Why do you think the idea of the American frontier or "wild West" became such a popular part of the American identity?

Explanation:

The American frontier, also known as the Old West or the Wild West, includes the geography, history, folklore, and culture in the forward wave of American expansion that began with European colonial settlements in the early 17th century and ended with the admission of the last few western territories as states in 1912. This era of massive migration and settlement was particularly encouraged by President Thomas Jefferson following the Louisiana Purchase, giving rise to the expansionist attitude known as "Manifest Destiny" and the historians' "Frontier Thesis". The legends, historical events & folklore of the American frontier have embedded themselves into United States culture so much so that the Old West, and the Western genre of media specifically, has become one of the defining tenets and prides of American national identity.

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answer:

d. because dr. laguardia selected a widely used measure, his study generalizes to has strong external validity. external validity has limited external validity. other applications of this measure.

the widely used measure that he applied to his experiment was the benton facial recognition test. it employed on to football players and as result, only players who suffered brain concussions did not pas this test. and although it has an external validity, it is limited because it isn't general to apply to other population.