Steven Pinker doesn't care whether self interest or morality is fueling the decline in violence:
With any humanitarian advance, there are always cynics who insist that no one … ever acts out of true morality, that there always must be some self-serving interest (the Quakers opposes slavery because they were bankers who financed the industrial revolution; the British stopped the slave trade because their French rivals were getting rich from their Caribbean plantations, and so on). …
I don’t care. My book is about the decline of violence, not a putative increase in virtue. I don’t think the chickens [who are more humanely treated] (or the slaves) care about whether their better treatment was motivated by an altruism that is pure in the eyes of God or other moralistic judges, as long as they suffer less. And if we set up institutions that allow people to be less cruel and destructive as they pursue their interests, that is a sign of progress–God help us if every advance in human welfare depended on Christ-like levels of moral purity.
Earlier posts on Pinker's book here, here, here, here, here, here, here, here, here and here.
The Dish 