When Jem asks Nathan Radley why he filled the knothole with cement, Nathan lies to Jem by telling him that the tree is sick. Jem then asks Atticus about the tree's health and discovers that Nathan lied to him. Jem loses his innocence by recognizing that adults can be dishonest and untrustworthy. He also realizes that his opportunity of interacting with Boo Radley is shattered. Later that evening, Jem remains on the porch and Scout notices that he has been crying. Scout says,

He [Jem] stood there until nightfall, and I waited for him. When we went in the house I saw he had been crying; his face was dirty in the right places, but I thought it odd that I had not heard him (65).

It’s like bein' a caterpillar in a cocoon, that’s what it is...Like somethin’ asleep wrapped up in a warm place. I always thought Maycomb folks were the best folks in the world, least that’s what they seemed like (Lee, 219).

I think I’ll be a clown when I get grown...Yes sir, a clown...There ain’t one thing in this world I can do about folks except laugh, so I’m gonna join the circus and laugh my head off (220).

As a bildungsroman, a novel of maturation, To Kill a Mockingbird is replete with instances in which the Finch children lose their innocence and learn the hard realities of life. 

1. Scout loses some of her idealism. When Scout discovers that her first-grade teacher is new to the area, she tries to help her by informing her of the socioeconomic backgrounds of certain students. Having been given voice in her home, Scout does not realize that Miss Caroline perceives her as being insolent. Scout is then shocked when the teacher menacingly grabs her by the collar, saying, "You're starting off on the wrong foot in every way. . . Hold out your hand." Scout is hit several times with Miss Caroline's ruler.

Scout is also baffled by Miss Caroline's reaction to her proficiency in reading aloud. Instead of being praised, Scout hears Miss Caroline derogate Atticus for teaching her: "Your father does not know how to teach" (Ch.2).

2. Scout learns the truth about Boo Radley and Mr. Radley's and Nathan's cruelty toward him. One day when Scout asks Miss Maudie about Boo, the kind woman explains that Mr. Radley is a "foot-washing Baptist" who believes that "anything that's pleasure is a sin." Further, she tells Scout that Boo lives in "a sad house." Later, Atticus scolds Jem and Scout about going near the Radley house, ordering them to stop "tormenting that man." The children have not considered that what they have been doing is "tormenting" Boo and "making fun" of him, but Atticus explains to his children the reality of what they are doing to Boo.  

They also learn of Boo's brother, Nathan. He is the one who cruelly seals the tree's knothole with cement, the only avenue of communication Boo has with the children. This cruel act prevents Boo from leaving small gifts for the children (Ch.5).

3. Scout is disillusioned about her Aunt Alexandra. While Scout is at Finch's Landing for Christmas dinner, her cousin Francis taunts Scout with disparaging remarks that his grandmother (Alexandra) has said about Atticus and his family.

"Grandma says it's bad enough he let you all run wild, but now he's turned out a n****r-lover we'll never be able to walk the streets of Maycomb agin. He's ruinin' the family, that's what he's doin'." (Ch.9)

4. Scout and Jem learn about drug addiction. After Jem's vengeful act of damaging Mrs. Dubose's camellias, he is ordered by Atticus to read to her each day. It is not until after she dies that Jem learns of Mrs. Dubose's addiction to morphine. It has been because of this addiction that the old woman's behavior has been so erratic and the reason she has said things that no lady would say. After her death, the Finch children learn that Mrs. Dubose was not in control of herself because of her drug addiction. They also are told that she demonstrated great courage and willpower by withdrawing from the drug before she died (Ch.11).

5. Scout and Jem witness herd/mob mentality. In the evening before the trial, Jem, Dill, and Scout find Atticus under a light bulb, propped in a chair that is in front of the jailhouse door.  The children become alarmed when the Old Sarum bunch drive up and stop in front of the jail. One man tells Atticus, "You know what we want. . . . Get aside the door, Mr. Finch." Other men speak from the shadows, and Scout realizes that she is amid a ring of people that she "has never seen before." There is a smell of whiskey about them, and they all seem the same with only one intention. These men are affected by what psychologists call "deindividuation," and because of this loss of individuality, they are less inhibited and restrained, a condition that can lead to the men's reacting with violent acts that they would not undertake on their own.

