A provocative and surprising exploration of the longest sustained relationships we have in life—those we have with our siblings.

Nobody affects us as deeply as our brothers and sisters. Our siblings are our collaborators and co-conspirators, our role models and cautionary tales. They teach us how to resolve conflicts and how not to, how to conduct friendships and when to walk away. Our siblings are the only people we know who truly qualify as partners for life.

In this perceptive and groundbreaking book, Jeffrey Kluger explores the complex world of siblings in equal parts science, psychology, sociology, and memoir. Based on cutting-edge research, he examines birth order, twins, genetic encoding of behavioral traits, emotional disorders and their effects on sibling relationships, and much more. With his signature insight and humor, Kluger takes science’s provocative new ideas about the subject and transforms them into smart, accessible insights that will help everyone understand the importance of siblings in our lives.