Electronic copy available at: http://ssrn.com/abstract=2776657

The Impact of Mass Shootings on Gun Policy

Michael Luca Deepak Malhotra Christopher Poliquin

Working Paper 16-126

Electronic copy available at: http://ssrn.com/abstract=2776657

Working Paper 16-126

Copyright © 2016 by Michael Luca, Deepak Malhotra, and Christopher Poliquin

Working papers are in draft form. This working paper is distributed for purposes of comment and discussion only. It may not be reproduced without permission of the copyright holder. Copies of working papers are available from the author.

The Impact of Mass Shootings on Gun Policy

Michael Luca Harvard Business School

Deepak Malhotra Harvard Business School

Christopher Poliquin Harvard Business School

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TheImpactofMassShootingsonGunPolicy

MichaelLucaHarvardBusinessSchool

<mluca@hbs.edu>

DeepakMalhotraHarvardBusinessSchool<dmalhotra@hbs.edu>

ChristopherPoliquinHarvardBusinessSchool<cpoliquin@hbs.edu>

October2016

Abstract There have been dozens of high-profile mass shootings in recent decades. This paperpresentsthreemainfindingsabouttheimpactofmassshootingsongunpolicy.First,massshootingsevokelargepolicyresponses.Asinglemassshootingleadstoa15%increaseinthe number of firearm bills introducedwithin a state in the year after amass shooting.Second,massshootingsaccountforasmallportionofallgundeaths,buthaveanoutsizedinfluence relative tootherhomicides.Ourestimates suggest that theper-death impactofmassshootingsonbills introduced isabout80 timesas largeas the impactof individualgunhomicides in non-mass shooting incidents. Third,when looking at enacted laws, theimpactofmass shootingsdependson theparty inpower.Amass shooting increases thenumber of enacted laws that loosen gun restrictions by 75% in states with Republican-controlled legislatures. We find no significant effect of mass shootings on laws enactedwhenthereisaDemocrat-controlledlegislature.

WethankJosephHallandJessicaLiforexcellentresearchassistance.

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1.Introduction

Recentdecadeshavewitnessedaseriesofhigh-profilemassshootingsthroughout

theUnitedStatesintownsrangingfromNewtown,CTtoKilleen,TX.Whilemosthomicides

receive little attention from the general public, mass shooting incidents are extremely

salient.Nonetheless, a common and frequently articulated view is that despite extensive

discussionaboutmassshootings,theyhavelittleinfluenceonpolicymaking.

Shouldwe expect policymakers to proposenew legislation in thewake of amass

shooting?Giventhatthevastmajorityofgundeathsdonotresultfrommassshootings,it

would be difficult to reconcile large responses to mass shootings with basic models of

optimalpolicyaimedexclusivelyatreducinggunviolence.However,massshootingsmay

have another effect – bringing attention to the issue of gun violence. Mass shootings

potentiallyleadtopolicychangesbyfocusingattentionongunviolence,eveniftheydonot

providenewinformationorchangepoliticians’preferences(whicharegenerallystaticand

alignedwithpartypreferences).Politicalscientistshavenotedthefactthatissuestendto

riseandfallwithinapolicyagenda,creatingperiodsinwhichspecificpolicieschangevery

rapidlyandotherperiodsinwhichtheydonotchangeatall(BaumgartnerandJones1993,

Kingdon 1984). In the context of gun violence, events like the Columbine shooting have

lead to both calls for new restrictions on guns and vehement reaction from gun rights

groupsopposedtosuchchanges(Goss2006,Spitzer2012).Moregenerally,massshootings

may create “policy windows” during which legislatures become receptive to change –

potentiallyduetoshiftsintheattentionofconstituents.Buttheextenttowhichthisoccurs,

andthedirectionofresultingchangesareempiricalquestions.

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Inthispaper,weexploretheimpactofmassshootingsongunpolicy,constructinga

dataset of all U.S. gun legislation andmass shootings over a period of twenty-five years

(1989-2014)–combiningdatafromavarietyofmediaandgovernmentsources.Webegin

by looking at the extent of deaths resulting from mass shootings relative to other gun

deaths.Overall, therearemorethan30,000gunrelatedfatalities intheUnitedStatesper

year. Roughly 56%of these are suicides and40%are homicides. The remaining 4%are

accidentsorincidentsofundeterminedintent.Massshootingsaccountedforabout0.13%

ofallgundeathsand0.34%ofgunmurdersbetween1989and2014.

Becausemassshootingsaresalientandplausiblyrandomoccurrences,weareable

to implementadifference-in-differencesstrategyaround the timingofmassshootings to

estimate theircausal impactongunregulation.Specifically,wecomparegun lawsbefore

andaftermassshootings,instateswheremassshootingsoccurrelativetoallotherstates.

Wethenpresentthreemainfindingsabouttheimpactofmassshootingsonpolicy.

First, mass shootings evoke large policy responses. A single mass shooting leads to an

approximately15%increaseinthenumberoffirearmbillsintroducedwithinastateinthe

yearafteramassshooting.Thiseffect is largestaftershootingswiththemost fatalities–

andholdsforbothRepublican-controlledandDemocrat-controlledlegislatures.

Second,althoughmassshootingsaccountforasmallportionofallgundeaths,they

haveanoutsizedinfluencerelativetootherhomicides.Ourestimatessuggestthattheper-

deathimpactofmassshootingsonbillsintroducedisabout80timesaslargeastheimpact

ofgunhomicidesinnon-massshootingincidents.

Third,whenlookingatenactedlaws,theimpactofmassshootingsdependsonthe

party in power. Amass shooting increases the number of enacted laws that loosen gun

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restrictions by 75% in states with Republican-controlled legislatures. We find no

significanteffectofmassshootingson lawsenactedwhenthere isaDemocrat-controlled

legislature.

These findingscontribute to theempirical literature thatusesapolitical economy

lens to explore the determinants of policymaking (Makowsky and Stratmann 2009,

Bardhan and Mookherjee 2010). Our results show that salient events – such as mass

shootings–can lead tosignificantpolicyresponses.Moreover,policymakersseemtouse

massshootingsasanopportunitytoproposebillsthatareconsistentwiththeir ideology.

This helps to shed light on the role of attention and salience in shaping policy and the

interactionbetweenissuesalienceandexistingpoliticalpreferencesinshapingthedegree

anddirectionofenactedpolicies.

2.BackgroundandData

Asdescribedabove,outoftheroughly30,000annualgundeathsintheUnitedStates,

fewerthan100occurinmassshootings.Forthepurposeofthispaper,wedefinea“mass

shooting” as an incident in which 4 or more people, other than the perpetrator(s), are

unlawfullykilledwithafirearminasingle,continuousincidentthatisnotrelatedtogangs,

drugs, or other criminal activity. This definition closelymatches theoneusedbyKrouse

andRichardson(2015) and the FBI’s definition of “massmurder” as 4 ormoremurders

“occurring during the same incident, with no distinctive time period between the

murders… typically involv[ing] a single location” (Morton and Hilts2008). We further

restrict our analysis to cases where at least three of the fatalities were individuals

unrelatedto,andnotromanticallyinvolvedwith,theshooter(s).Weincludespreemurders

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–homicidesatmultiple locationswithout a significantpausebetween incidents– if they

resultinfourormoredeaths.

