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Arms control treaties have a rich history in international relations. But do they work? More specifically, do formal arms control agreements promote peace by helping to short-circuit potentially dangerous arms races?

We investigated the effects of one prominent arms control treaty: the nuclear Nonproliferation Treaty (NPT). The NPT requires most states to refrain from building nuclear bombs and to accept international inspections designed to detect cheating (the treaty allows China, France, Russia, the United Kingdom, and the United States to possess nuclear arsenals). Has this nearly 50-year-old treaty limited the global spread of nuclear weapons?

This question has long been debated in scholarship and policy circles. One view holds that the NPT has restrained nuclear proliferation by preventing some key countries – for example, West Germany – from building nuclear bombs. Based on this line of thinking, the NPT helps explain why just 10 countries have built nuclear weapons to date – far fewer than have the capability to do so. The treaty worked, these scholars argue, by raising the costs of “going nuclear” and reducing uncertainty about adversaries’ capabilities and intentions, which together discouraged proliferation.

Others are more pessimistic about the treaty’s effects. The NPT has done relatively little to curb nuclear proliferation, they argue. Numerous states, such as Iraq, have cheated on their NPT commitments, suggesting that the treaty is incapable of reining-in determined proliferators. Moreover, these scholars question whether NPT members would have proliferated even in the absence of a treaty commitment.  They attribute the slow rate of proliferation to other factors, such as alliances that provide “nuclear umbrellas.” Japan, for instance, is nonnuclear because of its alliance with the United States – not because of the NPT.

Debate about NPT effectiveness persists, in part, because scholars have yet to design a study that can properly evaluate the effects of the treaty. Prior studies show that NPT membership is correlated with nonproliferation, but they cannot determine whether the treaty has caused states to remain nonnuclear. Countries decide whether or not to join the NPT. A relationship between treaty membership and nonproliferation, therefore, may emerge because states enter the agreement when they have already decided to refrain from seeking nuclear weapons. If that is the case, NPT commitments may be a consequence – rather than a cause – of preferences for a nonnuclear policy.

We designed a study that gets us closer to evaluating the causal effects of the NPT. To do so, we estimated the probability that countries will ratify the NPT based, in part, on their latent preferences for joining the treaty. We then made an “apples-to-apples” comparison: we linked states that are similar in terms of their affinity towards the NPT but make different decisions on treaty ratification. After matching NPT members (i.e., the “treatment” group) and non-members (i.e., the “control” group), we analyzed whether the two sets of countries seek nuclear weapons at a different rate.

We found that they do: states that joined the NPT are considerably less likely to seek nuclear weapons from 1970 to 2000 than their non-NPT counterparts. Thus, even after accounting for strategic selection into the treaty, we find that the NPT has restrained the spread of nuclear weapons.

Many scholars and policymakers are skeptical that treaties can change state behavior in the area of international security. To be sure, international cooperation on security issues can be difficult. Yet we provide new evidence that security-related treaties restrain states from pursuing policies they might otherwise prefer. International institutions may therefore play a greater role in promoting peace than many scholars believe.

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