Proof There's Nothing Quite Like A Sibling Bond

Much research on this type of bond has focused on twins. But what about the rest of us? Approximately four out of five Americans grow up with a brother or sister, after all. Most siblings only share approximately 50 percent of the same genes, but the lifelong connection they feel often has nothing to do with biology. The connection is hard to describe in a single word. It's the comfort felt when you sit in the same room with your brother and sister, in pure silence, yet you both know how the other is feeling. It's picking up right where you left off, even if it's been weeks, months or years since the last deep conversation. It's the knowledge that, at the end of the day, you'll always be able to call on that person for support. But what is at the root of these incredible sibling bonds?
 Here are seven possibilities that have nothing to do with genetics.