We assemble a list of mass shootings since 1989 from a variety of government and

media sources because there is no single, comprehensive government database ofmass

murders.Weextractallgun-relatedmassmurders(fourormoredead)thatarenotfelony

relatedfromtheFBIsupplementaryhomicidereports(SHR).Wethenverifyeachincident

intheSHRusingmediaaccounts;theSHRmaycontainerrorsinwhichseparatehomicides

in amonth are reported as a single incident, which is why it is necessary to verify the

incidentswithmedia coverage. Participation in the SHRprogram is voluntary andmany

lawenforcementagenciesdonotreportdetaileddatatotheFBI.Wethereforesupplement

the FBI data with mass shootings gathered from media accounts or compiled by other

researchers and journalists interested in the topic.We combine the SHRdatawithmass

shootingscollectedbytheMassShootingsinAmerica(MSA)projectatStanfordUniversity

(Stanford Geospatial Center and Stanford Libraries2015) and a list created by USA

Today(2013).Foreachshooting,wedeterminetheeventlocationaswellasthenumberof

victimfatalitiesandinjuries.Wealsoclassifyshootingsbasedontherelationship(ifany)

betweentheallegedshooter(s)andvictims.Previousworkonmassshootings(Duwe2007;

KrouseandRichardson2015)distinguishesbetweenpublicmass shootings thatoccur in

places frequented by the public, felony-related murders, and familicide. We categorize

shootingsbywhethertheyarepubliceventsorprimarilyrelatedtodomesticconflicts,and

we focuson incidents inwhichat least threepeoplenotrelatedorromantically involved

with the shooter died. This restriction filters out family-killings in residences aswell as

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family-related murders in public places.1Figure 1 shows the number of incidents and

fatalitiesinmassshootingsbyyear.Thedatashowaslightupwardtrendinthenumberof

incidentsandfatalitiesovertime,butbothincidentsandfatalitiesvarysubstantiallyfrom

yeartoyear.

2.2.GunLegislation

Stategovernmentsaretheprimaryregulatorsoffirearms.Federallawsestablisha

minimum level of gun control,which is thenaugmented to varyingdegreesby state and

local policies. Federal government has limited commerce, the possession of guns by

potentially dangerous individuals, and some types of firearms and ammunition. States

decideavarietyofgunpoliciesrangingfromwhocanpurchaseandpossessaguntowhat

typesof gunsareallowed indifferent situations tohowguns shouldbe storedandwhat

typesof trainingshouldbeundertakenbygunowners.Localordinancescanalsorestrict

firearmpossessionanduse,butstatestatutesenactedinthepastfewdecadeshavelimited

theimportanceoflocalgovernmentinthisarenabypre-emptinglocalregulations.

Wecreateacomprehensivedatasetofgunlegislationinallfiftystatesusingthebill

tracking reports service from LexisNexis, which includes all bills introduced in state

legislatures since at least 1990with a synopsis and timelineof eachbill’s progress.This

allows us to determinewhether bills pass the legislature and become law. We identify

firearm bills by searching for the firearm-related terms “firearm”, “handgun”, “pistol”,

“revolver”,“rifle”,“shotgun”,“long-gun”,and“assaultweapon.”Weidentify20,409firearm

billsand3,199lawsbetween1990and2014.Inotherwords,therewere20,409proposals

1A2006shootingatachurchinLouisianaisoneexample.Amankilledhiswifeandin-lawswhileabductingherandtheirchildrenfromachurch.Onlythewife’sfamilywaspresentatthechurchduringtheshooting.

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introduced and 3,199 laws passed in the twenty-five year sample period across all fifty

states.Thisincludeslawsthatloosenortightengunrestrictions,andmanythatdoneither

orboth.Weexcluderesolutions,executiveorders,andballotinitiativesfromtheanalysis.2

Toexplorewhetherguncontrol is tightenedor loosenedaftermassshootings,we

hired eightpeople tomanually code the summaryof bills that became law.Coderswere

giveninstructionsexplaininghowtocodelegislation,butwereotherwiseblindtothetopic

and design of the study. We presented bill summaries from LexisNexis to coders in

randomly chosen groups of 50. Twopeople coded each summary, andno coder saw the

same summarymultiple times. For each summary, coders decidedwhether the bill was

tightening (stricter gun control), loosening (weaker gun control), uncertain (insufficient

information),bothtighteningandloosening,orneithertighteningnorloosening(neutral).

There were therefore five possible labels for bills: tighten, loosen, both, neutral, or

uncertain.AppendixAshowsexamplebillsummariesandtheirexpectedlabels.

Tocross-validate(andincentivize)thebillcoding,wecodedasmallfractionofbills

ourselvesasabaselinecomparisonpoint.Forthisprocess,weblindedourselvesfromany

informationaboutwhenorwherethebillwasproposed.Wethenusedourscorestoassess

thequalityofcoders.Specifically,eachgroupof50billsgiventoacodercontainedfivebills

thatwehadalsocoded(theydidnotknowwhichbillswereandweren’tcoded,anddidnot

haveaccesstoanyofourassessmentsofwhetherabillwaslooserortighter).Coderswere

paiduptoa50%bonusbasedontheextenttowhichtheircodingmatchedours(whichwe

simplytoldthemwasa“goldstandard”ofknowncodes).

2Legislatorsinsomestatesfirstsubmitideasforbillsintheformofadraftrequestorsimilardocument.Weexcludethesefromouranalysesbecausetheyresultindoublecountingsomelegislation.Weinsteadfocusonlyonactualbills.

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Across all five categories, coders agreed with each other 52% of the time (the

agreementratewouldbe20%bychance)andagreedwith thegoldstandard71%of the

time. Coders performedworst on the neutral category, andbest on the tighten-only and

loosen-onlycategories;whenabill tightensguncontrol (accordingto thegoldstandard),

codersagreeon tightening67%of the time, andwhenabill loosensgun control, coders

agreeonloosening60%ofthetime.

Most importantly for the purposes of our analysis: when coders agreewith each

otherontightening,theyalsoagreewithourcoding93%ofthetime;whencodersagreeon

loosening, they are consistent with our scores 91% of the time. When analyzing the

direction of policy change, we leverage this high degree of reliability by restricting our

analysistobillsonwhichcodersagreedthatthelawwasdesignedtotightenorloosengun

control.Becausestatescanpasseither,none,orbothtypesoflawsinayear,ourdependent

variableisthecountoflawsineachdirection.

Figure2showsmeanbills introduced, lawsenacted,and tighteningand loosening

lawsbypoliticalcontrolofthestatelegislature.Republicansenactmorelawslooseninggun

control, and fewer laws tightening gun control, thanDemocrats.Republican,Democratic,

andsplit legislaturesenactasimilarnumberof totalgun laws.Thecoderswhoclassified

the legislationwere only given summaries of each bill; theywere not providedwith the

state,year,oranyinformationonpoliticalaffiliation.

2.3.ControlVariables Whileourempiricalstrategyallowsustocontrolforalltimeinvariantfactorsthat

mayaffectgunlegislation,wealsoaddtimevaryingcontrols.Theseincludeeconomicand

demographic factors such as unemployment, divorce rates, and rates ofmilitary service.

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Wealsocontrolforinstitutionaldifferencesbetweenlegislatures.First,wecontrolforthe

numberof lawmakersasameasureof legislature size.Larger legislaturesconsidermore

bills.Second,wecreateadummyforlegislaturesthatheldaregularsessioninagivenyear

becausenotalllegislaturesmeetannually.Third,wecontrolforwhetherbillsineachyear

carryover intosubsequentsessions; somechambersallow forcarryoverwhileotherskill

all unpassed bills at the end of each session. Fourth,we control for years inwhich bills

were restricted to specific topics; seven states restrict the scope of legislation

(e.g.appropriationsonly)inspecificyears.Fifth,wecontrolforthepoliticalpartyinpower.

Table1containssummarystatisticsforallvariablesusedintheanalyses.

3.TheImpactofMassShootingsonGunPolicy

3.1IdentificationStrategy We implement a difference-in-differences strategy that compares gun lawsbefore

andaftermassshootings,instateswheremassshootingsoccurrelativetoallotherstates.

Our dependent variables are counts of bills or enacted laws at the state-year level. We

studytheeffectofmassshootingsusingPoissonregressionswithconditionalmean:

E 𝑦!,! 𝛼!, 𝜆! , 𝑆ℎ𝑜𝑜𝑡!,!!!,𝑋!,! = exp 𝛼! + 𝜆! + 𝛽!𝑆ℎ𝑜𝑜𝑡!,!!! + 𝛾!𝑋!,!

where𝑦!,!isacountofbillsintroducedorlawsenactedinstate𝑠andyear𝑡;𝛼!and𝜆!are

stateandyearfixedeffects;𝑆ℎ𝑜𝑜𝑡!,!!!iseitheranindicatorforstateswithamassshooting

or the fatality count in mass shootings, and𝑋!,! is a vector of time-varying political,

economic,anddemographicfactors.Weestimatetheparametersviamaximumlikelihood

byconditioningonthesumof𝑦!,!withinstatesandincludingyearindicators.

