What was a common belief among historians in the early 20th century that makes their sources biased?

That entire groups of people can easily be classified by stereotypical traits.

Explanation:

This was a common belief among historians in the early 20th century. This behaviour was particularly noticeable when historians dealt with cultures outside of the Western canon or people who were not Caucasian. Because many historians believed this broad characterizations to be accurate, they often did not take the time to get to know individuals more deeply, which means that their findings were biased due to their prejudice.

if you could provide the poem below, i'd be happy to you!

explanation:

you didn't give us the poem, or the title of the poem. but it is essential for you to give us that, otherwise we can not detect when the poem has a tonal shift!