In any case, even if we were to grant that people deserve every dollar they earn, that doesn’t answer the question of what they should do with it. We might say that they have a right to spend it on lavish parties, private jets and luxury yachts, or, for that matter, to flush it down the toilet. But we could still think that for them to do these things while others die from easily preventable diseases is wrong. In an article I wrote more than three decades ago, at the time of a humanitarian emergency in what is now Bangladesh, I used the example of walking by a shallow pond and seeing a small child who has fallen in and appears to be in danger of drowning. Even though we did nothing to cause the child to fall into the pond, almost everyone agrees that if we can save the child at minimal inconvenience or trouble to ourselves, we ought to do so. Anything else would be callous, indecent and, in a word, wrong. The fact that in rescuing the child we may, for example, ruin a new pair of shoes is not a good reason for allowing the child to drown. Similarly if for the cost of a pair of shoes we can contribute to a health program in a developing country that stands a good chance of saving the life of a child, we ought to do so.

Perhaps, though, our obligation to help the poor is even stronger than this example implies, for we are less innocent than the passer-by who did nothing to cause the child to fall into the pond. Thomas Pogge, a philosopher at Columbia University, has argued that at least some of our affluence comes at the expense of the poor. He bases this claim not simply on the usual critique of the barriers that Europe and the United States maintain against agricultural imports from developing countries but also on less familiar aspects of our trade with developing countries. For example, he points out that international corporations are willing to make deals to buy natural resources from any government, no matter how it has come to power. This provides a huge financial incentive for groups to try to overthrow the existing government. Successful rebels are rewarded by being able to sell off the nation’s oil, minerals or timber.

In their dealings with corrupt dictators in developing countries, Pogge asserts, international corporations are morally no better than someone who knowingly buys stolen goods — with the difference that the international legal and political order recognizes the corporations, not as criminals in possession of stolen goods but as the legal owners of the goods they have bought. This situation is, of course, beneficial for the industrial nations, because it enables us to obtain the raw materials we need to maintain our prosperity, but it is a disaster for resource-rich developing countries, turning the wealth that should benefit them into a curse that leads to a cycle of coups, civil wars and corruption and is of little benefit to the people as a whole.

In this light, our obligation to the poor is not just one of providing assistance to strangers but one of compensation for harms that we have caused and are still causing them. It might be argued that we do not owe the poor compensation, because our affluence actually benefits them. Living luxuriously, it is said, provides employment, and so wealth trickles down, helping the poor more effectively than aid does. But the rich in industrialized nations buy virtually nothing that is made by the very poor. During the past 20 years of economic globalization, although expanding trade has helped lift many of the world’s poor out of poverty, it has failed to benefit the poorest 10 percent of the world’s population. Some of the extremely poor, most of whom live in sub-Saharan Africa, have nothing to sell that rich people want, while others lack the infrastructure to get their goods to market. If they can get their crops to a port, European and U.S. subsidies often mean that they cannot sell them, despite — as for example in the case of West African cotton growers who compete with vastly larger and richer U.S. cotton producers — having a lower production cost than the subsidized producers in the rich nations.

The remedy to these problems, it might reasonably be suggested, should come from the state, not from private philanthropy. When aid comes through the government, everyone who earns above the tax-free threshold contributes something, with more collected from those with greater ability to pay. Much as we may applaud what Gates and Buffett are doing, we can also be troubled by a system that leaves the fate of hundreds of millions of people hanging on the decisions of two or three private citizens. But the amount of foreign development aid given by the U.S. government is, at 22 cents for every $100 the nation earns, about the same, as a percentage of gross national income, as Portugal gives and about half that of the U.K. Worse still, much of it is directed where it best suits U.S. strategic interests — Iraq is now by far the largest recipient of U.S. development aid, and Egypt, Jordan, Pakistan and Afghanistan all rank in the Top 10. Less than a quarter of official U.S. development aid — barely a nickel in every $100 of our G.N.I. — goes to the world’s poorest nations.

Adding private philanthropy to U.S. government aid improves this picture, because Americans privately give more per capita to international philanthropic causes than the citizens of almost any other nation. Even when private donations are included, however, countries like Norway, Denmark, Sweden and the Netherlands give three or four times as much foreign aid, in proportion to the size of their economies, as the U.S. gives — with a much larger percentage going to the poorest nations. At least as things now stand, the case for philanthropic efforts to relieve global poverty is not susceptible to the argument that the government has taken care of the problem. And even if official U.S. aid were better-directed and comparable, relative to our gross domestic product, with that of the most generous nations, there would still be a role for private philanthropy. Unconstrained by diplomatic considerations or the desire to swing votes at the United Nations, private donors can more easily avoid dealing with corrupt or wasteful governments. They can go directly into the field, working with local villages and grass-roots organizations.

Nor are philanthropists beholden to lobbyists. As The New York Times reported recently, billions of dollars of U.S. aid is tied to domestic goods. Wheat for Africa must be grown in America, although aid experts say this often depresses local African markets, reducing the incentive for farmers there to produce more. In a decision that surely costs lives, hundreds of millions of condoms intended to stop the spread of AIDS in Africa and around the world must be manufactured in the U.S., although they cost twice as much as similar products made in Asia.