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Our identificationallowsus tomeasure the impactofamassshootingwithin that

state, controlling forotherchanges thatarehappeningat thenational level.Because this

identification strategy does not identify national responses to mass shootings (which

would be absorbed by our year effect), our estimates of changes in gun policy may

underestimatethetotalimpactofamassshooting.Wecanaccountforpotentialspillovers

into neighboring states, but do not see significant spillover effects; these results are

presentedinAppendixB.

3.2TheEffectofMassShootingsonGunBillIntroductions Table2 shows that a mass shooting leads to a 15% increase in firearm bills

introduced.Fortheaveragestate,thisamountstoanadditional2.5firearmbillsintroduced

in the year following a mass shooting. Mass shootings with more deaths lead to larger

effects.Onaverage,eachadditionaldeathinamassshootingleadstoa2.5%increaseinthe

number of gun bills introduced. This result holds both for Republican-controlled and

Democrat-controlledlegislatures.3

3.3ComparingMassShootingsandNon-MassShootings

Table 3 shows that fatalities resulting from mass shootings lead to much larger

increasesingunbillintroductionsthangunhomicidesinnon-massshootingincidents.We

estimatethemodelsinthistableusingmassshootingfatalitiesandordinarygunhomicides

per 100,000 people to facilitate comparison between the two types ofmurder. Itwould

take approximately80peopledying in individualgunhomicide incidents to have asmuch

impactonbillsintroducedaseachpersonwhodiesinamassshooting.Ourestimatesimply

3Resultsonbillsproposedbrokendownbypoliticalaffiliationareavailableuponrequest.

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that, on average, a single mass shooting has as much impact on the number of bills

proposedaswoulda270%increaseinthenumberofgunhomicidesinastate.Giventhe

average number of gun homicides per year is roughly 260 per state, this would be

equivalenttoanadditional448homicidesperstateyear.

3.4TheRoleofPoliticalPartyonLawsEnacted Asmentionedpreviously,thetwomajorpoliticalpartiesintheUnitedStatesdiffer

dramaticallyintheirstancesonhowrestrictivegunpolicyshouldbe,withtheRepublican

Party favoring fewer gun restrictions.4To look at the impact of political parties on gun

policy,werestrictouranalysistoenactedlaws,allofwhichwerecodedforwhetherthey

loosenedortightenedgunrestrictions(seedatadescriptionformoredetails).

Table4 shows the effect of mass shootings interacted with Democrat and

Republican control of state government (divided government, inwhich the legislature is

not controlledby a singleparty, is theomittedgroup).The results show thatDemocrats

andRepublicansresponddifferentlytomassshootings.

When there is a Republican-controlled legislature, mass shootings lead to more

firearm laws that loosenguncontrol.Amassshooting in thepreviousyear increases the

number of enacted laws that loosen gun restrictions by 75% in stateswith Republican-

controlled legislatures.When there is a Democrat-controlled legislature, mass shootings

lead to a statistically insignificant reduction in laws that loosen gun control.We find no

significanteffectsofmassshootingsonlawsthattightengunrestrictions,buttheestimates

are imprecise.Summingacrossall legislatures (Republican,Democrat,andsplit), there is

4See,forexample,https://www.gop.com/platform/andhttps://www.democrats.org/party-platform.

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roughly a 10% increase in laws enacted after a mass shooting, but this estimate is

impreciseandstatisticallyinsignificant(AppendixC).

3.5RobustnessChecks

Inthissection,wepresentthreesetsofrobustnesschecks.First,weprovidesupport

fortheexogeneityofmassshootings.Second,weshowthatourmainresultsarerobustto

the inclusion of state-specific time trends. Third, we perform a falsification exercise in

whichweuserandomlygeneratedplaceboshootingsinsteadofactualshootings;weshow

therearenoeffectsusing theplacebo shootings,providing support forour identification

strategy.

3.5.1DeterminantsofMassShootings

Our ability to identify the causal impact ofmass shootings on policy rests on the

assumptionthattheyareplausiblyexogenoustootherfactorsthatwoulddriveguncontrol

inagivenyear.Giventheerraticnatureofmassshootings,thisisaplausibleassumption.

Nonetheless,onemightbe concerned thatbothmass shootingsandgunpolicyarebeing

drivenbya thirdvariable.Toprovidesupport forourassumptionand interpretation,we

regressan indicator forwhetheramass shootingoccursoneconomic,demographic, and

policyvariables.

Consistentwiththeassumptionthatmassshootingsareexogenouswithrespectto

potentialconfounds,theresultsinAppendixDshowthat,outof32variablesweconsider,

onlyunemploymentissignificantlyassociatedwithahigherprobabilityofmassshootings.

Becausehigherunemploymentisalsoassociatedwithareductioningunbillintroductions

(Table2), thepotentialbiasof thiswouldwork in theoppositedirectionofour finding–

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makingitunlikelythatthisisdrivingourresults.Tofurthersupportourinterpretation,we

control forunemployment in allmodels. Importantly, bills introduced, lawsenacted, and

majorgunpoliciesdonotpredictfuturemassshootings(AppendixD).

3.5.2State-SpecificTimeTrends

Anotherpotentialconcernisthatstateshavedifferentialtrendsinmassshootings,

and that these trends correlate with gun regulations, which would violate the parallel

trends assumption. As a robustness check, we run our main specifications with state-

specifictrends.AppendixEshowstheresultsofre-estimatingthemodelsinTables2and3

withstatespecific timetrends.The inclusionofstatespecific trendsdoesnotchangeour

main results fromTables 2 and 3.We are unable to estimatemodelswith state specific

trendsforouranalysesoftighteningandlooseninglawsbecausethelikelihoodfunctionis

discontinuouswhen including the additional parameters due to some states having very

few laws that we can identify as tightening or loosening. We can, however, conduct a

placeboanalysistoaddressanyresidualconcerns.

3.5.3PlaceboTests

Asafinalrobustnesscheck,weperformafalsificationexercisebasedontheinsights

ofBertrand,Duflo,andMullainathan(2004)andDonaldandLang(2007).Specifically,we

randomly assign placebo mass shootings to state-years in which no actual shooting

occurredwithprobabilityequaltoeachstate’sfrequencyofshootings,andrandomlydraw

afatalitycountfromtheempiricaldistributionoffatalities.AppendixFshowspercentiles

of the test statistic based on 1,000 repetitions of this procedure and our actual test

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statistics from Tables 2 and 4. The results suggest our tests do not over-reject the null

hypothesisthatmassshootingshavenoeffectongunpolicy.

4.Discussion

Massshootingsaccountforasmall fractionofgundeaths intheUnitedStates,but

haveasignificantimpactongunpolicy.Moregunlawsareproposedintheyearfollowinga

mass shooting, a result that holds for both Republican- and Democrat-controlled

legislatures.Notably,massshootingshavemuchlargereffectsonpolicy,perfatality,than

ordinarygunhomicides.

However, we also find evidence that Democrat- and Republican-controlled

legislaturesdiffersignificantlywhenitcomestoenactinggunlaws.Republicansaremore

likelytoloosengunlawsintheyearafteramassshooting.TheeffectforDemocrats,which

tends toward less loosening of gun restrictions after a mass shooting, is statistically

insignificant.Thisisconsistentwithsurveyevidencesuggestingthatevenwhenamajority

supportsaguncontrolproposal,thoseopposedtoincreasedguncontrolaremorelikelyto

take actions likewriting a letter or donatingmoney to support their side (Schuman and

Presser1981).

Our results are consistent with qualitative research that has hypothesized the

possibility of mass shootings precipitating change. For example, Godwin and

Schroedel(1998) argue that the Stockton schoolyard massacre in 1989 led to the

enactmentof California’s assaultweaponsban.We find empirical evidence that sporadic

eventssuchasmassshootingscanleadtomajorpolicychanges.Thisraisesthequestionof

other factors thatmight drive policy, and conditions underwhichwemight expect such

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effects. For example, Egan andMullin (2012) show that extreme temperatures influence

beliefs about global warming in the short-term. Might we expect a greater impact of

randomeventsinsomepolicycontexts(e.g.,theeffectofaterroristattack)thaninothers

(e.g.,theeffectofanEbolaoutbreak)?

