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The Correlates of Nuclear Proliferation: A Quantitative TestAuthor(s): Sonali Singh and Christopher R. WaySource: The Journal of Conflict Resolution, Vol. 48, No. 6 (Dec., 2004), pp. 859-885Published by: Sage Publications, Inc.Stable URL: http://www.jstor.org/stable/4149798 .

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The Correlatesof Nuclear ProliferationA QUANTITATIVETEST

SONALISINGHBainandCompanyCHRISTOPHERR. WAYDepartmentof GovernmentCornellUniversity

Fearsofroguestates,withdrawalofcoldwar-erasecurityguarantees,afallingtechnologicalthreshold,andavailabilityto terroristorganizationsensurethatnuclearweaponsproliferationremainsa centralsecu-rityissueandthatdevelopinganadequatetheoryof proliferationrankshighon theagenda.A dataset onnuclearproliferationis constructedthatidentifiesthreedifferentstageson thepathto theweaponizationofnuclearweaponstechnology.Hazardmodelsandmultinomiallogitareusedtotesttheoriesof nuclearprolif-eration.Resultssuggestthatnuclearweaponsproliferationisstronglyassociatedwiththelevelofeconomicdevelopment,theexternalthreatenvironment,lackof great-powersecurityguarantees,anda low levelofintegrationintheworldeconomy.Keywords: Nuclear weapons proliferation;security; economic development; great-powersecurityguarantees;hazardmodels

Since the adventof the atomicage, nuclearweaponsproliferationhas been one of themajorsecurityissues facing the world. Afterthe end of the cold war,concerns aboutproliferationhavegrownratherthan subsided:thewithdrawalof superpowersecurityguaranteeshas created incentives for smallerpowers to acquirenuclearweapons, ahandfulof "rogue"states havesoughtnucleararms,Pakistanand Indiahavejoined theranks of overt nuclear powers, the technological threshold necessary to developatomic weapons is in reachof ever more nations,and the possibility of new nuclearpowersselling weaponsto terroristorganizationshas focused concerns.Controversyragesaroundthe worldoverU.S. plansto builda nationalmissile defense (NMD) sys-tem to fendoff emergingnuclearthreats,andscholarsdebate whether NMD will fanthe fires of proliferationor reduce the incentive for more states to acquirenuclear

AUTHORS'NOTE: Thanksto BenjaminGoldsmith,QuanLi, Alex Montgomery,KathleenO'Neill,KarthikaSasikumar,andMariaZaitsevaforhelpfulcommentsandto Efe Cummings,MitchellFagen,JuliaLevy,ChristinaPawroczenko,andArgilio Rodriguezforresearchassistance.All dataandcomputerfiles areavailableat www.yale.edu/unsy/jcr/jcrdata.htm.JOURNALOFCONFLICTRESOLUTION,Vol.48 No. 6, December 2004 859-885DOI:10.1177/0022002704269655? 2004 Sage Publications

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860 JOURNALOF CONFLICTRESOLUTION

weapons.Thus,more thanahalfcenturyintothe nuclearweaponsera,theproblemofproliferationis even morepressingthan at the dawnof the atomic age.This is troubling, given our lack of reliableknowledge about the determinantsofnuclearproliferation.Althoughthere is no shortageof academic theoriesto accountfor the spreadof nuclearweapons, few agreeon the validityorgeneralizabilityof thevariousalternatives.Policy makersand scholarsof internationalrelations suffer froman embarrassmentof riches in the diverse attemptsto explain decisions to acquirenucleararms,matchedby a correspondingpovertyof consensus aboutthe empiricalsupportenjoyed by variousperspectives.Authorsfrequentlyfind existing explana-tions unable to account for the details of a case of particularinterestand then seek toredresstheshortcomingby offering yet anotheralternative.Even as explanationspro-liferate,we do not know whichof theseperspectivesprovidesthe bestguide to under-standingdecisions to "go nuclear"and for forecasting potentialfutureproliferators.Weenterthedebateby suggestingthataquantitativetest of theoriesof nuclearpro-liferationcan providea usefulcomplementto thequalitative,comparativecase studymethodsthatpredominatein thisresearchagenda.'Wehighlightthreereasons aquan-titativetest is anappropriate,andindeedperhapsnecessary, supplementto theprolif-erationliterature.First,by samplingon thedependentvariable,mostqualitativestud-ies ignoreor underemphasizethe largenumberof countriesthat have neverpursuednuclear weapons. Ignoring nonproliferatorsruns the risk of underestimatingthestrengthof causal effects or, more rarely,erroneouslyaccepting a relationshipthatdoes not hold up in a widersample(CollierandMahoney,1996;Dion 1998;Geddes1990;King,Keohane,and Verba1994).Forexample,as Sagan(2000) pointsout,ana-lysts can almostalways identifyan ex-antesecuritythreatafter the fact of a prolifera-tionepisode. Yettheyoften fail to acknowledgethatsecuritythreatsareubiquitousbythe ratherelastic standardsoften employed by realists.In this situation,samplingonthe dependentvariablemay create a bias towardoveremphasizingthe explanatorypower of securitythreats.More prosaically,butequally important,samplingon thedependentvariable simply discards much valuable informationuseful in drawinginferences about the correlatesof proliferation.Quantitativeanalyses that includeobservationscoveringthefull rangeof varianceon boththedependentandindepend-ent variablescan providea useful complement to qualitative approachesthatdelvedeeply into a limited numberof cases.Second, theories of nuclearweapons proliferationoften offer, eitherexplicitly orimplicitly,probabilistichypotheses,yet theories arefrequentlytested as if theymakedeterministicclaims. For example, the simplest realist claim-that the more severeandimmediatea securitythreat,the morelikely a stateis to pursuenucleararms-isclearly based on a probabilisticlogic. Yet studies of nuclearproliferationoften findrealismwantingby identifyingone or a handfulof cases thatfail to conformtothereal-ist logic. Statisticalmodels basedon a probabilisticlogic of inference offera betterfitwith theoreticallogic thanthe deterministiclogic associatedwith theMillian methodsunderpinningcomparativecase studies (Lieberson1992, 1994).

1. Quantitativelarge-nstudies of nuclearweaponsproliferationhavebeen scarce,with few follow-ing up on the early efforts of Kegley (1980) andMeyer(1984) to providesystematicstudies.


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Singh, Way/ THE CORRELATESOF NUCLEARPROLIFERATION 861

