Pinker states his postulate at the very beginning: the mind was shaped by evolution, like any organ of the body, and some aspects of the human mind (from predatory violence to revenge) lead to selfish, violent behavior, while others (like empathy and self-control) lead to altruistic, peaceful behavior. The fact that violence has been declining over time means that changes in the social and cultural environment (such as the judiciary, commerce, cosmopolitanism, rationality and feminism) have tended to reward the peaceful and altruistic traits over the violent and selfish ones.

Studying the archeological and historical record, Pinker has deduced that there was a significant decrease in violent deaths five thousand years ago, when the first agricultural civilizations and the first governments appeared. This is credible as a general theory, although the data are not quite as convincing as Pinker wants us to believe. I do believe that governments reduced the homicide rate of the pre-state era, and perhaps it also reduced endemic tribal warfare (in our times check how many people died under a brutal tyrant like Saddam Hussein and how many are dying now that the Iraqi government is weaker) although i suspect that governments introduced warfare (government vs government) on a bigger scale with more lethal weapons (in our times check out how many people got killed during Saddam Hussein's war against Iran). Governments often "legitimate" global murder while suppressing personal and tribal homicide.

Pinker's estimates are based on wildly approximate figures coming from studies by experts who estimated them as well as it can be done, but obviously we have no accurate headcount of ancient warfare comparable with today's statistics. He mentions the Semai tribe as an example that proves the opposite (a primitive tribal society that was never involved in warfare). It is easier to agree that brutal empires tend to bring peace to the areas that they control: Rome, the Arabs, the Mongols, Britain and now the USA created long periods of relative peace in the huge areas that they controlled (as did Saddam Hussein inside Iraq). But that goes up and down, so it would not be a straight declining line.

One has to be careful investigating Pinker's colossal bibliography. His thesis that blood feud was pervasive in ancient times is based on a study by Karen Ericksen and Heather Horton ("Blood Feuds") that was used by Douglas Fry in his book "Beyond War" (2007) to prove almost the opposite: "only" 34.5% of societies engaged in blood feud. Fry reached the conclusion that "Even in societies where kin group vengeance was socially permitted by no means was it always carried out". Pinker's conclusion based on the same study: "Kin-against-kin blood feuds were common in tribal societies". Pinker then quotes Keith Otterbein's "How War Began" (2004) and summarizes it in one sentence, preferring it to Otterbein's more recent "The Anthropology Of War" (2009). Pinker's one-line summary does not do justice to Otterbein's 300-age study. Here is the publisher's summary: "He identifies two types of military organization: one which developed two million years ago at the dawn of humankind, wherever groups of hunters met, and a second which developed some five thousand years ago, in four identifiable regions, when the first states arose and proceeded to embark upon military conquests."

So it sounds like Otterbein would lean towards an "up and down" course of warfare rather than a straight declining line. Pinker quotes Steven Spitzer's 1996 article "Punishment and Social Organization: A Study of Durkheim's Theory of Penal Evolution" as proving that "complex societies are more likely to criminalize victimless activities like sacrilege, sexual deviance, disloyalty, and witchcraft, and to punish offenders by torture, mutilation, enslavement, and execution"; which is totally credible; but Spitzer's was really a study on Durkheim's theory of penal evolution. And Laura Betzig's book "Despotism and Differential Reproduction" (1986) is summoned to show "that complex societies tend to fall under the control of despots", a rather vague one-line summary and a rather odd choice to summarize a highly controversial subject (Betzig's background seems to be in sexual studies, not in history of civilizations). I wonder if democratic Athens makes the list of "complex societies" or not.