Ourfindingsraiseanumberofadditionalquestions,andsuggestseveraldirections

forfutureresearch. First,ourestimatesfocusontheimpactonpolicywithinthestatein

which each shooting took place. Somemass shootings get national media attention and

potentially affect policy nationwide, which would not be identified by our fixed effects

model. One direction for future research is to develop strategies to identify national

responses. With respect to our findings, this suggests that the total impact of mass

shootingsongunpolicymaybeevenlargerthanourestimates.

Second,futureresearchmightfurtherexploretheroleofsalienceinshapingpolicy

byexaminingtheconditionsunderwhicheventsaremoreorlessinfluential.Forexample,

sometypesofevents(e.g.,schoolshootings)mayhavelargereffectsthanothers,something

wecouldnottestgiventherelativeinfrequencyofsuchevents.Salienteventsmighthavea

greater impact if they occur at a time when few other events are competing for media

attention(e.g.,EisenseeandStrömberg2007),orduringelections,whenpublicattentionis

more focused on such issues (e.g., Bouton, Conconi, and Pino2015). Finally, the role of

interestgroupsthattrytopromotetheirpreferredpoliciesintheaftermathofsuchevents

deservesfurtherexploration.

Third, future research might directly explore the preferences of politicians. Do

RepublicanlegislaturesloosengunrestrictionsbecauseRepublicanpoliticiansthemselves

prefer looser restrictions or due to pressure from constituents or interest groups? For

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example,ifconstituentpreferencesaredrivingresults,wemightexpectthatresultsdiffer

in areaswith high versus low rates of gun ownership. To provide exploratory evidence,

Appendix G shows the results from adding a proxy for gun ownership to the models.

Following Cook and Ludwig (2006), we calculate the percentage of suicides that are

firearm related as a proxy for gun ownership and interact this variable with the mass

shootingindicator.5Thecoefficientonthisvariable isnotsignificanteither in isolationor

when added to the specification with all political interactions. This suggests that the

tendencyofRepublicans to loosenguncontrol isnotentirelydrivenbyhighratesofgun

ownership (andpresumablyhigh ratesof support for lessgun control), but representsa

distincteffectofpoliticalaffiliation.

Fourth, there is a large literatureon the impact of gunpolicies on crime (Duggan

2001, Ludwig andCook2000, Ludwig andCook2003,Abrams2012),whichhas yielded

mixedresults.Therelationshipwefindbetweenmassshootingsandgunpolicyraisesthe

possibilityofusingmassshootingsasaninstrumentalvariabletostudytheimpactofgun

laws on gun deaths. Unfortunately, in our sample, mass shootings are not a sufficiently

stronginstrumenttoestimatetheeffectsofgunpolicyongundeaths,duetotheirrelative

infrequency.(AppendixHpresentsresultsofthisanalysis.)Thisleavesopenthepossibility

of using salient and plausibly random events to instrument for policy changes in future

research.

Our findings suggest that while much attention has been rightfully devoted to

understanding the impact of policy, there is a lot to be learned from exploring the

5Wesmooththepercentageofsuicidesthatarefirearm-relatedbytakinganaverageoftheprevious5-yearstoreducetheinfluenceofshort-termfluctuationsingunsuicides.Theresultsarequalitativelysimilarwithoutthisadjustment.

17

determinants of policy change aswell.We find that even random and infrequent events

that account for a relatively small portion of total societal harm in a domain might

nonethelessbecrucialleversforpolicyconsiderationandchange.Thisdoesnotimplythat

politicians and policymakers are over-reacting; itmay be that on issueswhere there is

usually political deadlock, salient events create opportunities for change that has been

sought all along. Whether these changes reflect appropriate responses to the problem

remainsanopenquestion.

18

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Finlay,Keith,andLeandroM.Magnusson.2009.“ImplementingWeak-InstrumentRobustTestsforaGeneralClassofInstrumental-VariablesModels.”TheStataJournal9(3):398–421.

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Godwin,MarciaL.,andJeanReithSchroedel.1998.“GunControlPoliticsinCalifornia.”InTheChanging Politics of Gun Control, edited by John M. Bruce and Clyde Wilcox, 88–110.Lanham,MD:Rowman&LittlefieldPublishers,Inc.

Goss,KristinA.2006.Disarmed:TheMissingMovementforGunControlinAmerica.Princeton,NJ:PrincetonUniversityPress.

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Krouse, William and Daniel J. Richardson. 2015.Mass Murder with Firearms: Incidents andVictims, 1999-2013 (CRS Report No. 7-5700). Washington, DC: Congressional ResearchService,2015.https://fas.org/sgp/crs/misc/R44126.pdf.

Levitt, Steven D. 1996. “The Effect of Prison Population Size on Crime Rates: Evidence fromPrisonOvercrowdingLitigation.”TheQuarterlyJournalofEconomics111(2):319–51.

Ludwig,Jens,andPhilipJ.Cook.2000.“HomicideandSuicideRatesAssociatedwithImplementationoftheBradyHandgunViolencePreventionAct.”JAMA284(5):585-91.

Ludwig,Jens,andPhilipJCook.2003.EvaluatingGunPolicyEffectsonCrimeandViolence.Washington,D.C.:BrookingsInstitutionPress.

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Moreira,MarceloJ.2003.“AConditionalLikelihoodRatioTestforStructuralModels.”Econometrica71(4):1027-48.

Morton,RobertJ.andMarkA.Hilts,eds.2008.“SerialMurder:Multi-disciplinaryperspectivesfor investigators.” Serial Murder Symposium, San Antonio, TX, August 29-September 2,2005. Washington, DC: Federal Bureau of Investigation, http://www.fbi.gov/stats-services/publications/serial-murder/serial-murder-july-2008-pdf.

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Spitzer,RobertJ.2012.ThePoliticsofGunControl.5thEd.Boulder,CO:ParadigmPublishers.

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20

USA Today. 2013. “Behind the Bloodshed: The Untold Story of America’s Mass Killings,”http://www.gannett-cdn.com/GDContent/mass-killings/index.html.

21

Figures

Figure1Massshootingincidentsandfatalitiesbyyear,1989-2014.Theupperpanelshowsthenumberoffatalitiesinmassshootingsinwhichatleast3peoplenotrelatedorromanticallyconnectedtotheshooterwerekilled.Thebottompanelshowsthenumberoftheseincidents.Washington,D.C.isnotincludedinthesample.

Figure 2Comparisonofannual legislation introducedbypoliticalparty.Points represent themeanand linesare95%confidenceintervals.Legislaturecontrolmeansonepoliticalpartyincludesbothchambersofthelegislature.Thecountsoftighteningandlooseninglawsarebasedonlawswithcoderagreement(seesection2.2oftext).

