This is part 1 of my two-part series discussing theories and evidence on whether the NPT has Limited the Spread of Nuclear Weapons. The second part will be published soon. The Puzzle At a news conference in 1963, President Kennedy proclaimed: “I see the possibility in the 1970’s of…a world in which 15 or 20 or 25 nations may have these (nuclear) weapons. I regard that as the greatest possible danger and hazard” (Kennedy 1963). Fortunately, Kennedy’s dire prediction has not come to pass. In 1963, four countries were members of the nuclear club (US, USSR, UK, and France). Today, only five additional countries are members of the nuclear club (China, Israel, India, Pakistan, and North Korea). A critical puzzle is why there has been so little nuclear proliferation compared to early dire predictions. There are currently dozens of states that have the financial, scientific, and technical ability to build a nuclear bomb (ElBaradei 2004; Hymans 2006), yet few of these countries have produced their own nuclear weapons. NPT Optimists A wide range of scholars have argued that the Nuclear Nonproliferation Treaty (NPT) has played a significant role in limiting nuclear proliferation (Coe and Vaynman 2015; Nye 1981; Walsh 2005). On the surface, this assessment appears to be correct: more than 190 countries have signed the treaty since it entered into force in 1970. In the 50 years since then, only one country (North Korea) has withdrawn even though the treaty has an article specifying that countries can leave. The treaty requires countries to conclude comprehensive safeguards agreements that open them to International Atomic Energy Agency (IAEA) inspection. No country has successfully been able to produce nuclear weapons while subject to the NPT and its safeguards. NPT optimists point to two major reasons why the NPT reduces the risk of nuclear proliferation (Fuhrmann and Lupu 2016): Like other international institutions, the NPT regime facilitates the exchange of information: it reduces uncertainty about others’ behavior, capabilities, and intentions. Because of this, it increases the costs of cheating, which helps to bring states into compliance.Domestic politics also aid the treaty’s effectiveness. International agreements can empower domestic actors who have a vested interest in compliance. The NPT may enable operators of nuclear power plants to pressure leaders to remain in compliance, so a country’s civilian nuclear program is not disrupted. NPT Pessimists NPT pessimists argue that the NPT has done relatively little to curb the spread of nuclear weapons, and have cast doubts that the NPT has done little more than “screen” participants (Betts 2000; Solingen 2007; Hymans 2006). Proponents of this perspective make two primary related arguments: They argue that the NPT is an effect of nonproliferation, not a cause of it (Betts 1999, 69). States join the NPT because they have little or no intention of pursuing nuclear weapons, i.e. their preferences are aligned ex-ante with the treaty’s requirements. Therefore, the treaty screens for proliferation rather than constrain it.The NPT doesn’t have an actual enforcement mechanism. The NPT is little more than a “scrap of paper” that cannot constrain states when their national security is on the line. Weighing the Arguments To begin with, it is important to emphasize that we are unlikely to definitively identify the true causal effect of the NPT on nuclear proliferation empirically because history does not provide us with a counterfactual world in which there was no NPT (see part 2 in this series Statistically Identifying the NPT’s Effect on Nuclear Proliferation: Is it a Fundamentally Unidentified Question?). Despite this, the weight of the evidence suggests that the NPT likely has had a positive effect on constraining nuclear proliferation. It is difficult to quantify the extent to which the NPT has constrained proliferation, but it has at least made nuclear proliferation harder to do and easier to stop. The first argument made by nuclear optimists is especially difficult to refute. The comprehensive safeguards agreements (CSAs) that are required of non-nuclear NPT member states and undertaken by the IAEA have clearly increased transparency into states’ activities. While it is true that the NPT and IAEA safeguards have limitations, it is hard to argue that IAEA safeguards have not helped reveal information about state nuclear programs that would not have been revealed in absence of the NPT. History provides many examples of states who attempted to cheat the NPT by covertly pursuing proliferation activities. These include Libya, Iran, Syria, North Korea and Iraq. IAEA inspections were helpful in revealing information about some of these programs, especially in the North Korean and Iranian cases: as Paula DeSutter, the Bush administration Assistant Secretary for Verification and Compliance, stated: “The international community knew little about Iran’s vast secret nuclear program until it was exposed to the public, and until much of it was described in alarming detail in a series of IAEA reports beginning in early 2003.” Of course, IAEA safeguards are not perfect. The clearest example of the safeguards regime failing was the early 1990s Iraqi case: in 1991, coalition forces found evidence of a covert Iraqi nuclear program that had made significant progress towards producing a nuclear weapon. They found that the Iraqi program had a complete nuclear weapon design and roughly 36.3 kilograms of weapons-grade uranium (>90% U-235) (Nuclear Threat Initiative 2017). The IAEA defines that 25 kg of weapons-grade uranium is a significant quantity of weapons-grade uranium, i.e. enough to make a nuclear weapon. Arms control experts have since concluded that Iraq might have been less than a year away from their first nuclear test at that time. Although the 1991 Iraq war was fought to liberate Kuwait after the Iraqi invasion, it also may have had the unintended consequence of stopping Iraq from joining the nuclear club. In response to this case, however, the IAEA significantly strengthened its safeguards regime. For the IAEA, the 1991 Iraqi revelation was a wakeup call (Rockwood 2014). Between 1991 and 1993, the IAEA board made a number of decisions reaffirming the agency’s right and obligation to ensure that in a state with a comprehensive safeguards agreement, no nuclear material, whether declared or undeclared, is diverted to nuclear weapons or other nuclear explosive devices. In other words, the IAEA was now committing itself to verify not just the correctness of state declarations of nuclear material, but also verifying the completeness of state declarations. Correctness means that declared state facilities are not being used for weapons production. Completeness means verifying that there are no hidden facilities or undeclared stockpiles of materials that could be used for nuclear weapons, in particular