As Martin Daly and Margo Wilson showed in "Homicide" (1988), over the last thousand years in Europe the rate of homicide has declined dramatically, and Pinker credits the consolidation of the centralized national state for this achievement. This is what German sociologist Norbert Elias discussed in "The Civilizing Process" (1939), and Pinker hails that book. One would imagine that the old simple rural life translated into lower crime rates, but instead crime declined when people moved to overcrowded cities. The commercial, industrial and cosmopolitan world indirectly discouraged crime. Elias argued that the change took place first among the aristocrats, and paralleled a shift in etiquette towards "good manners" (basically, self-control instead of impulsive reaction). This new state of mind slowly percolated down to the rich bourgeoisie and eventually to the broader public. Elias pointed at two factors that contributed to this shift towards more peaceful attitudes: the nationalization of justice (and monopolization of violence) by the new centralized monarchies; and the fact that people engaged in commerce are better served by cooperation (e.g. craft guilds) than by violence. The British criminologist Manuel Eisner's study "Long-Term Historical Trends in Violent Crime" (2003) confirmed Elias' thesis. Pinker also shows figures according to which the "civilizing process" followed industrialization: wherever industrialization took hold, crime declined.

As is typical of the rest of the book, Pinker mixes intriguing psychological analyses (quoting a huge number of specialists) and huge amounts of data (taken from all sorts of sources). For example, Donald Black in "Crime as Social Control" (1983) introduced the idea that the border between crime and noncrime is blurred: the definition of "justice" depends on whether you are the law enforcer or the criminal. From the viewpoint of the criminal frequently what s/he does is not "crime". It is just her/his job. Some activities are illegal but not viewed as amoral by those who practice them. The state, clearly, will not defend these "criminals" (because they are doing something illegal), and therefore they have to defend themselves. It is illegal to kill your wife and your wife's lover, but the cheated husband might not feel that way: he might feel that he has just served justice by killing both. The wife, in turn, might feel that it is "justice" to get rid of a husband she hates and who stands in the way of her happiness. And the lover might simply be someone who honestly loves that woman and feels that he is helping rectify a wrong by loving her.

It is not completely true that the Mafia, gangs, druglords and so forth are "criminals". Within their world there are ethical rules and a member can be punished for breaking them. It is just that they don't perceive the state laws as the ultimate laws. They perceive those rules as we perceive weekend rain or heavy traffic: an annoyance that we have to cope with. Besides, history has proven that many "illegal" activities have later become perfectly legal, from heresy (for which one used to be burned at the stake) to alcohol (forbidden during the Prohibition). Today's drug smugglers could be tomorrow's neighborhood store owners. In fact, many crimes are not viewed as such by the neighbors of the criminal. Those crimes are tacitly accepted by a sizeable minority if not by the majority. The Mafia was a good example of a criminal organization that was tolerated, respected and even admired by ordinary people. In fact, it would not have survived without that passive acquiescence. As Tacitus worded it: "A shocking crime was committed on the unscrupulous initiative of few individuals, with the blessing of more, and amid the passive acquiescence of all".

Obviously a lot of what is discussed in this chapter on homicide comes from British statistics: we know homicide rates in England very well. The one book that Pinker singles out in support of the theory that there is a general decline in homicides, not limited to Britain, is "A History of Murder" (2008) by the Dutch historian Pieter Spierenburg. However, we don't know homicide rates in most of the world and we can only guess. I have seen first hand how safe life tends to be under brutal regimes (whether the Soviet Union or Saudi Arabia), but it is difficult to prove it numerically. Democracy is, in fact, a mixed blessing for public safety.

Gary LaFree's and Andromachi Tseloni's "Democracy and Crime" (2006), which analyzed homicide trends in 44 Countries between 1950 and 2000, shows that the shift from tyranny to democracy frequently results in a crime wave (Russia, Papua, South Africa, Iraq...) One can argue that the state is weaker after the transition from tyranny to democracy. Later crime rates tend to decline as the democratic state becomes stronger. There are, however, many factors at play, and the statistics about the USA are a good example. It is difficult to make sense of homicide rates in the USA (several times higher than in Europe) without considering a) the wide availability of firearms; b) who and when invested in crime prevention and fighting crime (e.g. New York's mayor Giuliani); c) economic booms and busts; d) laws about recreational drugs; e) single mother pregnancies; and many other factors. Incidentally, statistics show that the rural South tends to be more violent than the industrial North.

