At the end of chapter 23, Jem forms a new theory why Boo Radley has never left his house in years. What is his theory?

Scout and Jem try to resolve exactly what separates and distinguishes the categories of white people. Background doesn't seem to matter, because all the families are equally old. Jem thinks these class definitions have to do with how long the family has been literate. Scout disagrees and thinks, "there's just one kind of folks. Folks." Jem says he used to think so as well, but he doesn't understand why they despise one another if that's the case. Jem seems very frustrated with society, and adds that maybe Boo Radley stays inside because he wants to. There could certainly be truth to this. Boo has seen human nature and what it is capable of. Perhaps he finds society just too stressful.