Discrimination. Let us remember that Dill dares Jem to run and touch Boo Radley's house. Before this, we are given the children's version of the various myths and urban legends that feature Boo Radley. We are told that he is a "malevolent phantom" and that he is a terrifying figure who tortures and eats animals such as squirrels and cats. The way in which the children demonise Boo Radley and how we as adults can obviously identify an exaggerated story that has been developed and passed down and gossiped about shows us that there is obviously more to Boo Radley's story than meets the eye.
The children, however, start off by treating Boo Radley as a terrifying ogre and as less than a human. Through the course of the story they come to realise that he is a human just like them who is worthy of love and respect. This of course is yet another example of discrimination in a novel where the biggest example of discrimination is the way that Tom Jones is treated.