Perhaps because most societies place so much emphasis on the parent-child relationship, there is a relative lack of information on the relationships among brothers and sisters. Yet sibling relationships can play essential roles in our development as individuals and often bring just as much, if not more, joy, rage, pain, frustration, and pleasure. Relationships with brothers and sisters are typically also the longest that we will experience in our lives, since most siblings outlive parents and they begin earlier than those we establish with friends. The dynamics among brothers and sisters are so complex and so diverse that they almost defy any attempt at scientific observation. Some scientists have made efforts to characterize sibling behavior and psychology and codify it into well-defined themes, but in general, these relationships remain understudied and, therefore, somewhat mysterious. The fact that sibling relationships vary widely according to children's status within...