DPRIETI Discussion Paper Series 13-E-035

The Effects of Birth Weight:Does fetal origin really matter for long-run outcomes?

NAKAMURO MakikoKeio University

UZUKI YukaNational Institute for Educational Policy Research

INUI TomohikoRIETI

The Research Institute of Economy, Trade and Industryhttp://www.rieti.go.jp/en/

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RIETI Discussion Paper Series 13-E-035 April 2013

The Effects of Birth Weight:

Does fetal origin really matter for long-run outcomes? 1

NAKAMURO Makiko (Keio University)2 UZUKI Yuka (National Institute for Educational Policy Research)

& INUI Tomohiko (Nihon University / RIETI)

Abstract

This paper investigates whether birth weight itself causes individuals’ future life chances. By using a sample of twins in Japan and controlling for the potential effects of genes and family backgrounds, we examine the effect of birth weight on later educational and economic outcomes. The most important finding is that birth weight has a causal effect on academic achievement at around the age of 15, but not on the highest years of schooling and earnings. Keywords: Birth weight, Nutritional intake, Identical twins, Endogeneity JEL classification codes: I10, I20

RIETI Discussion Papers Series aims at widely disseminating research results in the form of professional papers, thereby stimulating lively discussion. The views expressed in the papers are solely those of the author(s), and do not represent those of the Research Institute of Economy, Trade and Industry.

1 This study was conducted as part of a project titled “Research on Measuring Productivity in the Service Industries and Identifying the Driving Factors for Productivity Growth” of the Research Institute of Economy, Trade, and Industry (RIETI). We gratefully acknowledge that this research was financially supported by Grant-in-Aid for Scientific Research (A) titled “The Assessments of the Quality and the Productivity of Non-marketable Services” (Research Representative: Takeshi Hiromatsu, No. 3243044). 2 Corresponding author: Makiko Nakamuro, Faculty of Policy Management, Keio University, e-mail: makikon@sfc.keio.ac.jp. The authors would like to thank Atsushi Nakajima, Masahisa Fujita, Masayuki Morikawa, Keiichiro Oda, Yoshimichi Sato, Shinji Yamagata, Daiji Kawaguchi, Andrew Griffen and other participants of Tokyo labor economics workshop for their insightful comments and suggestions on the draft of this paper. All the remaining errors are ours.

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Introduction

Itwouldbegenerallybettertohaveasmallbabyatbirthandthenraise

him/hertogrowbiglaterinlife–thishaslongbeenbelievedtobegoodpracticein

child‐bearinginJapan.BeforeC‐sectiondeliveryorotherobstetricprocedures

becamepopularthroughoutsociety,peopleperhapsaimedtoreducetheriskof

endangeringmothers’livesbygivingbirthtoasmallbaby.Thiswidespreadbelief

seemstobestilldominanttoday.However,itisunexpectedlylittleknownthat

thereisahiddenriskofhavingasmallbaby:recentresearchhasfoundthatlow

birthweightissignificantlyassociatedwithbothshort‐andlong‐runadult

outcomes,suchasinfantmortality,studentachievements,andadulthoodhealth

(Conley&Bennett(2000);Linnetetal(2006);Currie&Hyson(1999),etc.).

Whyisthishappening?Lowbirthweightiscausedbypretermdeliveryor

lowfetalgrowththatmayreflectvariationinnutritionalintakeinthewomb.

Low‐birthweightisthusrecognizedastheleadingindicatorofpoorhealthamong

infants,whichmaydelaybrainandsomaticdevelopmentandthenaffectawide

rangeofsubsequentoutcomeslaterinlife.Thismechanismhasbeenalsorapidly

revealedasanobjectofepigenetics(e.g.,Petronis,2010etc.).Liketheresults

drawnfromdatainU.S.,DenmarkandEngland,Kohara&Ohtake(2009)used

officialstatisticsfromtheVitalStatisticsandtheNationalAssessmentofAcademic

AbilityinJapanandfoundanegativecorrelationbetweenbirthweightand

academicachievementsmeasuredbystandardizedtestscoresinG6andG9atthe

prefecturelevel.Ifthisisthecase,cantherebeanydoubtthattheGovernmentof

Japanmustshapeapolicyagendatoincreasethebirthweightofnewbornbabies,

forexample,throughimprovementsinthehealthofpregnantmothers?

Unfortunately,however,thereisnosimpleanswertothisquestion.

Whilemuchisknownaboutthecross‐sectionalcorrelationbetweenbirth

weightandadulthoodoutcomes,littleisknownregardingtheextenttowhatwould

havehappentoanindividualoutcomeifapersonwhowasactuallybornwitha

heavierbirthweighthadbeenbornwithalighterbirthweight.Inotherwords,itis

highlypossiblethatobserveddifferencesinbirthweightsamongnew‐borninfants

maysimplyreflectunobservedparentalcharacteristicswhicharealsocorrelated

withadulthoodoutcomesofanindividual:aselectionbiasariseswhenpartof

individualoutcomescanbeexplainedbyunobservedparentalcharacteristics.

