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Journal of Conflict Resolution
http://jcr.sagepub.com/content/53/2/161
The online version of this article can be found at:
DOI: 10.1177/0022002708330287
2009 53: 161 originally published online 30 January 2009Journal of Conflict Resolution
Matthew Kroenig
Proliferation
Importing the Bomb: Sensitive Nuclear Assistance and Nuclear
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161
Author’s Note: I would also like to thank the University of California’s Institute on Global Conflict and
Cooperation, the Center for International Security and Cooperation at Stanford University, and the Project
on Managing the Atom and the International Security Program at the Belfer Center for Science and
International Affairs at Harvard University for their support. Replication data are available online at
http://jcr.sagepub.com/supplemental.
Journal of Conflict Resolution
Volume 53 Number 2
April 2009 161-180
© 2009 SAGE Publications
10.1177/0022002708330287
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hosted at
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Importing the Bomb
Sensitive Nuclear Assistance and
Nuclear Proliferation
Matthew Kroenig
Department of Government
Georgetown University, Washington D.C.
Why do nuclear weapons spread? Using new data on sensitive nuclear transfers, this
article analyzes the determinants of nuclear proliferation. The author first describes a
simple logic of the technical and strategic advantages that potential nuclear
proliferators can gain by importing nuclear materials and technology from more
advanced nuclear states. The author then shows that sensitive nuclear transfers are an
important determinant of nuclear proliferation. In broader terms, the author find strong
support for a supply-side approach to nuclear proliferation. States that are better able
to produce nuclear weapons, due to either international assistance or domestic capacity,
are more likely to do so.
Keywords: nuclear proliferation; nuclear weapons; sensitive nuclear assistance;
supply side; nuclear energy
Why do nuclear weapons spread? Politicians, policy makers, and pundits often
worry that nuclear-capable states will provide sensitive nuclear assistance to
other states or terrorist networks, contributing to the international spread of nuclear
weapons.1
The idea that states that get help with their nuclear programs will be more
likely to acquire nuclear weapons has intuitive appeal, but international nuclear transfers
may have no meaningful effect on nuclear proliferation. Indeed, existing scholarly
approaches to nuclear proliferation have examined why states want nuclear weapons
(e.g., Sagan 1996/1997) and the relationship between domestic capacity and nuclear
acquisition (e.g., Singh and Way 2004; Jo and Gartzke 2007) but have not examined
the relationship between international nuclear transfers and the spread of nuclear
weapons. This raises an interesting question about the sources of nuclear proliferation:
does international nuclear assistance contribute to the spread of nuclear weapons?
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To answer this question, I begin with a simple logic of the technical and strategic
advantages that potential nuclear proliferators can gain by importing sensitive
nuclear materials and technologies from more advanced nuclear states. I argue that
states that receive sensitive nuclear assistance can better overcome the common
obstacles that states encounter as they attempt to develop a nuclear-weapons arsenal.
They can leapfrog technical design stages, acquire tacit knowledge from more
advanced scientific communities, economize on the costs of nuclear development,
and avoid international pressure to abandon a nuclear program.
Drawing on a new data set on the international transfer of sensitive nuclear materials
and technology, this article demonstrates that sensitive nuclear assistance is an
important determinant of nuclear proliferation. States that receive sensitive nuclear
assistance from abroad are more likely to acquire nuclear weapons. I also find that states
above a certain level of industrial development are more likely to acquire nuclear
weapons. Taken together, these findings provide strong support for the supply-side
approach to understanding the causes of nuclear proliferation advocated in this journal
issue (Gartzke and Kroenig 2009, this issue). States that are better able to produce
nuclear weapons, due to either international assistance or domestic capacity, are more
likely to do so. Cross-national variation in nuclear proliferation outcomes is best
explained not by analyzing which states want nuclear weapons, but by understanding
which states are able to get them.
Explaining Nuclear Proliferation
There is a vast scholarly literature on the causes of nuclear proliferation. DongJoon
Jo and Erik Gartzke (2007) have recently categorized this research into two
camps: arguments that focus on a state’s willingness to acquire nuclear weapons
(demand-side approaches) and those that privilege a state’s opportunity to acquire
nuclear weapons (supply-side approaches). The bulk of scholarly research on
nuclear proliferation has focused on demand. This school has sought to identify the
factors that drive states to pursue and abandon nuclear-weapons programs. Scott
Sagan (1996/1997) argues that there are three primary reasons why states seek
nuclear weapons. Sagan maintains that states in competitive security environments
desire nuclear weapons as a means to deter external aggression, that domestic political
lobbies (primarily the domestic nuclear complex) can encourage states to pursue
a national nuclear-weapons program for parochial reasons, and that international
norms of prestige or opprobrium associated with nuclear weapons can influence
states’ nuclear decisions. Sagan concludes that none of these causes is dominant but
that they are each in operation to varying degrees in different cases.
Other scholars have suggested additional factors that may influence a state’s demand
for nuclear weapons. Etel Solingen (1994, 1998, 2007) maintains that domestic
political coalitions and their associated economic development strategies determine
a state’s demand for nuclear weapons. “Liberalizing coalitions” are internationalist,
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pursue export-oriented industrialization strategies, and will be reluctant to jeopardize
international trade and investment on controversial foreign policies such as the
pursuit of nuclear weapons. On the other hand, states controlled by “inward looking,
nationalist, and radical-confessional coalitions” oppose liberalization, choose an
autarchic path to economic development, and are more likely to pursue nuclear weapons
because they face fewer international economic costs to doing so and because they are
more beholden to nationalist appeals.
Individual psychological drivers have also been invoked to explain a state’s willingness
to acquire nuclear weapons. Jacques Hymans (2006) argues that leaders’
conceptions of their countries’ national identities is the key to explaining state
demand for nuclear weapons. Other research has drawn on these and other factors to
explain why states pursue and abandon nuclear-weapons programs (e.g., Quester
1973; Paul 2000).