Not only are hypotheses aboutnuclearweapons proliferationbest thoughtof asprobabilisticstatements,but it is also likely that there aremultipledeterminantsandcombinationsof factorsresponsiblefor decisions to pursuenucleararms.Yet manystudies implicitly rely on monocausal logics of inference, comparing competingexplanationsas if lookingfor the"magicbullet"thatwill, by itself, account for allpro-liferationdecisions or setting up dueling explanationsin a winner-take-allcontest.Whenapplyingthis univariatestandard,implicit in theMillianmethods that form thebasisof manycomparativecase studies(Lieberson1992), it is notsurprisingthatexist-ing explanationsare repeatedlyfound inadequatewhen they fail to account for allobservationsorall nuancesof particularcases. As analternative,the multivariatelogicof inference embodiedin multivariatestatisticalapproachesseems moreplausible.We complementqualitativeanalyses by providingsystematicstatisticaltests of anumberof prominentperspectiveson nuclearproliferationagainsta data set thatcov-ers 154countriesbetweentheyears 1945 and2000. Weemploy new dataon key deci-sionsalongthepathtonuclearweapons.Conceptually,we argueforviewing prolifera-tion as a continuuminsteadof a dichotomy, defining four stages of proliferation:nonoticeable interestin nuclearweapons, serious explorationof the weapons option,launchof a weaponsprogram,andacquisitionof nuclearweapons.With these datainhand,we use survivalmodels and multinomiallogistic regressionsto test hypothesesderivedfromthreebroadapproachesto nuclearweapons proliferation:(1) technologi-cal determinants,emphasizingthe role of economic developmentand the decliningcosts of weapons;(2) externaldeterminants,emphasizingincentivesprovidedby thesecurity environment; and (3) internal determinants, emphasizing a variety ofdomestic factorsrangingfromregime type to economic policies.Topreviewourmainfindings,we concludethatcontraryto theimpressiongivenbymuch of the recentproliferationliterature,currentapproachescorrectlyidentify sev-eral statisticallysignificant and substantivelyimportantcorrelatesof proliferation.Thedevelopmentthresholdargumentfavoredby technologicaldeterministsprovidesconsiderableleverage:the likelihood of proliferationrises sharplywithgrowthat lowlevels of developmentbut levels off andeven declines at high levels aftera thresholdhas been passed.Securityfactorsplay a powerful,perhapscentral,role in explainingproliferationdecisions. Participatingin enduringrivalriesor taking partin more fre-quent militarizeddisputes strongly increase the chances a state will pursuenucleararms,but crediblesupportfrom a great-powerally dampensthe temptation.Finally,offeringsome supportfor more recentarguments,we find that domestic factors,suchas moreexternallyorientedeconomic policies, reduce the likelihoodof proliferation,although regime type appearsrelativelyunimportant.Thus, if we move beyond thesearchfor a deterministic,univariateaccount of proliferationdecisions, our conclu-sions aboutthecurrentstateof knowledgeareconsiderablyless pessimisticthanthoseof manyauthors:theconventionalwisdomtakesusquiteaways towardunderstandingnuclearweaponsproliferation.Thesecondsectionof thestudyoutlinesthreeperspectiveson the causes of nuclearproliferationandreviewstherelevantliterature.The dataon nuclearweaponsprolifer-ationandexplanatoryvariablesareintroducedin the thirdsection.We thenanalyzethe


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datausing survivalmodels andreportresults in the fourth section. The final sectiondiscusses implicationsfor policy and futureresearch.

THREE PERSPECTIVES ON PROLIFERATIONGiventhe vast destructivepowerof nuclearweapons, it is not surprisingthat con-siderablescholarlyattentionhas been focused on understandingstates' decisions topursuenuclear arms.2The rich literatureon proliferationhas not ignored this chal-lenge: on thecontrary,it offers a wide arrayof answers to these questions.Wedividethis diverse literatureinto three chief strandsby focusing on the source of theimpetustowardnuclearweapons:technological,external,or domestic determinants.

TECHNOLOGICAL DETERMINANTSThe essence of the technologicaldeterministliteratureis its emphasison technol-ogy as the drivingforce behindweapons development.Once a countryacquiresthelatent capacity to develop nuclear weapons, it is only a matterof time until it isexpected to do so. Accordingto this literature,a country's latentcapacityto acquirenuclearweapons is determinedby economic prosperity,literacylevels, and scientificdevelopment:as it becomes easierandcheaperfor a state to acquirenuclearweapons,

it becomesmore andmorelikely thatit will do so. Statesmayachievethecapabilitytoassemblenuclearweaponsby anexplicitintentionaleffort or as animplicitby-productof economicand industrialdevelopment(Meyer 1984,Schroeer1984). Howevertheyget there, once states cross the threshold that enables the development of nuclearweapons, "theuniversalappealof nuclear arms and the inabilityof individualsandorganizationsto resist technological change"will eventually impel states to acquirethesearms,regardlessof theiroriginalintent(Lavoy 1993, 194).Moreover,astechno-logical advancesinexorablyreducethe costs of acquiringnuclearweapons, eventuallyevery state will find the costs of building nuclear weapons so low as to make thetemptationirresistible.Thisperspectivehas fuelled muchpessimismabout thepossibilityof limitingpro-liferation and proved influential with policy makers,but technological imperativeargumentssuffer from an obvious lack of empirical support,at least in their strongform.Indeed,therearenumerousexamplesof states thathave hadthetechnicalcapac-ity to build nucleararmsfor severaldecades but have neverattemptedto do so.3How-ever,we arguethatit is animportantstartingpointbecause no nationcan build nuclearweapons without attaininga minimaleconomic/technological capacity.4Therefore,

2. This literatureon proliferationwas not as vast and varied 10 or 15yearsago. There was a fairlybroadconsensus thattheproliferationpuzzlehad an obvious answer:states thatface a strongsecuritythreatwill develop nuclearweapons. Explanationsfor proliferationand dissatisfactionwith the commonsenseanswer have mushroomedover the past 15 years; Ogilvie-White(1996) providesa nice surveyof the richarrayof answers.3. ExamplesincludeItaly,Japan,Belgium, Canada,andGermany.4. They may,of course,attempttobuythem insteadof developingthetechnologythemselves,apathtaken at varioustimes by bothLibyaand Australia.


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Singh, Way/ THECORRELATESOF NUCLEARPROLIFERATION 863

we expect this literatureto play an importantrole in understandingthe proliferationpuzzlebyemphasizingsomethingakin to anecessarybutinsufficientcondition for thepursuitof nuclear arms.EXTERNAL DETERMINANTS

In contrast,what we call the external determinantsliteratureemphasizes the will-ingness ratherthantheabilityof states to buildnuclearweapons.Althoughauthorsinthe realisttraditionhave presenteda varietyof arguments,the literatureemphasizestwo factors:the presence (or absence) of a security threatand a security guaranteefroma powerfulalliancepartner.Realist explanationsemphasize the threat environment:the probabilityof a statepursingnucleararmsincreaseswiththeseverityof externalsecuritythreats.Statesin aself-help systemwill feel pressureto balanceagainstthe nuclearcapabilitiesof a rivalstateby developinga similardeterrent(Waltz1979). In addition,statesmay exercisetheirnuclearoptionwhenconfrontedwith aconventionalthreatfrom apowerfulrivalas a way of achievingeffective parityagainsta strongernation(Kapur2001; Potter1982;Quester1973, 1977).Thus,realismexplainsnuclearproliferationas a responseto externalsecuritythreats,which arethoughtto be endemic to the internationalsys-tem due to the pervasivelogic of the securitydilemma.However,despite the intuitiveappealof this argument,it cannotby itself explainsatisfactorily the proliferation puzzle. Even though most states that have soughtnuclearweaponshave faced some sort of securitythreat,a largenumberof countrieshave not turnedto nuclear force for theirdefense, despite securitythreats(Ogilvie-White 1996).Moreover,as Sagan(2000, 26) hasemphasized,empiricalresearchsup-portingthis argumentsuffers frompotentialselection bias:observinga proliferationepisode, researchersworkbackwardto find the securitythreatthatcan rationalizethedecision.Leavingasidequalmsaboutevidence, thechoice for nuclearweaponsis alsotheoreticallyunderdetermined.Evengivenaclearthreat,acquiringnuclearweaponsisnot theonly,ornecessarilythebest,strategyavailabletoa statethatis seeking security.A statehas atleastthreepotentialchoices in the face of a securitythreatby a powerfuladversary.It can strive to develop its own nuclearability,it can publiclyrefrainfrommakinganyefforts to acquireits own deterrentso as to reassurepotentialrivals of itsbenignintentionsinthehopeof tamingthesecuritydilemma,oritcanforgean alliancewith a powerfulally. Because stateshave multipleoptions in respondingto securitythreats,such threats should be thoughtof as a probabilisticcause of proliferation.Although securitythreatsalmostsurelyincrease the probabilityof pursuitof nuclearweapons, the "multiple-exit"problem familiar to internationalrelations scholarsmeans thattheydo not determinea state'schoice.