Pinker mentions a "universal law of crime": that the majority of crimes are committed by males who are between 15 and 30 years old. And he speculates that women actually lower the crime rate by marrying those males and converting them to domestic life. Thus, for example, the Wild West was "wild" because it was mainly populated by single young men. When women started arriving, crime started declining. Pinker quotes a study by Robert Sampson, John Laub and Christopher Wimer, "Does Marriage Reduce Crime?" (2006), that shows this to be true in general.

Pinker's statistics are interesting in themselves (number of killings per 100,000 people per year): Russia (29.7) is three times more dangerous than Mexico (11.1) and 14 times more dangerous than China (2.2) but Russia is less dangerous than the District of Columbia (30.8), where the capital of the USA is located; Louisiana (14.2) is 5 times more dangerous than the northern states of the USA. And the USA is clearly split between African-Americans (an odd term that refers to people with a black skin, but not to African Americans like my friend Hassan who is from Morocco or my friend Gerard who is a white man from South Africa, both naturalized US citizens) and the rest of the country: the former's rate of homicide is 36.9 versus 4.8 for white people.

Homicide has definitely declined in the West over the last centuries. Pinker notes an exception but uses that exception to actually reinforce some of his main sociological points: violent crime spiked in the 1960s, at the time of the counterculture, of the hippies and of the student riots. The national rate went from 4.0 in 1957 to 10.2 in 1980. That trend is difficult to explain through the usual parameters of unemployment (that was very low in the 1960s) and income (that was relatively high). Pinker explains it as part of a general phenomenon of decivilization. Young people abandoned formalities like the tie and the hat. Aristocratic manners became the target of ridicule instead of admiration and emulation. The whole countercultute phenomenon ended up indirectly undermining the notion that polite manners are important and promoting casual manners as reflecting a more democratic attitude. The stars of rock'n'roll were role models of a different kind: poorly dressed, bad-mannered, unshaved, etc. Basically, the counterculture of the 1960s ended up undoing Elias' civilizing process. Pinker thinks that even the women's liberation movement contributed to this decivilizing process because it undermined marriage, which had been one of drivers of the civilizing process.

Pinker quotes Cas Wouters' "Informalization - Manners and Emotions Since 1890" (2007) but Wouters' book is really about how the evolution of manners since 1890 (not just the 1960s) has been towards more informal behavior. In a sense, Wouters' book refutes Pinker's thesis. Decivilization had been going on before the 1960s. Pinker probably sees a bigger break with the past in the 1960s because that was his generation.

Despite this blip in the charts, Pinker shows that the homicide rate has kept declining. It was 4.8 in 2010 in the USA. Neither unemployment nor income inequality help explain the decline in crime in the West (unemployment went up in many Western countries although not in the USA). Pinker mocks the theory popularized by John Donohue's and Steven Levitt's "Freakonomics" (that the legalization of abortion removed from society millions of unwanted children): the data prove it wrong. In any case the crime decline began during the generation born before the legalization of abortion, and then simply continued. Pinker's explanation is two-fold: on one hand the state got more powerful, and on the other hand the civilizing process resumed once the excesses of the 1960s showed their dark side.

Pinker's observations on Western phenomena are always interesting and well documented. Unfortunately, this is not as true of other continents, where a) data are not as easily available and b) he is probably less familiar with the local culture. Any traveler has observed how much safer countries like Japan and Singapore are. There is certainly a different mindset at work. It would be interesting to know not only that the mindset is a recent phenomenon but how it originated: why don't Japanese parents fear sending their children to school alone? Why is it so safe "there" and so "unsafe" here?