In contrast, the supply-side approach to nuclear proliferation recognizes that an
analysis of a state’s demand for nuclear weapons can only provide a partial explanation
for nuclear proliferation (Singh and Way 2004; Jo and Gartzke 2007; Meyer
1984; Lavoy 1993, 1995). Whether or not a state wants nuclear weapons is irrelevant
if it is unable to acquire them. States may badly desire nuclear weapons but lack the
technology, resources, and expertise required to build them. Moreover, opportunity
can shape willingness. States that could conceivably produce a nuclear-weapons
arsenal will face a great temptation to go nuclear. According to this view, “once a
country acquires the latent capacity to develop nuclear weapons, it is only a matter
of time until it is expected to do so” (Singh and Way 2004, 862). The supply-side
approach to proliferation claims that states with an advanced industrial capacity can
more easily create and maintain a nuclear-weapons program and are thus more likely
to acquire nuclear weapons than are less-developed states. This line of argumentation
has roots in earlier scholarship (Meyer 1984; Lavoy 1993, 1995) and has been
revived by recent quantitative analyses of nuclear proliferation (Singh and Way
2004; Jo and Gartzke 2007). The quantitative studies have found that measures of
economic development and industrial capacity are associated with a greater risk of
becoming a nuclear power. These authors do not consider, however, how the supply
of international nuclear assistance may advance a country’s ability to produce
nuclear weapons, nor do they explicitly examine the relationship between international
nuclear assistance and nuclear proliferation.
The literature on “proliferation rings” has argued that nuclear capabilities in
“second-tier” supplier states such as Pakistan, Iran, and North Korea could increase
the availability of nuclear materials and technology on the international marketplace,
which threatens the nuclear nonproliferation regime (Braun and Chyba 2004;
Chestnut 2007). Alex Montgomery (2005) has countered that without the tacit
knowledge that comes from deep experience with a nuclear-weapons production
program, states that receive nuclear assistance will still struggle to acquire nuclear
weapons. Yet, these scholars do not examine systematically the effect of nuclear
assistance on the spread of nuclear weapons.
Kroenig / Importing the Bomb 163
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Scholars have also examined the causes of international nuclear assistance.
Drawing on the nuclear deterrence literature, Matthew Kroenig (2009) has argued
that the spread of nuclear weapons is more threatening to relatively powerful states
than it is to relatively weak states. Applying this insight to the problem of sensitive
nuclear transfers, he finds that states are more likely to provide sensitive nuclear assistance
under three strategic conditions. First, the more powerful a state is relative to a
potential nuclear recipient, the less likely it is to provide sensitive nuclear assistance.
Second, states are more likely to provide sensitive nuclear assistance to states with
which they share a common enemy. Third, states that are less vulnerable to superpower
pressure are more likely to provide sensitive nuclear assistance. Matthew
Fuhrmann has examined why states trade in dual-use weapons of mass destruction
(WMD) technologies (2008) and why states sign civilian nuclear cooperation agreements
(2009, this issue). Fuhrmann also finds that strategic and not economic or normative concerns
drive states to export civilian nuclear technologies. Unlike this analysis, however,
these studies explore the causes but not the consequences of international
nuclear assistance.
Importing the Bomb
A state’s ability to produce nuclear weapons often hinges on the availability of
external assistance from a more advanced nuclear state. There are a number of common
hurdles that states face as they attempt to develop a nuclear-weapons program,
but sensitive nuclear assistance from a more advanced nuclear state can help a state
to overcome these technical and strategic challenges.2
First, the designs for many sensitive nuclear technologies, such as uraniumenrichment
plants and implosion-type nuclear weapons, are not available in the
public realm. States pursuing these technologies, without external assistance, must
fashion designs for these complicated and advanced technologies indigenously.
Second, the construction and successful operation of nuclear facilities requires much
trial and error. Previous scholarship has emphasized the importance of tacit knowledge
in successful nuclear-weapons programs (MacKenzie and Spinardi 1995; Montgomery
2005). For example, the operation of a gaseous-centrifuge uranium-enrichment plant
requires the spinning of large metal cylinders at a rate of 300 meters per second,
roughly the speed of sound. Inexperienced engineers often struggle to prevent the
cylinders from spinning out of control and crashing on the ground. The kind of trial and
error required for the indigenous development of advanced nuclear technology often
ends in failure. For example, from 1981 to 1991, Iraq tried and failed in multiple attempts
to produce highly enriched uranium using several different methods, including
gaseous centrifuge, chemical enrichment, ion exchange, and laser isotope separation,
before finally settling on electromagnetic isotope separation.3
Third, the development
of a nuclear-weapons infrastructure from scratch is an expensive enterprise.
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A state must, at a minimum, procure the relevant raw materials and technologies at
home or on the open market; develop an advanced industrial and nuclear infrastructure;
train and provide for a specialized cadre of physicists, mathematicians, engineers,
and metallurgists; and provide adequate finances to continue to develop and
support the program throughout its lifetime. For example, it is estimated that Iraq
spent many billions of dollars in its unsuccessful bid to develop nuclear weapons.4
Fourth, states striving for a nuclear-weapons capability must overcome these significant
technical challenges under intense international pressure. Other states, international
organizations, and nongovernmental organizations opposed to nuclear
proliferation apply a variety of economic, diplomatic, and military pressures to dissuade
states from fulfilling their nuclear ambitions. In 1981, for example, Iraq’s
nuclear reactor at Osiraq, then the centerpiece of Iraq’s nuclear program, was
destroyed by Israel in a preventive military strike.
