Frederick Turner, in one of the most influential theories of US history, argued that the ever moving "line" of the American frontier shaped the American character. He wrote that the frontier developed
coarseness and strength combined with acuteness and acquisitiveness... [a] practical inventive turn of mind, quick to find expedients; that masterful grasp of material things... restless, nervous energy; that dominant individualism...
First, the frontier was, by definition, not populated with an already established European population or civilization. It therefore took physical robustness and strength to "tame" it and cultivate it, leading Americans to develop those qualities. Second, it forced settlers to be practical to survive rather than focused on intellect, because survival was so difficult. Third, it developed individualism, because people were often alone, with wide spaces between homes, not crowded together as in cites. People therefore learned they could do what they wanted as individuals and grew to cherish that possibility. Fourth, it developed the American spirit of resourcefulness, as pioneers were thrown back on their own devices to find a way to survive, and fifth, it lead to the development of democracy, because people were equals on the frontier: family pedigree or background was far less important than having the right skills and character to survive and prosper in a new, hostile setting.
The US Census declared the American frontier "closed" in 1890, and Turner gave his speech outlining his frontier thesis in 1893 at the Chicago World's Fair, wondering where Americans could now find an outlet for their energies. It has been noted, too, that the thesis does not represent the view of native peoples and assumed the superiority of people of Western European descent.