Journalist Abigail Pogrebin is many things—wife, mother, New Yorker—but the one that has defined her most profoundly is “identical twin.” As children, she and her sister, Robin, were inseparable. But when Robin began to pull away as an adult, Abigail was left to wonder not only why, but also about the very nature of twinship. What does it mean to have a mirror image? How can you be unique when somebody shares your DNA?

In One and the Same, Abigail sets off on a quest to understand how genetics shape us, crisscrossing the country to explore the varied relationships between twins, which range from passionate to bitterly resentful. She speaks to the experts and tries to answer the question parents ask most—is it better to encourage their separateness or closeness? And she paints a riveting portrait of twin life, yielding fascinating truths about how we become who we are.