Steven Pinker, a professor of psychology at Harvard, begins his presentation with an image of corpses on a truck, being taken from Auschwitz concentration camp. The image is one of many characteristic of the 20th century, a century that included brutality under Stalin, Hitler, Pol Pot and the genocide in Rwanda. The 21st century, which has barely started, already includes the brutality of Darfur and the daily destruction in Iraq.

These sorts of images can lead us to thinking that modernity brings terrible violence. Perhaps native people lived in a state of harmony that we’ve departed from.

This, Pinker tells us, is bullshit. “Our ancestors were far more violent than we are.” We’re probably living in the most peaceful time of our species’s existence, a statement that seems almost obscene in light of Darfur and Iraq.

The decline of violence, he tells us, is a fractal phenomenon – we see it over the centuries, the decades and the years. That said, we see a tipping point in the 16th century – the age of reason – particularly in England and Holland.

Until 10,000 years ago, all humans were hunter gatherers. This is the group that some believe lived in primordial harmony – there’s no evidence of this. Studying current hunter-gatherer tribes, the percent of male adults who die in violence is extraordinary – from 20 to 60% of all males. Even during the vilent 20th century, with two world wars, less than 2% of males worldwide died in warfare.

Moving slightly further forward, we can see that violent punishment was common in the Bible – Moses tells his followers to kill all the men and married women of a village and rape the virgins. The death penalty was used for murder, idolatry, disrespecting your parents and “collecting sticks on the sabbath”.

The Middle Ages were filled with mutilation and torture as routine punishments for trangressions we’d punish with fines today. This was merely another charming feature of a time that featured passtimes like “cat burning”, dropping cats into a fire for entertainment puposes… Some of the most creative inventions of the Middle Ages were fantastically cruel forms of corporal punishment.

One on one death has plummeted through the middle ages, with an “elbow” of the curve in the 16th century. Despite a slight uptick in the 1960s – “perhaps those who thought that rock and roll would lead to a decline in moral values had it right” – we’ve seen two orders of magnitude fall in one on one violence from the middle ages to today. State sponsored violence has also fallen sharply – we’ve need a 90% reduction in genocide since the end of the cold war. State on state conflicts are dropping every decade.

So why do we so mispercieve the violence of our society? For one thing, our reporting is better. AP is more likely to cover a war somewhere on the planet than a 16th century monk. We’re subject to a cognitive illusion – memorable events (brutal murder) are judged to be more probable than they actually are. Finally, our standards tend to change faster than our behavior. We may be offended by capital punishment today because it no longer fits with our vision of ourselves, but it’s worth remembering that not long ago, that sort of punishment was exceedingly common and there wasn’t strong protection of rights in the courts to prevent it from taking place.

So why is violence becoming less common? He offers four explanations:

1) Hobbes got it right. “Life in the state of nature is solitary, poor, nasty, brutish and short.” In anarchy, there’s a temptation towards pre-emptive violence, hurting the other guy before he hurts you. But with the rise of the Leviathan – the State – there’s a monopoly on violence. This helps explain why we still see violence in the absence of the state – zones of anarchy, failed states, street gangs.

2) In the past, we had a widespread sentiment that life was cheap. As we’ve gotten better at prolonging life, we take life more seriously and are more reluctant to take life.

3) We’re seeing more non-zerro sum games, as people discover forms of cooperation that can benefit both parties, like trade and shared peace dividends. These zero-sum games come with technology, because it allows us to trade with more people. People become more valuable live than dead – “We shouldn’t bomb the japanese because they built my minivan.”

4) Finally, Pinker leans on Peter Singer to speculate about “the expanding circle”. By default, we empathize with a small group of people, our friends and family. Everyone else is subhuman. But over time, we’ve seen this circle expand, from village to clan to tribe to nation to other races, both sexes and eventually other species. As we learn to expand our circles wider and wider, perhaps violence becomes increasingly unacceptable.