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Bills Introduced Laws Passed Tightening Laws Loosening Laws

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Democrat

Split

22

Tables

Table1:SummaryStatistics

mean sd p5 p10 p50 p90 p95 N

Legislation BillsIntroduced 16.33 22.04 0 1 10 38 53 1,250LawsEnacted 2.56 3.35 0 0 1 6 9 1,250TighteningLaws 0.70 1.29 0 0 0 2 3 1,250LooseningLaws 0.25 0.62 0 0 0 1 1 1,250GunViolence MassShooting 0.12 0.32 0 0 0 1 1 1,250Fatalities 0.72 2.40 0 0 0 4 5 1,250GunHomicideRate 3.76 2.55 0.72 0.98 3.42 7.40 8.65 1,250PoliticalControls DemocraticLegislature 0.42 0.49 0 0 0 1 1 1,250RepublicanLegislature 0.34 0.47 0 0 0 1 1 1,250RepublicanGovernor 0.53 0.50 0 0 1 1 1 1,200InstitutionalControls RegularSession 0.94 0.24 0 1 1 1 1 1,250BillCarryover 0.27 0.44 0 0 0 1 1 1,250LimitedLeg.Topic 0.06 0.24 0 0 0 0 1 1,250LegislatureSize 148 59.3 62 82.5 144 200 236 1,250DemographicControls Elderly(65+)% 12.9 2.0 9.8 10.7 13.1 15.2 15.7 1,250Under25% 35.1 2.7 31.4 32.2 34.8 38.0 39.5 1,250Black% 10.3 9.5 0.6 0.8 7.4 26.4 30.1 1,250Hispanic% 8.3 9.2 0.8 1.2 5.1 20.3 29.9 1,250Unemployment% 5.7 1.9 3.1 3.5 5.4 8.1 9.3 1,250Incomepercapita 19.1 3.3 14.1 15.0 18.7 23.3 25.8 1,250HighSchool% 85.2 5.2 75.7 78.4 86.1 91.2 92.0 1,250Veteran% 11.8 2.4 7.9 8.8 11.8 15.0 16.1 1,250Divorced% 11.8 1.8 8.9 9.5 11.8 14.1 14.7 1,250

Note: Observations are state-years. Bills Introduced is the number of bills introduced in the legislature; LawsEnactedisthenumberofbillsthatbecamelaw.TighteningandLooseningLawsarenumbersofenactedlawsthattightened and loosened gun control respectively. Mass Shooting is an indicator for state-years with a massshootinginwhich3+peoplenotromanticallyinvolvedwithorrelatedtotheshooter(s)werekilled.Fatalitiesisthetotalnumberofdeathsinmassshootingsinastate-year.DemocraticandRepublicanLegislatureareindicatorsforpartycontrolofthestatelegislature.RepublicanGovernorisanindicatorforRepublicangovernors.RegularSessionindicateswhetherthelegislatureconvenedaregular(asopposedtospecial)sessiontoconsiderbills;somestatelegislatures onlymeet every other year. Bill Carryover is proportion of chambers in which bills are eligible forcarryovertothenextsession.LimitedLeg.Topicisanindicatorforlegislativesessionsduringwhichbillsarelimitedtospecifictopics(e.g.appropriations).LegislatureSizeisthenumberoflawmakersservinginthestatelegislature.Incomepercapitaismeasuredinthousandsof1987U.S.dollars;otherdemographicvariablesarepercentages.

23

Table2:TheEffectofMassShootingsonGunBillIntroductionsDependentvariable:numberoffirearm-relatedbillsintroducedinthestatelegislature.

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

MassShooting 0.154* 0.087 0.155** 0.147**

(0.088) (0.074) (0.065) (0.062)

Fatalities 0.023* 0.020** 0.023*** 0.023***

(0.013) (0.009) (0.007) (0.007)

Dem.Legislature -0.136* -0.134*

(0.074) (0.076)

Rep.Legislature 0.067 0.071

(0.064) (0.064)

Rep.Governor -0.022 -0.014

(0.054) (0.053)

InstitutionalControls No

No

Yes

Yes

No

No

Yes

YesDemographicControls No No No Yes No No No Yes

StateFixedEffects Yes Yes Yes Yes Yes Yes Yes YesYearFixedEffects No

Yes

Yes

Yes

No

Yes

Yes

Yes

N 1,250

1,250

1,250

1,250

1,250

1,250

1,250

1,250Note:Robust standarderrors clusteredby state in parentheses. Stars following coefficients representp-values less

than.10(*),.05(**)and.01(***).SeenotetoTable1forvariabledefinitions.

24

Table3:ComparingMassShootingsandNon-MassShootingsDependentvariable:numberoffirearm-relatedbillsintroducedinthestatelegislature.

(1)

(2)

(3)

(4)

MassShootingFatalities/100,000 1.678*** 1.332*** 1.303*** 1.316***

(0.428)

(0.240)

(0.223)

(0.202)

OrdinaryGunHomicides/100,000 -0.007

0.017

0.014

0.017

(0.024)

(0.032)

(0.032)

(0.035)

PoliticalControls No No No Yes InstitutionalControls No No Yes Yes DemographicControls No No No Yes StateFixedEffects Yes

Yes

Yes

Yes

YearFixedEffects No

Yes

Yes

YesN 1,250

1,250

1,250

1,250

Note:Robust standarderrors clusteredby state in parentheses. Stars following coefficients representp-values lessthan.10(*),.05(**)and.01(***).MassShootingFatalities/100,000isthenumberofdeathsinmassshootingsper100,000stateresidents.OrdinaryGunHomicides/100,000isthenumberofgunhomicidesnotinmassshootingsper100,000stateresidents.ControlvariablesaredefinedasinTable1.

25

Table4:MassShootingsandEnactedLawsDependentvariable:numberoffirearm-relatedlawsenacted(i.e.billsthatbecamelaw). TighteningLaws LooseningLaws (1) (2) (3) (4) (5) (6) (7) (8) MassShooting -0.030 0.246 (0.101) (0.181) Fatalities 0.015* 0.008 (0.008) (0.022) Rep.Leg.×Shooting -0.017 0.733*** (0.238) (0.255) Dem.Leg.×Shooting 0.037 -0.250 (0.140) (0.402) SplitLeg.×Shooting -0.216 0.168 (0.262) (0.340) Rep.Leg.×Fatalities 0.018 0.152*** (0.050) (0.031)Dem.Leg.×Fatalities 0.014 -0.047 (0.015) (0.054)SplitLeg.×Fatalities 0.015 -0.018 (0.013) (0.018)Dem.Legislature 0.110 0.105 0.070 0.105 -0.318 -0.303 -0.253 -0.259 (0.147) (0.148) (0.171) (0.165) (0.200) (0.198) (0.229) (0.222)Rep.Legislature 0.165 0.180 0.134 0.178 0.494*** 0.496*** 0.402* 0.318 (0.137) (0.137) (0.147) (0.142) (0.187) (0.190) (0.213) (0.202)Rep.Governor -0.046 -0.045 -0.047 -0.045 -0.113 -0.111 -0.093 -0.084 (0.086) (0.085) (0.085) (0.083) (0.168) (0.167) (0.166) (0.167)InstitutionalControls Yes Yes Yes Yes Yes Yes Yes Yes DemographicControls Yes Yes Yes Yes Yes Yes Yes Yes StateFixedEffects Yes Yes Yes Yes Yes Yes Yes Yes YearFixedEffects Yes Yes Yes Yes Yes Yes Yes Yes N 1,250 1,250 1,250 1,250 1,175 1,175 1,175 1,175 Note:Robuststandarderrorsclusteredbystateinparentheses.Starsfollowingcoefficientsrepresentp-valueslessthan .10 (*), .05 (**) and .01 (***). Models 3-4 and 7-8 show the effect of mass shootings in Republican,Democratic,andsplitlegislatures;theomittedgroupinthesemodelsisstateswithoutamassshooting.AllvariablesaredefinedasinTable1.

26

AppendixA:CodingGunLaws

id summary tighten loosen uncertain

1 Createsanewfelonyforfiringagunwithin1,000feetofaneducationalfacility. 1 0 0

2 Reducestheagelimitforpurchaseofahandgunfrom21to18. 0 1 0

3 Allowsparoleofficerstocarryaloadedfirearmwhilecommutingtoandfromwork. 0 0 0

4 Relatestotheuseoffirearmsinstateparksandcampgrounds. 0 0 1

5

Requiresalicensetooperateagunshow.Eliminatesthewaitingperiodforfirearmsalesifthepurchaserhasavalidpermittocarryaconcealedweapon.

1 1 0

Note: Table shows examples of coding gun laws based on bill summaries. Coders were given a full manual toexplain themeaning of “tighten”, “loosen”, “neutral,” and “uncertain” alongwith the following examples. ThistablemimicstheappearanceoftheExcelworkbooksusedbythecoders.Thefirstbillcreatesanewcrimerelatedto firearms. It tightens restrictions on firearms. The second bill makes it easier for people to acquire guns; itloosensrestrictionsonfirearms.Thethirdbillisexclusivelyaboutparoleofficers;itisneutralbecauseitdoesnotaffectthegeneralpublic.Thefourthbill isuncertainbecausethesummaryisagenericdescriptionthatdoesnotspecifywhether the law tightens or loosens restrictions on firearms. The fifth bill both tightens and loosens; itregulatesgunshows,butalsoeliminatesarestrictiononfirearmpurchasers.

