After the trial, Jem is very angry. He feels that Tom has been sentenced to death unfairly, and it really bothers him. He discusses his anger with Atticus. Atticus tells him about the jury system.
"Those are twelve reasonable men in everyday life, Tom's jury, but you saw something come between them and reason." (pg 220)
He reminds Jem of the night the men challenged them at the jail. They came as a mob, not as individuals. They arrived because they had the support of others. No individual would stand up on his own. The peer pressure kept them there. Most of them would not have been a part of it if they hadn't had peer pressure.
"When that crew went away, they didnt' go as reasonable men, they went because we were there." (pg 220)
Then Atticus tries to explain to Jem that there are social conditions, such as prejudice between the white man and black man, that make it difficult for a man to be fair.
"There's something in our world that makes men lose their heads --- they couldn't be fair if they tried." (pg 220)
He then tells Jem that if it is a black man on trial, and it is a white man's word against a black man, the white man will win every time. It isn't right, but that is the way things are. Jem will see more of it as he gets older. Jem argues that that isn't right and you can't convict a man just because he is black. Atticus agrees with him, but he also states that it happens and did happen to Tom Robinson. He tells Jem.
"....people have a way of carrying their resentments right into a jury box." (pg 220)
Then Atticus tells the children about other problems with choosing a jury. He gives the example of Mr. Link Deas. If he had to make a decision between Miss Maudie and Miss Rachel, he might lose their business, depending on how he voted. He would then ask to be excused from the jury and use the excuse that he needed to attend to his store, therefore avoiding the issue. Even though the vote is supposed to be secret, everyone knows how people voted.
"Serving on a jury forces a man to make up his mind and declare himself about something. Men don't like to do that. Sometimes it's unpleasant." (pg 222)