It's not Colbert's practice to get too deep into these things, but I wish he'd have asked Pinker why he thinks the decline in the percentage of people being killed in wartime is occurring so that I wouldn't have had to look it up.
A review at Amazon by Graham Seibert gives us a summary of Pinker's answer to this question. Seibert lists six significant trends Pinker claims have led to a decrease in wars and deaths in war since WWII:
1. Our evolution from hunter gatherers into settled civilizations, which he calls the Pacification Process.Of course, even if the statistics that Pinker bases his conclusions on are correct the reasons he offers are merely speculative. There's no way of knowing what's responsible for the alleged decline, but, nevertheless, I think a case can be made that the overriding cause, if a decline there be, is #4 plus the success of capitalism in creating decent living standards for so many of the world's people.
2. The consolidation of small kingdoms and duchies into large kingdoms with centralized authority and commerce, which he calls the Civilizing Process.
3. The emergence of Enlightenment philosophy, and it's respect for the individual through what he calls the Humanitarian Revolution.
4. Since World War II, violence has been suppressed, first by the overwhelming force of the two parties in the Cold War, and more recently by the American hegemony. Pinker calls this the Long Peace.
5. The general trend, even apart from the Cold War, of wars to be more infrequent, and less violent, however autocratic and anti-democratic the governments may be. Call this the New Peace.
6. Lastly, the growth of peace and domestic societies, and with it the diminishing level of violence through small things like schoolyard fights, bullying, and picking on gays and minorities. He titles this the Rights Revolution.
There has been less slaughter since WWII largely because the United States has been the dominant superpower and has had some success in meliorating conflicts around the world. Other powers have not used their might for fear of provoking American response. The recent case of Libya is a case in point. Had the U.S. not inserted itself in that internal conflict Qaddafi may well have killed tens of thousands of Libyans. Other examples are the India/Pakistan, the North/South Korean, and the Arab/Israeli conflicts in which nuclear war has been averted largely because fear of the United States has been an incentive to avoid overt, total war.
Moreover, American military technology has enabled the U.S., when it felt it had to fight, to do so with minimum harm to civilians. The ability to attack a single building without having to send a fleet of B-52s to carpet bomb a city has revolutionized modern warfare and considerably reduced the number of people who would otherwise have died.
Finally, the relative prosperity of much of the world since WWII has diminished the restlessness of people and reduced antagonisms so that nations are not as quick to go to war. All of this is a product of the American economic system that the left so deeply, and so ironically, despises.
What it's not a product of, I'm pretty sure, is an evolving sense of moral responsibility. It's peculiar that Pinker, an atheist, would think that the human race is becoming more moral even as naturalism makes deeper inroads into society, spreading its message that moral duties are pure illusion. It would be interesting to hear Mr. Pinker's explanation as to what survival advantage belief in moral duty confers that would cause natural selection to evolve the human race in that direction, even though, on atheism, such duties have no real world foundation.