Since 1914, we have endured world wars, genocides and government-sponsored famines, not to mention civil strife, riots and murders. Altogether, we have killed a staggering 100 million to 200 million of our own kind. But over the century, about 10 billion lives were lived — which means that just 1 to 2 percent of the world’s population died violently. Those lucky enough to be born in the 20th century were on average 10 times less likely to come to a grisly end than those born in the Stone Age. And since 2000, the United Nations tells us, the risk of violent death has fallen even further, to 0.7 percent.
As this process unfolded, humanity prospered. Ten thousand years ago, when the planet’s population was 6 million or so, people lived about 30 years on average and supported themselves on the equivalent income of about $2 per day. Now, more than 7 billion people are on Earth, living more than twice as long (an average of 67 years), and with an average income of $25 per day.

This happened because about 10,000 years ago, the winners of wars began incorporating the losers into larger societies. The victors found that the only way to make these larger societies work was by developing stronger governments; and one of the first things these governments had to do, if they wanted to stay in power, was suppress violence among their subjects.