27

AppendixB:EffectofShootingsinNeighboringStates

TableB1:MassShootingsinNeighboringStates,BillsandLawsDependentvariable:numberoffirearm-relatedbillsintroducedorlawsenacted BillsIntroduced LawsEnacted (1) (2) (3) (4) (5) (6) (7) (8) MassShooting 0.147** 0.158** 0.097 0.087 (0.064) (0.073) (0.067) (0.080) NeighborShooting -0.052 0.028 (0.041) (0.048) Cen.DivisionShooting -0.018 0.020 (0.042) (0.049) Fatalities 0.023*** 0.022*** 0.014* 0.011 (0.007) (0.008) (0.008) (0.009)NeighborFatalities -0.002 0.002 (0.004) (0.004) Cen.DivisionFatalities 0.002 0.004 (0.005) (0.004)PoliticalControls Yes Yes Yes Yes Yes Yes Yes Yes InstitutionalControls Yes Yes Yes Yes Yes Yes Yes Yes DemographicControls Yes Yes Yes Yes Yes Yes Yes Yes StateFixedEffects Yes Yes Yes Yes Yes Yes Yes Yes YearFixedEffects Yes Yes Yes Yes Yes Yes Yes Yes N 1,250 1,250 1,250 1,250 1,250 1,250 1,250 1,250 Note:Robuststandarderrorsclusteredbystateinparentheses.Starsfollowingcoefficientsrepresentp-valueslessthan.10(*),.05(**)and.01(***).Neighborvariablesrefertostateswithasharedborder;Cen.DivisionreferstostateswithinthesameCensusdivision.AllothervariablesaredefinedasinTable1.

28

TableB2:MassShootingsinNeighboringStatesandDirectionsofPolicyChangeDependentvariable:numberoffirearm-relatedlawsenacted TighteningLaws LooseningLaws (1) (2) (3) (4) Rep.Leg.×Shooting -0.015 -0.104 0.732*** 0.809*** (0.223) (0.260) (0.257) (0.275)Dem.Leg.×Shooting 0.072 0.082 -0.294 -0.169 (0.129) (0.122) (0.379) (0.416)SplitLeg.×Shooting -0.242 -0.057 0.174 0.090 (0.249) (0.224) (0.331) (0.383)Rep.Leg.×NeighborShooting 0.208 -0.059 (0.135) (0.236) Dem.Leg.×NeighborShooting -0.315** 0.370 (0.131) (0.246) SplitLeg.×NeighborShooting -0.157 -0.095 (0.202) (0.234) Rep.Leg.×Cen.DivisionShooting 0.166 -0.153 (0.154) (0.232)Dem.Leg.×Cen.DivisionShooting -0.071 -0.141 (0.112) (0.175)SplitLeg.×Cen.DivisionShooting -0.258 0.146 (0.170) (0.258)PoliticalControls Yes Yes Yes Yes InstitutionalControls Yes Yes Yes Yes DemographicControls Yes Yes Yes Yes StateFixedEffects Yes Yes Yes Yes YearFixedEffects Yes Yes Yes Yes N 1,250 1,250 1,250 1,250 Note:Robuststandarderrorsclusteredbystateinparentheses.Starsfollowingcoefficientsrepresentp-valueslessthan.10(*),.05(**)and.01(***).ModelsshowtheeffectofmassshootingsinRepublican,Democratic,andsplitlegislatures;theomittedgroupinthesemodelsisstateswithoutamassshooting.Neighborvariablesrefertostateswithasharedborder;Cen.DivisionreferstostateswithinthesameCensusdivision.AllothervariablesaredefinedasinTable1.

29

TableB3:FatalitiesinNeighboringStatesandDirectionsofPolicyChangeDependentvariable:numberoffirearm-relatedlawsenacted TighteningLaws LooseningLaws (1) (2) (3) (4) Rep.Leg.×Fatalities 0.014 0.002 0.151*** 0.150*** (0.047) (0.049) (0.031) (0.043)Dem.Leg.×Fatalities 0.016 0.023 -0.046 -0.018 (0.014) (0.014) (0.053) (0.054)SplitLeg.×Fatalities 0.013 0.029** -0.022 -0.023 (0.012) (0.014) (0.019) (0.029)Rep.Leg.×NeighborFatalities 0.017 -0.005 (0.013) (0.022) Dem.Leg.×NeighborFatalities -0.025** 0.015 (0.012) (0.017) SplitLeg.×NeighborFatalities -0.008 -0.035** (0.022) (0.018) Rep.Leg.×Cen.DivisionFatalities 0.020* 0.002 (0.011) (0.029)Dem.Leg.×Cen.DivisionFatalities -0.009 -0.031** (0.008) (0.015)SplitLeg.×Cen.DivisionFatalities -0.015 0.007 (0.021) (0.023)PoliticalControls Yes Yes Yes Yes InstitutionalControls Yes Yes Yes Yes DemographicControls Yes Yes Yes Yes StateFixedEffects Yes Yes Yes Yes YearFixedEffects Yes Yes Yes Yes N 1,250 1,250 1,250 1,250 Note:Robuststandarderrorsclusteredbystateinparentheses.Starsfollowingcoefficientsrepresentp-valueslessthan.10(*), .05(**)and.01(***).Modelsshowtheeffectofmassshootingfatalities inRepublican,Democratic,andsplitlegislatures;theomittedgroupinthesemodelsisstateswithoutamassshooting.Neighborvariablesrefertostateswithasharedborder;Cen.DivisionreferstostateswithinthesameCensusdivision.AllothervariablesaredefinedasinTable1.

30

AppendixC:LawsEnactedTableC1:LawsEnactedandMassShootingsDependentvariable:numberoffirearm-relatedlawsenacted(i.e.billsthatbecamelaw).

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

MassShooting 0.074 0.020 0.079 0.098

(0.118) (0.108) (0.075) (0.067)

Fatalities 0.011 0.007 0.011 0.014*

(0.013) (0.011) (0.008) (0.008)

Dem.Legislature -0.023 -0.020

(0.082) (0.085)

Rep.Legislature 0.269*** 0.275***

(0.087) (0.087)

Rep.Governor -0.002 -0.000

(0.051) (0.050)

InstitutionalControls No

No

Yes

Yes

No

No

Yes

YesDemographicControls No No No Yes No No No Yes

StateFixedEffects Yes Yes Yes Yes Yes Yes Yes YesYearFixedEffects No

Yes

Yes

Yes

No

Yes

Yes

Yes

N 1,250

1,250

1,250

1,250

1,250

1,250

1,250

1,250Note:Robust standarderrors clusteredby state in parentheses. Stars following coefficients representp-values less

than .10 (*), .05 (**) and .01 (***). Variables are identical to those in Table 2, except for the dependent variable,whichisthenumberoffirearm-relatedlawsenactedinsteadofthenumberofbillsintroduced.SeenotetoTable1forvariabledefinitions.

