Consider the salient truth about international concerns

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Our attitudes to risk are governed, not by their incidence, but by their salience. Subjective assessments emphasise the salience of risks. For politicians, salience is all and that is why we are at war on terror.

Last week Bill Clinton, former US president, brought the great, the good and, above all, the rich to New York to discuss important global problems. But what makes a global problem “important”?

We mostly judge risks by their salience. Alone at home a few weeks ago, I heard a noise and, on investigating, disturbed a burglar who had walked through an unlocked door. I have since installed a new and sophisticated alarm system. The objective risk has not changed but my perception of it has. Salience explains why we go for a check-up when we hear that a friend has cancer or heart disease and why we drive more carefully after passing the site of a road accident. Salient risks are those that everyone is talking about or that we have recently encountered.

The risk of terrorist attack was not salient enough before September 11 2001 and too salient after. But, as you stand in line at airport security, observe that you are more likely to be killed by an object from space than in an aircraft crash. If the asteroid that hit the planet 100 years ago had landed in Trafalgar Square, it would have destroyed London. Because it only flattened a forest in remote Siberia, the risk is not salient. A bigger collision in Mexico 65m years ago led to the extinction of the dinosaurs and most other species. One day a similar event will destroy life on earth as we know it, if North Korean weapons of mass destruction, or a new pandemic, have not got us first.

The asteroid risk was briefly made salient by the movie, Armageddon. For most of human history, there was not much point worrying about space objects because there was nothing we could do. But, as the film showed, we might now be able to avert such a disaster: a nuclear explosion could deflect the path of the object.

How probable is the event? How serious are the consequences? How feasible and how costly is counteraction? These criteria, not salience, should determine the importance of global problems. But they are not always easy to apply. Some scientists believe that a shower of quarks could become a strangelet and pull all other matter into it until the whole earth, you and me included, was compressed into a sphere of 100m diameter. This is a conceivable outcome of experiments in high-energy physics. Other scientists note that no strangelet has ever been observed and doubt that one ever will be. But the trouble with apocalyptic projections is that all, except the last one, will be falsified.

Strangelets so lack salience that they are hard to take seriously: I cannot imagine that a single reader of this newspaper will lie awake tonight worrying about strangelets. But if events can be too rare to be salient enough to merit our attention, they can also be too frequent to be salient enough to receive our attention.

Each year, a million children die from malaria. Those who live in areas where the disease is endemic, and survive, acquire a degree of immunity. They remain vulnerable through their lives to episodes of disease, which sap their energy and productivity. Perhaps a billion or more malarial episodes occur every year.

Almost every study that has asked the three key questions of global issues – how likely, how serious, how preventable – has put the communicable diseases of malaria and HIV/Aids at or close to the top of the list. HIV/Aids acquired salience by spreading through the bathhouses of San Francisco, and has lost salience as the developed world has brought its epidemic under control and this could be achieved in the developing world. Malaria, eliminated from Europe and North America in the last century, has never been salient. But it is largely preventable – sleeping nets treated with insecticide alone dramatically reduce its incidence, and the discovery of an effective vaccine is a wholly realistic prospect.

World leaders emphasise issues that are salient to them and to their voters. But Bill Gates and Warren Buffett, the richest men in the world, have instead asked the questions – how likely? how costly? how amenable to action? – and put disease control at the head of their list of global issues. That judgment demonstrates the power of philanthropy over politics, of individual over collective action, of decision over discussion.

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