After Elias' "civilizing process", Pinker focuses on the "humanitarian revolution". Since the Enlightenment there has been a growing movement to abolish social forms of violence such as slavery, tyranny, torture and death penalty. Pinker reminds us that torture was widespread everywhere on the planet and that it was a form of popular entertainment (i.e. everybody accepted it and even enjoyed watching it). Pinker is again at his best in analyzing the sociological changes that led to such a dramatic transformation in the way we, ordinary humans, perceive torture, not to mention human sacrifice. (My take on torture: The Genetics and Neuroscience of Torture). The Age of Reason, Pinker reasons, discovering the physical laws that move the world (not God's will), ridiculed the superstitions underlying practices such as the burning of witches.

Because of a more rational approach to matters, human life was suddenly worth more. Indirectly, people agreed that lives are more important than souls. Logical thinking led to Cesare Beccaria's "On Crimes and Punishments" (1764), that explained why the punishment should be proportional to the crime. The guillotine (1792) actually represented a step towards less brutal punishment because death was immediate. While gruesome to watch, it is actually much better than torture or burning at the stake. Pinker, however, like everybody else, has trouble explaining what caused the transformation: why did we become so disgusted with torture, slavery, cruelty, etc. Our ancestors accepted it and even enjoyed watching it. Where does the modern empathy come from? Pinker mentions a few implausible theories. Then he advances his own: that book production and literacy led to a humane sensibility. I find this explanation more credible than others: information, both factual and fictional, helped people better identify with other people. Pinker mentions the epistolary novel as a genre that was particularly influential at about the same time that the passion for cruelty started declining. A valuable source for this chapter is Lynn Hunt's "Inventing Human Rights" (2007).

Pinker then turns to what John Gaddis dubbed the "Long Peace" (1989): since World War II the major world powers have not fought any direct war, certainly the longest period of peace in Europe since the Roman era, and, since the Cold War ended, an increasing peaceful one in the rest of the world as mad dictators are forced to retire one after the other. He employs a vast arsenal of figures to show that, per capita, even the world wars killed fewer people than previous wars. One problem is that he is relying a lot on Rudolph Rummel's Death by Government - Genocide and Mass Murder (1994), Rudolph Rummel's Statistics of Democide (1997) and Matt White's website. I also used their studies (for my page on Worst dictators/genocides) but i have come to doubt many of them. For example, even Ukraine now admits that Stalin did not kill as many Ukrainians as we believed just a few years ago. If the figures for recent conflicts can be so wrong, imagine how wrong the figures can be for conflicts that took place centuries ago. By the end of the chapter (and thousands of statistics) Pinker convinced me that we really don't know. The book he quotes, such as Lewis Fry Richardson's "Statistics of Deadly Quarrels" (1960) and Michael Howard's "The Invention of Peace and the Reinvention of War" (2001), are confusing at best.

The future is never easy to predict. Pinker emphasizes that there have been no major conflicts in Europe since the end of World War II (except for the civil war in Yugoslavia) but this could simply be a side effect of the decline of Europe. We are living in a period of transition: power is shifting from the West to the East. Therefore the lull in warfare. We'll see what Asia does with its newfound power. Pinker hails the achievement of the United Nations that has kept stability around the world. Personally, i am ambivalent about the idea of protecting international borders and existing governments at all costs. An incredible number of wars are caused by the fact that people who hate each other are forced to live in the same country (think of Bosnia, Iraq, Syria). The contemporary world abhors secessions and partitions, and worships the borders decided by the old European colonial powers. It is debatable whether this system benefits peace in the long term as it does in the short term.

Pinker views the Long Peace as truly a "democratic peace" and a "capitalistic peace". In addition, international organizations such as the United Nations helped maintain peace. But more interestingly Pinker points at another factor in transforming our warmonger's mindset into a peace-loving mindset: just like the humanitarian revolution was probably accelerated by publishing and literacy, so the Long Peace may be an indirect effect of the proliferation of media after the 1950s, from television to the Internet.

Pinker also pays respect to Kant, whose essay "Perpetual Peace" (1795) prescribed three actions: "republican" regimes (in modern terminology, democratic), a "League of Nations" and "world citizenship" (protection by a state of other states' citizens).