Another strandof realistthought arguesthata crediblesecurity guaranteefrom apowerfulstatecan dull the desirefornuclearweapons,evenforastatefacinga signifi-cantthreat.In thisview,theacquisitionof nuclearweaponsandtheforgingof alliancesserve as substitutesin the quest for security(Betts 1993;Davis 1993; Thayer 1995).The supremacy of a superpower ally serves to deter attacks on its nonnuclearprotdgdes,negatingtheir need to pursuenuclearweapons. Building on this security


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guaranteeargument,some have arguedthatbipolarityinhibitsnuclearproliferation,whereas multipolarity induces proliferation (Frankel 1993; Mearsheimer 1990).Underbipolarity,statesgravitateinto two well-structuredalliance systems anchoredby the two dominantpowers,whichprovidea securityumbrellafor the weaker statesin theirrespectivecoalitions, obviatingtheir need for nuclearweapons. However,asthesesecurityguaranteesarewithdrawnor renderedless crediblein thepost-cold warsystem, states arethoughtto face greaterincentives to acquirenuclearweapons(Betts1993).DOMESTIC DETERMINANTS

A third strandof the literatureshares the emphasisthatthe externaldeterminantsapproachplaces on understandingthe motives behind state actions, but it shifts thefocus from externalto domesticdeterminantsof nuclearproliferation.Althoughana-lysts have described a rangeof potentialdomestic pro-proliferationforces, we focuson four domestic factors thatmay have an influence on the choice to pursuenuclearweapons: democracy,liberalizing governments,an autonomous domestic elite, andsymbolic/statusmotivations.Buildingon the democraticpeace literature,some have arguedthat the pacifyingeffects of democracyand thecomplex interdependencebetween stateswill reduce thenumberof states that feel "fearfulor ambitiousenough"to pursuenuclearweapons(Chafetz 1993). Chafetz (1993), for example, divides the world into "core" and"periphery"states,emphasizingthatin thecurrentsystem, the coreconstitutesliberaldemocracieswith shared normsand values, which foster internationalcooperation,tame the securitydilemma,anddampen dangersof a nucleararmsrace. In addition,democraticstateson the peripherymay no longer seek nuclearweapons because anincreasingnumberof governmentswant to reapthe benefitsof full integrationinto thecore economic-political system. Consequently,the spreadof democracyreduces thelikelihoodthat stateswill pursuenuclearweapons by enlarginga zone of peace.Incontrast,some have notedthatthewidespreadpopularsupportfornuclearweap-ons acquisitionin India and Pakistansuggests that democraticgovernmentsmay betemptedto panderto nationalistpopulationsin an effort to boost theirpopularityandretainpower(Perkovich 1999). These temptationsmay be greatestduringperiodsofdemocratizationas competingelites face incentives to stirupnationalism(MansfieldandSnyder1995;Snyder2000) andmayseek to score apopularnationalisttriumphbyjoining the nuclearclub.Turningfrompolitical regimetype to economic policies switches the emphasistothe economic componentof domestic liberalizationin reducingtheappealof nuclearweapons.Accordingto Solingen(1994, 1998), rulingcoalitionspursuingliberaleco-nomic policies are morelikely tojoin regionalnuclearnonproliferationregimesthaninward-lookingnationalistandradical-confessionalgovernments;liberalizingcoali-tionstradeawaytheopportunityto makethe bomb for theopportunityto makemoney,perceivinglittle benefitfrommaintaininganambiguousstance.Ina similarvein, Paul(2000) arguesthat as the benefitsof economic integrationandinterdependencerise,stateswill forgo highly sensitiveactivities, such as nuclearacquisition,which might


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generate uncertainty,negativerepercussions,and heightened internationaltensions.As thecost of placing tradingandinvestmentties atriskincreases,states will becomemore cautious aboutpursingnuclearweapons.Yet anotherdomestic-level argumenthighlights the degree of autonomyaffordedthedomestic elite inchoosing to pursuenucleararms.A statemay seeknuclearweap-ons when the nationalelite, who maywant to develop nuclearweapons for parochialreasons,publicizesthe state'sinsecurityor its poorinternationalstandingto popular-ize theidea thatnuclearweapons providemilitarysecurityandpolitical power (Lavoy1993). Likeotherbureaucraticpoliticsarguments,thisargumentemphasizesthe abil-ityof particularisticintereststouse securityconcerns and nationalisticrhetorictopro-mote theneedfor a nuclearprogramfor their own strategic,parochialgain (Elworthy1986;Sagan2000). It is only a shortstepfurtherto conclude thatelites will enjoy less

autonomy and that debate will be more transparent in democracies than innondemocracies.This generatesanothervariantof regime-typearguments;this onesuggests that democratizationmay foster restraintby reducing the authority andautonomy of elites who might otherwise pursue nuclear weapons for parochialreasons(Barletta1999).Possessionof nuclearweaponscan also playanimportantsymbolicrole in a state'sself-image.Therefore,the decision to acquirenuclearweaponsortoexercise restraintoffers a stateanimportantnormativesymbolof modernity(Sagan2000). Accordingtothisperspective,muchof a state's behavioris determinedby sharedbeliefs and normsaboutconductthatis deemedappropriateandlegitimatein internationalrelations.Thecold calculationsof leadersaboutnationalsecurityandparochialbureaucraticinter-ests may be of less consequence than concerns with "nuclearsymbolism."Nuclearweapons areoften imaginedas fulfilling functions similarto those of flags, airlines,andOlympicteams:theyformpartof what a numberof stateshave believedtheymustpossess to legitimize their statusas modern states or to lay claim to a "great-power"role in internationalpolitics(Sagan2000). Fromthisperspective,statessufferingfroma perceivedstatusinconsistencyor those seeking validationas modernandpowerfulstatesarestrongcandidatesto pursuenucleararms.

PROLIFERATION DATADEPENDENT VARIABLE

At first itmay appearthatthechoice betweenpursuinganuclearweapons programandexercisingnuclearrestraintis astraightforwardbinarydecision,butfurtherreflec-tionquicklyrevealsgreaternuances.It can oftenbe a long andwindingroadfromini-tial efforttoexplosionof anucleardevice,andtherearemanydifferentstoppingpointson thepathwaytoproliferation.Some statesendupwith nuclearweapons,some stateshavemade(oraremaking)serious effortsto build nuclearweaponsbut neveractuallyacquirethem,andothersseriouslyconsiderbuildingnuclearweapons yet stopshortofactuallytakingfirmstepsto do so. To be sensitive tothesediversestagesandallow forrobustnesschecks across indicators,we devised multilevel indicatorsof nuclearpro-


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liferation.Conceptually,ratherthanthinkingof nuclearweapons status as a dichoto-mous variable,we conceive of "degreesof nuclearness"arrayedalong a continuum,rangingfromabsolutelynoeffortorinterest,at one end,topossessionof a vast nuclearweapons arsenal,at the other end. Operationally,we divide this continuuminto fourstages: acquisition of weapon capability, substantialefforts to develop weapons,explorationof the possibility of developing/acquiringweapons, and no interest oreffortwhatsoever.(See the onlineappendixfor sources usedincodingeachcountry.)

Firstexplosion/assemblyof weapons. This is the most reliable and accuratemea-sureof nuclearproliferation.Accordingto this indicator,everycountrythat has everexploded a nuclear device or assembled a nuclear weapon is coded as a nuclearproliferatorfrom the yearof its firstexplosion or possession of a functionalnuclearweapon until the date thatit abandonsits program.5However,not all countries thatpursuenuclearweapons have gone all the way to test or assemble nuclear devices.Therefore,we have devised two additionalindicatorsof nuclearproliferation.

