There is some evidence to suggest that the original group known as a "mafia" was an unofficial organization of residents on the Italian island of Sicily who grouped together to form paramilitary groups in order to defend their small island home from invaders. This militaristic origin may be responsible for the formal organizational structure, which relies on a top-down leadership structure and strict adherence to the word of superiors. However, it may be difficult to know exactly what the origins of the Sicilian mafia are, due to the mafia's code of omerta, which demands that members keep mafia activities a secret from outsiders, particularly those in positions of legal authority.
The Sicilian mafia may have started engaging in criminal activity in the 19th century, when a political power vacuum in Sicily made it possible for independent paramilitary organizations to demand money from landowners in exchange for protection. Sicilian mafias operate as cohesive units much like a clan or family, and different mafia families may cooperate with or at least respect other mafia families' activities and territorial claims. Some of these Sicilians eventually immigrated to America, bringing their organized crime culture and lifestyle with them. However, though the two nations' mafias have cultural similarities, they operate as separate entities.