During the Cold War the superpowers encouraged dictators and civil wars, and tolerated genocide. After the Cold War ended, the same forces have often been used to remove dictators, suppress civil wars and prevent genocide. Peacekeeping missions (by NATO, United Nations, African Union) is much more prevalent than it used to be. At the same time the numerous projects for humanitarian aid have become much more effective.

Pinker also analyses genocide concluding that democracies are less likely to wage war, to get engulfed in civil wars and to commit genocide.

Then on terrorism Pinker notes that terrorism generally fails (Norther Ireland, Sri Lanka, Kashmir, Palestine, Kurdistan, Chechnya, Basques, etc), quoting Max Abrahms' article "Why Terrorism Does Not Work" (2006) and Audrey Cronin's book "How Terrorism Ends" (2011). Discussions on terrorism are also ambiguous because one man's terrorist is another man's hero. The men who started the war against Britain and declared independence and founded the USA were considered terrorists by the government that ruled over their lands at the time. Garibaldi, the hero who liberated and united Italy, was viewed as a terrorist by the Spaniards and French. Basically, Pinker is using a tautological argument. He calls "terrorists" those who lose and then "proves" that terrorists lose. Those who win are not terrorists but become the founding fathers of new nations. They won but they are not terrorists anymore.

Finally, Pinker mentions the Islamic world that, from Libya to Pakistan, is experiencing one of the most violent convulsions of its history, and seems to refute the thesis that the world has becoming more peaceful. His analysis is superficial and, alas, he uses a double standard: for the Islamic world he does not provide the same kind of minute analyses that he provided for the West. Perhaps he sensed that the data would have proven a vast increase (not decrease) in violence. He wrote the book before ISIS conquered big portions of Syria and Iraq.

He also seems to downplay the chance of a nuclear holocaust. The nuclear deterrent certainly help refrain the USA and the Soviet Union from attacking each other, or perhaps provided a strong rationale to preempt the military hawks in each country. We never ban a weapon from use. Pinker wrote the book before Assad used chemical weapons in the Syrian civil war. He writes that the world could get rid of nuclear weapons the way it got rid of chemical weapons, but Assad proved him wrong within a few months. Nuclear weapons are here to stay, and every day that goes by the chances that someone will use them increase, not decrease.

Pinker's only serious explanation for the decline in wars and genocides is the spread of information thanks to television and Internet. I would add one that he surprisingly neglected: the widespread adoption of the English language. It used to be difficult to communicate, even between nearby regions, because of the multitude of dialects. Worse: a dialect (an accent) was a way to discriminate friends from foes. The fact that an increasing number of people can speak the same language makes it easy to communicate, to understand each other and to identify with the "other".

And finally Pinker analyses progress in human rights that protect minorities (whether immigrants or homosexuals). He easily proves that (in the West) the last 60 years have witnessed progress in human rights that mirror declines in war and genocide. This chapter summarizes the recent history of civil rights, women's rights, children's rights, gay rights and animal rights, finding pretty much the same trend towards tolerance that one finds among countries. He only mentions in passing, though, that the peak of the civil rights movement and the birth of the gay rights movement correspond roughly to the period (the 1960s) when violent crime started increasing, a coincidence that is left unexplained. The 1960s also witnessed the reversal of roles: instead of antiblack whites rioting to lynch blacks it was blacks who started rioting (not to lynch whites but usually to loot stores).

The part on sex violence is interesting for Pinker's analyses from the perspective of evolutionary psychology. Because in mammals and birds the female is penalized by a lengthy period of gestation, and therefore reproduces more sloely than the male, she becomes a scarce resource over which males compete. Rape provides an evolutionary advantage to the rapist, so no surprise that it has been common in every culture. Worse: the human male is supposed to help raise the offspring, but, if his woman cheated on him, he may be raising someone else's genes, i.e. commit evolutionary suicide. Hence a man is likely to be aggressive in obtaining a woman and then aggressive in monopolizing her sexual activity. On the other hand, a woman's sexuality has a value. Unfortunately, this does not always translate into poems and flowers, but rather into the men of her family considering her private property.