International nuclear assistance can ease each of the challenges faced by potential
nuclear-weapon states. First, nuclear suppliers can provide the aspiring nuclearweapon
state with proven designs for nuclear technology. With a guaranteed design
in hand, scientists and technicians can leapfrog technical design stages and focus
their effort on replicating a model that has proven effective elsewhere. For example,
without access to Chinese nuclear bomb designs, it is believed that Pakistan would
have had great difficulty developing a design for the implosion-type nuclear weapon
that now constitutes its nuclear arsenal (Corera 2006, 46). Second, nuclear assistance
can reduce the amount of trial and error needed to successfully operate nuclear facilities.
States supplying nuclear assistance can construct and even operate nuclear
facilities for the recipient state. For example, when China provided Pakistan with
uranium-enrichment technology in the early 1980s, Chinese technicians remained in
Pakistan until the uranium-enrichment facility was fully operational (Jones et al.
1998, 50n57). In this way, the nuclear recipient benefits from the tacit knowledge
acquired by the scientific community in the more advanced nuclear state. Third,
importing sensitive nuclear technology can help states to economize on the costs of
nuclear development. Procuring sensitive nuclear assistance from abroad can be less
expensive than the indigenous development of a complete nuclear infrastructure. In
fact, previous research has demonstrated that states have often received substantial
amounts of sensitive nuclear materials and technology at little or no cost because
nuclear suppliers had a strategic interest in helping them to acquire sensitive nuclear
technology (Kroenig 2007). For example, from 1958 to 1960, the Soviet Union
“loaned” China the key component parts for the Lanzhou uranium-enrichment facility
and the Jiuquan plutonium-reprocessing plants, partly because Moscow feared a
U.S. attack on the Chinese mainland following the Second Taiwan Straits Crisis and
wanted to enhance China’s defensive and deterrent capabilities (Goncharenko 1998;
Lewis and Xue 1988). Fourth, and finally, sensitive nuclear assistance can help a
state to avoid international scrutiny. The receipt of sensitive nuclear materials and
technology from abroad can quickly remake a state without a nuclear-weapons
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program into a state with a latent nuclear-weapons capability, presenting the international
community with a fait accompli and preempting international efforts at dissuasion.
For example, France provided Israel with sensitive nuclear assistance from
1959 to 1965, transforming Israel from a state with a rudimentary, civilian nuclear
research program into a nuclear-weapon state in less than a decade. The United
States was strictly opposed to nuclear proliferation in Israel, but by the time U.S.
intelligence agencies recognized the extent of Israel’s nuclear program, the United
States had few remaining policy options to dissuade Israel from its nuclear course
(Richelson 2006; Cohen 1998).
The above discussion suggests that the receipt of sensitive nuclear assistance
increases the probability of nuclear proliferation. This logic leads us to the central
hypothesis of this article:
Hypothesis 1: States that receive sensitive nuclear assistance will be more likely to acquire
nuclear weapons.
There are clearly other, alternative explanations for why states acquire nuclear
weapons. I therefore control for a wide set of opportunity and willingness determinants
of nuclear proliferation. I discuss these variables in the below sections in
which I describe the data and examine the evidence for the above hypotheses.
Empirical Analysis
To examine the relationship between sensitive nuclear assistance and nuclear proliferation,
I use qualitative and quantitative research methods. Nuclear proliferation
and sensitive nuclear assistance are both rare events. From 1945 to 2000, the time
period under investigation, nine countries acquired nuclear weapons and, of these,
three (Israel, China, and Pakistan) received sensitive nuclear assistance.5
The relatively
small number of positive cases allows me to examine the role of sensitive
nuclear assistance in the positive cases and to compare these countries to other similar
countries that did not receive sensitive nuclear assistance. The qualitative analysis
is only the first step, however. To analyze the relationship between sensitive
nuclear assistance and nuclear proliferation in the entire universe of cases and to
control for potentially confounding factors, a large-N statistical analysis forms the
core of the empirical investigation.
Case Studies
A brief review of important cases of nuclear proliferation demonstrates that assistance
from abroad can be an important factor in determining whether a state eventually
acquires nuclear weapons. For example, in 1958, Israel’s nuclear program
consisted of nothing more than a national atomic-energy commission and a small
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research reactor at Nahal Soreq. From 1958 to 1965, however, France provided sensitive
nuclear assistance to Israel, which greatly enhanced Israel’s ability to produce
nuclear weapons. France constructed a large, plutonium-producing nuclear reactor
and a plutonium-reprocessing facility at Dimona, transferred a nuclear-weapon
design, trained Israeli scientists at nuclear facilities in France, and allowed Israeli
observers at French nuclear-weapon tests (Cohen 1998). By 1967, after seven full
years of sustained French assistance, Israel was able to assemble its first nuclear
weapon.