31

AppendixD:PredictingMassShootings

TableD1:LinearProbabilityModelforMassShootingusingControlVariablesDependentvariable:indicatorforstate-yearwithamassshooting. (1)

(2)

(3)

(4)

(5)

(6)

(7)

LagBillsIntroduced

0.000

(0.001)

LagLawsEnacted

0.000

(0.005)

LagTighteninglaws

-0.000

(0.009)

LagLooseninglaws

0.016

(0.014)

Dem.Legislature

0.006

0.011

0.010

0.010

0.010

0.012

(0.044)

(0.043)

(0.045)

(0.046)

(0.045)

(0.045)

Rep.Legislature

-0.030

-0.030

-0.027

-0.027

-0.027

-0.028

(0.036)

(0.035)

(0.037)

(0.037)

(0.037)

(0.038)

Rep.Governor

-0.012

-0.010

-0.010

-0.010

-0.010

-0.010

(0.020)

(0.020)

(0.021)

(0.021)

(0.022)

(0.021)

RegularSession

0.121

0.122* 0.122* 0.121

0.128*

(0.077)

(0.072)

(0.067)

(0.073)

(0.073)

BillCarryovera

0.057** 0.056* 0.055** 0.055** 0.056**

(0.026)

(0.029)

(0.027)

(0.027)

(0.027)

LimitedLeg.Topic

-0.044

-0.062

-0.062

-0.062

-0.065

(0.062)

(0.059)

(0.060)

(0.059)

(0.060)

LegislatureSize

0.002

0.002

0.002

0.002

0.002

(0.001)

(0.002)

(0.002)

(0.002)

(0.002)

LogPopulation -0.191 -0.131 -0.140 -0.210 -0.211 -0.210 -0.210 (0.279) (0.265) (0.273) (0.297) (0.298) (0.297) (0.299)Elderly(65+)% 0.001

-0.001

-0.002

0.004

0.004

0.004

0.003

(0.025)

(0.024)

(0.024)

(0.024)

(0.024)

(0.025)

(0.025)Under25% -0.002

0.001

0.001

0.002

0.003

0.003

0.002

(0.020)

(0.019)

(0.019)

(0.020)

(0.020)

(0.020)

(0.020)Black% -0.009

-0.009

-0.008

0.000

-0.000

-0.000

0.002

(0.015)

(0.015)

(0.015)

(0.017)

(0.018)

(0.018)

(0.017)Hispanic% -0.001

-0.003

-0.002

-0.001

-0.001

-0.001

-0.001

(0.015)

(0.015)

(0.015)

(0.016)

(0.016)

(0.016)

(0.016)Unemployment% 0.025** 0.025** 0.025** 0.024** 0.024** 0.024** 0.024**

(0.011)

(0.011)

(0.011)

(0.011)

(0.011)

(0.011)

(0.012)

Incomepercapita 0.013

0.012

0.013

0.013

0.013

0.013

0.013

(0.013)

(0.013)

(0.014)

(0.014)

(0.014)

(0.014)

(0.014)

HighSchool% -0.004

-0.003

-0.002

-0.003

-0.003

-0.003

-0.003

(0.006)

(0.006)

(0.006)

(0.006)

(0.006)

(0.006)

(0.006)

Veteran% -0.003

-0.001

-0.001

-0.004

-0.004

-0.004

-0.004

(0.012)

(0.012)

(0.012)

(0.013)

(0.013)

(0.012)

(0.012)

Divorced% -0.004

-0.003

-0.002

0.001

0.001

0.001

0.001

(0.010)

(0.010)

(0.011)

(0.011)

(0.011)

(0.011)

(0.011)

Constant 3.072

2.024

1.700

2.612

2.633

2.621

2.637

(4.410)

(4.218)

(4.221)

(4.627)

(4.636)

(4.620)

(4.645)

StateFixedEffects Yes

Yes

Yes

Yes

Yes

Yes

YesYearFixedEffects Yes

Yes

Yes

Yes

Yes

Yes

Yes

N 1,250

1,250

1,250

1,200

1,200

1,200

1,200aThereisnoapriorireasontothinkbillcarryoverwouldberelatedtomassshootings;thiscorrelationisinsignificantwhen

Virginia,whichunlikemoststates,allowscarryoverinevenyears,isdroppedfromthesample.FourofVirginia’ssixmassshootingshappenedinevenyears.

32

TableD2:LinearProbabilityModelforMassShootingusingPolicyVariablesDependentvariable:indicatorforstate-yearwithamassshooting.

(1)

(2)

HandgunWaitingPeriod(days) 0.002

0.002

(0.005)

(0.005)

Long-gunWaitingPeriod(days) -0.005

-0.005

(0.019)

(0.020)

Age18+forTransaction 0.031

0.026

(0.026)

(0.028)

Age21+forTransaction -0.067

-0.080

(0.057)

(0.056)

HandgunPermitSystem -0.137

-0.136

(0.094)

(0.100)

BackgroundCheck,AllHandgunSales -0.066

-0.066

(0.080)

(0.085)

BackgroundCheck,AllFirearmSales 0.019

-0.005

(0.116)

(0.127)

AssaultWeaponsBan 0.042

0.050

(0.051)

(0.053)

ShallIssueConcealedCarry -0.010

-0.006

(0.038)

(0.039)

NoPermitNeededConcealedCarry 0.162

0.181

(0.188)

(0.184)

LogPopulation -0.494 -0.445 (0.325) (0.310)PoliticalControls No

Yes

DemographicControls Yes YesStateFixedEffects Yes YesYearFixedEffects Yes

Yes

N 1,250

1,250Note:HandgunWaitingPeriodisthenumberofdayspurchasersmustwaitbeforeacceptingdeliveryofahandgun.

Long-gunWaiting Period is similarly defined for long-guns (e.g. rifles and shotguns).Age 18+ Transaction is anindicator for laws that prevent vendors from selling handguns to minors or prevent minors from purchasinghandguns. Age 21+ Transaction is defined the same way for persons under 21.Handgun Permit System is anindicator for states that require permits to purchase a handgun. Background Check, All Handgun Sales is anindicator for requiring a background check for all handgun transactions (including private sales). BackgroundCheck, All Firearm Sales is an indicator for requiring a background check for all firearm transactions (includingprivatesales).AssaultWeaponsBanisanindicatorforstatesthatbansometypesofassaultriflesorpistols.ShallIssueConcealedCarry isanindicatorforstatesthatrequirethepermittingauthoritytograntalicensetoanyonemeetingtheminimumstatutoryqualifications(i.e.donotpermit lawenforcementdiscretion in issuingpermits).NoPermitNeededConcealedCarryisanindicatorforstatesthatallowconcealedcarrywithoutapermit.

33

AppendixE:State-SpecificTimeTrendsTableE1:TheEffectofMassShootingsonGunBillIntroductionsDependentvariable:numberoffirearm-relatedbillsintroducedinthestatelegislature.

(1)

(2)

(3)

(4)

(5)

(6)

MassShooting 0.074 0.151** 0.157**

(0.075) (0.069) (0.063)

Fatalities 0.020** 0.024*** 0.024***

(0.010) (0.008) (0.007)

InstitutionalControls No

Yes

Yes

No

Yes

YesPoliticalControls No No Yes No No Yes

DemographicControls No No Yes No No YesStateFixedEffects Yes Yes Yes Yes Yes YesState-SpecificTrends Yes Yes Yes Yes Yes YesYearFixedEffects Yes

Yes

Yes

Yes

Yes

Yes

N 1,250

1,250

1,250

1,250

1,250

1,250Note:Robust standarderrors clusteredby state in parentheses. Stars following coefficients representp-values less

than.10(*),.05(**)and.01(***).VariablesareidenticaltothoseinTable2anddefinedinthenotetoTable1.

TableE2:MassShootings,OrdinaryGunHomicides,andBillIntroductionsDependentvariable:numberoffirearm-relatedbillsintroducedinthestatelegislature.

(1) (2) (3) MassShootingFatalities/100,000 1.504*** 1.481*** 1.409***

(0.323)

(0.261)

(0.195)

OrdinaryGunHomicides/100,000 0.010

0.005

0.007

(0.058)

(0.055)

(0.049)

InstitutionalControls No Yes Yes PoliticalControls No No Yes DemographicControls No No Yes StateFixedEffects Yes Yes Yes State-SpecificTrends Yes

Yes

Yes

YearFixedEffects Yes

Yes

YesN 1,250

1,250

1,250

Note:Robust standarderrors clusteredby state in parentheses. Stars following coefficients representp-values lessthan.10(*),.05(**)and.01(***).VariablesareidenticaltothoseinTable3anddefinedinthenotetoTable1.

