Answer: In chapter 15, Atticus is confronted by a lynch mob, but Scout's presence influences Mr. Cunningham and the Old Sarum bunch to disperse. The next day, Atticus discusses the previous night's events with his children and defines mob mentality to explain the men's hostile emotions. He begins his definition of a mob by stating that each one consists of individuals who behave like wild animals.
The effect of mob mentality influences the members to act against their natural disposition to behave civilly. They transform into hostile people, who lose their individuality, think with a collective mindset, and respond violently. Despite their uncivil, hostile demeanor, Scout was able to remind the Old Sarum bunch of their humanity and influence them to exercise sympathy for her father.
According to Atticus, a mob is made up of people, and in Maycomb's small town, everyone is familiar with its members. By referring to them as familiar people, Atticus is humanizing members of a mob, which helps him to empathize with Mr. Cunningham. Instead of castigating Mr. Cunningham for leading the lynch mob and threatening his life, Atticus exercises tolerance and does not judge him for his transgressions.
Atticus's definition of a mob and explanation of their mentality enhances his children's perspective on their community and teaches them tolerance. He humanizes members of the mob and encourages his children not to judge Mr. Cunningham for being their leader. His definition also sheds light on the psychological effects of a mob and explains why people act differently around their peers.