Other states with nuclear arsenals received substantial assistance from abroad. In
fact, much of the history of nuclear proliferation can be read as a history of a chain
of cases of nuclear assistance. From 1958 to 1960, the Soviet Union provided China
with key component parts for uranium-enrichment and plutonium-reprocessing
plants and trained Chinese technicians, contributing to China’s ability to conduct its
first nuclear-weapon test in 1964 (Lewis and Xue 1988). Thereafter, China itself
became a nuclear supplier. From 1981 to 1986, China transferred significant quantities
of highly enriched uranium, uranium-enrichment technologies, and a nuclearweapon
design to Pakistan (Corera 2006). While Pakistan refrained from testing a
nuclear device until 1998, it is believed that with China’s assistance, Pakistan assembled
its first nuclear weapon in 1990 (Jones et al. 1998, 132, 140n). More recently,
from 1987 to 2002, Pakistan, with the help of Pakistani nuclear scientist A. Q. Khan,
distributed sensitive nuclear technology and materials to Iran, Libya, and North
Korea.6
Since the end of cooperation in 2002, Libya has agreed to give up its nuclear
program, but North Korea tested its first nuclear device in 2006, and Iran is making
steady progress on developing its nuclear capability. In fact, according to a 2007
U.S. National Intelligence Estimate, Iran may be able to produce nuclear weapons
as early as 2009, thanks in large part to uranium-enrichment assistance from
Pakistan.7
On the other hand, states with a persistent demand for nuclear weapons but that
were unable to acquire substantial international assistance failed to sustain national
nuclear-weapons programs. Egypt, during the course of many decades, has been
rebuffed in numerous attempts to secure an international nuclear supplier and to this
day lacks a nuclear-weapons arsenal. Beginning in the 1960s, Egypt sought sensitive
nuclear assistance, first from the Soviet Union and then from China, but was denied
by both states.8
There is also evidence to suggest that Egyptian officials may have met
with representatives from the A. Q. Khan network, but Egypt never received sensitive
nuclear assistance from Pakistan.9
Unlike Israel and other current nuclear-weapon
states that received substantial imports of sensitive nuclear materials and technology,
Egypt presently maintains a rudimentary civilian nuclear program. Other states that
have shown a historical interest in nuclear weapons but that have not yet acquired the
bomb include states that have received little sensitive nuclear assistance from abroad,
including Iraq and Taiwan, and states that have received no sensitive nuclear assistance
whatsoever, such as Argentina, Saudi Arabia, Syria, and South Korea.10
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Nuclear Proliferation Data
To test the effect of sensitive nuclear assistance on the spread of nuclear weapons,
I construct an original sensitive-nuclear-assistance data set. The data set contains
yearly information for all states in the international system from 1945 to 2000. The
unit of analysis is the country-year. I also draw on data from Singh and Way (2004)
and Jo and Gartzke (2007) to construct other nuclear proliferation variables.
Dependent Variable
The dichotomous dependent variable is nuclear proliferation. This variable measures
whether a state acquires nuclear weapons in a given year.11
To construct this
variable, I draw on the nuclear proliferation dates from Gartzke and Kroenig (2009,
this issue). A state is coded as acquiring nuclear weapons when it first explodes a
nuclear device, or if it does not immediately conduct a nuclear test, when it first
assembles a deliverable nuclear weapon.
Independent Variable
I construct independent variables to test the hypotheses about the effects of
nuclear assistance explicated above. Sensitive nuclear assistance is a dichotomous
variable measuring whether a state has ever received the key materials and technologies
necessary for the construction of a nuclear-weapons arsenal from a capable
nuclear-supplier state.12
Sensitive nuclear assistance takes three forms. States receive
sensitive nuclear assistance when they receive assistance in the design and construction
of nuclear weapons, receive significant quantities of weapons-grade fissile
material, or receive assistance in the construction of uranium-enrichment or plutoniumreprocessing
facilities that could be used to produce weapons-grade fissile material.13
Sensitive nuclear assistance excludes other types of nuclear cooperation less relevant
to the development of a nuclear-weapons program. I exclude the receipt of the
platforms that could potentially be used to deliver nuclear weapons, such as bombers
and ballistic missiles. The receipt of nonsensitive nuclear assistance, such as scientific
exchanges, assistance in the surveying and mining of natural uranium, fuelcycle
services, and the construction of research and power reactors, does not qualify
as sensitive nuclear assistance.
To code the sensitive-nuclear-assistance variable, I began with an online nuclearweapons
database maintained by the Nuclear Threat Initiative. I also drew on prominent
reviews on the proliferation of nuclear weapons and on historical studies of
countries’ nuclear-weapons programs. To be included in the data set, a case of sensitive
nuclear transfer had to be verified by at least two sources.14
A list of the cases
of sensitive nuclear assistance can be found in Table 1.15
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Control Variables
I also include a number of variables to control for other factors thought to influence
the likelihood of nuclear proliferation. All control variables are drawn from
Singh and Way (2004), unless otherwise specified.16
To assess a country’s domestic
capacity to produce nuclear weapons, I include a measure of economic development.
GDP is measured as a country’s GDP per capita in constant 1996 dollars. To test for
a nonmonotonic relationship between level of economic development and nuclear
acquisition, I include a squared term, GDP squared.17
Industrial capacity is a
dichotomous variable that measures whether a country produces steel domestically
and has an electricity-generating capacity greater than 5,000 MW. States above a
certain threshold of industrial development may be better able to support a nuclearweapons
program.
Scholars (e.g., Sagan 1996/1997) have argued that states may pursue nuclear
weapons to improve their security. If this is the case, we may expect that states in threatening
security environments may be more likely to acquire nuclear weapons. To test the
effect of a state’s security environment on its risk of acquiring nuclear weapons,
I include a rivalry variable that measures whether a state is involved in at least one
enduring rivalry (Diehl 1998; Bennett 1998). Alliance is a dichotomous variable that
assesses whether a state is in a defense pact with a nuclear-armed state. States under an
ally’s nuclear umbrella may have fewer incentives to develop nuclear weapons.
Kroenig / Importing the Bomb 169
Table 1
Cases of Sensitive Nuclear Assistance
Year of
Recipient First Assistance Supplier(s) Type of Assistance
China 1958 Soviet Union Plutonium reprocessing, uranium enrichment
Israel 1959 France Plutonium reprocessing, nuclear-weapon design
Japan 1971 France Plutonium reprocessing
Pakistan 1974 France, China Plutonium reprocessing, uranium enrichment,
nuclear-weapon design
Taiwan 1975 France Plutonium reprocessing
Iraq 1976 Italy Plutonium reprocessing
Brazil 1979 Germany Plutonium reprocessing, uranium enrichment
Egypt 1980 France Plutonium reprocessing
Iran 1984–1995 China, Pakistan Plutonium reprocessing, uranium enrichmenta
Algeria 1986 China Plutonium reprocessing
Libya 1997 Pakistan Plutonium reprocessing, uranium enrichment,
nuclear-weapon design
North Korea 1997 Pakistan Plutonium reprocessing, uranium enrichmenta
a. It is widely suspected that Pakistan provided a nuclear-weapon design to Iran and North Korea,
although, as of yet, there is no firm evidence to prove it.
