Many mothers are fearful that they were the victim of a hospital mistake that resulted in them taking home a baby who was not theirs. Although there are many hospital methods to stop this from happening, it is a rare occurrence.

In the 1970s, in Spain's Canary Islands, an only child was switched at the hospital with a baby girl who had an identical twin. This baby girl, Delia, was raised as an only child by parent who she was not biologically related to, meanwhile the switched baby, Beatriz, grew up believing she had a fraternal twin, Begona, and was raised by Begona's biological parents. It was not until the girls were 28 years old that Begona and Delia were confused by a store clerk, who was a friend of one of the girls, and they and their families learned of the mistake.

AAAS member Nancy Segal is a psychologist and director of the Twins Study Center at California State University at Fullerton. She herself has a twin, and she has spent much of her career studying twins and what they can teach us about genetic predispositions and help us answer the question of nature vs nurture.

In this podcast, Dr Segal reads from her recent book, Someone Else's Twin; the True Story of Babies Switched at Birth about what happened to the girls in the Canary Islands. This opens up a number of questions about the safety of babies and mother's in hospitals, adoption issues and questions about identity and twin relationships. For more information about the book, check out Prometheus Books' website and listen to NPR's interview with Dr. Segal.

Segal also published Entwinted Lives; Twins and What They Tell Us About Human Behavior, published in 2000, about twins and why they are such an important window into human nature. Twins are often excellent subjects for psychological studies because identical twins share 100percent of their DNA so differences between them often give clues to the role of environment, both before and after birth, in shaping human personalities, traits, intelligence and identity.

In 2005, Segal published Indivisible by Two: Lives of Extraordinary Twins. This book talks about some of the most amazing sets of twins. It mixes science with human stories to provide a unique look at what it means to be a twin.

Coming in 2012, Segal will publish Born Together- Reared Apart; the Landmark Minnesota Twin Study. This book will talk about the famous study at the University of Minnesota that provided an in-depth look at many twin pairs. It features a discussion about the similarities found between identical twins, even when they were raised in different places. But it also looks at the many differences twins have even when they are raised together. The study answered many questions about what aspects of our personalities are designed by genetics and what are determined by your surrounding environment, but it also raised many new questions. Also stayed tuned for our podcast with Dr. Segal about this book in the spring of 2012.