The Radleys were very religious and private people. Miss Maudie called them foot-washing Baptists, which meant they were very strict. She said,
[That] is a sad house. I remember Arthur Radley when he was a boy. He always spoke nicely to me, no matter what folks said he did. Spoke as nicely as he knew how (Chapter 6).
As a teenager, Boo ran with a wild crowd ("some of the Cunninghams from Old Sarum"). When he and his friends were arrested, the other boys were sent to industrial school, ironically “the best secondary education to be had in the state.” Boo’s parents prevented him from going, but Boo was later arrested for stabbing his father in the leg with a pair of scissors.
Boo's parents then had to decide what to do with him.
Boo wasn’t crazy, he was high-strung at times. It was all right to shut him up, Mr. Radley conceded, but insisted that Boo not be charged with anything: he was not a criminal. The sheriff hadn’t the heart to put him in jail alongside Negroes, so Boo was locked in the courthouse basement (Chapter 1).
The Radleys brought Boo home, but kept him inside. There were rumors that the Radleys used some “form of intimidation” to keep Boo locked in the house. Jem’s theory that he was chained to the bed was likely an exaggeration. Arthur Radley became the legend of Boo Radley. Adults created stories about him peeking in windows, and children told stories about him eating animals.
Dill, Jem, and Scout encourage Arthur Radley to come out of his house. Dill says that if Boo just had someone to talk to, he might feel better. Slowly, Boo starts to come out of his shell. He leaves gifts for the children, and we learn his brother Nathan seems to have taken on the role of making sure Boo has no contact with the world. He cements the tree hole Boo used to leave gifts for the children.