How do Scout and Jem lose their innocence in the novel "To Kill a Mockingbird"?
The most important theme of Harper Lee's novel is the exploration of human nature - both the good and bad. Through the eyes of Scout, the reader is able to see the world of Maycomb through the eyes of an innocent child. At the beginning of the novel, both Jem and Scout see the world from this childhood perspective because neither have seen evil, so they believe that people are essentially good. The miscarriage of justice in the case of Tom Robinson’s trial and the subsequent racial backlash towards the Finch family are some of the first moments in which Jem and Scout are introduced to a very different and cruel Maycomb. The ultimate act of evil that Jem and Scout face is their brutal attack at the hands of Bob Ewell. For these two children, realizing that an adult could hurt a child is most surprising. It is interesting to note that while Jem becomes ultimately disillusioned with the state of affairs in Maycomb, Scout retains her basic faith in the good of people, telling Jem one evening that “No…I think there’s just one kind of folks. Folks” (Chapter 23).
TKAM is a "coming of age" story, meaning that in its pages, the main character reaches a point of understanding or maturity. In this case, Scout and Jem come to an understanding of society's darker side.
Because they become exposed to the goings-on surrounding Tom Robinson's trial and the racially-motivated conflicts that ensue as a result, both characters are forced to learn very adult lessons.
What's more, when the children are attacked by Bob Ewell, Scout loses the perception that all adults are kind and competent toward children. The idea that an adult would attack a child shatters some of her naivete as she learns this lesson.
In the end, Scout and Jem have both made many "real-world" realizations, and this is what causes their loss of childlike innocence in the novel.