If Steven Pinker is right about a historical decline of violence among state societies -- and I believe he is -- then what is the cause? He is almost certainly correct that divinely-prescribed moral codes have little to do with it. His answer: "The most important psychological contributor to the decline of violence over the long term may instead be reason: the cognitive faculties, honed by the exchange of ideas through language, that allow us to understand the world and negotiate social arrangements."
Reason allows one to make the connection between "It's bad for you to hurt me" and "It's bad for me to hurt you," says Pinker.
And why an enhancement of reason? "The most likely causes are increases in the duration and quality of schooling, the spread of symbol-manipulation into work and leisure, and the trickling down of scientific and analytical reasoning into everyday life."
That all sounds good to me.
I would guess that we are programmed by natural selection for altruism toward "us" and violence -- especially by males -- toward "them."
Originally, the "them" were those with whom we did not share a close genetic affinity.
With the evolution of human culture, the separation of "us" and "them" was reinforced by language, religion, and shared histories, with a gradual widening of the circle of those we do not kill.
To be sure, Pinker says as much, and believes that reason helps us debunk the myths by which we define the "us" and demonize the "them."
There is still the matter of empathy. Why in the educated developed nations do we now refrain from drawing-and-quartering murderers and chopping the hands off thieves? The popularity of slasher movies and violent video games suggests we have not altogether lost our taste for gore. How, then, to explain the growing revulsion to cruel and unusual punishment? Why our increasing reluctance to inflict pain even on laboratory animals and the creatures we harvest for food? Without the possibility of reciprocity, the Golden Rule doesn't apply.
Pinker doesn't say much about this in the Nature essay; maybe more in the book. Could it be an entirely cultural meme, reinforced by liberal Enlightenment values? Another example of a culturally defined "us" versus "them"?
We are superior, better, more "civilized" than them when we are not inflicting pain.