Observedcorrelationsusingcross‐sectionaldatainpreviousliteraturethusdidnot

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provideafulldescriptionoftheeffectofbirthweightandresultinbiasedand

inconsistentestimates.

Inthisresearch,wewouldliketoanswerthequestionsofwhetherbirth

weightitselfcausesindividuals’adulthoodoutcomeslateroninlife.Causalityis

thusobviouslythekey.Oneoftheinnovativemethodsthatsocialscientistshave

employedinrecentyearsaddressthecausalrelationshipbetweenbirthweightand

adulthoodoutcomesistouseasampleoftwins(orsometimessiblings).Infact,

manyeconomists,suchasBerhman&Rosenzweig(2004),Royer(2009),Almond

etal(2005)fortheU.S.,Milleretal(2005)forAustralia;Lawloretal(2006)for

Scotland;Oreopoulosetal(2006)forCanada,andBlacketal(2007)forNorway

(seeCurrie(2009)foramorecomprehensivesurvey)useadatasetcontaining

informationontwin‐pairsandattempttocopewiththeproblemofunobserved

differencesinabilityandfamilyenvironments.Theseconsiderableeffortshave

beendedicatedtouncoveringtheeffectofbirthweightonadulthoodoutcomes:

previousresearchreachedaconsensusthatbirthweightdoesmatterbothinthe

short‐andlong‐run.

Wealsofollowthisapproachtodealwiththeaforementionedbias,

comparingthedifferencesbetweentwin‐pairstoisolatethepureeffectofbirth

weightontheadulthoodoutcomes,holdinginnateabilitiesandfamily

environmentsconstant.Anotheradvantageofusingasampleoftwinsisthat,

becausetwinpairshavethesamegestationlength,thedifferencesinbirthweight

betweentwinsareattributedsolelytodifferencesinfetalgrowthrates.Themain

researchquestionofinterestinthispaperisthus:doesnutritionintakeinutero

reallymatterforone’slifechance?Ifso,whichstageofone’slifeisthemost

affected?

Tothebestofourknowledge,thecaseofJapanisrelativelyunexploreddue

tothedatalimitation.Thisisunfortunategiventherecentvariablefindingsin

Japanthatlowbirthweightisassociatedwithparentalsocioeconomicfactors,such

asthemother’ssmokinghabitsandemploymentstatus(Tsukamotoetal,2007;

Kawaguchi&Noguchi,2012betc.).Theunderstandingofwhetheranindividual

inheritshis/herparentalsocioeconomicstatusatfetaloriginwouldcontributeto

furtherdiscussionoftheintergenerationaltransmissionmechanismofsocial

stratification,towhichpolicycirclesmaypayconsiderableattention.Inthisstudy,

wetakeadvantageoftheuniquetwins‐datasetsthattheauthorshavecollectedin

Japanthroughaweb‐basedsurvey.

Toanswerourresearchquestion,wefollowtheprotocolofprevious

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literatureandoutlineatwin‐fixedeffectstrategyusingasampleofmonozygotic

twins(hereafter,MZtwins)whoaregeneticallyidentical.However,interestingly,

thereisavariationinbirthweightbetweentwin‐pairsingeneral.Aspointedoutby

AshenfelterandRouse(1998),first‐borntwinsareusuallyheavierthantheir

second‐bornsiblingsatbirth.Thissettingallowsustocreateacounterfactual

situationofwhatwouldhavehappenedtoadulthoodoutcomesofapairoftwin

whowerebornwithalowerbirthweightifs/hehadbeenbornwithaheavier

birthweightinstead.Wethensetupfivemainoutcomestobeexamined:(i)

participationinprivate(ornational)middleschools;(ii)studentperformanceat

theageofaround15;(iii)rankingatthecollegeattended;(iv)yearsofschooling;

and(v)earnings.Thesignificantfindinginthispaperisthatbirthweightonly

causesacademicachievementundertheageof15.Unlikesomeoftheevidence

fromwesterncountries,thiseffectsubsequentlydisappears.Ourempiricalresults

showthatfetalgrowthmayaffectstudentperformanceinyoungchildren,butit

doesnotdirectlyaffecthis/heradulthoodoutcomesinlaterlife,suchas

educationalattainmentsandearnings.

Therestofthispaperisorganizedasfollows:thenextsectionreviews

relevantliteraturetosortoutinformationonwhatwestilldonotknowand

explainshowwetackledthemethodologicalproblemsinpreviousresearch.The

followingsectionsintroducetheempiricalspecificationstobeestimated,identify

thepotentialbiasemergingintheeconometricanalysis,anddeterminethe

analyticaltechniquestobeusedtoidentifythecausalimpactofbirthweighton

adulthoodoutcomelaterinlife.Theninthefinalsection,wedescribetheunique

twinsdatasetusedforempiricalanalysisandpresenttheempiricalresults.