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A number of variables gauge the institutional and economic determinants of
nuclear acquisition. Scholars have argued that democratic states, because of their
position in the “core” of the international system, may feel more secure and may be
less likely to pursue nuclear weapons (e.g., Chafetz 1993). On the other hand, democratic
states may be more beholden to nationalist appeals and to domestic political
lobbies that favor nuclear proliferation (Sagan 1996/1997). I include regime type,
which measures a country’s domestic political regime type, drawing on data from the
Polity IV index (Jaggers and Gurr 1995). Scholars have also argued that states that
are open to the international economy or that are pursuing a strategy of economic
liberalization are less likely to seek nuclear weapons, because they are reluctant to
risk international trade and investment on controversial foreign policies (Solingen
1994, 1998, 2007; Paul 2000). Openness assesses a state’s openness to the international
economy and is calculated as a country’s trade ratio (exports plus imports,
divided by GDP). Liberalization measures changes in a country’s trade ratio over
spans of three, five, and ten years.
Data Analysis
My central hypothesis concerns the importance of international nuclear assistance
for understanding nuclear proliferation. I use Cox proportional-hazard models
to test claims about the correlates of nuclear acquisition (Box-Steffensmeier and
Jones 1997). Robust standard errors are adjusted for clustering by country.18
Several types of statistical analyses prove useful in exploring the evidence for or
against each of the hypotheses described earlier. To begin the investigation, I examine
the simple bivariate relationship between sensitive nuclear assistance and
nuclear proliferation (Table 2, model 1). To control for potentially confounding factors,
I then evaluate the effect of sensitive nuclear assistance after including the control
variables (Table 2, model 2).19
I then estimate a trimmed model that includes
only the variables that were statistically significant in the previous model (Table 2,
model 3). To assess the relationship between sensitive nuclear assistance and
nuclear proliferation among the states that actively pursued nuclear weapons, I use
a censored hazard model of the risk of nuclear acquisition contingent on a state’s
possessing a nuclear-weapons-production program (Table 2, model 4), as measured
by Jo and Gartzke (2007).20
I first evaluate the hypothesis that sensitive nuclear assistance is positively related
to nuclear acquisition. According to Hypothesis 1, states that receive sensitive
nuclear assistance will be more likely to acquire nuclear weapons than are similar
states that do not receive sensitive nuclear aid. Turning to the hazard models, we see
that the relationship between sensitive nuclear assistance and nuclear proliferation
is positive and statistically significant in each and every model. There is strong
empirical support for the causal significance of sensitive nuclear assistance for
understanding nuclear proliferation.21
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Next, I examine the control variables to assess the relative support for the supplyside,
as opposed to the demand-side, approach to nuclear proliferation. International
assistance and domestic capacity are the primary means by which a state acquires
the capability to produce nuclear weapons. I have already found a relationship
between sensitive nuclear assistance and nuclear proliferation. GDP and GDP
squared are statistically significant and have positive signs on the coefficients in two
of the three models in which they are included, which provides some support for the
existence of a nonmonotonic relationship between economic development and
nuclear proliferation. Furthermore, we see that industrial capacity is positive and statistically
significant in every model, which demonstrates that states above a certain
level of industrial development are more likely to acquire nuclear weapons. Taken
together, the results provide strong support for the supply-side approach to understanding
nuclear proliferation. States that can more easily produce nuclear weapons,
because of international assistance or domestic capacity, are more likely to do so.
Turning to the demand variables, we find that rivalry is positive and statistically
significant in two of the three models in which it is included. Consistent with securitybased
approaches to nuclear proliferation and the findings of previous quantitative
studies (Singh and Way 2004; Jo and Gartzke 2007), states in a threatening security
environment are more likely to acquire nuclear weapons. Next, we find that the protection
provided by a nuclear umbrella appears to mitigate a state’s demand for
nuclear weapons. Alliance is negative and statistically significant in two of the three
Kroenig / Importing the Bomb 171
Table 2
Hazard Models of Nuclear Proliferation
Model
Independent Variable 1 2 3 4
Sensitive nuclear 3.323**** (0.951) 2.093**** (0.641) 2.024*** (0.786) 1.478** (0.694)
assistance
GDP 0.649*** (0.240) 0.625*** (0.227) 0.609 (0.378)
GDP squared –5.13e–5**** (1.54e–5) –5.69e–5*** (2.03e–5) –4.60e–5 (3.02e–5)
Industrial capacity 3.430**** (0.387) 3.606**** (0.497) 3.276**** (0.756)
Rivalry 2.382* (1.367) 2.371* (1.252) 1.517 (1.651)
Alliance –1.800* (1.061) –1.705* (0.945) –.8253 (0.835)
Regime type 0.114** (0.050) 0.112** (0.055) 0.112** (0.050)
Openness –0.022 (0.018) –0.027 (0.026)
Liberalization 0.028 (0.026) 0.059** (0.028)
Log likelihood –32.669 –18.784 –19.260 –15.413
Number of countries 156 156 156 18
Total observations 5,901 5,901 5,901 398
Note: Statistically significant parameter estimators are denoted by *(p = .10); **(p = .05); ***(p = .01); ****(p = .001).
Coefficients are estimates for Cox proportional hazard models; robust standard errors, adjusted for clustering by country,
are in parentheses. GDP = gross domestic product.
