According to the Stockholm International Peace Research Institute (SIPRI), worldwide military expenditures have been growing annually for the past 15 years, and between 15 and 20 major armed conflicts—yes, wars—are in progress as you read this. All told, upward of 175 million people died in war-related violence during the 20th century, plus another eight million because of conflicts among individuals.

Even so, according to a weighty new book, The Better Angels of Our Nature: Why Violence Has Declined (Viking Adult, 2011), by Harvard University psychologist Steven Pinker, the "better angels" of human nature have actually brought about a dramatic reduction in violence during the past few millennia. Yes, the absolute number of victims has been rising, but relative to the world’s population, the numbers look good.

The shift toward nonviolence, he says, has been driven by many factors, such as the spread of agriculture and the rise of feminism and democracy. Such trends have led to a reduction in institutionalized torture and execution and slavery and, especially in recent years, to an increase in the rights of women, homosexuals, children and animals.

Pinker acknowledges that one's immediate experience belies these facts to the point where you might even want to call him "hallucinatory." Yet the wealth of data he presents cannot be ignored—unless, that is, you take the same liberties as he sometimes does in his book. In two lengthy chapters, Pinker describes psychological processes that make us either violent or peaceful, respectively. Our dark side is driven by a evolution-based propensity toward predation and dominance. On the angelic side, we have, or at least can learn, some degree of self-control, which allows us to inhibit dark tendencies.

There is, however, another psychological process—confirmation bias—that Pinker sometimes succumbs to in his book. People pay more attention to facts that match their beliefs than those that undermine them. Pinker wants peace, and he also believes in his hypothesis; it is no surprise that he focuses more on facts that support his views than on those that do not. The SIPRI arms data are problematic, and a reader can also cherry-pick facts from Pinker's own book that are inconsistent with his position. He notes, for example, that during the 20th century homicide rates failed to decline in both the U.S. and England. He also describes in graphic and disturbing detail the savage way in which chimpanzees—our closest genetic relatives in the animal world—torture and kill their own kind.

Of greater concern is the assumption on which Pinker's entire case rests: that we look at relative numbers instead of absolute numbers in assessing human violence. But why should we be content with only a relative decrease? By this logic, when we reach a world population of nine billion in 2050, Pinker will conceivably be satisfied if a mere two million people are killed in war that year.

The biggest problem with the book, though, is its overreliance on history, which, like the light on a caboose, shows us only where we are not going. We live in a time when all the rules are being rewritten blindingly fast—when, for example, an increasingly smaller number of people can do increasingly greater damage. Yes, when you move from the Stone Age to modern times, some violence is left behind, but what happens when you put weapons of mass destruction into the hands of modern people who in many ways are still living primitively? What happens when the unprecedented occurs—when a country such as Iran, where women are still waiting for even the slightest glimpse of those better angels, obtains nuclear weapons? Pinker doesn’t say.