RelevantLiterature

Evidencetoshowwhetherandtowhatextentincreasingthebirthweight

ofnewborninfantscanimprovetheirfuturelifechanceswouldbeusefulfor

framinganappropriatepolicydirectionregardingthenutritionalintakeof

expectantmothers.Agrowingbodyofresearchhasattemptedtoidentifythe

causaleffectsofbirthweightonnotonlyshort‐termbutalsolong‐termoutcomes

bytheuseoftwindata.Suchdataenablesresearcherstoruleoutthepotential

influencesofgeneticmakeupandfamilybackgroundsthataffectbothbirthweight

andlateroutcomesandtoobtainbetterestimatesofthecausaleffectsofbirth

weightthanthosederivedfromconventionalcross‐sectionalanalysis.Inthis

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section,wereviewtherelevantliteratureinvestigatingthecausaleffectofbirth

weightoneducationalandeconomicoutcomes,inparticularbyusingtwindata.

Regardingeducationaloutcomes,thereisevidencebasedontwinstudies

thatbirthweighthasalong‐termimpact.BehrmanandRosenzweig(2004),Black

etal.(2005)andOreopoulosetal.(2008),usingtwindatafromMinnesota,Norway

andManitoba,respectively,foundthatbirthweighthasapositiveeffectonhigh

schoolcompletion.LinandLiu(2009)analyzedTaiwanesetwindataandfound

thatbirthweightincreasedgradesatage15.ItisnoteworthythatBehrmanand

Rosenzweig(2004)andLinandLiu(2009)showedthattheOLScoefficientsfor

birthweightwithoutcontrollingforgenesandfamilybackgroundsare

underestimatedby50%,whileBlacketal.(2005)foundthatOLSestimatesand

twin‐fixedeffectsaresimilarinsize.BehrmanandRosenzweig(2004)andLinand

Liu(2009)arguethattheirfindingssuggestthatparentsmayinvestmoreinlighter

twinstomakeupfortheirdevelopmentaldisadvantage.

However,somestudiessuggestthattheeffectofbirthweightonyearsof

schoolingisrathersmall(Royer,2009)orthatthereisnosignificantrelationship

betweenbirthweightandeducationalattainmentorcognitiveabilitymeasuredby

languagetestscores(Milleretal.,2005;Oreopoulosetal.,2008).Itremains

unclearwhetherthismixedevidenceisduetodifferencesinmeasuresregarding

educationaloutcomesordatasources.

Thereisrelativelylessevidenceonthedirecteffectofbirthweighton

economicoutcomes.Milleretal.(2005)areoneoftheexceptionalgroupsof

researcherswhofoundapositive,directeffectofbirthweightonearnings,but

arguethatbirthweightplaysonlyaminorroleindeterminingearnings,witheach

additionalounceofbirthweightincreasingearningsby0.4%.Otherstudiesthat

examinetheeffectofbirthweightoneconomicoutcomesincludeRoyer(2009)and

Oreopoulosetal.(2008).However,Royer(2009)didnotfindevidencetoshowthat

birthweightisassociatedwithneighborhoodincomelevelsinadulthood.

AlthoughOreopoulosetal.(2008)foundthatbirthweightaffectssocialassistance

takeupandlengthinadulthood,butgiventhattheyalsofoundtheeffectofbirth

weightonhighschoolcompletion,itisunclearwhetherthebirthweighteffecton

economicoutcomeswouldremainaftercontrollingforthemediatingeffectof

educationaloutcomes.

InJapan,noresearchhasusedtwindatatoinvestigatethecausaleffectof

birthweightoneducationalandeconomicoutcomesinadulthood.Themost

importantreferenceworkisanon‐twinstudyconductedbyKawaguchiand

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Noguchi(2012b),analyzingearlychildhooddatafromtheLongitudinalSurveyof

Babiesinthe21stCentury.Theyfoundthatlowbirthweight,definedasaweight

below2,500grams,isassociatedwithadelayindevelopmentatagetwoandahalf,

butnotwithbehaviorsatagesixandahalf.Apossiblereasonfortheassociation

disappearingatanolderagemaybethefactthatJapaneseparentsinvestmorein

theirchildreniftheirdevelopmentisobservedtobeslowatanearlierstage.

Therefore,thisevidencedoesnotconfirmwhetherthebirthweighteffectremains

onlyforashortperiodoftimeinJapan.Furthermore,littleisknownaboutthe

effectofbirthweightonmuchlatereducationalandeconomicoutcomesinJapan.

