Introduction
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A year after my first book was published, my editors called me in to talk about ideas for my next project. I remember they asked me what I found myself thinking most about, which subject had always preoccupied me. I blurted out, “Twins.”

   And then I immediately regretted it. Because writing about twins felt like I was volunteering to do a public striptease. Because being a twin goes to the core of who I am and I was wary of examining that. Because I knew that my twin sister, Robin, would be both supportive and hesitant; not only is she more private than I am but she writes for the New York Times and always wants to maintain a reporter’s remove. Because I knew that as exhilarating as our twinship was growing up, its impact on Robin’s sense of self was more complicated than mine. Because I knew that for me to be honest about my twinship and ask others to be honest about theirs was not to tell the perfect quaint story of how we all dressed alike, tricked people, or swapped boyfriends.

   Being an identical twin—I can’t speak for fraternals—is intense. It’s all the clichés: feeling like you have an unwavering partner in life, knowing exactly what another person feels, wanting to tell her a story before anyone else, confiding with unrestrained—sometimes shocking—candor, valuing her opinion above anyone else’s, taking on someone else’s pain to the point of vicarious depression, being incapacitated by any minor dispute.

   To say that someone’s “always been there for me” was never meant so literally. Robin was there while I was taki...