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models in which it is included. States that are in a defense pact with a nuclear-armed
state are less likely to acquire nuclear weapons. The sign on the coefficient of regime
type is positive and statistically significant in each model. This finding provides support
for the idea that democratic states may be more prone to nuclear proliferation
because they may be subject to pressure from domestic constituencies that favor
nuclear development. The alternate hypothesis that democratic states will be less
likely to acquire nuclear weapons because they form the secure core of the international
system does not find support. There is no discernable relationship between economic
openness and nuclear proliferation. Openness is not statistically significant in
any of the models in which it is included. States that are open to the international
economy are neither more nor less likely to acquire nuclear weapons. Neither is there
support for the idea that liberalizing states will seek to avoid controversial foreign
policies such as nuclear-weapons proliferation. Liberalization is statistically significant
in model 4 only, but the sign on the coefficient is positive. This suggests, contrary
to theoretical expectation, that liberalizing states may be more, not less, likely
to acquire nuclear weapons. Taken together, I find modest support for demand-side
approaches to the study of nuclear proliferation. Security environment and domestic
politics appear to play some role in shaping the likelihood that a state will acquire
nuclear weapons, but a state’s relationship to the international economy does not.22
Table 3 interprets the substantive effect of the variables that were statistically significant
in all of the above models on nuclear proliferation, using the results from
the uncensored hazard model reported in Table 2, model 2, and the censored hazard
model reported in Table 2, model 4 (see Table 3). The entries represent the percentage
change in the baseline hazard ratios of nuclear acquisition for a given change in
the independent variable. Focusing my comments on the results from the uncensored
model, Table 3 reveals that providing a state with sensitive nuclear assistance
increases the risk that it will acquire nuclear weapons by more than 700 percent.
Sensitive nuclear assistance has not just a statistically significant effect but also a
substantively significant effect on nuclear proliferation. Turning now to the substantive
effect of the control variables, Table 3 shows that industrial capacity has a substantive
effect on nuclear proliferation. States above a certain threshold of industrial
capacity have a hazard ratio of nuclear proliferation that is more than twenty-nine
times greater than the hazard ratio for similar states below the industrial-capacity
threshold. In contrast, regime type has a smaller substantive impact. Increasing a
state’s level of democracy by one point on the twenty-point scale increases the risk
that it will acquire nuclear weapons by only 12 percent.
The hazard analysis is only the first step, however. To address problems related
to nonrandom assignment of the treatment, I use nonparametric, matching techniques
as recommended by Ho et al. (2007).23
It is possible that the findings presented
above are biased, because states that receive sensitive nuclear assistance are
quite different from those that do not. Sensitive nuclear assistance is not randomly
assigned. If countries that received sensitive nuclear assistance and those that did not
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Kroenig / Importing the Bomb 173
Table 3
Substantive Effects of the Explanatory Variables on
the Likelihood of Nuclear Proliferation
Percentage Change in the Hazard Ratios
Variable Uncensored Censored
Sensitive nuclear assistance +711 +338
Industrial capacity +2,986 +2,546
Regime type +12 +12
Note: Hazard ratios on whether a state acquires a nuclear weapon are based on the hazard models reported
in Table 2, models 2 and 4.
are very different, the above findings could be largely the result of extrapolations
from the available data.
To correct for this problem, Ho et al. (2007) recommend preprocessing data using
matching techniques in which treated cases are matched with similar untreated
cases. Observations within the control group (in this study, states that did not receive
sensitive nuclear assistance) are matched as closely as possible with the treated cases
(states that did receive sensitive nuclear assistance) to form a matched subsample of
data. This allows the researcher to make inferences about the causal effect of sensitive
nuclear assistance based on a comparison of the most similar cases. Matching
reduces the role of functional form and specification assumptions of the parametric
model, resulting in more reliable causal inferences. When comparing cases in which
other causal variables are as similar as possible, any remaining differences between
the cases can be attributed to the treatment. To adjust for any remaining imbalances,
Ho et al. (2007) recommend using the same parametric model one would have
applied to the entire data set on the matched subsample of data.
To begin the analysis, I first identify the confounding factors on which to match
observations. Confounding factors are those variables that may influence the dependent
variable conditional on treatment, may be correlated with the treatment variable,
and are causally prior to treatment. According to Ho et al. (2007, 216), “All
variables in Xi that would have been included in a parametric model without preprocessing
should be included in the matching procedure.” I include, therefore, as confounding
factors, the control variables detailed above: GDP, GDP squared, rivalry,
alliance, regime type, openness, and liberalization.
Next, to preprocess the data, one-to-one nearest neighbor matching with replacement
was used, using GenMatch (Sekhon forthcoming; Sekhon and Diamond 2008;
Sekhon and Mebane 1998). Table 4 presents the before and after balance statistics,
using five standard indicators of balance: the difference in means; the p values from
a t-test on the difference of means; where possible, the p values from a K-S test of
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174
Table4
BalanceStatistics
MeanMeant-testK-StestVar.ratioMean
Std.
VariableTreatedControlpvaluepvalue(Tr/Co)eQQDiff.
GDPBeforematching7,057.7005,452.5000.0000.0001.0770.114
Aftermatching6,943.9006,608.6000.5740.1441.0630.046
GDPsquaredBeforematching86,540,299.00063,991,650.0000.0310.0000.9760.105
Aftermatching84,853,891.00078,135,099.0000.6250.1441.1540.045
IndustrialcapacityBeforematching0.7460.2290.0001.0790.259
Aftermatching0.7510.7260.5710.9400.012
RivalryBeforematching0.7610.2690.0000.9290.246
Aftermatching0.7660.7410.5640.9340.012
AllianceBeforematching0.4620.4660.9151.0040.002
Aftermatching0.4530.5320.1110.9950.040
RegimetypeBeforematching–0.523–0.2740.6470.1240.9680.035
Aftermatching–0.692–0.5570.8600.1140.9360.049
OpennessBeforematching39.03052.4320.0000.0000.3700.096
Aftermatching38.30438.6210.9020.2731.2390.028
LiberalizationBeforematching–0.1952.5870.0080.1950.6970.033
Aftermatching–0.1850.0440.8690.3301.0370.029
Note:GDP=grossdomesticproduct.
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similar distributions; the ratio of the variances of the treated and control samples;
and the mean standardized differences from the QQ plot (Imai, King, and Stuart
2006; Sekhon and Diamond 2008).