WeaimtofurthertheliteraturebyusingJapanesetwindatatobringout

newfindingsinthefollowingthreerespects.Firstlyandmostimportantly,wewill

estimatetheeffectsofbirthweightbyaddressing,forthefirsttimeinJapan,

potentialendogeneitybiasesduetotheeffectsonbirthweight,post‐natal

developmentandlateroutcomesofgenesandparentalbehaviorsusually

associatedwithafamily’ssocio‐economicstatus.Secondly,wewillinvestigate

longer‐termeducationalandeconomicoutcomesashasbeendoneinrelevant

previousresearchinJapan,whichmayleadtoinsightintohowlong‐lastingthe

birthweighteffectswouldbe.Lastly,wewillexamineeducationaloutcomes

measuredinseveralwaysforthesamesample,whichmaycontributetoclarifying

whicheducationaloutcomesshouldbehighlighted,giventhecurrentmixed

evidenceacrosstheworldontheeffectsofbirthweightoneducationaloutcomes.

EmpiricalSettings

Inordertoaddressourresearchquestionsofhowbirthweightaffects

adulthoodoutcomes,webegintheanalysisusingaconventionalOLStoreportthe

cross‐sectionalcorrelationswiththeentiretwinsample,inwhichmanyprior

studieshavefoundthatthebirthweightisstronglyassociatedwithawiderangeof

adulthoodoutcomes.Followingthepreviousliterature,weoutlinethesimple

educationproductionfunctionillustratingtheinput‐outputrelationshipathomeor

schoolthatparticularlyhighlightstheroleofchildhealth.Themodelcanbe

formallyexpressedinthefollowingmathematicalequationwhereyisoutcomes

andisafunctionofthebirthweight(bw)andunobservables(A),suchasgenetic

makeupormaternal/pre‐natalcareincombinationwithothercharacteristics(X)

andrandomdisturbancewithmeanzeroandconstantvariance(e)asspecifiedin

theequation(1)and(2)below.Notethatthefirst‐borntwinisdenotedas1bya

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subscriptandthesecond‐bornas2.

′ (1)

′ (2)

Thecoefficientofβreferstotheeffectofthebirthweightonoutcomevariables

holdingotherobservedcharacteristicsconstant.Aswediscussedearlier,the

cross‐sectionalestimateofβmaybebiasedandinconsistentbecause

unobservables,Aij,affectboththebirthweightandoutcomes.However,identical

twinsenableustosetupA1j=A2j,giventhattheysharegeneticmakeupandfamily

environments.Wewillthustakeatwinfixedeffectapproachtakingthedifference

betweenequation(1)and(2)toobtainawithin‐twinfixedeffectsestimateofβ,

yielding:

) (3)

Inequation(3),unobservable,Aj,iseliminated,relievingusoftheconcernthatthe

outcomesarepartlyexplainedbyindividualunobservedcharacteristics.Giventhe

assumptionthattheerrortermisanidiosyncratic,whichisindependentofall

othertermsintheequation,βisconsideredastheconsistentestimate.

Data

Thedatausedforourempiricalanalysiswascollectedthrougha

web‐basedsurveyinJapanbetweenthemonthsofFebruaryandMarch2012(see

Nakamuro&Inui(2012)formoredetailedinformationonthedatacollection

strategy).Weconductedthesurveythroughaweb‐basedsurveycompany,Rakuten

Research,withover2.2millionmonitors.Inordertoanalyzetheeffectofbirth

weightonadulthoodoutcomes,oursampletargetedtwinswhoarenon‐students

betweentheagesof20and60.Throughthisweb‐basedsurvey,onememberofa

twinpairisresponsibleforreportingregardinghim/herselfandhis/hertwin

siblingatonetime,andtheresultsaredesigneddifferentlyfromthoseoftheother

twinsurveyfilledoutbybothmembersofthetwinpair3. 3 Onemayquestionthat,inoursurvey,theremayexistsubstantialmeasurementerrorsinself‐reportedbirthweightandotheroutcomesbyoneofthetwinpairs,insteadofboth.Itisimportanttonotethatwehave23twinpairs,eachmemberofwhichwasincludedinthissurvey.Whenwechecktheirresponses,wefindoutthattheirresponsesreportedbyeach

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Oncethemonitor(s)filledoutthequestionnaires,theywouldbegivena

certainamountofcash‐equivalent“points”thatcouldbespentonRakutenonline

shopping.Inordertoexclude“fake”twins,whopretendtobetwinstocollectthe

cash‐equivalentpoints,wecarefullydevelopedthefollowingdatacollection

strategy:wedidnotinformrespondentsthatthepurposeofoursurveywasto

collectdatafromtwins.Furthermore,westartedwithfivequestionsonfamilyand

siblingsthatwerenotrelatedtotwinstatusandthen,inthesixthquestion,forthe

firsttime,askedwhetherornotarespondentwasatwin.Iftherespondent

answered“No”inthisquestion,s/hewouldbeautomaticallyexcludedfromthe

survey.Wediscovered23twinpairs,eachmemberofwhichwasincludedinthis

survey,thenthoroughlycheckedtheresponsesofbothtwins,andeliminatedoneof

thetwinsrandomlyfromoursample.Ourweb‐basedsurveyovercamethe

disadvantagesofthedatacollectioninpreviousliterature,suchassmallsample

sizeordataattrition.Consequently,wecollected2,360completepairsoftwins

(4,720individuals)while1,371twinpairs(2,742individuals)aremonozygotic

(seeTable1).Tothebestofourknowledge,thisisoneofthelargestdatabasesof

twinscompiledinJapannationwide,anditconveysawiderangeofsocioeconomic

information.