Pursuitof weapons.Forvariousreasons,not all states thatpursuenuclearweaponsend up acquiringthem. Accordingly,we count every countrythat has ever made anactive effort to pursuenuclearweapons as a nuclearproliferatorfrom the year of itsfirsteffort.6Thus,all countrieswith nuclearweaponsprograms,withoutregardto thesize orstageof development,are coded as nuclearproliferatorsuntil the date thattheyabandontheirefforts.7To warrantinclusion in this category,states have to do morethansimplyexplorethepossibilityof aweaponsprogram.Theyhave totakeadditionalfurtherstepsaimed atacquiringnuclearweapons,suchas a politicaldecision by cabi-net-level officials,8movementtowardweaponization,or developmentof single-use,dedicated technology. Thus, Sweden, for example, is excluded from this categorydespite serious explorationof the nuclearoption over a numberof years because itfailed eitherto take anexplicitpoliticaldecisiondemonstratinga strongwillingness toacquireweaponsor to move beyonddual-use researchthat was necessaryfor atomicpowerbutalso useful for a potentialweaponsprogram(see sources for Sweden in theonline appendix).

5. In practice,the distinctionbetween assembly and explosion is importantin four cases. SouthAfricaassembledworkingnucleardevices butdeclined to test them. Pakistanis widely consideredto haveassembledready-to-usenucleardevices longbeforefinally testingthemin 1998,resultingintwo alternativecodings: one for assembly and one for first tests. We follow a similar process of includingtwo alternatecodings withIndia,which,accordingto severalsources,hadweaponsreadyforquickassemblyin 1988. Wealso code Indiaas crossing this thresholdtwice: once with the test in 1974, after which we judge it to dropbackdown,andagainin 1988.Clearly,consequentialpoliticaldecisions aboutcrossinga significantthresh-old andopeningupthecountrytocriticismwereentailedby both actions.Althoughtherangeof variationisnarrow,thereis some disagreementaboutwhen precisely Israelgainedfull-fledgedweapons capability.Inall cases, we assess sensitivityof ourfindingsto the particularcoding employed.6. The Soviet successor states that inherited nuclear arms and ultimately renounced them areexcluded from ouranalysis. They madeno independentpolitical decision to pursuenuclearweapons andthereforedo notqualify as proliferators.Intheconclusion,we discuss thedesirabilityof studyingtwo-waytransitions,which would includethese states' decisions to renounce their inheritedarsenals.7. Hence, this indicator includes countries that have ongoing nuclearprogramsbut that have nottested weaponsor preparednucleardevices for quick assembly.8. We also count the decisions of seniormilitaryleadersin the case of a militarygovernment.


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Explorationof weapons. Castingthe net even broader,we can include countriesthat seriously considered building nuclearweapons, even if they never took majorstepstowardthatend.This finalandmostcomprehensiveindicatorof nuclearprolifer-ation codes everycountryas anuclearproliferatorfromtheyearthatit firstconsideredbuildingnuclearweapons, as demonstratedby political authorizationto explore theoption or by linking research to defense agencies that would oversee any potentialweaponsdevelopment.Therefore,this indicatorincludes all statescoveredby the firsttwo indicatorsas well as all countries thatconsidereddeveloping nuclearweaponswithoutfollowing throughwiththeirplans andcrossingthe thresholdto level 2. Tak-ing Sweden as anexampleonce again,we include it because (1) atomicresearchwasconductedby a semipublic companyexplicitly linkedto andreportingto the defenseministry,and(2) decisions were explicitly madeby cabinet-level officials to developdual-use technology with high potentialutility for any futureweaponsprogram(seesources for Sweden in the country appendix).For eachcountry-year,everystatein oursampleis assignedto one of the four cate-gories. States thatclearlyandconvincinglyrenounceweapons or stop seriouslycon-sideringthem canmove backdown,from theexplorationlevel to the no-interestlevel,for example. Thus, when Sweden ceases to seriously consider nuclearweapons, itdropsback down to the no-interestcategory.EXPLANATORY VARIABLES

We group proxy explanatoryvariablesunder the headings of the three types ofexplanationssurveyedabove. TableI summarizesthe includedvariablesandtheoreti-cal expectations.9Technological Determinants

To assess a country's ability to develop and construct nuclear weapons, weemployed severalvariablesthattappedthe level of economic and industrialdevelop-ment.Wefocus on indicatorsof thegenerallevel of economic and industrialdevelop-ment,eschewingthose that arelikely to be endogenousto aninterestin nuclearweap-ons (suchas numberof nuclear scientists or metallurgists).'o

Gross domestic productper capita. Gross domestic product(GDP) provides aroughandreadyindicatorof the level of economic development.Whereasaggregateeconomic size indicates total resourcesavailable, per capita GDP more accurately9. We omit ratificationof the NuclearNonproliferationTreaty(NPT) because of obvious and sub-stantialendogeneity.Assessing the causalrole of the NPT in affectingdecisions towardnuclearweapons

poses a host of complex problemsthatwe planto treatseparatelyin a new study.10. Wealso constructeda timeseries of known uraniumdeposits,reasoningthatdomesticavailabilityof uraniumis likely to decreasethedifficultiesof mountingaweaponsprogram.However,incollecting thesedata,itbecameclear thaturaniumexplorationis endogenousto interestin nuclearweapons.Superficially,itmight appearthatdiscoveryof domestic sourcesof uraniumspursthe startof a nuclearweapons program;closer examinationof the cases reveals that an interestin nuclearweapons often spursintensive uraniumprospecting.


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TABLE 1TheoreticalExpectationsand MeasuresAnticipatedDirection

ExplanatoryVariable of Effect OperationalizationsTechnologicaldeterminismLevel of development Positive Grossdomestic product(GDP) percapita;energyconsumption per capitaIndustrialcapacity Positive Index based on steel productionand electrical-generatingcapacity;aggregateandper capitaelectricityand steel productionExternaldeterminants

Securitythreat Positive Participationin enduringrivalry;frequencyofmilitarizedinterstatedispute(MID) involvementSecurity guarantee Negative Alliance with great powerInternaldeterminantsDemocracy Negative Polity IV democracyscaleDemocratization Uncertain Change in Polity IV democracyscale (3-, 5-, and10-year periods)Global democracy Negative Percentageof democraciesamongstates in systemExposureto global economy Negative (Exportsand imports)/GDPEconomic liberalization Negative Change in trade ratio(3-, 5-, and 10-yearperiods)Dissatisfaction/symbolic Positive S score or Tau-bwith eitherglobal or regional

motivations hegemon

reflects level of economic development,which is most closely linked to the sophisti-cated technical,engineering,andmanufacturingknowledge necessaryfor the devel-opment and constructionof nuclear arms. Purchasing-powerparityGDP data aretakenfrom version 6.1 of the PennWorldTables(PTW) (Heston,Summers,andAten2002)." Toallow for thepossibilitythat the relationshipbetween economic develop-ment and thepursuitof nucleararms is curvilinear(e.g., becauseof a thresholdeffectwhereby achieving moderatelevels of developmentallows countries to act on latentnuclearambitions,althoughadditionalincrementsof wealth areunlikely to increasethe temptation),we also include a squaredterm.

Industrialcapacity index. To tap the level of industrialcapabilitiesuseful for anuclearweapons program,we createda dichotomous variable based on electricitygenerationand steel production.This variabletakes on a value of I if a countrybothproducessteel domesticallyandhas installedelectricity-generatingcapacity greaterthan5,000 MW and 0 otherwise.1211. Forcountries not covered by the Penn WorldTables (PWT), we turnedto Angus Maddison's(2002) dataof purchasing-powerparitygrossdomesticproduct(GDP)covering 124 countriesfrom 1950 to1998,convertinghis data frombase year 1990 to 1996 forcomparabilitywith thePWT data. Forcountriesstill not covered,we drew on Gleditsch's (2002) expansionof the InternationalMonetaryFund's (IMF's)GDP data.12. To create the index, we used data on electricity-generatingcapacity from the United Nations(EnergyStatisticsYearbook,variousyears;StatisticalYearbook,variousyears)andon steelproductionfrom


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Energy, electricity, and steel production and consumption.To supplement theindustrialcapacityindex, we used dataon bothaggregateandper capita energy con-sumption,electricity productionandgeneratingcapacity,and steel production.13EXTERNAL DETERMINANTS

