Why has violence declined so much?

Steven Pinker wrote an article last April about the rapid decline in the level of violence throughout history. The essay is adapted from his talk at TED. We've gone from a 15-60% chance that you would die by being killed by another person in hunter/gatherer times to less than a 1% chance in the 20th century. The question is why. Pinker presents four possibilities:

The first is that the 17th-century philosopher Thomas Hobbes got it right. Life in a state of nature is nasty, brutish, and short—not because of a primal thirst for blood but because of the inescapable logic of anarchy. Any beings with a modicum of self-interest may be tempted to invade their neighbors and steal their resources. The resulting fear of attack will tempt the neighbors to strike first in preemptive self-defense, which will in turn tempt the first group to strike against them preemptively, and so on. This danger can be defused by a policy of deterrence—don't strike first, retaliate if struck—but to guarantee its credibility, parties must avenge all insults and settle all scores, leading to cycles of bloody vendetta.

These tragedies can be averted by a state with a monopoly on violence. States can inflict disinterested penalties that eliminate the incentives for aggression, thereby defusing anxieties about preemptive attack and obviating the need to maintain a hair-trigger propensity for retaliation. Indeed, Manuel Eisner attributes the decline in European homicide to the transition from knightly warrior societies to the centralized governments of early modernity. And today, violence continues to fester in zones of anarchy, such as frontier regions, failed states, collapsed empires, and territories contested by mafias, gangs, and other dealers of contraband.

James Payne suggests another possibility: that the critical variable in the indulgence of violence is an overarching sense that life is cheap. When pain and early death are everyday features of one's own life, one feels less compunction about inflicting them on others. As technology and economic efficiency lengthen and improve our lives, we place a higher value on life in general.

A third theory, championed by journalist Robert Wright, invokes the logic of non-zero-sum games: scenarios in which two agents can each come out ahead if they cooperate, such as trading goods, dividing up labor, or sharing the peace dividend that comes from laying down their arms. As people acquire know-how that they can share cheaply with others and develop technologies that allow them to spread their goods and ideas over larger territories at lower cost, their incentive to cooperate steadily increases, because other people become more valuable alive than dead.

Then there is the scenario sketched by philosopher Peter Singer. Evolution, he suggests, bequeathed people a small kernel of empathy, which by default they apply only within a narrow circle of friends and relations. Over the millennia, people's moral circles have expanded to encompass larger and larger polities: the clan, the tribe, the nation, both sexes, other races, and even animals. The circle may have been pushed outward by expanding networks of reciprocity, à la Wright, but it might also be inflated by the inexorable logic of the Golden Rule: The more one knows and thinks about other living things, the harder it is to privilege one's own interests over theirs. The empathy escalator may also be powered by cosmopolitanism, in which journalism, memoir, and realistic fiction make the inner lives of other people, and the precariousness of one's own lot in life, more palpable—the feeling that "there but for fortune go I."

Personally, I tend to think the last possibility from Singer the strongest possibility. I agree that our instinctive morality seems to have a double standard: what is evil to do to one's group is not evil to do to those outside one's group. And we do seem to think in more international terms these days.

The question would then be, if that's the reason, then why have our polities/groups expanded? The increase in journalism/memoirs/fiction and ideologies of cosmopolitanism as Singer suggests? Perhaps other factors such as increased trade? more traveling? cheaper long-distance communication? Perhaps all of these?