Variables

Theindependentvariableofinterestis,ofcourse,birthweightforwhich

wesetupseveralvariantsinthefollowingway:theprimarymeasureisthebirth

weightwhichisself‐reportedbyoneofthetwin‐pair.Theresponsecategoryinthe

originalquestionnairerangedfrom1(=lessthan1,500grams)to7(=morethan

4,500grams)and8(=don’tknow).Wesettheminimumto1,500andthe

maximumto4,500grams.Thenwetookthemid‐valueforthecategoriesbetween

2(=1,750grams)and7(=4,250grams).Basedonthisvariable,wecreatetwo

variantsofthekeyindependentvariables:avariantisthenaturallogarithmofthe

otherarequiteaccurate:thecorrelationsbetweenself‐reportedandcross‐reportedbirthweightis91.2%.Notonlythebirthweightbutalsootheroutcomesshowover90%ofcorrelations.Furthermore,wecheckwhetherthereexistsignificantdifferencesbetweenresponsesonhim/herselfandonhis/hertwinsiblings;forexample,onemaybedoubtfulthatrespondentsarepronetopretendthattheirearningsoreducationarehigherthanthoseoftheirtwinsiblings.However,accordingtotheresultdrawnfromtwosamplet‐testsfordifferenceofthemeans,thereisnodifferencebetweenthem.Asafurtherrobustnesscheck,weincludearespondentdummyinallspecifications,butthedummiesarestatisticallyinsignificant.

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birthweight.Theotherisdefinedasadichotomousvariablemeasuringthelow

birthweight,codedas1ifthebirthweightismorethan2,500gramsandzero

otherwise.

ThedescriptivestatisticssummarizedinTable2showthattheaverage

twinsinoursampleweighed2,441gramsatbirthandmorethanhalfofthemwere

categorizedasinfantswithlowbirthweights.Assuggestedinpreviousliterature,

ourdataillustratesadisparityinbirthweightbetweenthefirstandsecondbornof

twins:theaveragebirthweightofthefirstbornis2,464gramswhilethatofthe

secondbornis2,423grams,andthisdifferenceisstatisticallysignificantata5%

level.Furthermore,ourdatashowsthat24.8%ofMZtwinpairsweredifferentin

weightatbirth,whichiscruciallyimportanttoensureanaccurateestimateofthe

twinfixedeffectsmodel.IfwerestrictthesampletothosewhoareMZtwins,we

havequitesimilarresults(seeTable1‐a).Werunseparateregressionsforeach

variantofthebirthweight:theexplanatorypowerassessedbyR2statisticsfrom

withintwinfixed‐effectsestimationshelpsustochoosethebestpossibleoption

amongthesethreevariantsofbirthweight,aspresentedinTable3.

Wethencharacterizesixoutcomesrangingfromtheperiodofchildhoodto

adulthood.Thefirstoutcomevariableisatypeofmiddleschoolattended,which

aimstomeasurescholasticabilityinearlychildhood.Sometwelve‐year‐old

childrenenrollinJapanprivatejuniorhighschoolsinsteadofpublicschools

becausemostprivatejuniorhighschoolsrequirechildrentopassentrance

examinations,whichareoftencompetitiveandselective.AccordingtotheSchool

BasicSurveyadministeredbyMinistryofEducation,Culture,Sports,Scienceand

Technology,theenrollmentrateofprivatejuniorhighschoolswas8%4 in2012,

withaconsiderablegeographicalvariation.Whilethemajorityofchildrenand

parentsdonotconsiderchoosingprivateschoolsatall,anon‐negligibleproportion

ofchildrenandparentsinJapanhaverecognizedtheentranceexaminationsof

privatejuniorhighschoolsasthefirstscreeningprocessthrougheducational

institutions.Inoursurvey,weaskwhichtypeofmiddleschooltherespondentand

his/hertwinsiblingsattended.17.1%ofrespondentswerestudentsofprivateand

nationaljuniorhighschools,whichisslightlyhigherthanthepercentageshowed

byofficialstatisticsnationwide.Thismayinpartbeduetothecharacteristicofour

surveythatitismorelikelytogatherinformationfromresidentsinlarge

4 Thisnumberincludestheenrollmentrateofnationaljuniorhighschools.Nationaljuniorhighschoolsaregovernment‐ownedjuniorhighschools,mostlyaffiliatedwithnationaluniversities.Theseschoolsalsorequireapplicantstopassentranceexaminationstoenroll.