However, because Atticus understands the psychology of the mob, he individualizes Walter Cunningham and speaks to him directly, addressing him by his first name. Also, when Scout sees Mr. Cunningham, she innocently talks to him. Because both Atticus and Scout call his name and individualize him, Mr. Cunningham becomes uncomfortable and embarrassed as his conscience awakens. Consequently, Mr. Cunningham cannot perpetrate violence against the lawyer who has graciously accepted vegetables as payment for his legal services. Now embarrassed, Mr. Cunningham gets into his car and drives away.

6. Scout and Jem learn of the evil of racial prejudice.  To excuse their behavior, Bob and Mayella Ewell give false testimony against the innocent Tom Robinson, and they seem unconcerned that he can receive the death penalty. Because of the place and the times in which they live (the Jim Crow South), an innocent man is found guilty of rape and sentenced to death (Ch. 21).

7. Scout witnesses hypocrisy. In her third grade class, Scout's teacher, Miss Gates, tells the class of the atrocities of the Nazis and their prejudice against the Jews. That same day, Scout overhears Miss Gates speaking to Miss Stephanie as they come out of the courthouse in the evening:

"It's time somebody taught 'em a lesson, they were gettin' way above themselves, and the next thing they think they can do is marry us." (Ch.26)

Furious upon hearing this, Jem tells Scout he does not want to ever hear about the courthouse again. It is apparent that Jem has been deeply affected by the injustice of Tom Robinson's trial.

8. Scout and Jem learn firsthand about the evils of hatred and vindictiveness.

As the Finch children walk home from the Halloween pageant, they hear footsteps behind them. When they nearly reach the road, the pursuer runs after them. Jem urges Scout to run, but she falls, hampered by her clumsy ham costume.  After she falls, Scout hears the metal mesh being crushed. Then, Scout hears crunching sounds and a terrible scream from Jem. When she runs to help Jem, she is grabbed and squeezed. Soon, however, the assailant is forcibly pulled away from her by someone else. When she recovers, Scout searches for Jem, but he is gone; only an unshaven man reeking of whiskey lies where Jem has been. As she starts for home, Scout sees under the streetlight a man carrying Jem.

Once home, Scout learns that Jem's arm has been broken and they were attacked by Bob Ewell, who now lies dead. Boo Radley saved their lives from the vindictive man as he tried to kill the children of Atticus Finch, the attorney who proved him a liar (Chapters 28-30).

LOSS OF INNOCENCE IN TO KILL A MOCKINGBIRD

1. Scout's First Day at School (Chapter 2).  Scout has "never looked forward more to anything in my life," but her first day is a disaster. Her teacher, Miss Caroline, treats her badly because she reads so well, and she claims that "your father does not know how to teach."
2. The Cementing of the Knothole (Chapter 7).  When Jem discovers the secret knothole filled with cement, he asks Nathan Radley about it. Boo's brother lies to him, telling Jem the tree was diseased. But when Atticus explains the tree is fine, Jem realizes that Nathan deliberately closed the hiding place to prevent Boo from communicating with the children.
3. The Blanket (Chapter 8).  When Scout discovers that it was Boo that put the blanket on her shoulders on the night of Miss Maudie's house fire, she and Jem find out once and for all that Boo is their friend, not the neighborhood monster.
4. Ol' One Shot (Chapter 10).  The children believe Atticus feeble and untalented, but they discover a hidden talent they could never have imagined: Atticus was once the best marksman in Maycomb County.
5. Mrs. Dubose (Chapter 11).  Jem gets a lesson in real courage and how some people are not always what they seem when he spends a month reading to the cranky Mrs. Dubose. When she provides Jem with a present--a prize camellia--after she dies, the confused Jem calls her an "old hell-devil."
6. Dill (Chapter 19).  Dill is forced to leave the courtroom in tears after the prosecutor's poor treatment of Tom Robinson. It is his first time in a courtroom, and he has never witnessed a lawyer cross-examining a witness before. "Hasn't anybody got any business talkin' like that--it just makes me sick."
7. Jem and the Jury (Chapter 22).  The jury verdict leaves Jem in tears, and he wonders if juries shouldn't be abolished. He also has second thoughts about the caliber of people who live in Maycomb.
8. Scout and Boo (Chapters 28-31).  Scout's fantasy comes to life when Boo Radley saves her life as well as Jem's from the attack by Bob Ewell. She sits with Boo and then walks him home; later, she relates Boo to a story Atticus has been reading her, telling her father that

"... he was real nice."
"Most people are, Scout, when you finally see them."