Therefore virginity is prized in every culture because it is a guarantee that the woman is not bearing another man's child. Pinker points out that in no culture are people obsessed with the groom's virginity: a man's infidelity (whether before or after the wedding) has less devastating consequences (evolutionarily speaking) than a woman's. A consequence of this mindset is that the woman is viewed as property of a man (father or husband). Rape has traditionally been treated like "a tort for damaged goods" (Pinker's cute expression), the victim being either the father or the husband, That's why rape of a woman by her husband made no sense in traditional societies. And that's why to this day the father gives away the bride to the groom. To this day statistics show that boyfriends and husbands frequently end a relationship after the woman has been raped. Pinker's statistics show a steady decline of rape in the West. It is impossible to verify the same statistics for other countries (as i write, the world is horrified at the news of brutal rapes and murders of girls in India).

The history of children's rights is similar in that for millennia children were considered private property of their parents, and the parents could dispose of their children at will. The idea that the state should protect children from abuses picked up in the late 19th century.

The Western bias in Pinker's statistics is obvious when, for example, he shows that vegetarianism is increasing. There are more vegetarians in the West, but a lot fewer in India. Each new generation of Indians tends to be less strict about not eating meat, especially outside of the high castes. Since India had the vast majority of vegetarians in the world, i suspect that globally vegetarianism is declining, and very rapidly. It could be that, just with all the other movements, the West will eventually influence the rest of the world and cause vegetarianism to become more popular. In the short term, however, it is more likely that vegetarians will become a tiny minority of the world's population.

After proving his point, Pinker asks again what caused the transformation: why modern humans care so much for people's rights (and even animal rights) after millennia during which they didn't care at all? He comes up with something similar to his explanations for the Long Peace (television and mass media in general) and for the Humanitarian Revolution (the printing press, literacy, book publishing). The Rights Revolution may be caused, or at least accelerated, by the proliferation of media and faster communications that, together, connect people more tightly than in the past. His explanation has the enviable advantage of providing an explanation for why all of this doesn't seem to happen in the Islamic world: that is the part of the world that was hostile to the printing press, where countless books (even the Divine Comedy) were not published until very recently, and where reading a book is not a common passtime. The media facilitate an understanding of other people but also provide knowledge that can make people more rational. In other words, they increase both empathy and rationality. Pinker notes that the Rights Revolutions (plural) did not require much violence, if any at all.

I have a slightly different take on this, and i am surprised that Pinker did not frame it this way. An enlightened elite forced the new morality on its masses. The death penalty was abolished in Britain and France when a majority still supported it. Much progress in the integration of the European Union has happened with scant support from the public (in fact, strong opposition from the British public). International organizations such as the United Nations would be disbanded overnight if referendums were held in all the member nations. What happened in all of these cases is that a small elite of intellectuals (some of them with no political or military power) sponsored an idea and kept pushing it until it prevailed.

There was no need for violence because intellectuals employ two powerful weapons that are more efficient than weapons: the printed word and the educational system. Intellectuals constitute a tiny percentage of the population but are the ones who publish books, magazines and newspapers (and who make movies and write songs); and they are the ones who teach in schools. Therefore they simultaneously influence public opinion and raise the future generations. Recently, gay marriage and the liberalization of marijuana, who have long been opposed by the majority of the nation, have been promoted in the USA by a minority of intellectuals, and their insistence, year after year, is eventually creating the majority needed to pass those laws. Women's rights in Islam (and even democracy itself) are slowly being introduced by a minority of Westernized leaders (and sometimes by foreign powers), not by the will of the majority.

The intellectual is really the engine behind much of the social change that took place in the West since the Enlightenment. The rest of the world has mostly looked at the West for inspiration. Much of the world was a colony of the Western powers, and still views the Western powers as the role models. World War II and the Cold War ended the same way: with the triumph of the USA. Inevitably most of the world views the USA as a role model, at least when it comes to social matters. Whatever becomes popular in the West tends to spread to the rest of the world because the rest of the world suffers from this inferiority complex, whether justified or not.