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metropolitanareas,suchasTokyoandOsaka.Moreover,10.9%ofMZtwins‐pairs

attendeddifferenttypesofschool:forexample,oneattendedaprivateschooland

theotherapublicschool.Thesecondoutcomevariableisstudentperformanceat

middleschool,whichismeasuredona5point‐scale(1=lower;2=belowaverage;

3=average;4=aboveaverage;5=upper)basedonasubjectiveevaluationof

respondents’andtheirtwin‐siblings’academicachievements.

Thethirdoutcomeistherankingofthecollegeattendedwitharestricted

sampleofcollege‐educatedrespondents.Oursurveyasksthenameofthehigh

schoolwhererespondentsandtheirtwinsiblingsgraduated.Weconvertthis

informationintoameasureofdeviationvalue(“hensachi”inJapanese),which

representstherankingofeacheducationalinstitutionwithameanof50and

standarddeviationof10byusingaseriesofdeviationvalues.Wematchthename

ofcolleges/universitiesandthedeviationvalueofeachdepartmentofeach

college/universitybyusingthedataset,“KawaijukuCollegeRankings”,calculated

andreleasedin2011byKawaijuku,oneofthelargestcramschoolsinJapan.

CollegeadmissionsinJapanaredeterminedalmostentirelybyperformancein

writtenexaminationsadministeredbyeachinstitutionexceptforsomespecial

admissionprograms,suchasathleticscholarshipprograms.

Thefourthoutcomeisyearsofschooling.Toavoidthepossibilityof

institutionalmisreporting,inouroriginalquestionnaire,welisteverytypeand

levelofeducationalinstitution(26categories,including“don’tknow”),andthen

askrespondentstoselectthehighestdegreeobtained.Thechoiceof“dropoutor

stopped”wasinsertedbetweenthequestionsoneachtypeandlevelofinstitution

inordertodisentanglecasesofleavingschoolwithoutadiploma.Incalculatingthe

yearsofeducation,weassumethatthosewho“droppedout”or“stopped”finished

halfoftheminimumrequiredyearstocompletetheeducationalinstitutionlast

attended(e.g.,dropoutfromhighschool=10.5yearsofschooling).

Itisimportanttonotethatourdatashowsthatthewithin‐twinvariations

ineducationalexperiencesbecomelargerwiththepassageoftime:only5.9%or

10.9%oftwinpairsattendeddifferenttypesofschoolswhentheywereprimaryor

middleschoolstudents.Eventually,38.4%oftwinpairsinoursampleacquired

differentnumbersofyearsofeducation.Somemaywonderwhytwins,whoshare

thesamegeneticmakeupandfamilyenvironment,eventuallyendupsodifferently

intermsofeducationalbackgrounds.BoundandSolon(1999)andNeumark

(1999)pointedoutthatevenifthewithin‐twinestimateisabletoremovethe

effectofgeneticendowment,itdoesnotnecessarilymeanthatthewithin‐twin

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estimatecompletelyeliminatesallaspectsofabilitybiastotheextentthatability

goesbeyondgenes.Thepotentialendogeneitycouldbecausedbyunobserved

differencesinabilitybetweentwin‐pairswhichmayresultfromdifferent

experiencesinearlierchildhood.However,accordingtooursurvey,parentsare

unlikelytotreattwinsdifferently,particularlyintermsofeducationalinvestment.

Forexample,thereislittledifferenceinexperiencesofshadoweducationbetween

twin‐pairs,whichmaybeacrucialpartofhouseholdexpendituresoneducationin

Japan5.Oursurveyshowsthatthewithin‐twinvariationsinshadoweducationis

quitesmall,asthewithin‐twinvariationsinformaleducationbecomelargerwith

thepassageoftime(seeTable1‐b).Therefore,itcanbesaidthatwithin‐twin

variationsdonotreflectanysystematicdifferenceintheparentaleducational

strategyforeachtwin,whichoftenleadstodifferentexperiencesinearlier

childhood.

Thefifthoutcomeislabormarketperformancemeasuredbythenatural

logarithmofannualwagebeforetaxdeductionsduringthe2010fiscalyear6.The

responsecategoryintheoriginalquestionnairerangedfrom1(=noincome)

through16(=morethan15millionJPY).Wesettheminimum(1=noincomeand

2=lessthan0.5millionJPY)tozeroandmaximum(16=morethan15millionJPY)

to15millionJPY.Then,wetakethemidvalueforcategoriesbetween3(=0.5

million‐0.99millionJPY)and15(=10million‐14.99millionJPY).

EmpiricalResults

Table4presentstheresultsestimatedbyaconventionalOLStoreplicate

thecorrelationalstudiesinpreviousliterature.Regardlessofthevariantof

dependentvariables,birthweightsarestronglyassociatedwithadulthood

outcomes,exceptforthetypeofmiddleschoolandrankingofcollegeattended.

