Why is Boo Radley kept hidden in his house for 15 years?

That’s a really good question because Harper Lee never tells us the exact circumstances surrounding Boo’s reclusive existence.  Is it forced or a choice that Boo makes?  We do know that it starts when Boo is arrested for some fairly harmless teenager behavior and sent to the state school for juveniles.  Since then, he has been hiding away in the Radley house.  The rumors run wild in Maycomb about his behavior and character, and we...

That’s a really good question because Harper Lee never tells us the exact circumstances surrounding Boo’s reclusive existence.  Is it forced or a choice that Boo makes?  We do know that it starts when Boo is arrested for some fairly harmless teenager behavior and sent to the state school for juveniles.  Since then, he has been hiding away in the Radley house.  The rumors run wild in Maycomb about his behavior and character, and we are led to believe that Boo’s father has locked up Boo in the house for years.  When Mr. Radley dies, Boo’s brother, Nathan, seems to carry on the family tradition.  However, we do know that Boo gets out and about Maycomb.  He places items in the knothole of a tree for Scout and Jem, he covers Scout with a blanket during Miss Maudie’s house fire, and he saves Jem and Scout’s life when they are attacked by Bob Ewell. 

So, it really comes down to whether Boo has lived his life abused by his father and brother’s embarrassment of his past, or if it has been a choice for Boo to shut himself away from the world.  I think there is a good argument for the idea that Boo’s reclusiveness is self-imposed.  Perhaps Boo can’t understand or live in the world the surrounds him.  We know that Boo is a kind, sensitive person, and maybe, the world is too harsh a place for him to survive.  In reaction, he may choose to divorce himself from the evil of the world but still have a place in his heart for the innocent like Scout and Jem.  No clues are given in the novel, and it is up to the reader to infer what keeps Boo, an innocent mockingbird, sequestered away from life.