Steven Pinker has written a book (summarized here) called The Better Angels of our Nature (from a quote by A. Lincoln) which investigates the history of violence, and concludes that it has been almost universally in decline for a long time. The question is why? He has an explanation, and also purports to explain more specific facts, such as - Why there is more violence in the Southern US than elsewhere (is that true, I wonder?)
Part of Pinker's argument is very economic: 'Trading surplus goods instead of simply feeding yourself with what you have hoarded changes life: Now you view your trading partner as someone more valuable to you alive than dead.'