Theresultscoupledwiththepositivecoefficientssuggestthatbirthweightaffects

themajorityofeducationaloutcomeslaterinlife,suchasstudentperformanceat

theageofaround15andthehighestyearsofschooling,aswellasthelabormarket

5 AccordingtotheofficialstatisticsoftheMinistryofEducation,Culture,Sports,ScienceandTechnology,alargepartofhouseholdexpendituresoneducationhasbeenspentonshadoweducationinJapan.TheBenesseEducationalResearchandDevelopmentCentershowedin2009thatapproximately20%ofelementaryandhighschoolstudentsand50%ofjuniorhighschoolstudentsaccessedshadoweducation,suchascramschoolsorprepschools. 6 Thissurveyaskedaboutearningsduringthefiscalyearof2010,insteadof2011,becauseearningsduringthefiscalyearof2011couldhavebeenaffectedbytheGreatEastJapanEarthquakethatoccurredonMarch11,2011.

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outcomemeasuredbyannualearnings.Theeffectseemsquitelarge:forexample,

anextra100gramsinbirthweightraisesearningsinthelabormarketby5.3%on

average.Takenasawhole,ourresultsareconsistentwiththemainstreamof

previousliteratureshowingapositivecorrelationbetweenbirthweightand

adulthoodoutcomes.

Thenweanalyzetheeffectofbirthweightonadulthoodoutcomes

concerningwhetherthedifferenceinweightatbirthissubstantialorreflectsthe

factthatinfantswithlighterbirthweightsfundamentallydifferfromthoseoftheir

counterpartswithheavierbirthweights.Asexplainedearlier,weemploythe

twin‐fixedeffectstocomparethedifferencesbetweenMZtwinstoisolatethepure

effectofbirthweightontheadulthoodoutcomes,holdinginnateabilitiesand

familyenvironmentsconstant.Table5showsaquitedifferentstoryfrom

conventionalOLSestimates.Theeffectofthebirthweightontheadulthood

outcomesinthelongerrun,suchasthehighestyearsofeducationandearnings,

appearstobestatisticallyinsignificantacrossmodels.However,theeffectstill

remainsineducation:birthweightcausesacademicachievementattheageof

around15.Itmayalsocausetheprobabilityofpassingaprivatejuniorhighschool

andtherankingofcollegeinwhichtheyparticipated,althoughtheevidenceto

supportthisisweak.Thetwin‐fixedeffectsestimatesaresubstantiallylargerthan

thecross‐sectionalones.

Thispaperhasthussucceededinansweringtheresearchquestion.Like

theevidencegeneratedfromwesterncountries,ourfindingalsosuggestthatbirth

weightaffectseducationaloutcomes,particularlystudentperformanceattheage

ofaround15.Ontheotherhand,theeffectisnotlong‐lasting,andbirthweight

doesnotaffectlonger‐runoutcomes,suchasthehighestyearsofschoolingand

earnings.Strictlyspeaking,inthissense,birthweightitselfisnottheculpritwith

respecttothemoredisadvantagedadulthoodoutcomes.

Conclusion

Thispaperhasinvestigatedthequestionofwhethertheeffectsofbirth

weightonlatereducationalandeconomicoutcomes,ifany,arecausal.Byusing

datafromasampleoftwinsinJapan,wehaveprovidedbestavailableestimatesfor

thecausaleffectsofbirthweightonacademicachievementattheageofaround15.

Wewouldberathercautiousaboutconcludingthatbirthweightdoesnotaffect

longer‐runoutcomes,suchashighestyearsofschoolingandearnings.Future

13

researchshouldexaminewhetherthisisalsothecasewhereeducational

attainmentmeasuredbyeducationalqualificationsobtained,orwhereearningsare

measuredwithsmallermeasurementerrorsthanwithself‐reportedearnings.

Giventhatthemostplausiblefactoraffectingbirthweightdifferences

betweenMZtwinsisintrauterinenutrientintake,ourfindingssuggestthat

improvingpregnantmothers’nutrientintakemayleadtotheimprovementof

educationaloutcomesatage15,therebyimprovingfuturelifechances.

KawaguchiandNoguchi(2012a)arguedthatthedecreaseintheaveragebirth

weightbetween1990and2005couldbepartlyexplainedbymedicalinstructions

offeredtopregnantmothers.Thisimpliesthatincreasingbirthweightthrough

improvingpregnantmothers’nutrientintakewouldbepolicy‐relevant.

Inordertobringoutmoredetailedandsolidimplicationstopolicy,future

researchneedstotestwhetherthefindingsofthispaperbasedonatwinsample

canbegeneralizedtonon‐twinpopulations.Itisalsonecessarytoinvestigatefor

whomincreasingbirthweightismosteffectiveinimprovingfuturelifechances,for

instance,bydetectingatwhichpointinthebirthweightdistributiontheeffectof

birthweightisstrongest.

