An interesting analysis of cause and effect. A couple thoughts occurred as I was reading.
Firstly, it seems that a lot of the decline, has had more to do with threat and the realization of threat. Ie, we have replaced violence with the threat of violence. For example, say you have a society of 5. If one person kills another, then literally 20% of your society is gone on a single choice. Not only that, but it would require at least 20% of society to stop that choice. As society grows the impact of that one choice lessens (in a statistical sense). So is the violence declining or are we just literally outgrowing it?
But as to the threat-realization aspect. In that society of five, if I was to be the Bad Guy, I would have to figure what are the odds that the other 4 can stop me? Would they all be present? Will I get caught at all? What will I lose if I'm caught?
As society and technology grows, new factors occur. They can stop me from a distance with locks, gates, guns, etc... They (the non-offenders) can retaliate and communicate and organize. So the odds of getting away with anything decrease because of perceived threat. That is why MAD was an effective policy when dealing with 'rational' nations. They KNOW the threat and they VALUE the threat.
So as society has increased the likelihood and the scale of consequence has increased. Just a side thought, different way of looking at it. Not disagreeing with him.
My other thought was, has violence really decreased? If you limit violence to simply death or classified harm, then you could make the statistical case that violence is declining. But take, capital punishment vs life imprisonment. Is one really doing less violence? The prisoner doesn't stay by choice, it is forced upon them. They are receiving the violence of captivity. Would they probably choose that over execution? Possibly. But that might be because they still have hope (even if never realized) that they won't actually spend their whole life there. Or it might be at the beginning that they don't fully comprehend the weight of spending their whole life in a prison. The only way to know if the choice was better would be to take a prisoner in prison about to die of natural causes and ask if they were to do it over which would they choose? And then how could you know the answer was true even then because you're asking them to deny the value of their entire life?
Just wondering then what is the value in the loss of violence? You could have a completely peaceful world by sedating every human being from birth until death, and would it be any good? You could have a completely peaceful world simply by restraining evil-doers from committing the evil that remains in them, imprisoning them with their own darkness and no one ever actually knowing what would happen if they were free to act.
Just some philisophical musings if you will...