Four of the seven lion cubs (Panthera leo) recently born at the National Zoo in Washington, D.C. Lions boast a special form of sister and brotherly alliance: sisters stay together all their lives in prides, and brothers form coalitions to conquer other prides. Mehgan Murphy/Smithsonian Institution hide caption

itoggle caption Mehgan Murphy/Smithsonian Institution

A Chinese family coaxes their son to pose in a "little emperor" costume at a park in Beijing last year. China introduced its one-child policy for urban Chinese in 1976, resulting in a generation of sibling-less offspring. Wang Zhao/AFP/Getty Images hide caption

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Daphne Beal (right) and her sister Cecily, who is developmentally disabled and legally blind. Daphne says that when they're together, they revert to the things they've done since they were kids. Courtesy of Daphne Beal hide caption

itoggle caption Courtesy of Daphne Beal

Tressa Borick has six siblings: one full, three halfs, and two steps spanning age from 31 to 13. Courtesy of Tressa Borick hide caption

itoggle caption Courtesy of Tressa Borick

Monica Hanson (top left) is the eldest of four daughters and says she fell into the role of firstborn naturally. Her sisters (from left to right) are Elena Lynn, Maria Godoy (a senior editor at NPR.org) and Olga Czekalski. Also pictured (bottom left) is her daughter, Erica, and Erica's cousins Kelsey and Taylor Lynn. Courtesy of Monica Hanson hide caption

itoggle caption Courtesy of Monica Hanson