A gang is a group or society of associates, friends or members of a family with a defined leadership and internal organization that identifies with or claims control over territory in a community and engages, either individually or collectively, in illegal, and possibly violent, behavior. Gangs arose in America by the middle of the nineteenth century and were a concern for city leaders from the time they appeared. Some members of criminal gangs are "jumped in" (by going through a process of initiation), or have to prove their loyalty and right to belong by committing certain acts, usually theft or violence. A member of a gang may be called a "gangster".