Why does parental language input style predict child language development? A twin study of gene–environment correlation

Why

does

parental

language

input

style

predict

child

language

development?

A

twin

study

of

gene–environment

correlation

Philip

S.

Dale

*

,

Maria

Grazia

Tosto

,

Marianna

E.

Hayiou-Thomas

,

Robert

Plomin

a

DepartmentofSpeech&HearingSciences,UniversityofNewMexico,UnitedStates

b_{DepartmentofPsychology,TomskStateUniversity,Russia}

c_{DepartmentofPsychology,UniversityofYork,UnitedKingdom}

d

Social,GeneticandDevelopmentalPsychiatryResearchCentre,King’sCollegeLondon,UnitedKingdom

JournalofCommunicationDisorders57(2015)106–117

ARTICLE INFO

Articlehistory:

Received7February2015

Receivedinrevisedform31July2015

Accepted31July2015

Availableonline4August2015

Keywords:

Childlanguage

Input Genetics

ABSTRACT

Therearewell-establishedcorrelationsbetweenparentalinputstyleandchildlanguage development,whichhave typicallybeen interpretedasevidencethattheinputstyle causes,orinfluencestherateof,changesinchildlanguage.Wepresentevidencefroma largetwinstudy(TEDS;8395pairsforthisreport)thattherearealsolikelytobeboth child-to-parent effects and sharedgenetic effectson parent and child. Self-reported parentallanguagestyleatchildage3andage4wasaggregatedintoan‘informallanguage stimulation’factoranda‘correctivefeedback’factorateachage;theformerwaspositively correlatedwithchildlanguageconcurrentlyand longitudinallyat3,4,and4.5years, whereasthelatterwasweaklyandnegativelycorrelated.Bothparentalinputfactorswere moderatelyheritable,aswaschildlanguage.Longitudinalbivariateanalysisshowedthat the correlation between the language stimulation factor and child language was significantlyand moderatelydueto sharedgenes. Thereissomesuggestiveevidence from longitudinal phenotypic analysis that the prediction from parental language stimulationtochildlanguage includesbothevocativeandpassivegene–environment correlation,withthelatterplayingalargerrole.

Learningoutcomes:Thereaderwillunderstandwhycorrelationsbetweenparental languageandrateofchildlanguagearebythemselvesambiguous,andhowtwinstudies canclarifytherelationship.Thereaderwillalsounderstandthat,basedonthepresent study,atleasttwoaspectsofparentallanguagestyle–informallanguagestimulationand correctivefeedback–havesubstantialgeneticinfluence,andthatforinformallanguage stimulation,asubstantialportionofthepredictiontochildlanguagerepresentstheeffect ofsharedgenesonbothparentandchild.Itwillalsobeappreciatedthatthesebasic researchfindingsdonotimplythatparentallanguageinputstyleisunimportantorthat interventionscannotbeeffective.

ß2015TheAuthors.PublishedbyElsevierInc.ThisisanopenaccessarticleundertheCC BYlicense(http://creativecommons.org/licenses/by/4.0/).

*Correspondingauthorat:DepartmentofSpeech&HearingSciences,UniversityofNewMexico,1700LomasBoulevardNE,Albuquerque,NM87131,

UnitedStates.

E-mailaddress:[email protected](P.S.Dale).

ContentslistsavailableatScienceDirect

Journal

of

Communication

Disorders

http://dx.doi.org/10.1016/j.jcomdis.2015.07.004

0021-9924/ß2015TheAuthors.PublishedbyElsevierInc.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/

1. Introduction

SincetheseminalworkofSnow(1972),anextensivebodyofresearchhasbeenconductedonthecharacteristicsofspeech directedto,orproducedininteractionwith,youngchildrenintheearlystagesoflanguageacquisition,typicallybelowthree yearsofage(seeHoff,2006;Rowe,2012,forreviewsofthiswork).Acentralquestionhasbeentheextenttowhichthese featuresofchild-directedspeech(CDS)arepredictiveofchildren’srateoflanguagedevelopment.Inrecentyears,aprimary focusofattentionhasbeenonquantityofinput.Althoughnumerousearlierstudiesfoundapositiverelationshipbetween quantityofinputandrateoflanguagedevelopment,thisresultwasmademorevividtoawiderpopulationbytheworkof

HartandRisley(1995),whodocumentedwidevariationsintheamountofspeechtoyoungchildren,andtheextenttowhich

thesewerereflectedinvocabularydifferencesinthechildren.Butbeyondthequantitativedifferences,numerousqualitative differenceshavealsobeenshowntopredictchildlanguagebothconcurrentlyandlongitudinally,generallywith weak-to-moderateeffectsizes.Thesequalitative,facilitatingaspectsofCDSarefoundinallthesubdomainsoflanguage(phonology, lexicon,syntax,semantics,andpragmatics),andincludegreatervocabularydiversity,appropriatelyscaffoldedmeanlength ofutterance(MLU),increasedrepetition,exaggeratedprosody,promotionofjointattention,proportionof conversation-elicitingspeechasopposedtobehaviorregulation,semanticcontingency,decontextualizedlanguageusesuchasnarratives, ‘grammaticaltutorials’suchassentencerecastsandexpansions,andothers(Hoff,2006;Rowe,2012).

ThesefeaturesofCDSarenotallequallyimportantatvariousstagesofdevelopment.Promotionofjointattentionanduse ofexaggeratedintonation,arelikelytobemostimportantintheearlieststagesofdevelopment,whilevocabularydiversity anduseofsentencerecastsarelikelytobemoreimportantlater.Averyclearexampleofthisshiftwasdemonstratedin

Rowe’s(2012)studyofpredictorsofvocabularygrowthbetween18and42months.Quantityofparentalinputwasmost

importantduringthesecondyearoflife,whilediversityofparentalvocabularywasmoreimportantinthe3rdyear,andthe useofdecontextualizedlanguagesuchasnarrativesandexplanationswasmostimportantinthe4thyear.Thepresentstudy focusesonabroaddimensionofCDSwhichcombinesquantityandquality,namelythetendencytoincorporatetalkwith childrenintoeverydayroutines,aswellasaseconddimension,thepotentialtendencytoprovidecorrectivefeedback.

MostoftheidentifiedfacilitativefeaturesofCDSareintercorrelated,andalsocorrelatedwithsocioeconomicstatus(SES), usuallyassessedasparentaleducation.Nevertheless,Rowe(2012)andothershavefoundthatmeasuresofspecificfeatures ofCDS,representingtheproximalenvironment,addsignificantlytothepredictionofchildlanguagebeyondtheprediction fromthedistalvariableofSES.InpartthisreflectsthefactthatwithinlevelsofSES,thereisgreatvariabilityinparental behavior.Morebroadly,environmentalinfluencesonlanguagedevelopmentareextensiveandwidelyvaried,including,for example,prenatalcareand nutrition,exposuretotoxins anddisease,caregivereducationand mentalhealth,typeand quality ofchildcare,and multilingualvs.monolingual context(seeotherpapers inthis specialissuefor somecurrent examples).Inmanycases,itislikelythattheeffectoftheseenvironmentalvariablesismediatedatleastinpartbyparental languageinput.