Enduringrivalry.Although perceptionsof securitythreatscan vary substantially,participationin anenduringrivalrycan be safely takenas an indicatorof a significantsecuritythreat,especially because the vast majorityof wars and militarizeddisputesoccur in thecontextof enduringrivalries(Diehl 1998). Drawingon Bennett's(1998)codingforenduringrivalriesand theirdates,we createadichotomousvariableindicat-ing whethera statewas involvedin one or moreenduringrivalriesin a given year.Frequencyof dispute involvement.As an alternatemeasure of the intensityof thesecuritythreat,we calculatedthe5-year moving averageof the numberof militarizedinterstatesperyear in which a state is involved,drawingon version 3.0 of the milita-rized interstatedispute(MID) data set (Ghosnand Palmer2003).Securityguarantee.The allureof nuclearweaponsas an avenue to security maybeattenuatedby a securityguaranteein theform of a defense pactfrom a nuclear-armedgreatpower.14Drawingon Singerand Small's(1982) standardlist of greatpowers,we

count the United States,the Soviet Union/Russia,the United Kingdom(from 1952),France(from 1960), and China(from 1964) as nuclear-capable,great-powerallies.Basingourcodingon version3.0 of theCorrelatesof Waralliancedataset (GiblerandSarkees2002), we countonlydefensepactsasprovidinga significantsecurityguaran-tee, deeming ententes and neutralitytreaties insufficiently reassuringto elide thetemptationfor an insecurestateto pursuenuclearweapons.15INTERNAL DETERMINANTS

Democracy and democratization.We use the Polity IV data (Jaggers and Gurr1995) to createthree differentvariablesrelated to argumentsaboutregime type andproliferation.One variablemeasuresdemocracyfor each country-year:we create aderivedmeasureof the level of democracywithin each stateby combining the twothe Correlatesof Warproject's Composite Index of Capabilities (Singer, Bremer,and Stuckey 1972),extractedusing EUGene (BennettandStam2000) andupdatedthrough2000, using the UnitedNations'sStatistical Yearbook.13. Energyconsumptiondataare from the Correlatesof Warproject'sCompositeIndexof Capabili-ties (Singer,Bremer,andStuckey1972),extractedusingEUGene(BennettandStam2000). Populationdatacoveringtheperiodfrom 1945to 2000 weregatheredfrom the PennWorldTable6.1 (Heston,Summers,andAten2002), the Correlatesof Warproject,Maddison(2002), and the UnitedNations(StatisticalYearbook,variousyears).14. Althoughmany authors,buildingon this logic, have emphasizedbipolarity,because bipolaritycharacterizesmostof ourtime period,we focus on the presenceor absenceof anexplicit bilateralsecurityguaranteeinsteadof systemicvariablestoassessargumentsemphasizinggreat-powersecurityassurances.15. Alliance data were extractedusing the EUGene softwareprogram(BennettandStam2000).


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separate11-pointscales fordemocracyandautocracyfromPolityIV:demi = democi -autoci. However,periodsof transitiontowarddemocracymay prove particularlyvola-tile, makingnew or unconsolidateddemocraciesmore aggressiveand warprone.Toallow for this possibility, we create a democratizationvariable,demch,= democ, -democt,_, thatmeasuresmovementtowarddemocracyover timespansof 3, 5, and 10years. Finally,asan indicatorof theprevalenceof democraciesinthesystem,we createa variablethat recordsthe percentageof states each year that receive a score of 7 orhigheron the democracymeasure.16

Economicinterdependenceand liberalization.Among possible measuresof expo-sure to the global economy-international capitalmobility,volume of foreign directinvestment,tariffand nontarifftradebarriers,and so on-the traderatio is the moststraightforwardandis availableforthe largestnumberof countriesandyears.Conse-quently,we use exportsplus importsas a shareof GDPas ameasureof exposureto theinternationaleconomy, drawingon dataprimarilyfromthe PWT(version6.1).17 Wealso createa measureof tradeliberalizationanalogousto ourdemocratizationvariableby calculatingthe change in traderatios over spansof 3, 5, and 10 years.

Status inconsistency/symbolicmotivations. Operationalizingconcepts, such asprestigedeficit, statusinconsistency,andsymbolic motivations,for adataset thatcov-ers 154 countriesposes nearlyinsurmountabledifficulties. Nonetheless, we suspectthatany countrysufferinga prestigedeficit is likely to be dissatisfiedwith the interna-tionalstatusquoestablishedby thedominantpowerinthesystemand thattheirdissat-isfaction should be observableintheirchosen policy portfolio.As aproxyfordissatis-faction with the international status quo, we employ two variants of Bueno deMesquita's (1975, 1981) measureof similaritybetween alliance portfolios and analternateversionproposedby Signorinoand Ritter(1999). The firstcompareseachcountrywith the UnitedStates,takento be theglobal hegemonthroughouttheperiodunderconsideration.However,because grievancesmay more often be regional thanglobal for second- and third-tierpowers,we also assess a measurethatindicates simi-larityof each state'sportfoliowith that of the leading regional power.'

METHODS AND RESULTSMETHODS

Weemploy event historymodels, supplementedwith multinomiallogistic regres-sions, to test claims about the correlatesof nuclearweapons proliferation.Event his-16. To createthis indicatorand to determineeligibility forour data set in general,we drawon version2002.1 of the Correlatesof Warstatesystem membershipdata(Correlatesof WarProject2003).17. Forcountriesnot coveredby thePWTproject,we useddatafrom theIMF,drawingon Gleditsch's(2002) extension of the datausing AMELIAinterpolationsoftware.18. Bothversionsof theglobalandregionalindicatorsof thedivergenceof preferencesarecomputedby the EUGene softwareprogram(Bennettand Stam2000).


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tory models-also called survival,hazard,or durationmodels-offer several advan-tagesof particularrelevanceto ourresearchquestionanddata.We needa method thatis bothwell suited to rareevents and able to model the effects of time (e.g., durationdependence),providingestimates of the likelihood that a country begins an effort topursuenuclearweaponsgiventhatit has not doneso untilthatpointintime.Moreover,because most countries never do pursue nuclear weapons, we need a method thataccountsfor this "rightcensoring"andthus avoids the selection bias resultingfromexcluding countriesthat nevereven seriously consideredgoing nuclear.Finally, weneed amodelallowingus to includeexplanatoryvariablesthatchangeinvalue over theobservationperiod.Severaltypes of event historymodels are ideal for these needs.Eventhistorymodels provideestimatesof theprobabilityof an event occurring-in ourcase, a stategoing nuclearorstartingdown thatpath-at aparticulartime,giventhatithas notyet happened(Allison 1984;Box-Steffensmeierand Jones 1997). Inthelanguageof eventhistory analysis,thisprobabilityis given by the hazardrate,whichtells us the "risk"that a countrywill go nuclear.Event history models can be eitherparametric,requiringthe specificationof a particulardistributionalform(such as theWeibull,exponential,orGompertz)for the baseline hazardfunction,orsemi-paramet-ric, allowing one to avoid makingsuch assumptionswhen thereis no stronga priorireasonto favorone distributionalformover another.To facilitatethe inclusionof bothtime-invariantandtime-varyingvariables,we estimateparametricdiscrete-timehaz-ardmodelsusinga Weibulldistributionto characterizethe baseline hazardfunction.19The hazardrate is thengiven by

h(tlxj)=ptP-'lexp (0 + xjBx),whereh(t) is thehazardrate,t is time,andPO+ xiBxare the estimatedcoefficients andvariables.P is ashapeparameterestimatedfrom thedata:whenp equals 1,the baselinehazardis constantover time;if p is less than 1, it decreasesmonotonically;andif p isgreaterthan 1, hazardincreases with time at risk.Becausesurvivalmodelsarenonlinear,interpretationof coefficients is notstraight-forward.Unlike those in standardordinaryleastsquares(OLS)regressionmodels,thebeta coefficients do not representthe marginaleffect on the dependentvariableof aone-unitchange in the independentvariable.To ease interpretation,we estimatethemodelsinboth standardandlog relative-hazardforms;inthelattercase, the coefficientcan be readas the numberby which we would multiplythe odds of, for example, theinitiationof a nuclearweapons programfor a one-unit increase in the independentvariable.For example, a coefficient of 4 on the enduring rivalry dummy variablewouldimplya 300% increasein thelikelihoodof startinga nuclearweaponsprogram(in otherwords,the chance is fourtimes as great).In interpretingresults,we presentthe standardcoefficients andtheirstandarderrorsin main tablesbut discuss these rela-tiverisksinthe textto ease interpretationof thesubstantivemeaningof thefindings.