The last section of the book returns to the premise: human nature is neither good nor bad, it is a mixture of the two as shaped by evolution. Pinker summarizes what science has discovered so far of human nature.

Pinker begins by following Roy Baumeister, author of "Evil: Inside Human Violence and Cruelty" (1999), in dispelling the notion that evil comes from a few horrible human beings: most evil acts are committed by ordinary people for relatively rational purposes. Pinker then spends quite a bit of time illustrating the neurophysiology and evolutionary psychology of violence, i.e. the brain circuits that preside over predation, dominance (mainly a male thing, proving again that women have always been mostly a pacifying force), revenge (that probably evolved as a deterrent), sadism (yes, common among ordinary folks, not only "sadists", particularly males) and ideology (which requires more psychology than neurology because the puzzling thing is how many people follow it and how many people acquisce), and for each one he explains what triggers the violent action.

Pinker then turns to the factors that helped reduce violence. He sounds "new age" when he emphasizes the trend to "expand global emphatic consciousness" but he has a good point. He is not too convincing in analyzing the neural circuits that generate empathy (incidentally, he seems to mostly discount the notion that mirror neurons are important for empathy). He pinpoints oxytocin, produced by the hypothalamus, as a major protagonist in the production of empathy, but i doubt that shots of oxytocin would turn genocidal dictators into chronic benefactors.

What is credible is the theory that he borrows from Peter Singer, author of "The Expanding Circle" (1981). Humans have been expanding the circle of sympathy, which originally only included pretty much themselves and their offspring and now includes even animals. Pinker, again, credits literacy as triggering this trend. He then argues that self-control is an inherited trait correlated with intelligence and that crime is inversely proportional to intelligence and to self-control. Elias had already speculated that a neurological deficit in the faculty of self-control is a major cause of violence.

Pinker makes the connection with intelligence, that valued law enforcement and economic cooperation. He mentions that the human race may have evolved (biologically evolved) better self-control, a hypothesis first advanced by Gregory Clark in "A Farewell to Alms" (2007) when, trying to explain why the industrial revolution happened in England, he speculated that Darwinian selection in the agrarian society created a new human race that naturally adapted to create the industrial revolution and capitalism. (Of course the question immediately arises whether this is true of other societies too). Pinker, though, argues that, while plausible, there is no need for this hypothesis to explain the decline in violence. Intelligence is the main factor that Pinker examines.

In the 1980s James Flynn started emphasizing that the intelligent quotient of people has increased dramatically since the 1930s, and his book "What is Intelligence?" (2007) popularized this notion that had come to be called the "Flynn effect". Pinker views the Long Peace and the Rights Revolution as side-effects of the Flynn effect: people used reason more often and were better at it, and therefore reached the conclusion that peace was better than war and that others deserved the same rights as theirs. The Flynn Effect was not the only factor, but it acted as an accelerator: the effect of reason is combinatorial. Adam Smith had already reached a similar conclusion in "The Theory of Moral Sentiments" (1759): "reason... is the great judge and arbiter of our conduct".

In the last chapter Pinker debunks various theories used to explain the course of human history: technological determinism (more deadly weapons resulted in less violence not more violence); resource determinism (violence does not always follow rare resources); wealth determinism (the correlation between affluence and nonviolence is dubious) and religious determinism (religion has caused many tragedies but also prevented violence). He then rephrases the Prisoner's Dilemma into a similar mathematical model, the Pacifist's Dilemma (in which unilateral pacifism is a losing strategy and joint peace is impossible), and argues that five developments have encouraged humans towards cooperation (positive sum outcomes): more powerful states, more trade (and so far that's Elias' theory), "feminization" (particularly in Europe), Singer's expanding circle and the Flynn effect If he is right, he finally found a rational explanation for the decline in violence over the centuries. The first factor that alone help explain the decline in violence are: reason