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But here's another interesting statistic: in the past 25 years, the proportion of Americans who own guns has fallen from about one in three to only one in five. However, the United States, unlike Yemen, is a rich country, and the average American gun-owner has four or five firearms. Moreover, he or she is utterly determined to keep them, no matter what happens.
What has just happened in Sandy Hook, Connecticut, is the seventh massacre this year in which four or more people were killed by a lone gunman. The fact that this time 10 of the victims were girls and boys 6 or 7 years old has caused a wave of revulsion in the United States, but it is not likely to lead to new laws on gun controls. It's not even clear that new laws would help.
Half the firearms in the world are in the United States. The rate of murders by gunfire in the United States is almost 20 times higher than the average rate in 22 other populous, high-income countries where the frequency of other crimes is about the same. There is clearly a connection between these two facts, but it is not necessarily simple cause-and-effect.
Here's one reason to suspect that it's not that simple: the American rate for murders of all kinds - shooting, strangling, stabbing, poisoning, pushing people under buses, and so on - is seven times higher than it is in those other 22 rich countries. It can't just be guns.
And here's another clue: the rate of firearms homicides in Canada, another mainly English-speaking country in North America with a similar political heritage, is about half the American rate and in England itself it is only one-thirtieth as much. What else is in play here?
Steven Pinker, whose book The Better Angels of Our Nature is about the long-term decline in violence of every kind in the world, is well aware that murder rates have not fallen in the United States in the past century. (Most people don't believe that violence is in decline anywhere, let alone almost everywhere. That's why he wrote the book.) And Prof Pinker suggests an explanation for the American exception.
In medieval Europe, where everybody from warlords to peasants was on his own when it came to defending his property, his rights and his ''honour'', the murder rates were astronomically high: 110 people per 100,000 in 14th-century Oxford, for example. It was at least as high in colonial New England in the early 17th century.
By the mid-20th century, the murder rate in England had fallen more than a hundredfold: in London, it was less than one person per 100,000 per year. In most Western European countries it was about the same. Whereas the US murder rate is still up around seven people per 100,000 per year. Why?
Prof Pinker quotes historian Pieter Spierenburg's provocative suggestion that ''democracy came too early'' to America. In European countries, the population was gradually disarmed by the centralised state as it put an end to feudal anarchy. Only much later, after people had already learned to trust the law to defend their property and protect them from violence, did democracy come to these countries.
This is also what has happened in most other parts of the world, although in many cases it was the colonial power that disarmed the people and instituted the rule of law. But in the United States, where the democratic revolution came more than two centuries ago, the people took over the state before they had been disarmed and kept their weapons. They also kept their old attitudes.
Indeed, large parts of the United States, particularly in the southeast and southwest, still have an ''honour'' culture in which it is accepted that a private individual may choose to defend his rights and his interests by violence rather than seeking justice through the law. The homicide rate in New England is less than three people per 100,000 per year; in Louisiana it is more than 14.
None of this explains the specific phenomenon of gun massacres by deranged individuals, who are presumably present at the same rate in every country. It's just that in the United States, it's easier for individuals like that to get access to rapid-fire weapons. And, of course, the intense media coverage of every massacre gives many other crazies an incentive to do the same, only more of it.
But only one in 300 murders in the United States happens in that kind of massacre. Most are simply due to quarrels between individuals, often members of the same family. Private acts of violence to obtain ''justice'', with or without guns, are deeply entrenched in American culture, and the murder rate would stay extraordinarily high even if there were no guns.
Since there are guns everywhere, of course, the murder rate is even higher. But since the popular attitudes to violence have not changed, that is not going to change, either.
• Gwynne Dyer is an independent London journalist.