This is an excellent question and one that critics have debated since the release of To Kill a Mockingbird in 1960. There is no direct evidence--in either Scout's narration or in the testimony given in court--that suggests definitively that Bob Ewell had sexual relations with his daughter. Atticus first hints at Bob's physical improprieties when he asks Mayella if her father has ever beaten her--"when he's been drinking... When he's... riled...?"
"My paw's never touched a hair o'my head in my life," she declared firmly. "He never touched me."
Of course, Atticus soon reveals Tom's crippled left arm--one that could not have possibly caused the bruises on Mayella's neck and face. So Atticus asks again:
"Who beat you up? Tom Robinson or your father?"
This time there is no answer. Later, Tom testifies that Mayella had told him that
"... she never kissed a grown man before an' she might as well kiss a nigger. She says what her papa do to her don't count."
The wording here is the strongest suggestion of possible incest, since Mayella does not say that her "papa's kisses don't count"--"do to her" being a suggestion of some physicality. When Bob suddenly peered through the window, he called his daughter a "whore" and threatened to kill her--hardly the words of someone who has never "touched a hair o'my head." In his final summation, Atticus asks the jury the question,
"What did her father do? We don't know..."
The suggestion of incest is subtle at best, and the reader has to read between the lines to even consider the possibility. Other hints within the story that might suggest such a relationship between Bob and Mayella include:
- Atticus' view that the "Ewells had been the disgrace of Maycomb for three generations." This is one of the few harsh admonitions Atticus has for anyone in the novel, and one must consider what led Atticus to making such a statement--and if Mayella is a part of the "three generations."
- Bob's wife is dead, and Mayella is 19--the only adult female in the home.
- Bob stays drunk most of the time, and he must have sexual urges, telling Link Deas when confronted about harassing Helen Robinson that he "ain't about to go with no nigger!"