Sachs stresses the ethical imperative of bold, concerted action and derides "armchair economists" who do not offer constructive alternatives. Easterly criticizes "planners" such as Sachs, seeing more promise in the "searchers" who have historically solved economic problems in a decentralized, piecemeal fashion. Sachs highlights sins of omission, Easterly sins of commission. (Both men are affiliated with my employer, the Center for Global Development, Sachs as a board member and Easterly as a fellow.)

Many of those working day to day on poverty reduction, in both rich and poor countries, express dissatisfaction with both sides in this argument. Sachs' exhortations do not convince many insiders because, according to the refrain, he promises too much and does not see the limits of money and technology. (One memorably described Sachs' book to me as reflecting the thoughts of a "great preacher, mediocre theologian, and lousy minister.") Yet even many of the battle-hardened assert that Easterly's doubts go too far; they see him as a wise but blithe naysayer, comforting cynics who would happily leave poor countries to their fates.

Paul Collier, an economist at Oxford, aims to fill the gap between the two poles. His new book, The Bottom Billion: Why the Poorest Countries Are Failing and What Can Be Done About It, is meant to surmount the Sachs-versus-Easterly debate by remedying what Collier sees as the limitations of each view. Collier wants to temper Sachs' grandiose claims by arguing that a worldwide aid plan will be ineffective and that assistance should be more focused in both space and time. And he seeks to temper Easterly's aid skepticism by arguing that less should be expected and that the instruments of assistance should extend far beyond mere aid. As he puts it, rich countries must "narrow the target and broaden the instruments."

Collier's first step is to show that the people most in need of our attention are not all those who are poor but those who are both very poor and caught in countries whose economies are not growing. This includes about a billion people living in the 58 countries that have low per capita incomes and have not grown over the long term. Most of these countries are in Africa, but they also include Bolivia, Cambodia, Haiti, Laos, Myanmar, Yemen, and several Central Asia countries. There are many very poor people in other places, such as India, but at least that country is starting to generate opportunities for larger numbers of people. The poorest people everywhere might need humanitarian assistance, but development assistance -- something distinct, for Collier -- should go primarily to those countries currently achieving no sustained growth.

For the countries of the bottom billion, the long-term problem is not that they have lacked "sustainable, pro-poor growth ... it is that they have not had any growth."

Clemens, befitting a Foreign Affairs article and his own considerable expertise, goes into depth parsing out where Collier contributes new, useful thinking and where he merely equivocates between Sachs and Easterly. Exact answers are beyond any of these thinkers, including Clemens, as he notes in his conclusion:

Helping the bottom billion will be a very slow job for generations, not the product of media- or summit-friendly plans to end poverty in ten or 20 years. It will require long-term, opportunistic, and humble engagement, much of it through public action -- built on a willingness to let ineffective interventions die and on a sophisticated appreciation of the stupendous complexity of functioning economies. The grievous truth is that although a range of public actions can and should help many people, most of the bottom billion will not -- and cannot -- be freed from poverty in our lifetimes.