Writer. Mother. Wife. New Yorker. Abigail Pogrebin is many things, but the one that has defined her most profoundly is "identical twin." Pogrebin's relationship with her sister---both as children, when they were inseparable, and today, when she longs for that uncomplicated intimacy---inspired her to examine the phenomenon of twinship, to learn how other identical pairs regard their doubleness and what experts are learning about how DNA impacts our sense of identity and shapes our lives.