According to Atticus in chapter 20, what is the thing that Mayella has done wrong in To Kill a Mockingbird?
In Chapter 21, we hear of the decision of the jury. As everyone is waiting, there is a feeling of uncertainty. The children, particularly Jem, believe that the verdict will be innocent, but the adults know that Tom Robinson will most likely be found guilty.
Reverend Sykes tells the optimistic Jem these words:
“Now don’t you be so confident, Mr. Jem, I ain’t ever seen any jury decide in favor of a colored man over a white man...”
In light of this point, the wrong that Mayella committed against Tom Robinson is to blame him for a crime he certainly did not commit. She was covering up her shame, because she knew that she could get away with it, even if all evidence was against her, simply because she was a white woman and Tom Robinson was a black man. She knew the people, the town, and the outcome. So she worked the system to her favor.
In Chapter 20 Atticus gives his rationale more clearly.
“The state has not produced one iota of medical evidence to the effect that the crime Tom Robinson is charged with ever took place. It has relied instead upon the testimony of two witnesses whose evidence has not only been called into serious question on cross-examination, but has been flatly contradicted by the defendant. The defendant is not guilty, but somebody in this courtroom is.
“I have nothing but pity in my heart for the chief witness for the state, but my pity does not extend so far as to her putting a man’s life at stake, which she has done in an effort to get rid of her own guilt.
“I say guilt, gentlemen, because it was guilt that motivated her. She has committed no crime, she has merely broken a rigid and time-honored code of our society, a code so severe that whoever breaks it is hounded from our midst as unfit to live with.
Chapter 20 of To Kill a Mockingbird is where Atticus gives his closing arguments in the rape case against Tom Robinson. Since there is no clear evidence to prove that Tom Robinson raped Mayella Ewell, Atticus does his best to discredit his white accusers. The whole case is ironic because the true victim is on trial for a crime he didn't commit; so, Atticus sheds light on the person who is the true offender in the case--Mayella Ewell. Atticus specifically says that Mayella didn't do anything illegal, per se; but she did do something socially wrong according to the current mindframe of the South in 1935. Atticus explains as follows:
"What was the evidence of her offense? Tom Robinson, a human being. She must put Tom Robinson away from her. Tom Robinson was her daily reminder of what she did. What did she do? She tempted a Negro" (203).
No socially respectable white woman of the time period would even consider tempting, or throwing herself at, a black man. Mayella is low-class, lonely, and not respectable. That must be why she thought she could get away with setting Tom up for her to kiss him. When her father caught her, they had to blame it on the black man in order to save Mayella's reputation, no matter how low it was.
Her crime, as Atticus says in Chapter 20, is that she "was white, and she tempted a Negro."