34

AppendixF:PlaceboMassShootingAnalysis

TableF1:PlaceboAnalysisforBillIntroductions(MirrorsTable2)

PercentilesofPlaceboTestStatistic

Actual 1st 5th 10th 90th 95th 99th

ShootingIndicator(model4) 2.37 -3.69 -2.80 -2.33 0.71 1.06 1.90ShootingFatalities(model8) 3.29 -4.01 -2.81 -2.40 0.94 1.35 2.32

TableF2:PlaceboAnalysisforEnactedLaws(MirrorsTable4)

PercentilesofPlaceboTestStatistic

Actual 1st 5th 10th 90th 95th 99thTighteningLaws(models1-4)

PooledShooting -0.30 -2.19 -1.40 -1.04 1.82 2.29 3.02PooledFatalities 1.88 -2.39 -1.64 -1.24 1.80 2.28 3.44Rep.Leg.×Shooting -0.07 -2.93 -1.71 -1.18 1.63 2.18 3.28Dem.Leg.×Shooting 0.26 -2.47 -1.63 -1.33 1.45 1.96 2.60SplitLeg.×Shooting -0.82 -2.65 -1.46 -0.98 2.13 2.60 4.14Rep.Leg.×Fatalities 0.36 -2.80 -1.86 -1.33 1.68 2.46 3.72Dem.Leg.×Fatalities 0.93 -2.81 -1.80 -1.47 1.52 2.02 3.49SplitLeg.×Fatalities 1.15 -2.79 -1.65 -1.09 2.26 2.94 4.82

LooseningLaws(models5-8)PooledShooting 1.36 -3.19 -2.28 -1.89 0.84 1.26 2.11PooledFatalities 0.36 -3.04 -2.25 -1.82 0.93 1.46 2.58Rep.Leg.×Shooting 2.87 -2.98 -2.27 -1.97 0.68 1.05 1.69Dem.Leg.×Shooting -0.62 -2.69 -1.79 -1.40 1.23 1.66 2.80SplitLeg.×Shooting 0.49 -3.10 -2.12 -1.70 1.40 1.86 2.89Rep.Leg.×Fatalities 4.90 -2.80 -2.24 -1.89 0.93 1.35 2.22Dem.Leg.×Fatalities -0.87 -2.89 -1.90 -1.48 1.47 1.94 2.87SplitLeg.×Fatalities -1.00 -3.22 -2.12 -1.75 1.47 2.04 3.32

Notes:We randomly assign placebo mass shootings to state-years in which no actual shooting occurred withprobabilityequal toeach state’s frequencyof shootings,and randomlydrawa fatality count fromtheempiricaldistributionof fatalities.Wethenre-runthemodelsandcalculatetheteststatistic fortheplaceboshootingandfatalitycoefficients.Theabovepercentilesarebasedon1,000replications.The“Pooled”rowsinTableE2mirrormodels1-2and5-6ofTable4(themodelswithoutinteractioneffects).Thelegislatureeffectsmirrormodels3-4fortighteninglawsand7-8forlooseninglaws.

35

AppendixG:GunOwnership,MassShootingsandEnactedLaws

TableG1:EffectsofMassShootings,withGunOwnershipProxyDependentvariable:numberoffirearm-relatedlawsenacted(i.e.billsthatbecamelaw).

TighteningLaws LooseningLaws (1) (2) (3) (4) (5) (6) MassShooting -0.209 -0.469 -0.325 0.189 0.720 0.527 (0.260) (0.584) (0.511) (0.347) (0.786) (0.710)Rep.Leg.×Shooting 0.193 0.169 0.533 0.580 (0.399) (0.385) (0.484) (0.482) Dem.Leg.×Shooting 0.255 0.243 -0.385 -0.382 (0.308) (0.295) (0.467) (0.440) GunSuicidePct.×Shooting 0.005 0.005 -0.010 -0.005 (0.009) (0.010) (0.014) (0.014)GunSuicidePercent 0.037 0.036 0.035 0.106* 0.109* 0.116* (0.027) (0.027) (0.027) (0.060) (0.061) (0.061)Dem.Legislature 0.062 0.062 0.100 -0.255 -0.261 -0.322* (0.171) (0.170) (0.149) (0.229) (0.225) (0.194)Rep.Legislature 0.145 0.149 0.177 0.423** 0.413* 0.510*** (0.143) (0.143) (0.133) (0.216) (0.218) (0.192)Rep.Governor -0.040 -0.046 -0.046 -0.092 -0.082 -0.108 (0.086) (0.088) (0.089) (0.164) (0.162) (0.166)InstitutionalControls Yes Yes Yes Yes Yes Yes DemographicControls Yes Yes Yes Yes Yes Yes StateFixedEffects Yes Yes Yes Yes Yes Yes YearFixedEffects Yes Yes Yes Yes Yes Yes N 1,250 1,250 1,250 1,175 1,175 1,175 Note:Robuststandarderrorsclusteredbystateinparentheses.Starsfollowingcoefficientsrepresentp-valueslessthan.10(*),.05(**)and.01(***).GunSuicidePct.isthe5-yearmovingaverageofthepercentageofsuicidesthatarefirearm-relatedandisusedtoproxyforgunownership(CookandLudwig2006).Models1and4showthatthemain result does not change with adding this control variable; the other models suggest that the response ofRepublicanlegislaturescannotbeexplainedbyratesofgunownership.AllvariablesaredefinedasinTable1.

36

AppendixH:MassShootingsasanInstrumentforGunPolicy

Inthisappendixweusemassshootingsasaninstrumentalvariabletostudytheimpactof

gunlawsongundeaths.Westartwiththefollowingmodel:

ln 𝐷!" = 𝛼! + 𝜃! + 𝛽 𝐺𝑢𝑛 𝐶𝑜𝑛𝑡𝑟𝑜𝑙!" + 𝛿!𝑍!" + 𝜖!"

where𝐷!"isnon-massshootinggundeathsper100,000peopleinstate𝑠andyear𝑡,𝛼!and

𝜃!are stateandyeareffects,𝐺𝑢𝑛 𝐶𝑜𝑛𝑡𝑟𝑜𝑙!"is an index representing the strictnessof gun

policy,and𝑍!"isavectorofcontrols–demographic,political,andeconomicfactors–that

potentiallyaffectgundeaths.WeusethesamevariablesasLevitt (1996)ascontrols,but

also include dummies for Republican and Democratic trifectas or legislatures, and a

dummyforRepublicangovernors.

We do not directly observe𝐺𝑢𝑛 𝐶𝑜𝑛𝑡𝑟𝑜𝑙!"; instead, we observe the enactment of

newlawsthatchangegunpolicy.Therefore,weestimatetheequationinfirstdifferences:

𝛥 ln 𝐷!" = 𝜆! + 𝛽 𝑁𝑒𝑤 𝐺𝑢𝑛 𝐿𝑎𝑤𝑠!" + 𝛥𝑍!"𝛿 + 𝛥𝜖!"

where 𝑁𝑒𝑤 𝐺𝑢𝑛 𝐿𝑎𝑤𝑠!" = 𝛥𝐺𝑢𝑛 𝐶𝑜𝑛𝑡𝑟𝑜𝑙!"isnegativeforlawsthatloosenguncontroland

positiveforlawsthatincreaseguncontrol(accordingtoourcoders,seedatadescription).

Basedonourmainresults,weinstrumentforgunlawsusingthefirstlagsofmassshooting

fatalitiesandtheinteractionof laggedmassshootingfatalitieswithRepublicancontrolof

stategovernment.Theformershouldbepositivelycorrelatedwithnewlawsandthelatter

negativelycorrelatedwithnewlaws.

We estimate the model using Fuller’s (1977) modified LIML with𝛼 = 1(Baum,

Schaffer,andStillman2007).Firststageresultssuggesttheinstrumentsareweakdespite

being jointly significant(𝐹 = 5.98)with the expected sign (Stock and Yogo 2005). The

37

coefficientsontheexogenousinstrumentsinthereducedformequationforfirearmdeaths

are not significant, but also have the expected signs (negative for laggedmass shooting

fatalities and positive for the interaction of lagged fatalities with Republican control of

government).Ourestimate𝛽is−0.016withstandarderror0.013.Aconditionallikelihood

ratiotest(Moreira2003;Andrews,Moreira,andStock2004;FinlayandMagnusson2009)

cannot reject the null hypothesis that𝛽 = 0 (𝑝 = 0.24). We also estimated models that

includeproxiesforgunownership.Includingthepercentageofsuicidescommittedwitha

gun(CookandLudwig2006)doesnotchangeourinference.