutton, 1999, NY: Plume 2000). Dr. Segal is also the senior editor of the volume Uniting Psychology and Biology: Integrative Perspectives on Human Development (1997).

Dr. Segal is an Associate Editor of Twin Research and Human Genetics, the official journal of the International Society for Twin Studies. She has authored over 200 scientific articles and book chapters. Her awards include the James Shields Award for Lifetime Contributions to Twin Research from the Behavior Genetics Association and International Society for Twin Studies, the “International Making a Difference” Award from Multiple Births Canada, the Mensa Foundation's Excellence in Research, and California State University’s campus-wide Wang Family Excellence Award. Dr. Segal will appear in the upcoming Ken Burns documentary, The Gene.

Dr. Segal received a B.A. degree in psychology (with distinction) and English literature from Boston University (1973), and M.A. (1974) and Ph.D. (1982) degrees in Social Sciences and Behavioral Sciences from the University of Chicago. Her website is