The horror has become almost routine. This time, the massacre site was a movie theater in Aurora, Colorado, where accused shooter James Holmes murdered and injured dozens of moviegoers. In 1999, the scene was nearby Columbine High School. By some estimates, there are more than 20 mass shootings per year in the United States. And always the same question: Why?

When the US is compared to the rest of the world, one reason becomes obvious: while America may not have more homicidally insane people than other countries do, homicidally insane people can get their hands on guns more easily in America than they can virtually anywhere else.

According to a 2007 survey, the United States is far ahead of the rest of the world in terms of gun ownership, with 90 guns for every 100 citizens. With 5% of the global population, America has between one-third and one-half of the world’s civilian-owned guns – around 270 million weapons. And many studies show that the US far surpasses other developed countries in deaths from gun violence – 30,000 per year, most of them suicides, but more than 12,000 of them homicides – while guns injure 200,000 Americans annually.

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With these casualty figures, one would think that gun-control laws would be a much higher national priority in America than the far more loudly hyped fight against terrorism. After all, ever since the terrorist attacks of September 11, 2001 left roughly 3,000 people dead, gun violence has killed almost 140,000 and injured more than two million.

But, when one looks more closely at why the US is so addicted to this unique kind of violence, the obvious is not so obvious. Why are gun-control laws so hard to pass?

One big reason is the gun lobby, which is one of the most heavily funded in America. Few legislators – Democrats and Republicans alike – care to take on the National Rifle Association. And many Americans believe that the US Constitution’s Second Amendment (“A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed”) permits individuals virtually unrestricted access to guns.

Indeed, many argue that the risk of gun-related deaths and injuries is the price that Americans must pay for the right to bear arms, which they regard as a powerful defense against tyranny. And, given how many ascendant tyrants have systematically disarmed the population they seek to control, it is difficult to dismiss this argument entirely.

But surely there can be a balance between Second Amendment rights and rational constraints on the ability of mentally unstable people to accumulate arsenals. For example, Colorado and many other states have sought to require more stringent background checks, aimed at preventing those with criminal records or obvious mental-health problems from arming themselves. But few such restrictions have been legislated – or have been left unchallenged by the gun lobby when they are.

Finally, opposition to reasonable gun-control laws in America is cultural, which is reflected in the many news reports following mass shootings that, refusing to admit that America could be wrong, downplay the striking contrast between US gun laws and those elsewhere. So, for example, journalists stress the rather pathetic high note of a grim reality: at least there are not more massacres and murders, and the numbers are stable.

Such coverage also tends to individualize and psychologize social pathologies – another deep-seated American trait, and one reinforced by the lone-cowboy frontier ethos that is central to US mythology (and to gun mythology). As a result, the media tend to focus on the need for better parenting and mental-health treatment. But little US coverage following a gun massacre assesses the impact of America’s health-care system, which is unaffordable to many, especially for those with mental-health problems.

That is why, in many US cities, it is common to see people with serious mental illnesses speaking to themselves and otherwise acting out, sometimes violently, on the street. This is a far less common sight in countries with functioning mental-health systems.

Many mental illnesses, such as schizophrenia and bipolar disorder, can cause auditory hallucinations that “command” the patient to commit acts of violence. Medication manages such psychotic symptoms. But proper diagnosis and treatment requires money, and funding is being cut.

Indeed, according to a report in February, US states have had to cut mental-health services by almost 10% in three years, threatening to “swamp emergency rooms and raise health-care costs for all patients.” But, if patients cannot get low-cost outpatient psychiatric care for chronic illnesses such as schizophrenia and bipolar disorder – which require continual management to adjust medication – there will also be more lethal violence, especially if guns are readily available.

Inpatient care, too, has been slashed. In recent decades, mental institutions and halfway houses have been closed in a wholesale way, often in the name of reforming care. But nothing has replaced these facilities, leaving many patients homeless and their severe psychotic symptoms untreated.

Despite the well-documented shortcomings of America’s mental-health services, few US policymakers are prepared to address the issue. Until they do, the easy availability of guns all but ensures that massacres like the one in Aurora remain a bitter American refrain.