Underlying these very different responses are opposing positions on two distinct issues. One is whether the countries that, over recent decades, have largely missed out on development, are now on a path that will lead to convergence with the rest of humanity. The other is whether, if they are not on such a path, international public action can do anything about it.

On the former issue, Edward Miguel has the most informed challenge. The Bottom Billion was based on data that barely extended beyond the millennium. As he notes, since then Africa has at last experienced modest growth, and this may be the beginning of the sustained acceleration that would be necessary for convergence. But we do not know how much of the recent upturn was due to the commodity booms, which have proved ephemeral, and how much was due to reforms that will prove more enduring. There were substantial improvements in macroeconomic management, while, according to widely accepted indicators, quality of governance is down and other aspects of the economy, such as business climate, have changed little. Whether the reforms that have occurred are sufficient to enable convergence is a matter of judgment.

I think that in the absence of effective external assistance, continued modest growth combined with continued divergence is likely. Miguel sounds more hopeful, but some economists are considerably less hopeful than even I am. Writing at the peak of the booms, Michael Clemens, in a long review of The Bottom Billion for Foreign Affairs, concluded: “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.” While we should hope that Miguel is right, we also should recognize the possibility of continued divergence. The latest theoretical developments, notably on the origins of state failure, provide a rigorous account of situations that will not be self-rectifying. As to the statistical evidence for autonomous recovery, the concerns about levels of statistical proof that have been directed against me can surely be turned even more forcefully against the proposition that divergence will be self-correcting.

In presenting his own optimistic case for autonomous recovery and convergence, William Easterly utterly misrepresents the argument of The Bottom Billion as being that of a “poverty trap” and then argues at length that there is no such trap. Indeed, he is so eager to criticize that he appears to have confused me with Jeffrey Sachs, the true evangelist of the poverty-trap thesis, which I explicitly reject. Certainly, growth is complicated. My argument is that a few salient factors have been important in the persistent divergence between rich and poor countries, while in no sense suggesting that this is an exhaustive account. An example is dependence upon natural-resource wealth, which makes the politics of development more difficult. Another is being landlocked without natural resources and surrounded by bad neighbors. Does Easterly deny that these are intractable problems that have contributed to the current stark disparity in living standards?

If we cannot be confident in autonomous recovery, then the second issue upon which the commentators focus is especially important: can international public action significantly assist convergence? In this context, those who are cheerily optimistic about autonomous recovery become dismissive pessimists. It is important to recognize that behind expressions of statistical fastidiousness lurks a recognizable philosophical hostility to public action that has no statistical foundation whatsoever. In critiquing the scope for international action, Easterly simply trots out disaster stories.

But since Easterly has mounted an aggressive critique, I will take a moment to defend myself. Eight years ago my colleague Anke Hoeffler and I proposed that three economic characteristics—low income, slow growth, and dependence upon natural resources—all made conflict more likely. Each of these propositions has since become considerably more robust. The balance of the statistical evidence suggests that the propositions are correct.

Many influential development experts agree with me that peacekeeping is one of those policies we should implement more vigorously.

As to method, my colleagues and I adopt the statistical approach of “general-to-specific,” in which insignificant variables are systematically and progressively deleted by the rule of stepwise deletion: this is not data mining. If it were, our results would not have been accepted by professionally refereed economics journals. Easterly’s twists on my remarks concerning new results—where a doubling of the sample and other improvements led to a minor refinement in our previous results on the effect of ethnic diversity—typify his bias. If doubling a data set should not lead to any changes in results, he should not have titled one of his papers, “New Data, New Doubts.” Our more recent results on peacekeeping are, as we readily admit, a first attempt. We very much hope to encourage or provoke other researchers to work on the question. But it should be said that in 2008 this work was assessed by a panel of Nobel-laureate economists who found it a sufficiently solid basis for their policy recommendation: peacekeeping was a good use of public money.

We should not, however, get bogged down in statistics. It would be absurdly naive to imagine that they alone, or even primarily, determine what policies are in order.

Many influential development experts agree with me that peacekeeping is one of those policies we should implement more vigorously. I was heartened to find during the Copenhagen Consensus 2 event that Andrew Mack, who leads the Human Security Report and who knows UN peacekeeping operations better than almost anyone, strongly supported the need for expanded peacekeeping. Similarly, in the above commentaries, Larry Diamond argues that we need to double or triple funding for multilateral peacekeeping operations. As he points out, “it has been an impressively successful and cost-effective type of international intervention.” Nancy Birdsall notes that the Center on Global Development includes peacekeeping as one of the criteria in the Global Development Index of commitment to development.

Nothing could better illustrate the true nature of the disagreement about peacekeeping than Easterly’s accusation of colonialism. This accusation is founded on coarse thought, not statistical rigor. Colonialism was an oppressive system in which non-democratic empires conquered territories and ran them according to the interest of their own elites. International peacekeeping is temporary, sanctioned by democratic governments whose electorates have no appetite for empire, and aimed at establishing governments that are accountable to their own citizens. Conflating peacekeeping with colonialism is too crude to constitute abuse.

Birdsall, Diamond, Stephen Krasner, and Mike McGovern all accept the case for international public action. Their comments get to the core of the serious range for policy discussion. McGovern’s position is that while there is sometimes a solid case for external security intervention, this cannot be generalized. McGovern is an anthropologist, so this argument from cultural specificity is scarcely surprising: it is the standard battleground between anthropology and other social sciences. His particular grouse is with my statement that heterogeneous countries lack the social unity needed for cooperation. His retort is that he knows of no empirical basis for such a claim. I refer him, among a mass of other literature, to the work of two economists named Miguel and Easterly. Unfortunately, there is considerable evidence that, where income levels are low, social heterogeneity limits the provision of public goods.

Birdsall, Diamond, and Krasner are all supportive of my contention that international public action is needed, though doubtless each proposes sensible alternatives to my specific recommendations. Birdsall perfectly captures my purpose. Do I imagine that as a result of a few calculations my policy proposals will be adopted? Of course not. Do I hope that I have opened a discussion on an issue that has to-date been too uncomfortable to face? Absolutely.

Structural difficulties in providing key public goods run so deep in certain countries that, in the coming decades, some of those countries will probably continue to diverge from global living standards. In this context, what international public action would be legitimate and effective? It will take a decade of debate to build a consensus answer to this question. While the policy discussion proceeds, citizens in the developed democracies should strive to better understand these issues: a more informed citizenry would improve the effectiveness of international action. If, by 2020, divergence has been replaced by autonomous recovery, Miguel will be vindicated. But an irrelevant debate does no harm, and if divergence persists, then, thanks to that debate, a basis for effective remedial international action will be available.