By historical measures, there’s really not all that much to be angry about. Since 1981, the proportion of the developing world living in extreme poverty has fallen from 50 percent to less than 20 percent, according to the United Nations. Infant mortality is down across the board; the number of girls in school is up. Terrorists and tyrants get their comeuppance with toe-tapping regularity. The chances of dying in war have never been lower. In 2011, the 7 billionth person was born into a world that’s richer, healthier, and safer than at any time in history.