To readers familiar with the literature in evolutionary psychology and its tendency to denigrate the role reason plays in human behavior, the most striking aspect of Pinker’s account is that the last of his “better angels” is reason. Here he draws on a metaphor I used in my 1981 book “The Expanding Circle.” To indicate that reason can take us to places that we might not expect to reach, I wrote of an “escalator of reason” that can take us to a vantage point from which we see that our own interests are similar to, and from the point of view of the universe do not matter more than, the interests of others. Pinker quotes this passage, and then goes on to develop the argument much more thoroughly than I ever did. (Disclosure: Pinker wrote an endorsement for a recent reissue of “The Expanding Circle.”)

Pinker’s claim that reason is an important factor in the trends he has described relies in part on the “Flynn effect” — the remarkable finding by the philosopher James Flynn that ever since I.Q. tests were first administered, the scores achieved by those taking the test have been rising. The average I.Q. is, by definition, 100; but to achieve that result, raw test scores have to be standardized. If the average teenager today could go back in time and take an I.Q. test from 1910, he or she would have an I.Q. of 130, which would be better than 98 percent of those taking the test then. Nor is it easy to attribute this rise to improved education, because the aspects of the tests on which scores have risen most do not require a good vocabulary or even mathematical ability, but instead test powers of abstract reasoning. One theory is that we have gotten better at I.Q. tests because we live in a more symbol-rich environment. Flynn himself thinks that the spread of the scientific mode of reasoning has played a role.

Pinker argues that enhanced powers of reasoning give us the ability to detach ourselves from our immediate experience and from our personal or parochial perspective, and frame our ideas in more abstract, universal terms. This in turn leads to better moral commitments, including avoiding violence. It is just this kind of reasoning ability that has improved during the 20th century. He therefore suggests that the 20th century has seen a “moral Flynn effect, in which an accelerating escalator of reason carried us away from impulses that lead to violence” and that this lies behind the long peace, the new peace, and the rights revolution. Among the wide range of evidence he produces in support of that argument is the tidbit that since 1946, there has been a negative correlation between an American president’s I.Q. and the number of battle deaths in wars involving the United States.

Reason also, Pinker suggests, moves us away from forms of morality more likely to lead to violence, and toward moral advances that, while not eschewing the use of force altogether, restrict it to the uses necessary to improve social welfare, like utilitarian reforms of the savage punishments given to criminals in earlier times. For reason does, Pinker holds, point to a particular kind of morality. We prefer life to death, and happiness to suffering, and we understand that we live in a world in which others can make a difference to whether we live well or die miserably. Therefore we will want to tell others that they should not hurt us, and in doing so we commit ourselves to the idea that we should not hurt them. (Pinker quotes a famous sentence from the 18th-century philosopher William Godwin: “What magic is there in the pronoun ‘my’ that should justify us in overturning the decisions of impartial truth?”) That morality can be grounded in some commitment to treating others as we would like them to treat us is an ancient idea, expressed in the golden rule and in similar thoughts in the moral traditions of many other civilizations, but Pinker is surely right to say that the escalator of reason leads us to it. It is this kind of moral thinking, Pinker points out, that helps us escape traps like the Cuban missile crisis, which, if the fate of the world had been in the hands of leaders under the sway of a different kind of morality — one dominated by ideas of honor and the importance of not backing down — might have been the end of the human story. Fortunately Kennedy and Khrushchev understood the trap they were in and did what was necessary to avoid disaster.

“The Better Angels of Our Nature” is a supremely important book. To have command of so much research, spread across so many different fields, is a masterly achievement. Pinker convincingly demonstrates that there has been a dramatic decline in violence, and he is persuasive about the causes of that decline. But what of the future? Our improved understanding of violence, of which Pinker’s book is an example, can be a valuable tool to maintain peace and reduce crime, but other factors are in play. Pinker is an optimist, but he knows that there is no guarantee that the trends he has documented will continue. Faced with suggestions that the present relatively peaceful period is going to be blown apart by a “clash of civilizations” with Islam, by nuclear terrorism, by war with Iran or wars resulting from climate change, he gives reasons for thinking that we have a good chance of avoiding such conflicts, but no more than a good chance. If he had been able to see, before his book went to press, a study published in Nature as recently as August of this year, he might have been less sanguine about maintaining peace despite widespread climate change. Solomon Hsiang and colleagues at Columbia University used data from the past half-century to show that in tropical regions, the risk of a new civil conflict doubles during El Niño years (when temperatures are hotter than usual and there is less rainfall). If that finding is correct, then a warming world could mean the end of the relatively peaceful era in which we are now living.