Animportantqualificationtothissummaryisthatitisbasedprimarilyonresearchinliterate,technological,primarily CaucasianWesternsocieties.Thereisalsomuchresearchthatparent–childverbalinteractionpatternsarequitedifferentin traditional, nonliterate societies,and also(in differentways) inEast Asiancultures (Johnston&Wong, 2002; Ochs&

Schieffelin,1984).Althoughnotasspecificallyfocusedonparentallanguageastheprecedingresearch,researchin

African-Americancommunitiesonparent–childinteractionanditspredictivesignificanceforlanguagedevelopmenthasshownthat boththemeaningandpredictiveeffectsofspecificbehaviors,e.g.,sensitivity,andnegative-intrusivebehaviors,canvaryby racial group(Pungello,Iruka,Dotterer, Mills-Koonce, &Reznick, 2009; see alsoDudley-Marling& Lucas,2009,for an alternative,criticalperspectiveonHart&Risley).Thereisconsiderabledebateatpresentabouttheextenttowhichdifferent patternsofverbalinteractioncansupportlanguagedevelopment(Lieven,1994;Hoff,2006),butthatisoutsidethescopeof thispaper.

1.1. Interpretingphenotypiccorrelations

Thebodyofresearchconfirmingcorrelationsbetweenaspectsofparentallanguageinputandchildlanguageoutcomesis extensiveandimpressiveintheconsistencyofthefindings(Hoff,2006;Rowe,2012).Themosttypicalinterpretationisa causalone:thesefeaturesoftheinputfacilitatelanguageacquisition.Basedonthatinterpretation,itisnaturaltotranslate positivecorrelationsintorecommendationsforparents,educators,andclinicians(e.g.,Finestack&Fey,2013;Girolametto,

Weitzman,&Earle,2013).Inthispaperwefocusonaninherentambiguityinsuchcorrelations,whichcomesfromthefact

thatmostofthisresearchisbasedonbiologicallyrelatedparentsandchildren,whosharebothgenesandenvironments. Analternativeexplanationforthecorrelationsbetweenparentalinputandchildlanguageisthatchildreninfluence parentsasmuchormorethanparentsinfluencechildren.Childrenwhoaremoretalkative,moreadvanced,and/ortalkin specificwaysmayelicitparentalspeech(orthespeechofotheradults)withthehypothesizedfacilitatingfeaturestoa greaterextentthanchildrenwhoarelesstalkative,lessadvanced,ortalkindifferentways(Bohannon&Marquis,1977;

DeThorne&Chanell,2007;Paul&Elwood,1991).Evenifthechildlanguageoutcomeismanifestedlaterindevelopmentthan

themeasuredparentalbehavior,thoseoutcomesmayrepresentastableaspectoftheirbehavior,andtheearlierbehavior mayhaveinfluencedparentalinput.Anotheralternative,familiartoanyonewhohastakenabasicstatisticsclass,isthe possibilitythatbothmeasuredvariablesareinfluencedbyanunmeasured‘thirdvariable’.Anespeciallyplausiblepotential candidateforthatthirdvariableisthesharedgeneticendowmentofparentandchild.ParentswhoadoptthestyleofCDS

discussedabovemaybegenerallymoresensitivetolanguage,andpassthatontotheirchildrengenetically.Indeed,thefirst alternativeexplanationmaybeduetothesecond,whenthefeaturesofchildrenthatinfluenceparentsarethemselvesat leastinpartduetosharedgenes.Thereis,infact,considerableevidenceforageneticinfluenceonchildren’srateoflanguage development,smallbutsignificantatage2,andgenerallyrisingacrossdevelopmenttoadolescence(Hayiou-Thomas,Dale,&

Plomin,2012;Spinath,Dale,Price,&Plomin,2004).

Bothchild-to-parenteffectsandsharedgeneticeffectsleadtogene–environmentcorrelation,acorrelationbetweenthe environmentthatchildrenexperienceandtheirgeneticendowment.Caseswherechildreninfluenceparentsarecalledin behavioral geneticsevocative gene–environment correlation, becausethe correlationis theresultof children evoking a particularkindofparentallanguageresponse.Whena‘thirdvariable’ofsharedgenesinfluencesbothparentandchild directly, this is passive gene–environment correlation, because the correlation between the child’s genes and his/her environmentdoesnotrequireanyactiveresponseonthepartofeitherthechildortheparent.

Thesethreepossibilities,the‘standardcausal’oneandthetwoalternatives,areofcoursenotmutuallyexclusive.Tothe extentthateachmayplayarole,amorecomprehensiveviewofthelikelyroleofgenesandenvironmentissummarizedin

Fig.1.

1.2. Identifyingandmeasuringgene–environmentcorrelationasanexplanationforphenotypiccorrelations

Anexamplefromthedevelopmentofliteracycanclarifythisresearchapproach,inparticular,howgene–environment correlationsaredetected.Inthepreschoolyears,therehasbeenmuchemphasisonearlyliteracyexperienceasafacilitative factorforemergentandearlyliteracy.Oliver,Dale,andPlomin(2005)examinedtheinformationprovidedbyparentson theirchildren’spreliteracyknowledge(letter,wordandrhymeknowledgeat4),earlyliteracyexperience(book-reading activities,numberofchildren’sbooks,etc.)andreadingability(teacherrating)attheendofthesecondyearofschool.Both preliteracyknowledgeandearlyliteracyexperiencewerepredictorsofreadingability(r=.29and.19,respectively).Not surprisingly,bothpreliteracyknowledgeandreadingabilityshowedgeneticinfluenceasshownbythediscrepancybetween theintraclasscorrelations(ICCs;closelyrelatedtothefamiliarPearsoncorrelation)betweentwinsinmonozygotic(MZ)and dizygotic(DZ)twinpairs.Butsodidearlyliteracyexperience,anominallyenvironmentalvariable,withgeneticvariance accountingfornearlyaquarteroftheobservedvariance(h2_{=.22).Theevidenceforthisgene–environmentcorrelationis}

thattheintraclasscorrelationwithintwinpairsforearlyliteracyexperiencewasalsohigherforMZpairs,whohaveidentical DNAsequences,thanforDZpairs,whohave50%similarsequencesonaverage.1_{Inotherwords,MZtwinpairsexperienced}

moresimilarenvironmentsthanDZtwinpairs;becausetheparentsandotherfamilyfactorswerethesameinbothkindsof twinpairs,theonlypossibleexplanationisthegeneticdifferencebetweenthetwokindsoftwinpairs.Inthesecondstage, the extent to which the gene–environment correlation is responsible for the observed phenotypic correlation was determined.Thisinfluenceismeasuredbybivariateheritability,theproportionofthephenotypiccorrelationwhichisdueto genesinfluencingboththeenvironmentalvariableandthechildoutcome.Ittooisestimatedbycomparingcorrelationsfor MZtwinpairswiththoseforDZpairs,butinthiscasethecorrelationsarebetweentheenvironmentalmeasureandthechild

Fig.1.Theinterplayofgeneticandenvironmentalfactors,particularlyparentallanguageinput,inlanguagedevelopment.Causalinfluencesdepictedas

lighterredlinkscontributetogene–environmentcorrelation.

