The brutal murder of 20 children and seven adults in Newtown, Connecticut, has shaken Americans to the core as individuals, and requires a response from all citizens.
The United States seems to reel from one mass gun killing to another - there has been about one a month this year. Easy access to guns leads to horrific murder rates compared to those in other educated and wealthy societies. America needs to find a better way.
Other countries have done so. Between the mid-1970s and the mid-1990s, Australia had several mass shootings. After a particularly horrible massacre in 1996, the new prime minister, John Howard, clamped down. He required would-be gun owners to submit to a rigorous application process, and to document why they would need a gun.
Conditions for gun ownership in Australia are now very strict; registration and approval can take over a year. Mr Howard's government also implemented a "buy-back" policy, in which the government purchased weapons people no longer wanted.
The policy worked. Violent crime has not ended, but Australia's murder rate is down and since 1996 there has not been even one case in which three or more people have died (the common definition of a "mass shooting"). Before the policy change, there had been 13 such massacres in 18 years.
Yet the US refuses to act. The gun lobby remains powerful, and politicians are afraid to counter it. Given the shooting of then-Congresswoman Gabrielle Giffords in 2011, perhaps they even fear that they, too, might be targeted.
There can be little doubt that some societies are more steeped in violence than others, even controlling for obvious factors like income levels and education. The US homicide rate is roughly four times that of comparable societies in Western Europe. Latin America's homicide rates are even higher. Why?
US violence is rooted in US history. North and South American countries are "conquest" societies, in which Europeans ruled over multiracial populations.
In many of these countries, including the US, the European conquerors and their descendants nearly wiped out the indigenous populations, partly through disease, but also through war, starvation, and forced labour.
The US and many Latin American countries were also slaveholding societies, and slaves were commonly murdered.
The US also developed a populist belief that gun ownership is a vital protection against tyranny.
The US was born in a citizens' revolt against British imperial power, and so the right of citizens to organise militias to resist tyranny was a founding idea of the new country, enshrined in the Second Amendment to the US Constitution, guaranteeing "the right of the people to keep and bear arms".
Today citizens' militias are anachronistic, but gun owners use the amendment to defend individual gun ownership, as if that somehow offers protection against tyranny. A reckless, right-wing Supreme Court has agreed with them.
As a result, gun ownership has become perversely linked to freedom in a vast gun-owning American subculture.
But, instead of protection of freedom, Americans are getting fear, and a high death toll. The claim that gun ownership ensures freedom is absurd, given that most of the world's vibrant democracies have long since cracked down on private gun ownership. No tyrant has arisen in Australia since Mr Howard's reforms.
In the 21st century freedom does not depend on unregulated gun ownership. Indeed, America's gun culture is a threat to freedom, as witness the assassinations of presidents, senators, and other public leaders - and many more assassination attempts - over the years.
Yet US gun culture remains pervasive. America reels from one shooting disaster to the next, and on nearly every occasion, politicians dutifully declare their continued devotion to unregulated gun ownership.
No one even knows how many guns Americans hold. The number is estimated to be around 270 million, in a country of about 315 million people. One recent poll indicated that 47 per cent of households have a gun in them.
The horrific and heartbreaking shooting in Newtown was part of an increasingly common pattern - a specific kind of murder-suicide that has been carefully studied by psychologists and psychiatrists.
Loners, often with paranoid tendencies, commit these heinous acts as part of their own suicide. They plan carefully to kill the innocent as a way to take revenge on society and to glorify themselves as they take their own lives.
The perpetrators are not hardened criminals; many have no previous record. Many of them have struggled with mental instability for much of their lives. They need help - and society needs to keep guns out of their reach.
The US has suffered around 30 shooting massacres over the past 30 years, including this year's deadly dozen. Each time, gun owners scream that freedom would be eliminated if they were unable to buy assault weapons and 100-round clips.
The bloodbath in Newtown is the time for America to stop feeding this gun frenzy. Other countries show how to do it: regulate and limit gun ownership. America's real freedoms depend on sane public policy.
Jeffrey Sachs is director of the Earth Institute at Columbia University in New York and special adviser to the United Nations secretary general on the millennium development goals
* Project Syndicate