19. Because parametricmodels derivedfrom contendingdistributionsare nonnested,we used theAkaike informationcriteriato assess the relativeappropriatenessof models using exponential,Weibull,Gompertz,log-logistic, andlog-normaldistributions.


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RESULTSToinvestigatethe correlatesof nuclearweapons proliferation,we estimatedaseriesof models employing each of the threedependentvariablesin turn.20For the hazardmodels,durationsconsistof stringsof country-years.When acountrycrosses overthethresholdin questionfor a given model, it exits theriskpool and thusthe analysis. Acountrycan,however,reentertheriskpool if itmakesa clear andconvincingrenuncia-tionof itspreviousnuclearambitions:at thispoint,it is againdeemed"atrisk"andcanchoose to reigniteits interestin nuclearweapons at any time. Sweden, for example,exits theriskpool forexploringnuclearweaponsonce it startsdownthatpathbutlaterreentersit aftercompletelystoppingseriousconsiderationof the nuclearoption.Simi-larly,SouthAfrica actuallyacquiresnuclearweapons but later reentersthe risk poolafter it destroysthemalong with all remnantsof its program.Table2 presentsthe estimatesof the hazardmodels featuring,in turn,each of thethree outcomes of interest:exploring the option, pursuingweapons, and acquiringweapons. Startingwith "level 1"proliferation-the decision to explore seriously thenuclearoption2'-as thedependentvariable,model I reveals thatGDPpercapitaandindustrialcapacityhavestrongandsignificanteffects on the hazardrate.22The patternof coefficients on GDP percapitasupportsthe nuclearthresholdinterpretationof thetechnologicaldeterminismapproach:at low levels of GDP,furthereconomic growthsteadily increasesthe likelihoodthat a countrywill explore the nuclearoption;yet athigh levelsof development,theeffect levels off and,infact,reversesbecauseveryhighlevels of income areassociatedwith a falling hazardrate.23This suggests thatat lowlevels, greaterdevelopmentrenderstheacquisitionof nuclearweaponsmorefeasible,allowing countries to act on their previously latent ambitions. At higher levels ofdevelopment,thosethathavenotalreadyinitiatedaprogramareunlikelyto be swayedby the smallmarginalreductionin opportunitycost arisingfrom furthergrowth.Thecoefficient on the industrialcapacity index indicatesthe importanceof achieving aminimal level of industrialcapacity in enabling nuclear weapons development intermsof relativerisk ratios.The next groupingof potentialcorrelatesof proliferationfocuses on the externalsecurityenvironment.Participationin an ongoing enduringrivalryand the frequency

20. In survivalanalysis, identifyingentryinto the riskpool is important.Entryshould be defined sothatif twosubjectshave identicaltime atriskvalues,the risktheyfacewouldbe identicalif theyhadthe samevalueson allof theexplanatoryvariables.Given datalimitationsandthe adventof the nuclearera,we setriskonset time zero as 1944 for all countriesthat were independentsovereignstates at that time. Forcountriesthatgainedtheirindependenceat laterdates,we use their firstyearof existence, as identifiedby the Corre-lates of WarProject(2003), as time zero.21. We code 24 instancesof countriescrossingthis first-levelthresholdat some time:Algeria,Argen-tina,Australia,Brazil,China, France,Iran,Iraq,India, Israel,Libya,NorthKorea,Pakistan,Romania,Rus-sia/USSR, Sweden, Switzerland,South Africa, South Korea,Taiwan(twice), United Kingdom, UnitedStates,andYugoslavia.See the online appendixfor sources:http://falcon.arts.cornell.edu/crw12/.22. TheenergyvariablesarehighlycollinearwiththeGDPvariables;usingeitherenergyorGDPpro-duces similarresults.23. Thethresholdoccurs at about$7,700 percapitaincome in 1996 U.S. dollars.All else being equal,priorto thatlevel, additionalincrementsof income increasethe hazardrate,althoughby smallerandsmalleramountsas the inflectionpointis approached;afterthatlevel, additionalincrementsof income decreasethehazardrate.


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TABLE2The Correlatesof NuclearWeaponsProliferationDependent Variable

IndependentVariable Explore Pursue AcquireTechnologicaldeterminantsGDP per capita 0.00052.119 0.001.017 0.0002.378(0.0003) (0.0004) (0.0003)

GDP squared -3.66e-08"094 -7.92e-08017 -2.36-08l(2.19e-08) (3.1 le-08) (1.43e-08)Industrialcapacityindex 1.89.016 1.46046 3.19


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explorenuclearweapons acquisition.Sheer level of percapitaincome plays a some-what smaller role here, which is not surprisingwhen one recalls the low levels ofincome at which China, India, Pakistan,and the Soviet Union exploded their firstweapons.28However,althoughGDPper capitais somewhat less importantthanin theprecedingmodels,29the industrialcapacityindexlooms large,with a verylargecoeffi-cient anda vanishinglysmallp value.30Evenin theface of this strongeffect, variablestappingthe securityenvironmentretaintheirpower.Enduringrivalriesarepowerfulspurs,notjust to explorationanddevelopmentbut to testinganddeploymentas well.This is not surprisingwhen one recalls that of the countries to acquirenuclearweap-ons, all buttwo (SouthAfrica andFrance)are coded as participatingin an enduringrivalryat the time of acquisition. By alternatebutdefensible coding rules, one couldmake the case thatbothof these exceptions were involved in enduringrivalries.Fre-quencyof disputeinvolvementprovidesa morenuancedindicatorbecause it actuallyvariesovertime,evenwithinrivalries,anditprovesbothstatisticallyandsubstantivelysignificantonce again.Clearly,notjust the existence of a rivalrybut also the ebb andflow of thehostilitylevel also playanimportantrole in pushingstates overthe nuclearthreshold.Moreovera countrywith an alliancehas a hazardrate thatis a fractionofone withouta great-powersecurity guarantee,althoughit falls shortof significance.Finally,thebatteryof internaldeterminantsvariablesfalls below significancein thesemodels.Thedemocracyvariable,however,does approachsignificance in these mod-els andwitha positivecoefficient.3'Evencontrollingforlevel of incomeandeconomicdevelopment,countries that score high on the democracy scale are more likely toacquirenuclearweapons.Finally,economic openness loses its significance.32Althoughmanyof the variablesattain statisticalsignificance, how significantarethey substantivelyin shaping the likelihood that a country explores and acquiresnuclearweaponscapability?Drawingon relativeriskratios,Table3 interpretsthesub-stantiverole played by each variable for decisions to explore and acquire nuclearweapons.The entriesrepresentthepercentagechangein thebaseline hazardrate for agiven change in the explanatoryvariable.Forexample, a countrywith a great-powermilitaryalliancehas a hazardrate for exploringthe nuclearoptionthatis 49% lowerthana similarcountrywithoutanalliance,aswell as ariskof acquiringweaponsthatis

28. In 1996 U.S. dollars,the averageGDP percapitaat which countriesexploded/deployednuclearweaponsis $5,275. India,China,Pakistan,and(to a lesser degree)the Soviet Union were all considerablybelow thataverage.No countryhas ever gone nuclearwhen its GDP per capita was above the $11,000threshold.29. Ifthe 1998 testdates are used as alternatecodingsforIndiaandPakistan,thenGDPpercapitaandits squarearesignificantat betterthanthe 5% level.30. Everycountrytoacquirenuclearweapons,with theexceptionof Pakistan,was abovethe thresholdembodiedin the index.31. When theexplosiondatesareusedas an alternatecodingfor PakistanandIndia,the coefficient ondemocratizationfalls below statisticalsignificance.32. Wealso ranmodelsaskinga slightlydifferentanddistinct butrelatedquestion:giventhata coun-tryhasexploredthe nuclearoption,whatdetermineswhetherit exercises thatoption?This greatlyreducesthe numberof durationsforstudy.Inourview, this makesit less useful andtractablethan theapproachfol-lowed here but yields some interestingresults.Althoughdevelopmentand securitystill play a role, mostnotableis thattradeliberalizationremainssignificant,whereasdemocracycontinues to have no discernibleeffect. The multinomiallogit models reportedbelow allow us to deal with the kinds of issues raisedby thismultiple-stepquestion, thuscomplementingthe hazardmodels.