1

MZtwinsareidenticalintermsofDNAsequencevariation(atleastasmuchasourDNAsequenceatoneageisidenticaltoourDNAsequenceatanother

age),whereasDZtwinsareonly50%similaronaverageforDZvariantsthatoperateinanadditivemanner.Epigeneticandothereffectsongeneexpression

mayalsoaddvariabilitybutareconceptualizedwithinbehavioralgeneticresearchasenvironmentaleffects,aviewwhichissupportedbymanyrecent

findingsofepigeneticinfluencessuchasmethylationresultingfromtheorganism’srecentenvironmentalexperience.

P.S.Daleetal./JournalofCommunicationDisorders57(2015)106–117

outcome(Plomin,DeFries,Knopik,&Neiderhiser,2013).Inthiscase,thegeneticcontributionwassmall,butsignificant;the bivariateheritabilityestimateof.05wasjustoverone-quarterofthephenotypiccorrelation.

Althoughthetwinmethodhasbeenusedtoexaminegene–environmentcorrelationfor manyaspectsofcognitive, academic,andsocioemotionaldevelopment(cf.Plominetal.,2013),itsapplicationtothestudyofearlyfirstlanguage developmentfacestworelatedmajorchallenges.Child-specificmeasuresoftheenvironmentareessential,notjust family-level measures, and large samples of twins are needed. The most common environmental measures for language development(e.g.,MLU,semanticcontingency)arederivedfromthetranscriptionandanalysisoflanguagesamples,which isahighlytime-andlabor-intensiveprocess.Fewprojectscanundertakeobtainingandanalyzingtwolanguagesamplesfor eachofhundreds,preferablythousands,offamilies.Toourknowledge,onlyasingletwinstudyofchildlanguageinputand developmentbasedonlanguage-samplederivedmeasuresoftheinputhasbeenconducted(DeThorne&Hart,2009).Atotal of 207twinpairs (mean age7 years)from theWesternReserve Reading Project(Petrill, Deater-Deckard,Thompson,

DeThorne,&Schatschneider,2006)participated.Eachtwininapairparticipatedinaconversationwithaseparate,unrelated

examinerduringplaywithmodelingclay.Theuseofunrelatedexaminersmadeitpossibletofocusspecificallyonevocative gene–environmentcorrelation.Althoughsubstantialgeneticeffectsonallconversationalmeasures–talkativeness,mean length ofutterance, vocabularydiversity, and grammatical complexity – werefound, this was nottrue for examiner language.Thatis,therewasnoevidencethattheexaminersweremodifyingtheirlanguageinresponsetothechild,and thereforenone forevocativegene–environmentcorrelation, sotherewasno needtoconductthesecondphase ofthe analysis. However, this wasa single study, conducted at a specific, later developmental period than early language development.Inaddition,itwasnotfocusedonparentalinput,whichisoftenthelargestandalsothemoststudiedsourceof languageinputinearlydevelopment.

Anotherpotentialsourceofinformationonthelanguageenvironmentisparentalself-report.Althoughparentreporthas beenwidelyusedasameasureofchildlanguagedevelopment,itsusetomeasuretheinputisnot,andmayseemquite radical.Thismethod,however,isnotuncommoninthesocioemotionaldomain.Forexample,Parentetal.(2014)foundthat parentalreportsofnegativeparentingbehavior(thoughnotpositiveparenting)werevalidintermsofcorrelationwith observations.Similarly,Plomin,Riess,Hetherington,andHowe(1994)foundthatparents’ratingoftheirownpositivityand negativitytowardtheiradolescentchildrengavesimilarresultstothoseprovidedbytheadolescentsthemselves. 1.3. Thepresentstudy

Inthepresentstudy,wetookadvantageofinformationintheTwinsEarlyDevelopmentStudy(TEDS)onchildlanguageat 3,4,and(forasubsample)4.5years,andalsoonreportedparentlanguageinputstyleat3and4yearsforaverylargenumber offamilies.ThegeneticallysensitiveandlongitudinaldesignofTEDSmakesitpossibletoaddressthequestioninthetitleof thispaperintwoways:first,bydecomposingthecorrelationbetweenparentlanguageinputandchildlanguageintogenetic and environmental factors; andsecond,by comparing thecross-correlationfromparentlanguageinputto laterchild languagewiththatfromchildlanguagetolaterparentlanguageinput.

Ourspecificresearchquestionswere:first,dotheseparentinputmeasurespredictchildlanguageatthesameageand later?Second,isthereevidenceforgene–environmentcorrelationintheformofageneticinfluenceontheparentinput measures?Third,whatproportionofthephenotypiccorrelationbetweenparentalinputandchildlanguageisduetothe geneticfactorssharedbyparentandchild?(Thesequestionsareaddressedthroughthefirst,behavioralgeneticmethod describedabove.)Andfourth,althoughthetwindesigndoesnotitselfenablediscriminationofevocativevs.passivegene– environmentcorrelation,istherephenotypicevidencefordirectionalityofeffect?(Thisisaddressedthroughthesecond, longitudinalphenotypicmethod.)

2. Method 2.1. Participants

TheTwinsEarlyDevelopmentStudy(TEDS)isaverylarge,longitudinalpopulation-representativestudyoftwinsbornin 1994–1996inEnglandandWales.FamiliesoftwinswereidentifiedbytheUKOfficeofNationalStatisticsfrombirthrecords andwerecontactedwhenthechildrenwere1yearold.Ofallfamilies(n=16,810)whorespondedthattheywereinterested inparticipatinginTEDS,over12,000familieshavebeeninvolvedinTEDSsinceitsinception,atleastforoneassessment point.Giventhesizeofthesample,parentreportservedasthemajormeasurementmethodinthepreschoolyears.TEDShas remainedreasonably representativeofUKcensusdatawithrespecttopercent ofwhitefamilies, parentaleducational qualifications,andpercentofmaternalemployment,especiallyduringtheearlyyearswhicharethefocusofthepresent study(Haworth,Davis,&Plomin,2013).

