Toward the end of How the Mind Works, Steven Pinker makes the following observation about the relative levels of violence inside and outside of civilizations:
Why don't we see periodontists or college professors dueling over a parking space? First, they live in a world in which the state has a monopoly on the legitimate use of violence. In places beyond the reach of the state, like urban underworlds or rural frontiers, or in times when the state did not exist, like the foraging bands in which we evolved, a credible threat of violence is one's only protection. Second, the assets periodontists and college professors, such as houses and bank accounts, are hard to steal. "Cultures of honor" spring up when a rapid response to a threat is essential because one's wealth can be carried away by others. They develop among herders, whose animals can be stolen, more often than among crop-growers, whose land stays put. And they develop among people whose wealth is in other liquid forms, like cash or drugs.
Just a note toward some future essay of mine on the rise of civilizations...