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TABLE 3SubstantiveEffects of the ExplanatoryVariableson the Likelihoodof ExploringNuclearWeaponsPercentageChangefromBaseline HazardRate

Variable Explore AcquireGreat-powermilitaryalliance -49 -64Participationin ongoing enduringrivalry +382 +743Increasein frequencyof MIDs (two more/year) +38 +52Industrialcapacitythreshold +563 +2,340Increasein tradeopenness -72 -2Increasein percapitaGDP-$500 at verylow level +26 +12Increase in percapitaGDP-$500 at high level -20 -17Satisfaction +40 -82Increasein democracy +25 +94NOTE:MID = militarizedinterstatedispute;GDP = gross domesticproduct.

54% lower.3 Even more striking, participationin an enduring rivalryincreases thehazardratenearlyfourfold (382%) comparedto a countrynot so engaged, and theeffect for the actualacquisitionof weapons is even greater(at 743%). Frequencyofmilitarizeddisputeinvolvementalso producesapowerfuleffect: increasingthe5-yearmovingaverageof the numberof disputesper year by two yields a 52% increasein thelikelihood thatacountrywill go nuclear.Takentogether,these threeexamplesindicatethat the securityenvironmenthas notjust a statistically significantbut,more impor-tant,a substantivelysignificanteffect on decisions to explorenuclearweapons acqui-sition, as realistshave long emphasized.Yet, the next four items remindus thateco-nomic factors also play a substantial role. Moreover, the process of economicliberalizationis associated with a reducedlikelihoodof exploringnuclearweapons:acountrythathas expandedits openness by 20 points over the past half decade has a72% lower hazardrate, although a level of economic openness 20 points higherreducesthepredictedhazardof acquiringweapons by only 2%.The levelof economicdevelopmentplays an even largerrole, althoughthese effects are nonlinearover thelevel of GDP percapita.Two examples,chosen to reflect differentends of the spec-trum whererelativelysmall changes in per capitaGDP yield relativelylargeeffects,illustratethe relationship.At very low levels of GDP, increasing per capitanationalincome by $500 producesa fairlydramaticrise in the hazardrate for exploration,by26% (and 12%for acquisition).34However,once a countryis already very wealthy,furtherincrementsof income only reduce the hazard;the same increasenow yields a33. Inotherwords,the likelihoodthatitexploresnuclearweaponsis about51 %thatof a similarcoun-trywithout an alliance.34. This example correspondsroughlyto the change in India'sper capitaGDP from 1958 to about1983.


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TABLE4Pathwaysto Proliferation:MultinomialLogit Models

LevelIndependentVariable I (Explore) 2 (Pursue) 3 (Acquire)TechnologicaldeterminismGDP percapita 0.0003


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multinomial logit models produce results quite similar to the survival models,althoughoffering greatersupportfor variablesassociatedwith internaldeterminantsperspectives,enhancingour confidence in the findings.PUZZLING MISSES

These results suggest that,contraryto what some scholars have argued, existingargumentsabout the determinantsof nuclearweapons proliferationdo a reasonablejob of accountingfor the data. One furtherway of seeing this is to examine instanceswhen the models miss by a fairlywide margin:thatis, which countriesdid notexplorethe nuclearoption, althoughthemodelssuggest theyshouldhave,and whichcountriesdid explore the option, even though the model produces relatively low predictedhazards.Tothisend, Table 5 lists the countries that had a high predictedhazardfor severalyears, yet never(tothebestof ourknowledge) seriously exploredthenuclearoption.Itis reassuringthat the list correspondswith the countriesthatanalystsoften identifyaspuzzlingnonproliferators.Saudi Arabia'spresence may surprisesome, but its combi-nationof a high threatenvironment,substantialwealth,andminimallysufficient eco-nomicandscientific infrastructuremake it a likely suspect.39Still, ourcoding of secu-rity guarantees is based on formal alliances and thus probably overstates thetemptationfacingSaudiArabia.Althoughitenjoys no formalalliance withthe UnitedStates,the factthatSaudiArabiahas(atleast untilrecently)a de factosecurityguaran-tee wasamplydemonstratedin 1990 and 1991.Ina similarvein,Syria'sinclusionmaysurprisesome due to its low level of economic development,butouranalysessuggestthat its frequentdispute involvement,participationin an enduringrivalry,relativelylow economic interdependence,and(barely)adequatelevel of economic developmentmadeit a strongcandidatefor nuclearweaponsproliferation.40 JapanandGermany'spresencecomes as no surprise:theyarewidely seen as powerful,economicallydevel-oped statesfacing strong securitythreatsthatonly foresworenuclearweapons underduress and with the reassuranceof a highly credible Americansecurity guarantee.4'Although lackingthe samelevel of industrial/scientificdevelopment,ItalyandSpainare similar cases; moreover, rumorsof interest in nuclear weapons have swirledaroundboth countries at times.42Egypt'schallenging securityenvironmentprovidesstrongincentivescounterbalancedby relativelylow levels of economic development.Bulgariais more of a surprise,but its status as a relativelydeveloped economy on thefront lines of the cold war gave it a combination of strongincentives and sufficient

39. Contraryto widely held images, Saudi Arabiaproducessteel domestically (since 1976, and infairly large quantitiessince 1984), has a large and modernelectrical-generatingcapacity (well over the10,000 MW thresholdsince 1983), and has a well-educateduppertierof researchers.40. Indeed,Syria is also a "nearmiss" in ourcoding of proliferators.Despite some suggestiveevi-dence,we ultimatelydecided thatthereis notenoughfirminterestin serious or sustainedexplorationof thenuclearoptionto code Syriaas a level 1proliferator.41. AmongothereconomicallyadvancedWesterncountriesnot on this list, Finlandprovidesthebig-gest outlier butis not a surprisingmiss for fairlyobvious reasons.42. However,theevidencewas too slim orthelevel of interesttooephemeralto warrantcodingas level1 proliferators.