The balance statistics indicate that excellent balance was achieved. The p values
on all t-tests are greater than .56, with the exception of the t-test on alliance, which
is still well balanced at .111. QQ statistics improve in all cases except alliance and
regime type, which were both well balanced before matching (p = .915 and .647 on
the t-tests, respectively) and continued to be well balanced after matching (p = .111
and .860 on the t-tests, respectively).
Next, I analyze the preprocessed data, using a Cox proportional hazard model.
Table 5 presents the effect of sensitive nuclear assistance on nuclear acquisition as
estimated by the Cox regression in the matched sample. I only present the coefficients
for the treatment variable in Table 5. The coefficients for the confounding factors
are substantively meaningless because I matched on those variables. The results
of the Cox estimation on the matched subsample provide further support for
Hypothesis 1. The sign on the coefficient is positive and statistically significant,
which demonstrates that states that receive sensitive nuclear assistance are more
likely to acquire nuclear weapons. Furthermore, the analysis on the matched data
suggests that sensitive nuclear assistance may have an even larger substantive effect
than indicated by the analysis performed on the unmatched sample. On the
unmatched sample, we saw that sensitive nuclear assistance increased the risk of
nuclear proliferation by more than 700 percent. In the matched sample, however,
sensitive nuclear assistance increases the risk of nuclear proliferation by more than
1,200 percent (not shown). If anything, it appears that not correcting for the nonrandom
assignment of sensitive nuclear assistance underestimates the effect of sensitive
nuclear assistance on nuclear proliferation.
Robustness Checks
I explore the robustness of my findings by examining the extent to which my
results depend on the coding of the dependent variable, model specification, and the
nuclear-proliferation behavior of a few key states. It is difficult to define precisely
when some states acquired nuclear weapons. For states that conduct a nuclear test,
the date of nuclear acquisition is quite clear. For nuclear-weapon states that did not
conduct nuclear tests, however, the date of the first assembly of nuclear weapons
requires an examination of the countries’ historical records of nuclear development
and some guesswork. Robustness checks performed using alternate codings of
nuclear proliferation reveal that the results are not sensitive to different measurements
of the dependent variable. Next, to ensure that my results were not being driven
by the inclusion of specific control variables, I reran dozens of models, omitting
right-hand-side variables one at a time. Again, the core results were not affected.
Finally, to assess whether the findings are being driven by the proliferation behavior
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of particular states, I dropped the observations containing certain key countries and
repeated the analysis. Sequentially removing the observations containing China,
Israel, and Pakistan and reestimating the models did not affect the findings.24
Conclusion
This article sought to explain why states acquire nuclear weapons. I found that in
order to explain patterns of nuclear proliferation, one must look to international
transfers of sensitive nuclear materials and technology. States that receive sensitive
nuclear assistance from more advanced nuclear states are more likely to acquire
nuclear weapons than are similar states that do not receive sensitive nuclear assistance.
The receipt of sensitive nuclear assistance helps potential nuclear proliferators
overcome the common obstacles that states encounter as they attempt to develop a
nuclear arsenal. By importing the bomb, states can leapfrog technical design stages,
benefit from tacit knowledge in more advanced scientific communities, economize
on the cost of nuclear-weapons development, and avoid international scrutiny.
In broader terms, this article provided strong support for the supply-side approach
to nuclear proliferation advocated in this issue (Gartzke and Kroenig 2009, this
issue). States that have the ability to produce nuclear weapons, either through international
assistance or domestic capacity, are much more likely to do so. In contrast,
this article found only modest support for key demand-side variables. This may be
because nuclear weapons provide states with a variety of security and diplomatic
benefits (Gartzke and Kroenig 2009; Gartzke and Jo 2009; Horowitz 2009;
Rauchhaus 2009; Beardsley and Asal 2009, all in this issue), which mutes demandside
differences across states. The scholarly study of nuclear proliferation should
place less emphasis on understanding which states want nuclear weapons and focus
greater analytical attention on examining which states can produce them.
The argument of this article began with the simple insight that the ability to construct
nuclear weapons spreads from state to state. As such, this argument about the
relationship between nuclear assistance and nuclear acquisition treats nuclear proliferation
as a transnational phenomenon. Scholarly approaches to nuclear proliferation
have focused largely on the characteristics of individual states and have failed to
176 Journal of Conflict Resolution
Table 5
Hazard Model of Nuclear Proliferation, Postmatching
Matched observations 280
Number of countries 48
Coefficient 2.552
Standard error 1.029
p value .013
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theorize fully the international dimensions of nuclear proliferation. Academic nonproliferation
studies have been criticized for a “tendency to isolate individual states
and to examine their unique motives for going nuclear prevent(ing) us from giving
due importance to the varieties of international collaboration that were common and
indispensable to all early developers of nuclear programs” (Abraham 2006, 55).
Indeed, it is somewhat peculiar that studies about the “proliferation,” “diffusion,”
and “spread” of nuclear weapons have not explicitly recognized that nuclearweapons
technologies and materials literally spread from state to state. A complete
understanding of nuclear proliferation, therefore, requires further research on the
causes and consequences of nuclear assistance.
Notes
1. See, for example, President Bush’s Statement on North Korea Nuclear Test, October 9, 2006,
available at http://www.whitehouse.gov/news/releases/2006/10/20061009.html.
2. I define sensitive nuclear assistance as the international transfer of the key materials and technologies
necessary for the construction of a nuclear weapons arsenal to a nonnuclear weapon state. A
complete list of sensitive nuclear materials and technologies is provided in the data section of this article.
3. Nuclear Threat Initiative (NTI) Web site, “Iraq Country Overview,” available at http://www.nti.org/
e_research/profiles/Iraq/Nuclear/index.html.
4. Ibid.
5. North Korea received sensitive nuclear assistance in the 1990s, but did not acquire nuclear
weapons, as defined here, during the time period under investigation. For data on nuclear proliferation
dates, see Gartzke and Kroenig 2009, this issue.
