"THE PAST," STEVEN PINKER DECLARES, "is a place where a person had a high chance of coming to bodily harm." In 13th century Eurasia, the slaughter of men and rape of women by invaders was so thorough that today 8 percent of men in the former Mongol Empire possess DNA from Genghis Khan's lineage. Timur Lenk, who took up Khan's mantle, marked successful conquests with minarets built from victim's skulls, leaving a body count of 17 million in his wake. Earlier burial grounds and charnel houses also brim with evidence of assault, butchery, murder, and war. Prehistoric mass graves from Egypt to Germany to the American Great Plains tell the same story: skulls severed from spines during ritual sacrifice and skeletons embedded with arrowheads and stone slugs. Traces of human blood tarnish ancient cooking pots and this grisly legacy is inscribed on our genomes, which contain defenses against the diseases that often accompany cannibalism.

Though history is a chronicle of atrocities, Pinker's The Better Angels of Our Nature: Why Violence Has Declined centers not on brutality itself, but on the forces that curtail it. The book ranges across human prehistory, examines the effects of growing cities and consolidating states, lauds the Enlightenment, attempts to explain the 20th century's World Wars ("extremely bad luck"), and zooms in on the conflicts and resolutions of recent decades, including the expansion of human and even animal rights. Pinker's central contention is that violence has declined to an unprecedented extent in the modern era. Conflicts and cruelty remain. But the common condemnation of bloodshed and will to avoid it are relatively recent developments. "Today," he writes, "people might be dumbfounded when asked whether we should burn heretics, keep slaves, whip children, or break criminals on the wheel, yet those very debates took place several centuries ago." Pinker argues for examining cooperation and peace alongside exploitation and war: instances of the former may prove to be among the most extraordinary achievements of our species.

Pinker is a cognitive scientist best known for books like The Blank Slate, How the Mind Works, and The Language Instinct, all of which examine how human cognitive capabilities have been shaped by the exigencies of human evolution. The "Hobbesian salt" of evolutionary psychology flavors The Better Angels of Our Nature too: human brutality is a given; emotions and social behavior are the legacy of hunter-gatherer forbearers; individuals shift between strategies of altruism, betrayal, deception, and vengeance based on benefits to themselves, kin, and tribe. While detractors of this line of thinking often cast it as reactionary, arguing that to emphasize the indelible in human nature militates against change and individuality, Pinker is generally known for advocating a nuanced, humanistic view. He emphasizes that though people are born with significant faculties and predisp...