Jem loses his innocence after witnessing racial injustice for the first time during the Tom Robinson trial. Throughout the trial, Jem believes that Tom will be found innocent. Jem is naive to think that a prejudiced jury would rule in favor of an African American man in the racist community of Maycomb. At the end of Chapter 21, Judge Taylor reads the guilty verdict, which shocks and upsets Jem. Jem loses his childhood innocence, and Scout mentions that each "guilty" seems to stab Jem between his shoulders. After the guilty verdict is read, Jem begins to cry and repeatedly says, "It ain't right" (Lee, 131). As the novel progresses, Jem expresses his disgust at Maycomb's prejudiced judicial system. Before hearing the final verdict, Jem naively believed that the Maycomb jury would rule in favor of Tom Robinson. After Tom Robinson is pronounced guilty, Jem loses his childhood innocence and becomes jaded towards his prejudiced neighbors.
Two events in the first part of the novel stand out as defining moments of an awakening within Jem. When Jem returns to the Radley fence to retrieve his lost pants following the children's raid on their neighbor's back porch, he finds them waiting for him--folded and crudely mended. He may not have understood the implications at that moment, but he must have realized that only Boo Radley could have done this. But it is the act by Boo's brother that completely robs Jem of his innocence. When Jem finds the secret knothole of the Radley oak sealed up, he questions Nathan about it. Boo's brother assures Jem that the oak is sick.
"Tree's dying. You plug 'em with cement when they're sick. You ought to know that, Jem." (Chapter 7)
But when Jem asks Atticus about it, his father points out that
"... the leaves, they're all green and full, no brown patches anywhere--
"That tree's as healthy as you are, Jem." (Chapter 7)
Jem not only discovers that adults lie when it is necessary, but that Nathan's reason for sealing the knothole--the place where Boo's gifts are found and the children's only way of communicating with him--is strictly out of meanness.