weapons-grade uranium and plutonium. In addition, theIAEA developed an Additional Protocol approved in 1997 that has since been concluded by 133 countries that further strengthened the safeguards regime. The safeguards regime still has shortcomings, in particular in verifying completeness in countries that have not concluded an Additional Protocol. For instance, the IAEA largely missed the Al-Kibar facility in Syria that was preemptively targeted in a strike by Israel in 2007. Despite this, the safeguards regime is now the strongest that it has been in its history. It is important to note that the IAEA has been useful historically not just in detecting and sounding the alarm bell on cheating the NPT, but also in verifying compliance after cheating has been detected by the IAEA or by other means. This was the case when the IAEA oversaw the destruction and disarmament of Iraq’s nuclear facilities in the 1990s, when the IAEA oversaw Libya’s compliance with the NPT after it admitted to and renounced its nuclear program in 2003, and in verifying Iran’s compliance with the 2015 Joint Comprehensive Plan of Action (i.e. the Iran nuclear deal). The IAEA is able to play this important disarmament verification role in large part because the NPT is now ubiquitous. Under Article III of the NPT, non-nuclear weapons states are required to open their nuclear facilities to the IAEA to verify that they are being used for peaceful purposes. The near-universal ratification of the NPT creates an international norm that puts extra pressure on states that have been accused of cheating, even if not by the IAEA, to open themselves to further IAEA inspection. When North Korea claimed that the IAEA was a tool of the United States to uncover North Korean military facilities and defense technologies, this largely fell on deaf ears in the international community because of the reputation that the IAEA had developed in working with a large portion of the world’s countries to verify compliance with the safeguards agreements required by the NPT. This reputation for independence was further cemented when Hans Blix, the former head of the IAEA, led the United Nations Monitoring, Verification and Inspection Commission (UNMOVIC) in Iraq in the early 2000s and found that there was no evidence of active weapons of mass destruction programs in contradiction to the Bush administration’s claims. The first argument made by NPT pessimists is that in a counterfactual world with no NPT, there would still be the same number of proliferators, and therefore the NPT has no causal effect on limiting proliferation. This argument is feasible, but to make it more convincing, pessimists need to make a case for why the first argument made by NPT optimists discussed above does not hold. On multiple occasions historically, NPT/IAEA safeguards appear to have helped provide information that was acted on in the form of sanctions and threats of force and in verifying disarmament after cheating had been detected. In addition, the presence of IAEA safeguards deters countries from undertaking proliferation activities in the first place because these activities are more likely to be detected, resulting in further scrutiny, sanctions, and possible conflict. We cannot observe this deterrence effect of the NPT historically because it is silent evidence (Taleb 2007). Unfortunately, the constraining effect of deterrence is difficult to prove because it is defined by the absence of actions that would have taken place in the counterfactual world without deterrence, and we cannot observe actions that do not happen. North Korea is the only case of an NPT member that was ultimately not deterred from producing and testing nuclear weapons. In that case, they pulled out of the NPT three years before their first nuclear weapons test in 2006. Clearly, North Korea thought that it was more beneficial to pursue a nuclear weapon program without IAEA safeguards present to provide information on their activities as they raced towards and ultimately developed nuclear weapons. In regard to the second argument made by nuclear pessimists, it is correct that the NPT was not designed with its own enforcement mechanism; it is up to individual countries and coalitions to provide enforcement. However, the NPT provides countries with information on when and where enforcement mechanisms should be used. Key tools in the toolbox of shaping the costs and benefits of potential proliferators are sanctions and the threat and use of force (Miller 2014; Fuhrmann and Kreps 2010; Kreps and Fuhrmann 2011). Historical examples of compliance enforcement include Israel’s pre-emptive strikes on the nuclear programs in Iraq (in the 1981 attack on the Osirak reactor) and in Syria (in 2007 attack on the Al-Kibar facility) and the international sanctions on the Iran before the JCPOA was signed. As Paula Desutter stated: “Compliance enforcement is about shaping the cost-benefit calculations of those who would consider engaging in proliferation.” Just because the treaty does not have its own enforcement mechanism does not imply that it is unhelpful with enforcement. In short, in a world without the NPT, it would be easier for countries to covertly proliferate as the Soviets did in 1948 and the Chinese did in 1964. Indeed, it was the experience of Chinese proliferation in 1964 that caught both the United States and the Soviet Union by surprise and caused the two superpowers to work together to create the NPT in the first place (Gavin 2012). The weight of the evidence discussed above suggests that a counterfactual world without the NPT may have had more proliferation than the world with the NPT, or at least nuclear proliferation would be easier to do and harder to stop. The Need for Future Research Despite evidence that the NPT has been helpful in restraining proliferation, there is still work to be done to understand how and why the NPT continues to work and under what conditions it may no longer work. Andrew Coe and Jane Vaynman present a convincing formal model and logic for how collusion between the Soviet Union and the United States and subsequent constraint of their allies played a role in restraining countries from pursuing nuclear weapons under the NPT during the Cold War (Coe and Vaynman 2015). However, the Cold War has now been over for over 30 years and the NPT has continued to go strong. Important international players including China, France, Argentina, and Brazil decided to join the NPT after the Cold War ended. In addition, NPT parties decided to extend the treaty indefinitely in 1995 five and a half years after the Berlin Wall came down. For these reasons, there appears to be a logic to the NPT that goes beyond superpower collusion. Discovering such a logic is a promising area for future research. Works Cited Betts, Richard K. 2000. “Universal Deterrence or Conceptual Collapse? 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