Zygosityoftwinpairswasdeterminedusingparentquestionnairesofphysicalsimilarityadministeredwhenthechildren were18months,3years,and4years;DNAtestingwasconductedwhenzygositywasnotclearfromphysicalsimilarityorthe parentsrequestedit(seeKovas,Haworth,Dale,&Plomin,2007,fordetailsonthisdetermination).Priortoanalysis,the followingexclusioncriteriawereapplied:specificmedicalsyndromessuchasDownsyndromeandotherchromosomal abnormalities,cysticfibrosis,andcerebralpalsy;severehearingloss;autismspectrumdisorder;organicbraindamage; extremeoutliersforbirthweightandgestationalage;heavymaternalalcoholconsumptionduringpregnancy;andintensive

careafterbirth.Inaddition,onlyfamiliesinwhichEnglishwastheprimaryhomelanguagewereincludedinthepresent study.

Thesecriteriaresultedinatotalsamplesizeof8395pairs,2819monozygous(MZ)pairs,2842same-sexdizygotic(DZ) pairs,and2734opposite-sexDZpairs.Thesetotalsincludeallindividualswhocontributedatleastonedatapoint.Inaddition tothemainTEDSassessmentwhichwasdoneatages3and4,asubsetofTEDStwinswasalsogivenanin-depth,in-home assessmentof verbaland nonverbalabilityat4.5 years (Hayiou-Thomas etal., 2006). Because notallmeasures were administeredtoeachbirthcohort,thenumbersvaryforspecificanalyses;therelevantsamplesizesareincludedinthetables tobepresented.

2.2. Measuresofchildlanguagedevelopment 2.2.1. Childlanguageat3years(parentreport)

This measure is a UK-adaptation of the CDI-III (Fenson et al., 2007) which included a 100-item checklist of expressivevocabulary,andagrammarscaleconsistingofasetof12sentencepairsforwhichtheparentchosethe memberthatmostsoundedlikethechild’scurrentlanguage.ExamplesareThatmytruckvs.That’smytruck,andWhy herunawayvs.Whydidherunaway?Informationonthevalidityofthismeasureandthelanguagemeasureat4years is summarized in Dale, Price, Bishop, and Plomin (2003). The vocabulary and grammar scores were separately standardizedandthenaveragedtoyieldasinglescoreforlanguageat3years,whichwasnormallydistributedwith noskew.

2.2.2. Childlanguageat4years(parentreport)

AnewmeasurewasdevelopedspecificallyforthisageinTEDS.Thefirstpartwasa48-itemexpressivevocabulary checklist.Thesecondpartaskedtheparentstoindicateonascaleof 1–6aglobalratingofthecomplexityof their child’slanguage,from‘notyettalking’to‘talkinginlongandcomplicatedsentences’.Thevocabularyandgrammar scoreswerestandardizedandaveraged toyielda singlescoreforlanguageat 4years.Thisscoreshowednegative skewof .75.

2.2.3. Childlanguageat4.5years(restrictedsamplewithin-hometesting)

Theverbalsubtestsofthebatterywereusedtodefinealanguagecomposite;theyincludedmeasuresoforalvocabulary (McCarthy Scales of Children’s Abilities Word Knowledge subtest), verbalfluency (MCSA Verbal Fluency), expressive semantics(RenfrewBusStoryInformationscore),expressivesyntax(RefrewActionPictureTestGrammarscore),receptive syntax(BASVerbalComprehensionsubtest),verbalmemoryformeaningfulmaterial(MSCAVerbalMemoryWordsand Sentences),FollowingtheanalysisofHayiou-Thomasetal.(2006;seealsoKovasetal.,2005),scoresonthesetestswere standardizedandaveragedtoyieldasinglescoreat4.5years.Itshouldbenotedthatthissamplewasidentifiedandrecruited tooversamplelowperformance;asaresult,thesamplewasnormallydistributed,withlittleskew(0.11inthepresent sample),butwithmeanscores.5–.75SDbelowthemeanforthesetests.

2.3. Measuresofparentallanguageinputstyle

Parentscompletedaquestionnaireabouttheirverbalinteractionswitheachtwin,whentheirchildrenwere3yearsold andagainwhentheywere4.Questionsaboutsevenbehaviorswereutilizedinthepresentanalysis.Althoughthebehaviors hadnotbeenselectedonthebasisofacomprehensivereviewofchild-directedspeech,eachofthemhasbeenidentifiedand studiedaspotentiallyrelevant,positivelyornegatively,forlanguagedevelopment.Asshownbelow,parentsfirstresponded withrespecttotheeldertwin,utilizingafivepointscale(‘neverorrarely’to‘almostalways’).Thentheywereaskedforeach behavior,‘Doyoudothismoreorlesswithyour2ndborntwin?’,respondingonafivepointscale(‘alotmore’to‘alotless’). Thescorefortheeldertwinwasthestandardizedresponsetothefirstquestion;theresponsefortheyoungertwinwasthat scoreminusthestandardized‘moreorless’response(minuswasappropriatetoreversethescoringdirectionofthesecond question).FollowingAsbury,Wachs,andPlomin(2005),affirmativeresponsestofourofthequestionsweresummedand standardizedasreflectingareportedinformallanguagestimulationapproach(‘informalparent–childcommunication’in Asburyetal.),andaffirmativeresponsestothreeotherquestionsweresummedandstandardizedasreflectingareported correctivefeedback approach (‘instructive parent–child communication’ in Asbury et al.).The two patterns werenot assumedtobemutuallyexclusive.Asburyetal.estimateinternalconsistency(alpha)of.50forthelanguagestimulation factor,and.86forthecorrectivefeedbackfactor.Informallanguagestimulationshowedsubstantialnegativeskewatboth3 ( .6)and4( .68)years;whilecorrectivefeedbackwaslessskewed(.00and.24atthetwoages,respectively),andshoweda morenearlyuniformdistribution.

2.3.1. Informallanguagestimulation

1.Doesyour1stborntwintakepartinnurseryrhymes,simplesongs,orprayers?

2.Doyouteachyour1stborntwinaboutdirectionsandlocations(forexample,leftandright,wheretheshopsare)?

P.S.Daleetal./JournalofCommunicationDisorders57(2015)106–117

3.Doesyour1stborntwinreadbooksorlookatbookswithyou?

4.Doyoutalktoyour1stborntwinwhenyouaredoinghouseholdchores?

2.3.2. Correctivefeedback

5.Doyouevercorrectwordsthatyour1stborntwinpronounceswrongly(forexample,ifs/hesays‘boon’for‘spoon’? 6.Doyouevercorrectyour1stborntwin’ssentencestructure(forexample,iss/hesays‘menotwantit’insteadof‘Idon’t

wantit’?

7.Doyouevercorrectyour1stborntwinifs/hesaysthewrongwordforsomething(forexample,ifs/hecallsacowahorse)?

2.4. Dataanalysis

Becausethesharedage,andintwo-thirdsofcasesthesharedgender,withintwinpairsmightinflatetwincorrelations, ageandsexwereregressedoutofallmeasuresusedforgeneticanalyses,asistypicaloftwinresearch.Inaddition,outliers morethan3SDfromthemeanofeachmeasurewereremoved.

