Broadly defined, Arizona, California, Colorado, Nevada, New Mexico, Oklahoma, Texas, and Utah are all part of the Southwestern United States. Alternatively, Arizona, western New Mexico, western Texas, southern Colorado, and southern Utah comprise the core of the Southwestern United States under narrower definitions. Because different governmental departments of the United States have designated different states to be part of the Southwestern region, no official, uniform definition exists.
Historically, as the United States expanded its territory westward, the definition of "Southwest" moved with that expansion. This history of westward expansion changing the boundaries of the literal southwest portion of American territory has contributed to the present lack of a clear, universally-accepted definition of the Southwest. For example, prior to Texan independence and subsequent statehood, the Southwest was understood to include Arkansas, Louisiana and Mississippi.
Polls have found that, among residents of states that make up the most broadly defined Southwest, a majority consider themselves to be Westerners, with the exception of Texans and Oklahomans, who are more likely to consider themselves Southerners. This difference in self-identification suggests that Texas and Oklahoma may be considered culturally distinct from the rest of the Southwest states. Some geographers have opted to categorize Texas and Oklahoma as its own sub-region, containing many features of the Southwest but also incorporating much of the cultural heritage of the South.