6. Pakistani assistance to Iran, Libya, and North Korea from 1987 to 2002 was state-sponsored,
according to any reasonable definition of the term. Recent evidence reveals that senior government officials,
including heads of state and Army chiefs of staff, actively supported the policy of nuclear transfer
(Corera 2006; Langewiesche 2007; Sagan 2006, 53).
7. Office of the Director of National Intelligence, Iran: Nuclear Intentions and Capabilities,
November 2007, available at http://www.dni.gov/press_releases/20071203_release.pdf.
8. Nuclear Threat Initiative (NTI) Web site, “Egypt Country Overview,” available at http://www.nti
.org/e_research/profiles/Egypt/index.html.
9. See, for example, “The A.Q. Khan Network: Case Closed?” Hearing before the Subcommittee on
International Terrorism and Nonproliferation of the Committee on International Relations, House of
Representatives, May 25, 2006.
10. From 2001 to 2007 North Korea may have helped Syria construct a nuclear reactor. Nuclear reactors
are not considered sensitive nuclear assistance. Some have questioned whether North Korea may have
also provided Syria with plutonium reprocessing capabilities, but there is no firm evidence that these
countries engaged in any sensitive nuclear cooperation.
11. I do not include variables measuring state decisions to explore, pursue, or possess nuclear weapons,
because my theoretical interest is limited to the effect of nuclear assistance on nuclear acquisition.
Nevertheless, robustness tests performed using these alternate measures of nuclear proliferation produce
similar results.
12. The line between nonsensitive and sensitive nuclear assistance is often fuzzy in practice, yet there
is a fairly widespread scientific consensus that sensitive fuel-cycle facilities, such as uranium-enrichment
facilities, represent a direct nuclear proliferation threat, while other less sensitive, civilian technologies
are relatively resistant to proliferation. By drawing the line between nonsensitive and sensitive nuclear
assistance at sensitive fuel-cycle facilities, my definition follows this preexisting consensus.
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13. Nuclear-weapons experts have long recognized that a necessary, and the most difficult, step to
building a nuclear arsenal is the acquisition of the weapons-grade fissile material that forms the core of
the nuclear device. International Agency Atomic Energy regulations assume that 8 kg of plutonium and
25 kg of weapons-grade, highly enriched uranium are sufficient for the construction of a basic nuclear
device. Assistance on fuel-cycle facilities includes the construction of complete facilities or the transfer
of key component parts for the construction of such facilities, such as centrifuges for uranium-enrichment
plants or hot cells for plutonium-reprocessing plants. Assistance on uranium enrichment includes assistance
on any of the various types of uranium-enrichment processes including jet-nozzle, gaseous diffusion,
gas-centrifuge, and laser isotope enrichment. For a primer on nuclear weapons and their construction, see
Jones et al. (1998, 317-22).
14. The secretive nature of sensitive nuclear transfers raises the potential for a missing-data problem.
It is difficult, however, for countries to maintain a secret nuclear program, and sensitive nuclear transfers
that countries attempt to conduct in secret generally become known within a few years. Given the time
frame of this study, which ends in 2000, I assess that missing data do not pose a significant problem to
this analysis.
15. See Kroenig (2009) for a description of the cases of sensitive nuclear assistance, a list and description
of selected cases that do not qualify as sensitive nuclear assistance according to the above definition,
an explanation of key coding decisions, and the sources used in coding the variable.
16. To date, Singh and Way (2004) is the only study that uses quantitative methods to assess the correlates
of nuclear acquisition. Jo and Gartzke’s study on nuclear proliferation (2007) examines the determinants
of nuclear possession, not nuclear acquisition.
17. Including a variable and its squared term in the model is a common method for testing for a nonmonotonic
relationship (Ramsey and Schafer 2002, 244-45).
18. Using logit or probit estimators or hazard models with a Weibull distribution to characterize the
baseline hazard function produces virtually identical results.
19. I conceive of the demand-side and supply-side variables as additive in their effects. In other words,
a state that has a strong desire for nuclear weapons will be more likely to acquire nuclear weapons, holding
all other factors constant, because the benefits of doing so may be higher. Similarly, a state that has
the ability to produce nuclear weapons will be more likely to proliferate, ceteris paribus, because the costs
of doing so may be lower.
20. As a robustness check, I replicated the findings of Singh and Way (2004) and Jo and Gartzke
(2007) and then added the sensitive-nuclear-assistance variable to their models. In each and every model,
sensitive nuclear assistance was positive and statistically significant.
21. To assess whether other less sensitive types of nuclear assistance also contributed to the international
spread of nuclear weapons, I created a dichotomous variable measuring whether a country had ever received
foreign assistance on the construction of a nuclear research or power reactor. Perhaps surprisingly, the sign on
the coefficient of this variable was negative in all of the models in which it was included and was statistically
significant in some specifications. This may suggest that countries that receive international help on the construction
of a nuclear reactor are actually less likely to acquire nuclear weapons. The other results were unaltered.
A possible explanation for this finding is that one of the grand bargains of the Nuclear Nonproliferation
Treaty (NPT) may be paying off. Countries may be willing to trade the opportunity to develop nuclear
weapons in exchange for international assistance on basic nuclear research and energy production.
22. In various tests, I also included a dichotomous variable gauging whether a state was a member of
NPT. The variable was negative and statistically significant in all of the models in which it was included,
suggesting that countries that are members of the NPT are less likely to acquire nuclear weapons. The
other results remained unchanged.
23. Methodologists disagree about how best to apply matching techniques to time series data. Here, I
employ matching analysis merely as an additional check on the robustness of the findings presented above.
24. When I deleted any two of these three states and re-estimated the models, sensitive nuclear assistance
was no longer statistically significant. This result is not terribly surprising, given that dropping two
of these three states leaves only a single positive case of a country that received sensitive nuclear assistance
and then went on to also acquire nuclear weapons.
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