Phenotypicanalysesarereportedfirst,includingmeansbysexandzygosity,andcorrelationsamongtheparentalinput andchildlanguagemeasures.Analysesofvariance(ANOVAS)wereconductedtoassesstheeffectsofsexandzygosityonthe measures.Inthegeneticanalyses,wefirstreportintraclasstwincorrelations(ICCs)separatelyforeachmeasure.These correlationscanbeusedtomakeinitialestimatesoftheroleofgenetic,sharedenvironment,andnonsharedenvironment influences(Plominetal.,2013).MZcorrelationsgreaterthanDZcorrelationssuggestageneticeffect(symbolizedash2_ora2_,

andwhichmayberoughlyestimatedbydoublingthedifference),DZcorrelationsgreaterthanhalftheMZcorrelations suggestasharedenvironmentalinfluence(symbolizedasc2_{),andMZcorrelationslessthan1.0suggestsomeinfluenceof}

nonsharedenvironment(symbolizedase2_{),aswellasmeasurementerror.Morecomprehensiveandaccuratemeasuresof}

theseinfluencesareprovidedbystandardunivariatestructuralequationmodeling,usingspecializedsoftware(thisstudy usedOpenMx;Bokeretal.,2011).Modelingallowstheestimationofconfidenceintervalsforthreeparametersdescribed above,aswellascomparingthefitofalternativemodels.Ofparticularimportanceforthepresentpaperiswhetherthereis significantgeneticinfluenceonthereportedparentalinputmeasures,asthisconstitutesevidenceforgene–environment correlations.Evidenceforgeneticeffectonparentalbehaviorsuchaslanguageinteractionstylefromachild-basedtwin studymustreflectgeneticdifferencesbetweenthetwins.Theseanalysesarefollowedbybivariateanalysesoftheconcurrent and longitudinal correlations between parental input and child language measures (when those correlations are substantial). These analyses allow us to estimate the contribution of genetic, shared environment, and nonshared environmentinfluencestothosecorrelations.Forexample,theextenttowhichthecorrelationbetweentwomeasuresis higherforMZtwinsthanforDZtwins(thesearecross-trait,cross-twincorrelations,i.e.,measureAfortwin-1iscorrelated withmeasureBfortwin-2)isanindexofthecontributionofgeneticfactorsthatinfluencebothmeasuresinchildren.Inthe presentstudy,thislogicisappliedtothecorrelationbetweenparentinputandchildlanguagemeasures.Theoutcomeof theseanalysesare bivariateheritability, bivariateshared environmentality,and bivariatenonsharedenvironmentality, which addto1.0. Thefirstof theseisthemostimportantforthepresentresearchquestions.Inaddition,longitudinal phenotypicpartialcorrelationsprovideadditionalevidenceonpossiblecausalrelationships.

3. Results

3.1. Descriptivestatisticsforparentandchildmeasuresbygenderandzygosity

Table1summarizesmeansandstandard deviationsofthestandardizedmeasures(z-scores),alongwithanalysisof

varianceforsexandzygosity.Theanalysesarebasedononerandomlyselectedtwinperfamily,tomaintainindependenceof data.(Theuseofonlyhalfthesampleexplainswhythemeanisnotprecisely0,asitwouldbeifallparticipantswere included.)Parent-reportedlanguagestimulationwasveryslightlyhigher,anduseofcorrectivefeedbackveryslightlylower infemales.Althoughthelargesizeofthesamplerenderstheseeffectsandthoseofzygositysignificant,theyareallvery small,withthecombinedeffectofsexandzygosityneverexceeding1.3%ofthevariance.Forthisreason,datafromthetwo sexesarecombinedinthefollowinganalyses.

3.2. Doparentinputmeasurespredictchildlanguage?

CorrelationsamongthereportedlanguageinputandchildlanguagemeasuresarepresentedinTable2.Thesecorrelations arebasedonage-andsex-regressedmeasures.Childlanguageshowsmoderatestabilityfrom3to4.5years,withage-to-age correlationsexceeding.55.Thetwolanguageinputfactorsarealsostablefrom3to4,withcorrelationsof.52and.49,for languagestimulationandcorrective feedbackrespectively.Languagestimulationisconsistently andpositively,though

Table 1

Standardized parental input measures at 3 years, and child language measures at 3, 4, and 4.5 years.

Measures Means and standard deviation on standardized data ANOVA-effects of sex and zygosity

All Females Males MZ DZ Sex Zygosity Sex * zyg.

M (N) SD M (N) SD M (N) SD M (N) SD M (N) SD p h2 p h2 p h2 R2 1 Informal language stimulation at age 3 0.03 (N = 5465) 0.91 0.13 (N = 2778) 0.87 0.08 (N = 2687) 0.94 0.04 (N = 1857) 0.89 0.06 (N = 3608) 0.92 0.00 0.01 0.00 0.00 0.10 0.01 0.000 2 Corrective feedback age 3 0.02 (N = 5501) 0.97 0.05 (N = 2783) 0.99 0.08 (N = 2718) 0.95 0.08 (N = 1881) 0.93 0.02 (N = 3620) 0.99 0.00 0.03 0.00 0.03 0.09 0.00 0.000 3 Child language age 3 0.00 (N = 5065) 1.00 0.01 (N = 2584) 0.98 0.01 (N = 2481) 1.02 0.08 (N = 1712) 1.04 0.04 (N = 3353) 0.97 0.17 0.00 0.00 0.04 0.21 0.00 0.000 4 Informal language stimulation at age 4 0.03 (N = 7277) 0.93 0.12 (N = 3733) 0.91 0.07 (N = 3544) 0.94 0.02 (N = 2445) 090 0.05 (N = 4832) 0.94 0.00 0.01 0.00 0.00 0.07 0.00 0.013 5 Corrective feedback age 4 0.01 (N = 7319) 0.98 0.04 (N = 3740) 1.03 0.06 (N = 3579) 0.96 0.07 (N = 2466) 0.95 0.03 (N = 4853) 1.00 0.00 0.00 0.00 0.00 0.16 0.00 0.005 6 Child language age 4 0.00 (N = 4373) 1.00 0.00 (N = 2220) .096 0.00 (N = 2153) 1.03 0.08 (N = 1481) 1.05 0.05 (N = 2892) 0.96 0.95 0.00 0.00 0.00 0.49 0.00 0.003 7 Child language at age 4.5 0.00 (N = 805) 0.99 0.01 (N = 370) 1.05 0.02 (N = 435) 0.94 0.14 (N = 286) 0.96 0.09 (N = 519) 1.00 0.43 0.00 0.00 0.01 0.21 0.00 0.011

N = sample size based on one randomly selected twin in the pair; M = mean; SD = standard deviation; p = p-value of the effects of sex on variables;h2

= eta-squared and cleared of outliers scores (3 standard deviations). R2

= variance explained by sex and zygosity.

P.S. Dale et al. / Journal of Communication Disorders 57 (2015) 106–117 112

weakly,relatedtochildlanguage(r=.22–.27),whereascorrectivefeedbackisnegativelyandevenmoreweaklyrelated (r= .06to .02).Allthesecorrelationsarestatisticallysignificant.

