In many cases, Jem's growth has as much to do with his approaching puberty as the events that unfold around him. Jem is growing taller, hair is beginning to appear in unusual places, and his moodiness bewilders Scout. Jem is showing signs that he is outgrowing Scout as a companion, and he starts by distancing himself from her at school. By the end of the novel, Jem is in high school while Scout is still an elementary student. Jem and Scout are together on the night of the Halloween pageant only because Atticus and Aunt Alexandra are unable to go; Jem serves as her adult escort rather than as her playmate as he has in the past.
Both Jem and Scout mature faster than most children their age. They both discover that adults are capable of dishonesty (Nathan Radley lies to Jem), gossip (Miss Stephanie), secretly drinking (Miss Rachel), child neglect (Dill's parents), hate (Bob Ewell), racism (the jury), mental disease (Boo) and hypocisy (the missionary circle, Miss Gates). Bearing witness to the trial of Tom Robinson gives them insight into an adult world that few children their age would ever see, and the deaths of Tom and Mrs. Dubose affect them personally. Scout does not always understand everything that she sees and hears, but Jem has grown enough to recognize the seriousness of many of these actions.
As a novel of maturation, a bildungsroman, To Kill a Mockingbird traces the physical, emotional, and psychological growth of the two children Scout and Jem Finch. This growth is the result of both nature and experience.
- On Scout's first day of school, she attacks Walter Cunningham in the school yard during their lunch break because she has been punished on his account. Jem pulls her off Walter, and in a gesture of reconciliation, he invites Walter to their home for "dinner." Obviously, Jem has outgrown the impulsive aggression of a younger age.
- Later, however, Jem does act impulsively when he tears the blooms from Mrs. Dubose's camellia bushes. But, after her death, he learns that she was not herself because she was on morphine. Jem matures as he realizes that Mrs. Dubose was not in control of her thoughts, but she bravely took control before dying as she withdrew from the morphine.
- When Scout and Jem find the soap figures that Arthur Radley has skillfully carved from a bar of soap, he stores them with other well-loved memorabilia, having gained a respect for Boo.
- During the trial of Tom Robinson, Jem believes in the ideal of justice being blind, but despite the excellent cross-examinations by Atticus that prove the falsity of the Ewells' charges against Tom, the biased jury convicts the innocent man. After this disillusionment, Jem becomes rather cynical about people in Maycomb, disclaiming his earlier judgment of Maycomb. "I always thought Maycomb folks were the best folks in the world," he says as he tells Miss Maudie that it was a "heathen jury" who convicted Tom. Nevertheless, he matures in the sense that he becomes more realistic about life.
- Jem retains his inner goodness as he accompanies Scout to her school's Halloween program. When Scout is attacked, Jem bravely tries to defend his sister.
- Although Jem is disappointed in human nature, his father believes that his son will "be himself again."
- Scout possesses the naïveté of a child, so her ingenuousness gets her into trouble on the first day of school when she tries to be helpful and inform Miss Caroline about the backgrounds of her students. After getting her hand hit by a ruler and having to stand in the corner, Scout is confused about how attempts at good behavior result in punishment.
- She retaliates against Walter Cunningham after she gets into trouble for trying to explain his economic position. But Jem teaches her to be kind, as does Calpurnia, who spanks her for making fun of Walter's eating habits. Scout learns her lesson and matures.
- From Miss Maudie, Scout learns about the sad history of Boo Radley; consequently, she begins to "climb into his skin" as Atticus has encouraged Scout to do in order to understand people. As she accompanies Jem when he reads to Mrs. Dubose, she acquires knowledge of the elderly lady just as Jem does and changes her opinions.
- In her naïveté, Scout rushes to Mr. Cunningham and speaks to him when the mob appears at the jailhouse. Her actions deflate the hostile atmosphere, and she saves Atticus.
- At the trial of Tom Robinson, Scout as narrator comments perceptively about Mayella--she whispers to Jem, "Has she got good sense?"--then further in the trial, she comments that she begins to see the pattern of Atticus's questions. As a lawyer's child, Scout is perceptive beyond her years.
- Like Jem, Scout is disillusioned by the results of the trial. However, she has learned from Dill's distress at the rude manner of Tom's interrogation; when Dill commented on how the prosecution called Tom "boy," Scout originally replied to Dill, "after all he's just a Negro." Scout changes her perception of the black people of Maycomb; she realizes that they are people with intrinsic rights.
- At the Missionary Tea, Scout recognizes pretentiousness in Mrs. Merriweather, and she appreciates how Miss Maudie subtly points out her hypocrisy. After this incident, Scout finally sides with Aunt Alexandra and they step into the parlor together.
- After the attack on her and Jem by Bob Ewell, Scout finally meets Boo, who has saved their lives. She walks Boo home later on and stands on his porch, realizing that appearances are often deceptive. She has gained much wisdom from her experiences.