During the testimony of Mayella Ewell in Chapter 18 of To Kill a Mockingbird, both Jem and Scout wondered if she was mentally challenged.
I whispered to Jem, "Has she got good sense?"
Jem responded that "--oh, I don't know." After watching Mayella "burst into tears" during Atticus's questioning, Judge Taylor was equally curious, asking her
"What's the matter with you?"
Scout finally came to the conclusion that Mayella was simply unused to being out in public or being around people. An outcast who rarely left her house, Scout saw a comparison with Boo Radley. After listening to Tom's testimony, when he explained that Mayella had gotten rid of the Ewell children and invited him into her house to perform a few chores,
... it came to me that Mayella Ewell must have been the loneliest person in the world. She was even lonelier than Boo Radley, who had not been out of the house in twenty-five years.