3.3. Isthereevidenceforgene–environmentcorrelation?

Table 3 includes intraclass correlations (ICCs) and estimates of the genetic, shared environment, and nonshared

environmentinfluencesforeachofthereportedparentalinputandchildlanguagevariables,basedonstructuralequations modeling.Standardtestsofmodelfit,summarizedinTable4(seetablenotefordetails),confirmedthatineverycase,the best-fittingmodelincludedsignificanteffectsforallthreeinfluences(ACEmodels).Thelanguagestimulationfactorwas approximately30%heritableatbothages(i.e.,geneticinfluencesaccountedforapproximately30%ofthevarianceinthis measure);correctivefeedbackwasapproximately40%heritable.Thatis,thesenominallyenvironmentalvariableswere

Table2

Correlationsbetweenparentalinputmeasuresandchildlanguageoutcomes,andstabilitycorrelations.

1 2 3 4 5 6 9

1 Informallanguage

stimulationatage3

1

N 5465

2 Correctivefeedbackage3 .06** ₁

N 5438 5501

3 Childlanguageage3 .27**

.09** 1 N 5011 5043 5065 4 Informallanguage stimulationatage4 .52** .04* .21** 1 N 4410 4439 4118 7277

5 Correctivefeedbackage4 .03 .49** _.08** _.07** ₁

N 4431 4468 4138 7227 7319

6 Childlanguageage4 .27**

.06** .65** .24** .07** 1 N 4324 4355 4047 4275 4297 4373

9 In-homelanguageonwholesample .25**

.12** .56** .22** .13** .58** 1 N 495 504 452 784 796 470 805

Variablescorrectedforageandsex,outliers3standarddeviationsexcluded.

** Correlationissignificantatthe0.01level.

* Correlationissignificantatthe0.05level(2-tailed).

Table3

Univariategeneticanalysesofparentalinputandchildlanguagemeasures.

Measures Intraclasscorrelations Parameterestimatesfrombestfittingunivariatemodel

MZ DZ a2_{(95%CI) c}2_{(95%CI) e}2_(95%CI)

ICC(95%CI)(N) ICC(95%CI)(N)

1 Informal language stimulation atage3 0.85(0.84;0.86)(N=1850) 0.70(0.69;0.72)(N=3583) 0.32(0.28;0.35) 0.55(0.52;0.58) 0.13(0.13;0.14) 2 Corrective feedback age3 0.84(0.83;0.86)(N=1877) 0.65(0.63;0.67)(N=3600) 0.45(0.41;0.49) 0.41(0.38;0.45) 0.14(0.13;0.15) 3 Child language age3 0.91(0.90;0.92)(N=1634) 0.74(0.73;0.66)(N=3180) 0.30(0.27;0.33) 0.60(0.57;0.63) 0.10(0.09;0.11) 4 Informal language stimulation atage4 0.86(0.85;0.87)(N=2429) 0.70(0.69;0.72)(N=4785) 0.32(0.30;0.35) 0.55(0.52;0.57) 0.13(0.12;0.14) 5 Corrective feedback age4 0.85(0.84;0.86)(N=2457) 0.67(0.65;0.68)(N=4816) 0.40(0.37;0.43) 0.46(0.43;0.49) 0.14(0.13;0.15) 6 Child language age4 0.88(0.87;0.89)(N=1441) 0.71(0.69;0.73)(N=2809) 0.27(0.24;0.31) 0.59(0.56;0.62) 0.14(0.13;0.15) 7 Child language atage4.5 0.77(0.70;0.80)(N=285) 0.54(0.48;0.60)(N=514) 0.44(0.31;0.57) 0.33(0.20;0.44) 0.23(0.20;0.28)

significantlycorrelatedwithindividualchildren’sgeneticmakeup(thesameforMZtwins;differentforDZtwins).Consistent withearlieranalysesofthisdataset(Spinathetal.,2004;Hayiou-Thomasetal.,2012),thechildlanguagemeasureswere significantlygeneticallyinfluenced,withheritabilityfiguresinthe.25–.40range.

3.4. Whatproportionofthephenotypiccorrelationsisduetosharedgeneticinfluences?

Thethirdresearchquestionaskstowhatextentthegene–environmentcorrelationidentifiedaboveinfactinfluencesthe observedphenotypic correlationsbetween reported parentalinput andchild language.Bivariateanalysesidentifythe proportionofthetotalphenotypiccorrelationthatisduetocommongeneticinfluences(bivariateheritability),common shared environmental influences (bivariate shared environment), and common nonshared environmental influences (bivariatenonsharedenvironment).Theseanalyseswereperformedonlyfortheinformallanguagestimulationfactor,asthe cross-trait,cross-twincorrelations(e.g.parentlanguageinputfortwin1correlatedwithchildlanguagefortwin2)were significantonlyforthatinputfactor.Thisisacriteriontoassurereliablecovariancefordecomposition.Theresultsofthese analysesarereportedinTable5,andillustratedinFig.2.Withrespecttothepredictionfromage3,approximately one-quarterofthepredictiontotheage3and4measureswasduetobivariateheritability,andmorethanthree-quartersofthe predictiontotheage4.5measureswasduetobivariateheritability.Thefiguresweresimilar,butsomewhatlowerwith respecttotheconcurrentcorrelationbetweeninputat4andchildlanguageat4and4.5.

3.5. Istherephenotypicevidencefordirectionalityofeffects?

Wecomputedthepredictionfrom3yearreportedlanguagestimulationto4yearlanguagepartiallingout3yearlanguage asanindexoftheeffectoflanguagestimulationonchildlanguage,andcompareditwiththepredictionfrom3yearlanguage

Table4

Summaryofmodelfit.

Univariatemodelfit

Measure Model 2LL df (D 2LL) AIC BIC p-Value ep

Informallanguagestimulationatage3 Saturated 24,211.33 10,933 – 2345.33 74,645.29 – 10

ACE 24,239.48 10,939 28.14 2361.48 74,671.40 0.00 4

Correctivefeedbackage3 Saturated 26,205.19 10,996 – 4213.19 73,221.09 – 10

ACE 26,250.95 11,002 45.76 4246.95 73,229.58 .000 4

Childlanguageatage3 Saturated 23,313.55 10,117 – 3079.55 68,164.77 – 10

ACE 23,348.70 10,123 35.15 3102.70 68,183.88 0.00 4

Informallanguagestimulationatage4 Saturated 32,545.06 14,532 – 3481.06 98,853.87 – 10

ACE 32,563.67 14,538 18.61 3487.67 98,889.51 0.00 4

Correctivefeedbackatage4 Saturated 34,805.25 14,628 – 5549.25 97,461.71 – 10

ACE 34,848.49 14,634 43.24 5580.49 97,472.72 0.00 4

Childlanguageatage4 Saturated 20,520.91 8734 – 3052.91 58,452.27 – 10

ACE 20,560.82 8740 39.91 3080.82 58,466.61 0.00 4

Childlanguageatage4.5 Saturated 4134.49 1603 – 928.49 10,359.90 – 10

ACE 4147.71 1609 13.23 929.71 10,400.93 0.04 4

2LL= 2Loglikelihood;df=degreesoffreedom; 2LL= 2Loglikelihood;D 2LL=differenceinlikelihoodbetweenthecomparedmodels;AIC=Akaike

InformationCriterion;BIC=BayesianInformationCriterion.SmallervaluesofBICandAICindexbetterfit;p-value=referstosignificantdropinlikelihood

valuebetweentheSaturatedandcomparednestedmodel;ep=estimatedparameters.