14

Table1:SampleCollectedthroughWeb‐BasedSurvey

MZTwins DZTwins Don’tKnow

2,742

(1,371pairs)

1,764

(882pairs)

214

(107pairs)

(Source)Authors’calculations

Table1‐a:DifferencesinEducationalOutcomesbetweenTwin‐Pairs

Birthweights Privateprimary

school

Privatemiddle

school

Highestyearsof

education

Total

MZTwins

DZTwins

24.8%

21.0%

32.1%

5.9%

5.6%

6.5%

7.5%

7.1%

7.7%

38.4%

32.0%

48.8%

Table1‐b:DifferencesinEducationalExpenditureonShadowEducationbetweenTwin‐Pairs

Preschool Primaryschool Middleschool Highschool

Total

MZTwins

DZTwins

2.4%

1.6%

3.2%

4.1%

3.1%

5.8%

6.2%

4.7%

9.0%

6.5%

4.6%

9.0%(Note)1.Shadoweducationrepresentsaccesstoprepschools,privatetutoring,anddistancelearning. 2.Privateprimaryandmiddleschoolsareincludingnationalschools.

(Source)Authors’calculations

15

Table2:DescriptiveStatistics

WholeSample MZTwins

Mean STDV Mean STDV

DependentVariables:

Birthweight

Log(birthweight)

Non‐LBW(1>2,500grams)

2,441.00

7.77

0.45

564.87

0.23

0.50

2413.76

7.76

0.50

551.51

0.23

0.50

IndependentVariables:

Typeofjuniorhighschool(1=privateornational)

Studentperformanceattheageof15

Rankingofcollegeattended

Yearsofschooling

Log(earnings)

0.171

2.57

52.00

14.38

5.96

0.376

1.10

9.84

2.33

0.74

0.16

2.60

51.72

14.42

5.98

0.37

1.08

9.76

2.30

0.74

(Source)Authors’calculations

16

Table3:GoodnessofFit

Independent

Dependent

Private

Middle

School

Student

Performance

(Age15)

Ranking

College

Highest

Yearsof

Schooling

Earnings

Birthweight

Log(birthweight)

Non‐LBW

0.0060

0.0054

0.0075

0.0052

0.0070

0.0014

0.0061

0.0057

0.0194

0.0002

0.0001

0.0008

0.0013

0.0016

0.0006

(Source)Authors’calculations

17

Table4:EmpiricalResults(OLS)

Dependent

Independent

Private

Middle

School

Student

Performance

(Age15)

Ranking

College

Highest

Yearsof

Schooling

Earnings

Birthweight

(/100)

Log(birthweight)

Non‐LBW

Observations

0.011

(0.016)

0.016

(0.038)

‐0.001

(0.017)

1,998

0.063*

(0.032)

0.140*

(0.079)

0.035

(0.035)

3,577

0.091

(0.442)

0.178

(1.082)

0.542

(0.517)

1,470

0.291***

(0.066)

0.810***

(0.167)

0.206**

(0.070)

3,631

0.053***

(0.017)

0.122***

(0.043)

0.035*

(0.019)

2,785 (Note)1.Thenumbersinparenthesesareheteroskedasticity‐robuststandarderrors.

2.***,**,and*represent1%,5%,and10%significancelevel,respectively.3.Otherindependentvariablesare(a)privatemiddleschool:gender,father’seducation;(b)studentperformance:gender,father’seducation,livingstandardattheageofaround15;(c)rankingofcollege:thesamewith(b);(d)highestyearsofschooling:thesamewith(b);(e)log(earnings):age,agesquared,gender,father’seducation,livingstandardattheageofaround15,highestyearsofschooling,maritalstatus,yearsoftenureatthecurrentemployment,hoursofworkperday.Mostofthecontrolvariablesarestatisticallysignificant.Thefullresultsareavailableuponrequest.

(Source)Authors’calculations

18

Table5:EmpiricalResults(Twin‐FixedEffects)

Dependent

Independent

Private

Middle

School

Student

Performance

(Age15)

Ranking

College

Highest

Yearsof

Schooling

Earnings

Birthweight

(/100)

Log(birthweight)

Non‐LBW

Observations

#oftwinpairs

0.050*

(0.029)

0.112*

(0.066)

0.053*

(0.030)

1,257

(641)

0.210*

(0.108)

0.575**

(0.249)

0.099

(0.088)

2,206

(1,138)

3.138

(3.090)

7.326

(7.149)

5.659*

(3.055)

918

(736)

‐0.077

(0.170)

‐0.137

(0.417)

‐0.146

(0.162)

2,234

(1,144)

0.065

(0.071)

0.177

(0.161)

‐0.042

(0.072)

1,832

(1,032) (Note)1.Thenumbersinparenthesesareheteroskedasticity‐robuststandarderrorsandclusteringatthefamilylevel.

2.***,**,and*represent1%,5%,and10%significancelevel,respectively.

(Source)Authors’calculations

19

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