The theme of a literary work is defined by its central or dominating ideas. The ideas that seem to control the narrative of To Kill a Mockingbird stem from misjudgments made by main characters. For example, the errors of judgment made by Jem and Scout serve to develop the dominant themes of maturation and knowledge.

Throughout the narrative of Harper Lee's novel, the young Finches incorrectly assume that people possess traits that are unlikable or distasteful. However, after they come to know these people better, the children's perception changes. Among those for whom this situation is true is Scout's first-grade teacher, Miss Caroline Fisher, who Scout assumes will appreciate her assistance. However, she does not understand that the teacher finds the informative Scout to be a threat to her control of the class. Scout and Jem misjudge Mrs. Dubose, assuming the older woman is merely mean and spiteful; they do not realize that she is addicted to morphine. They also make judgment errors about Aunt Alexandra, not realizing until the story's conclusion that Alexandra truly loves her brother. Then, too, Scout and Jem misjudge Boo Radley by believing at first that he is a malevolent spirit. 

In the end, after Arthur Radley saves the children from the malicious and vengeful Bob Ewell, Scout meets Arthur personally and later walks him home. Afterwards, when her father puts Scout to bed, she tells her father about a character from the book she has read, The Gray Ghost, remarking that the character was “real nice” when “they finally saw him.” Atticus, who thinks of the good, gentle, yet misunderstood recluse who has saved the lives of his children, replies, "Most people are, Scout, when you finally see them." This is a response that underscores the themes of maturation and knowledge.

The novel also tackles themes of prejudice and racism. Though the story ends on an uplifting note, readers can't forget that Tom Robinson is dead because of prejudice and misunderstanding. Lee emphasizes the redemptive power of empathy even as she points to the dire consequences of undefeated institutionalized prejudice.

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The main theme of To Kill a Mockingbird is innocence. We see this theme repeated over and over again. To start with, the title of the book is explained in the text. It is explained to Scout that no one should shoot a mockingbird because they are innocent and only want to please others. We see Scout relating this concept to Tom Robinson's trial and Atticus's defense of his client. We also see Scout start to realize this concept also applies to Boo Radley. Her own innocence puts a distance between her ideas and those of the towns people. It is because Scout is innocent herself that we are able to see the innocence of others through her eyes.