In 2004, I visited the Darfur area three times, trying to bear witness to the slaughter of children and the burning of villages. I stepped over the desiccated carcasses of camels and goats to interview survivors still in hiding. I interviewed people who had seen men pulled off buses and killed because of their tribe and skin color, and I spoke to teenage girls who had been taunted with racial epithets against blacks while being gang-raped by the Sudanese-sponsored Arab militia, the janjaweed.

I was enraged by what I found and, as a New York Times columnist, wrote time and again about these atrocities on the op-ed page. Yet at first the public reaction seemed to be a collective shrug: Too bad, but isn't that what Africa is always like? People slaughtering each other? Anyway, we have our own problems.

My frustration was multiplied when Manhattan erupted in a controversy showing that even cynical New Yorkers can brim with empathy—for a hawk. A red-tailed hawk dubbed Pale Male, one of the best-known residents of the Central Park area, had become embroiled in a housing dispute with the Upper East Side co-op on which he had a nest. The co-op removed Pale Male's nest, outraging New Yorkers and generating considerable news coverage. Now, don't get me wrong: I was on Pale Male's side, but I also dreamed that the plight of people driven from their villages in Darfur or Congo could get the same sympathy as a homeless bird. Clearly, something was wrong with the way I and other humanitarians were approaching Darfur.

So I turned to the field of social psychology, trying to understand how I could craft my writing so that it would generate a response rather than a turned page. Over the past 20 years, there have been many studies that shed light on this question, and, increasingly, I've come to believe that those of us who care about human rights and global poverty can do a far better job in our messaging. Like Pepsi, humanitarian causes need savvy marketing. Indeed, they need it far more than a soft-drink company.

Good people engaging in good causes sometimes feel too pure and sanctified to sink to something as manipulative as marketing, but the result has been that women have been raped when it could have been avoided and children have died of pneumonia unnecessarily—because those stories haven't resonated with the public. So for God's sake, let's learn how we can connect people to important causes and galvanize a robust public reaction.

THE RECENT RESEARCH in social psychology offers a couple of central lessons. The first is a bit surprising: We intervene not because of stories of desperate circumstances but when we can be cheered up with positive stories of success and transformation. For example, one experiment found that people are quite willing to pay for a water-treatment facility to save 4,500 lives in a refugee camp with 11,000 people in it, but they are much less willing to pay for the same facility to save 4,500 lives when the refugee camp is said to have 250,000 inhabitants. In effect, what matters is saving a high proportion of people, not just a large number of lives. Paul Slovic, a psychology professor at the University of Oregon who has pioneered this field of research, notes that saving a large proportion of a group is very satisfying, while saving a small proportion seems like a failure—even if it's a high number. All this fits in with a large body of research that suggests that people do good things in part because it feels good. The irony: Altruism creates its own selfish reward. Or, to put it another way, nobody gains more selfish pleasure than those who act selflessly.

Unfortunately, the most cost-effective aid interventions tend to be the kind that are incremental and save only a small proportion of lives—and are thus least satisfying to the giver. For instance, my wife, Sheryl WuDunn, and I have recently published a new book, Half the Sky, arguing that educating and empowering women is the most effective way to fight global poverty and extremism. In the book we call on the U.S. government to adopt a program to help poor countries iodize their salt. Right now, about one-third of families in poor countries don't get enough iodine, and the result is not so much goiters as diminished intellectual capacity. Iodine is essential to brain formation for a fetus in the first trimester, and if a mother lacks iodine her child may end up mentally retarded. More commonly, children in such areas lose 10 to 15 IQ points, with girls particularly affected for reasons that aren't fully understood. This is a lifelong intelligence deficit and a significant burden on poor countries, and it can be resolved very cheaply; iodizing salt costs a couple of pennies per person per year.

Studies have suggested that iodizing salt brings real economic returns of nine times the cost—and yet we don't do it. The reason is, I think, that the results are statistical, not visible. You can never look at a child afterwards and say, "This girl would have been retarded if it weren't for iodized salt." All you can do is note that retardation rates fall and that, a decade later, school performance improves significantly.