What's wrong with the United States of America? It's a question countless people are asking, in the United States and around the world, following the massacre at a movie theatre in Colorado. Why does the United States spawn such madness, so frequently? And why is it getting worse?

I'll get to that. But first, let's recall another massacre in Colorado, not far from last week's atrocity.

On April 20, 1999, two teenagers walked into Columbine high school and opened fire. One teacher and 12 students were killed; 24 more were injured. There was shock and anguish across the United States and around the world.

In the days and weeks that followed, people explained why it had happened. It was the dark youth sub-culture known as "Goth," some said. No, said others. It was marijuana. Family alienation. Prescription drug abuse. Violent video games and movies. On and on the explanations went. A decade later, the journalist David Cullen carefully parsed the evidence and showed in Columbine that most of the sup-posed facts people used to construct their pet theories were false, which meant their theories were false - and al-most all the chatter following the massacre was empty noise.

But there was also a more fundamental mistake made.

Everyone assumed school violence was getting worse.

It seemed so obviously true. Just the year before there had been a school massacre in Jonesboro, Arkansas, where two students had murdered five people. And incidents before that.

One month after Columbine, a poll found that half of American parents feared for their children's safety at school.

But what everyone knew to be true wasn't: Following the Jonesboro massacre, the U.S. Department of Justice had collected national statistics on school violence and discovered that violence wasn't soaring. It was plummeting.

Unfortunately, the justice department's report on school violence was ignored. And the whole passionate de-bate proceeded on the basis of a belief that was indisputably false. The explanation for this sort of mass misperception begins with some basic psychology.

All our judgments are strongly influenced by thought processes that are not consciously directed. One such process is particularly important when we try to judge how prevalent some-thing is. It's called the "avail-ability heuristic."

How easy is it to think of an example of something? If it's very easy, that thing must be common. But if you have to think hard to come up with an example, it must be uncommon. That's all there is to the availability heuristic.

But remember that we don't consciously reason our way through this, so we may not know why we concluded that this thing is common, or uncommon, and we may not know the important role played by the example. We just feel that the conclusion is true.

Obviously, a vivid horror like Columbine will be easily recalled, even if news reports are brief and understated.

But news reports about Columbine were not brief and understated. They are shrieking and constant. The Pew Research Center found that 70 per cent of Americans said they followed the Columbine story "very closely," making it by far the biggest story of 1999 and the third-biggest of the decade. The Jonesboro massacre was the biggest story of 1998. Combine saturation coverage with the avail-ability heuristic and you've gone a long way to explaining misperception.

But there's another big problem. It's what I call "de-nominator blindness."

Should we worry that violence is soaring if there are five murders in a month? Too often, people will answer that with an emphatic "yes!" But the correct answer is "it depends." If it's five murders in a village of 100 people, be afraid. But if it's five murders in a nation of 34 million people, five murders could mean a dramatic decline in violence. The denominator is critical.

When people looked at school violence in the United States, far too many considered the number of incidents but ignored the fact that there were almost 70 million American children - and thus dramatically over-estimated the risk of school violence to any one American child.

Now, to go back to the original question: In light of the movie theatre massacre, what's wrong with the United States of America?

That question is based on the assumption that mass murders are a uniquely American phenomenon, or at least a plague that is vastly more prevalent in the United States, and getting worse.

Is that assumption true? I don't know. As far as I can determine, a careful statistical analysis hasn't been done so there is no equivalent of the justice department report on school violence to tell us the answer.

But there is good reason to think our perceptions might again be out of alignment with reality.

All the elements that distorted reality after Columbine are in place now: A vivid tragedy, the availability heuristic, endless sensational news reports. And "denominator blindness."

The population of the United States is 311 million, far more than in any other developed country (which is the peer group within which meaningful comparisons can be made). For that reason alone, we should expect to see far more mass murders in the U.S. than elsewhere. Four times more than in Germany. Five times more than in Brit-ain. Nine times more than in Canada.

And rampages certainly do happen elsewhere. Nor-way, infamously. Germany, Australia, Japan. In 2010, a man in rural England murdered 12 people. In 2001, a man shot up the legislature in Zug, Switzerland, killing 14 people. In 2009, during a national holiday, a man slammed his car in-to a crowd, killing seven, in a quiet town in the Nether-lands. In 2006, Canada was torn by the Dawson College shooting.

Then there was the time a student went on a rampage at a college, killing 10 people. No, I don't mean the Virginia Tech massacre. I mean the 2008 incident in Finland, which happened less than a year after a shooting at a Finnish high school, which took the lives of eight people.

Most of us have forgot-ten these incidents, or never heard of them at all, be-cause the United States is the media centre of the world - particularly the English-speaking world - and murders outside the U.S. get a fraction of the attention de-voted to American tragedies in the American media and media around the world. (The sole exception was Anders Breivik's massacre in Norway, which featured an unprecedented number of fatalities and unsettling political overtones.) It's hard to exaggerate just how much that distorts reality.

Of course it's true that the homicide rate in the U.S. is much higher than in other developed countries. It's also true that guns are more prevalent. But we seldom hear that homicide rates and other measures of violence have been on a long-term downward trend in the U.S. since the 1970s. And most of us would be shocked to know that gun ownership has declined sharply with only one-third of American house-holds reporting that they own a gun today, compared to one-half in 1973.

Still, it may be true that the U.S. suffers a wildly disproportionate number of mass murders. What I have presented is not conclusive evidence that it does not. But it is reason to be skeptical of our perceptions and doubt that it does.

What's wrong with the United States of America? At least in this regard, the answer may be "nothing."