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TABLE5Dogs That Didn't Bark?CountriesThat Did Not SeriouslyExplorethe NuclearOption... but Should Have

Country Yearsof MaximumPredictedHazardSaudiArabia Mid-1980s to mid-1990sWestGermany Mid- 1950s to early 1960sJapan Mid-1950s to 1960sTurkey Late 1960s to 2000Bulgaria 1950sSpain 1960s to early 1970sGreece 1960s and 1980sItaly 1950s to early 1960sSyria Various

abilitythatled to a strong predictedhazard,despite the Soviet alliance.43 TurkeyandGreece areobvious, if not frequentlymentioned,possibilities.If we turnthequestionaroundto ask who soughtnuclearweaponsbut should nothave, the list is quiteshort,simplybecause ourmodel attributesa relativelyhigh haz-ard to nearly every state thatpursuednucleararms.Libya, Brazil,Algeria,and Paki-stanhadrelativelylow predictedhazardsat the timethey beganseriouslyexploringthenuclear option. Of these, Algeria and Libya provide perhaps the biggest surprisebecausebothfeaturerelativelyinfrequent(thoughnot inconsequential)MID involve-mentwith a moderateto low level of economic development, renderingthem some-what unlikely proliferators."Pakistanis primarilya surprisebecause of the remark-ably low level of economic development at which it began exploring the nuclearoption, and to this day, it stands out as the least developed country to pursueandacquirenucleararms.45Brazilprovidesanotherminorsurprise;althoughits relation-shipwithArgentinaoften rendereditssecurityenvironmentless thanbenign,ithasnotfaced a high-intensity threat environmentcompared to most other proliferators.Argentina,by contrast,does notappearas an outlierbecause of its moreperiloussecu-rity situation(tensions with Chile as well as Brazil,not to mention the United King-dom) andhigher level of economic development.Interestingly,the "Australiansur-prise" (Walsh 1997) is not much of a surprise to our model, which predicts acomparativelyhigh hazard for Australiain the 1950s and 1960s, precisely when itsecretly pursuedthe atomicoption.

43. Ties to the Soviet Union were notsufficient to renderacountryimmunefrom thenucleartempta-tion, as Romania'sflirtationwith indigenousnucleararmsin the 1980s indicates.44. However,it is worthnotingthat bothalso share a numberof unresolvedborderdisputes.45. China's per capita GDP, in 1996 U.S. dollars at purchasing-powerparity exchange rates,waslower, but it had a largedomestic steel industryand a much more extensive electricity industrythan didPakistan.


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ROBUSTNESS CHECKSSpecialattentionto therobustnessof the estimatesof the coefficients and theirvari-ances is warrantedbecauseof the relativelysmall numberof proliferatorsand due todisagreementsaboutcodingof thedependentvariablesforseveralcountries.Toassesstheinfluenceof particularcountries,we rana seriesof modelssequentially,deletinginturneachcase thatfeaturesa positiveoutcome on the dependentvariable.Thisproce-durerevealedno stronglyinfluentialcases, althougha few slightsensitivities areworthnoting. Deleting Chinaand/orIndiastrengthenstheimportanceof economic develop-ment variables, whose estimated effects are clearly attenuatedby the (successful)efforts of two of the world's poorercountries to develop nuclearweapons. Finally,omitting India and/or Pakistan elides the effect of the enduring rivalry variablesomewhat.Our second robustness check was to experiment with alternatecoding of thedependentvariables for countries where thereis some disagreementamong sourcesaboutthetimingof keyevents and decisions.Althoughfor mostcountries,thereis sur-prisingconvergencefromcrediblesourceson factual accountsandassessments,somecodings are based on more divergentand difficult-to-reconcilesources and evalua-tions, forcingus to make ajudgmentcall. NorthKoreaprovidesan extremecase, withbothits decisions toexplorenuclearoptionsand dates of first seriousefforts at a weap-ons programshroudedin secrecy and controversy.Iranprovides anotherexample;althoughthereis broadagreementwithin a 5- or 10-yearrange,choosing aprecise yearfor programinitiationis morecontroversial.In these cases, we createdalternatever-sions of thedependentvariables,usingyearsacross thetemporalrangeof estimatesinturn.Runningdozens of models basedon these variantsreveals thatthe results are notsensitive to codings thatdifferby upto 10 years for the contentiouscases.

CONCLUSIONFearsof rogue states, withdrawalof cold war-era security guarantees,a fallingtechnologicalthreshold,and concerns thatnew nuclearpowerswill provide weaponsto terroristsall ensurethatnuclearweapons proliferationremainsa centralsecurityissue andthatdevelopinganadequateunderstandingof thecorrelatesof proliferationrankshigh on the agendaof internationalrelations scholars. Yet, althoughscholarshaveofferedanabundanceof explanationsforproliferationdecisions, little consensusexists on the adequacyof varioustheories or whetherwe even possess a theory ofnuclearproliferation(Ogilvie-White 1996). We arguethat thisunsatisfactorystate ofaffairsderives at least partlyfroma mismatchbetween theoreticalarguments,whichtend to makeprobabilisticclaims and envisionmultiplecausalvariables,andthepre-dominantempirical methodology in the area,which tends towardcase studies thatimplicitlyapply deterministicstandardsbased on an (often implicit) univariatelogicof inference andsampleson the dependentvariable.Seeking to complementexistingresearch,we constructeda new dataset on nuclearweapons proliferationand usedhazardmodels to test theoriesof nuclearproliferation.Thedataanalysis suggests that


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existingtheoriesdeservemorecreditthantheyarefrequentlygiven:nuclearweaponsproliferationis reasonablywell accountedfor by the level of economic developmentand the externalthreatenvironment.Althoughthe inevitableuncertaintyentailed in coding cases of nuclearweaponsproliferationmakes us cautious about drawing sweeping conclusions, the findingsclearly suggest severalimplicationsforpolicy and futureresearch.Startingwithpol-icy, one of our more surprisingfindings offers some, albeit limited, supportfor theemphasisplaced on economic interdependenceand (less so) liberalizationin recentwork on proliferation(Paul2000; Solingen 1994, 1998). This empiricallinkbetweeninvolvementin theworldeconomy and nuclearabstinenceseems tobolsterthe case ofthose arguingfor a strategyaimed at tying down potentiallytroublesome states in amutuallybeneficialweb of economic interdependence.Yetthecausallinkageremains

somewhat opaque and unpersuasive:the direct economic costs and foregone eco-nomicopportunitiesof pursuingoreven acquiringnuclearweaponsdo not seem pro-hibitive,as therelativelymildand short-livedsanctionslevied againstIndiaandPaki-stan demonstrate.Adding nonproliferationto thelist of putativebenefitsof economicintegrationis prematurewithoutfirmertheoreticalandempiricalknowledgeaboutthecausal mechanismthatproducesthis relationship.Parsingout this linkageboth theo-reticallyandempiricallythusposes animportanttaskforfutureresearchandpointstothepossibilityof synergiesbetweenqualitativeandquantitativemethodsinexploringthis relationship.Our findings about the centralityof perceived threatsin proliferationdecisionshave implicationsfor debatesabout the proposedU.S. nationalmissile defense pro-gram (NMD). Whether NMD fans or dampens the fires of proliferationdependslargelyon howotherstatesperceiveit. IfaU.S. missiledefense is viewedas animplicitsecurityguaranteeby countriesunderits umbrella,thenit should function like analli-ance, substitutingfor home-grownsecurity measuresand reducingthe incentive toacquirenucleararmsas a deterrent.However,if some statesview NMD as threateningbecauseitneutralizestheirown nucleardeterrentsorallows theUnitedStatesto inter-vene whereverit chooses withvirtualimpunity,thenitmay spurthemtoacquirelarger

nucleararsenals,rushtoacquirenuclearweaponsbeforea defense canbedeployed,ordevelopalternativedeliverysystems.As Paul(2000) haspersuasivelyargued,when itcomes to nuclearweapons, the securitydilemmaseems to operatewith a vengeancebecauseattemptsbyrivalsto bolsternucleararsenalsorbuilddefenseareoftenviewedwith greatalarmand met with forceful responses.The question of the influence ofNMD on proliferationmay thus be one of net effect: states underits umbrellafacereducedincentivesto acquiretheirown deterrents,whereas statesoutsideitsreacharelikely to feel their security reduced and face heightened incentives to bolster orassemble nucleararsenals.These findings also offer considerablesupportfor the commonsense theory ofnuclearproliferation,in which statesgo nuclear"whentheyface a significantmilitarythreatto theirsecuritythatcannot be metthroughother means"(Sagan2000). Infact,thereare no cases of the determinedpursuitof nuclearweapons by countriesnotexpe-riencinga subjectivelythreateningsecurityenvironment.Given this fact, one has toquestion the wisdom of policies aimed at countering proliferationthat may well


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