I think it's likely that the civilizing effect of literature has done most of the work, and still continues to do. Look at Steven Pinker's book, The Better Angels of Our Nature: Why Violence Has Declined. It proves beyond any shadow of doubt that violence has declined dramatically throughout the centuries. There are various reasons for it: the rise of the state, Leviathan, the monopoly of violence, children's rights, animal rights. They're all positive signs.