Fitcomparisonbetweenthesaturatedmodel,fromobserveddata,andthebestfittinggeneticACEmodel.IntheACEmodeltheparametersareestimatedas

theeffectsofgenetic(A),shared-environmental(C),andnon-sharedenvironmental(E)factors.ThedropinlikelihoodbetweenthesaturatedandfullACE

modelissignificantinall7comparisons.However,inlargesamplestheBICindexisconsideredmorereliablethanAICinevaluatingthefitrelativeto

parsimony,asBICtakesintoaccountthesamplesize.TheBICindicatesabetterfitherefortheACEmodels,astheindexisconsistentlysmaller(more

negative)forthosemodels.

Table5

Bivariateanalysesofthephenotypiccorrelationsfromparentalinputtochildlanguage.

Measures Phenotypic correlations Bivariateparameters a2 (95%CI) c2 (95%CI) e2 (95%CI)

Informallanguagestimulationatage3withchildlanguage3 0.27 0.20(0.12;0.28) 0.76(0.69;0.83) 0.04(0.01;0.06)

Informallanguagestimulationatage3withchildlanguage4 0.27 0.25(0.16;0.35) 0.74(.65;0.81) 0.01( 0.01;0.04)

Informallanguagestimulationatage3withchildlanguage4.5 0.25 0.73(.29;1.0) 0.18( 0.35;0.57) 0.09( 0.02;0.21)

Informallanguagestimulationatage4withchildlanguage4 0.24 0.40(.07;077) 0.51(0.19;0.78) 0.09( 0.01;0.19)

Informallanguagestimulationat4withchildlanguage4.5 0.22 0.47(0.13;0.85) 0.42(0.07;0.71) 0.11(0.00;0.21)

Figuresinboldidentifysignificanteffects.

P.S.Daleetal./JournalofCommunicationDisorders57(2015)106–117

to4yearreportedlanguagestimulationpartiallingout3yearlanguagestimulationasanindexoftheeffectofchildlanguage onlanguagestimulation.Bothcorrelationsdroppedsubstantiallyfromthezero-ordervaluesinTable2,reflectingthehigh stabilityfromage3to4ofbothlanguagestimulation(.52)andverbalability(.65).Nevertheless,thepartialcorrelationswere stillsignificant(bothp<.001),andthecorrelationindexingtheeffectoflanguagestimulation(.13)wassignificantlyhigher thanthecorrelationindexingtheeffectofchildlanguage(.07)usingtheFisherr-to-ztransformation(p=.016,df=4002and 4078 respectively). Both the correlations and the difference are significant but small, suggesting influences in both directions,possiblylargerfromparenttochild.

4. Discussion

Theself-reportmeasuresofparentalinputusedinthisstudyaremodestlybutsignificantlyrelatedtochildlanguage outcomes.Asexpected,theinformallanguagestimulationfactorhasapositivepredictiontochildlanguage(.22–.27),while correctivefeedbackhasanegativeprediction( .06to .12).Furthermore,thepredictionsforinformallanguagestimulation are noticeably larger in absolute value. With respectto their reliabilityand validity, note that both measures were moderatelystable(rabout.5)from3to4.Thecorrelationsfromlanguagestimulationtochildlanguage,whilesignificant,are smallerthanthoseintheliteraturewhicharebasedondirectobservation(seeHoff, 2006,forareview).Thelanguage stimulationfactorherewasquitegeneral,whereasmostresearchinthisareahasexaminedmorespecificbehaviors,suchas sentencerecastsandexpansions;thosespecificfactorsmayleadtohigherpredictions.Itisalsonotablethatthecorrelations fromparentalinputquantityandrichnessofvocabulary,whichmaybeclosesttothepresentmeasure,tendtobesmaller thanotherparent–childcorrelationsinprevious research(Hoff,2006),andhenceshowtheleastdiscrepancy withthe presentresults.

Both dimensions of reported parental language input have significant, moderate heritability, confirming gene– environmentcorrelation.Asnotedearlier,thesecorrelationscanonlybebasedongeneticdifferencesbetweenthetwins.The significantheritabilityandsharedenvironmentparametersalsoprovideakindofconcurrentvalidityfortheinputmeasures, astheycanbeinterpretedasmeasuringthedegreeofrelationshipbetween theparentallanguageinputmeasuresand geneticandsharedenvironmentfactors,respectively.(Becausenonsharedenvironmentalsoincludesmeasurementerror,it isnotrelevantforvalidity.)

Asignificantproportionofeachofthephenotypicpredictionsfromparentalinputtoconcurrentorlaterchildlanguage wasduetosharedgeneticeffects(.20–.73,median.40).Interestingly,boththeunivariateandbivariateheritabilitiesare higherforthepredictionofthedirect,4.5yrmeasureofchildlanguagethanfortheparent-reportedmeasures.Shared environmentalfactorsalsoplayedasignificantandsubstantialrole.

Because parents provided information on both their children’s languageand their ownlanguage input,there is a possibilityofrater-bias(insufficientdistinctionbetweentheirownandtheirchild’sbehavior)orinsufficientdistinction betweentheirinputtothetwotwins.Ifeitherorbothoftheseprocessesweretooccur,thecorrelationsbetweenthetwins shouldbeinflatedforbothMZandDZtwins,whichwouldinflatetheestimatesinbivariatesharedenvironmentalfactors. Interestingly,thebivariatesharedenvironmentalitywasquitehighfortheparent-reportedchildlanguagemeasuresat3and 4,butnotforthein-personassessmentat4.5.Thustheremaybeinter-raterbiasfortheformer,butnot(orless)forthelatter, whichactuallyyieldsthestrongerresultswithrespecttoourresearchquestions.

