The Boo Radley story provides a parallel plot to the Tom Robinson story. Both are examples of prejudice. In the Robinson story, the white population of Maycomb automatically assumes that a white person must be believed over a black person. Therefore, when Mayella Ewell accuses Tom Robinson of rape, the white citizens in town don't feel they need a trial; they know who is in the wrong and what should be done. They resent Atticus for mounting an honest defense of Robinson, especially when he shows that Tom could not possibly have raped Mayella as described. They would rather condone the lie that Tom raped Mayella than have their prejudices challenged.
Likewise, Maycomb residents discriminate against Boo for his reclusive life in a rundown home. They don't need to actually have met him or know anything concrete about him: the rumor mill supplies the information that he is the root of all evil. If something goes wrong in the town, it must be the fault of Boo Radley.
Scout buys into theses...
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