Taken together, the heritability of the reported parental factors and the longitudinal bivariate results provide considerableevidencethatgeneticfactorsclearlyplayaroleinthenatureofparentalinputanditsobservedrelationship withlanguagedevelopment.However, itshouldalsobenotedthatthephenotypiccorrelations arenotentirelydueto genetics;theyarealsoinpartduetosharedenvironmentinfluencesonbothparentandchild.Theseresultsshouldbe regardedasinitialestimates,giventhelikelylimitedvalidityoftheinputvariableswithrespecttodistinguishingthetwins

Fig.2.Proportionofthephenotypiccorrelationofparentallanguagestimulationat3or4yearswithchildlanguageoutcomemeasureswhichisdueto

genetic(black),sharedenvironment(shaded),andnon-sharedenvironment(verticallines)influences.LS=informallanguagestimulation;Lg=child

andaccuracyofself-reportofbehavior.Directmeasuresoftheinputwouldbehighlypreferable,butmuchmore resource-intensive,especiallybecausetheymustbechild-specific,notfamily-general,effectivelydoublingtheeffort.

Canwedistinguishpassivefromevocativegene–environment(GE)correlationwiththepresentdata?No,theclassical twinstudycannotdothis;otherdesignsareneeded,e.g.,adoptionstudies,orchildrenoftwinsdesigns(Plominetal.,2013). SomesuggestiveevidencethatthereisbothevocativeGEcorrelationalongwithstrongerpassiveGEcorrelationcomesfrom thephenotypicpartialcorrelations,whicharebothsignificant,andsignificantly–butonlyslightly–strongerfromparentto childthanconversely.ThenegativeresultsfromDeThorneandHart(2009),whichfocusedonevocativeGEcorrelationisalso consistentwiththisimpression.However,strongergeneticallysensitivedesigns,suchasadoptionstudiesandthechildren oftwinsdesigns,willbeneededformoredefiniteconclusionsonthisquestion.

Asomewhatunexpectedaspectoftheresultswasthesubstantialheritabilityofthereportedcorrectivefeedbackfactor, indeedhigherthanthatforlanguagestimulation,combinedwithaveryweakpredictiontochildlanguage.Theheritabilityof correctivefeedback means that it reflectsgenetic influences in thechild. However, these arenot necessarily genetic influencesspecificallyonlanguage.Forexample,theymightberelatedtobehavioralissueswhichevokeanorientation towardbehaviormanagement,which‘spillsover’intocorrectingspeechandlanguage.Alternatively,parental‘teacherly style with concern for correct form’ might reflect a trait observable in the parent, which emerges only later in the developmentofthechild, perhapsin theformofmoreadvanced metalinguisticawareness.Researchwhich compares parentallanguageinteractionstylewithotheraspectsofparentalbehaviorisneededtoclarifythisresult.

4.1. Limitations

Themostimportantlimitationtotheconclusionsofthisstudystemsfromtheuseofparent-reportmeasuresoflanguage input,ratherthan someformofdirect observation. Asthismeasure wasinnovativetothepresentstudy,there is no independentevidenceconcerningitsreliabilityandvalidity.Abovewehaveprovidedsomeevidencethatthesemeasures havesignificantvalidity. In addition,theyarehighly globalmeasuresrather than morespecificallyfocused measures suggested by thecurrent literature, suchas vocabulary diversity orproportion of sentencerecasts. The low internal consistencyofinformallanguagestimulationcouldalsobeviewedasalimitation.Detailedexaminationofcorrelations revealed that all four component questions for informal language stimulation related approximately equally to the composite,andapproximatelyequallytothelanguageoutcomemeasures.Thispatternsuggeststhatinformallanguage stimulationmaynotbeunidimensional,butthatallcomponentscontributetotheoverallprediction.

Inaddition,theexactwordingoftheself-reportquestionswasnotidealforthepresentpurposes.Thebasicquestion frameofaskingaboutonetwinfirst,andthenaboutthedifferencebetweenthetwins(ratherthanaboutthesecondtwin directly)mayhaveinflateddifferencesbetweenthetwins.IfthiseffectoccurredequallyforbothMZandDZtwins,the estimateof geneticinfluence onparental behaviorwouldhavebeen spuriouslyloweredandtheeffectof nonshared environmentincreased.Alternatively,ifresponsesforMZtwinsweremoreaffected(becauseitledtheparentstosearch fordifferencesthatwereinfactminimal)thanforDZtwins,theestimateofgeneticinfluencewouldagainhavebeen diminished, and that for shared environment increased. Both cases thus would yield an underestimate of genetic influence.Afinalissuefor theuseofself-reportisthatparentresponsestoquestions#1and#3mayhavereflected individualdifferencesinchildrenaswellastheparents’ownbehavior,potentiallyinflatingthecorrelationbetweenparent andchild.

AnotherlimitationofthedesignisthattheyoungestageatwhichtheenvironmentalmeasurewasavailableinTEDSwas 3years.Itispossiblethataspectsofparentalinputareevenmoreimportantatyoungerageswhenitformsanevenlarger proportionofthetotalinputandtheneedforjointattentionisespeciallycritical.

5. Conclusion

Geneticresearchofthistypeiseasilymisunderstood.Thebroadestconclusionfromthisworkisnotgeneticdeterminism. Genesneedtheenvironmenttohavetheireffect.Achildwithastronggeneticendowmentformathematics,orformusic,or forathleticsdoesnotbecameexceptionallyskilledwithoutyearsofpractice;thegenesmayhavetheirlargesteffectby incliningthechildtospendhisorhertimeinthatpractice(cf.Detterman,2014,andassociatedpapersforadiscussionof practice,ability,andexpertise).Similarly,thegenessharedbyparentandchildleadtoformsof‘languagepractice’which,in theirquantityandtheirquantity,facilitatechildlanguagedevelopment.Thisgenotype-environmentcorrelationinnoway precludestheeffectivenessofinterventionsforparentsorforchildren,andmanyinterventionshavebeendevelopedwith bothshort-termandlong-termeffects(Finestack&Fey,2013;Girolamettoetal.,2013).Nevertheless,wesuggestthatitis importanttounderstandthatcorrelationsbetweenparentlanguageandchildlanguagedevelopmentarenotalwayscausal; theyofteninvolvecorrelationsbetweengeneticpropensitiesofparentsandtheirchildren,aconclusionwhichhasimportant implications for intervention and prevention as well as interpretation. Awareness of the ways in which genes and environmentinterplayareessentialfordevelopingbetterinterventions,forexample,inunderstandingthebidirectional transactionsbetweenparentlanguageandchildlanguagedevelopmentandthegenotype-environmentfeedbackloops.It hasoftenbeenpointedoutthatgeneticallysensitivedesignssuchastwinstudiescanprovidethestrongestpossibleevidence forenvironmentaleffects.Forexample,Hardy-Brown,Plomin,andDeFries(1981)concludedfromtheiradoptionstudythat maternalresponsivenesstoinfantvocalizationsplayedanimportant,purelyenvironmentalroleinmakingtheauditory

P.S.Daleetal./JournalofCommunicationDisorders57(2015)106–117

speechchannelsalienttoinfants,andthereforeitwouldbeagoodfocusforintervention.Beyondsuchspecificlessons, researchongene–environmentcorrelationsprovidesalessoninrespectingtheindividualityeachperson–parentandchild –bringstothelanguagelearningcontext.

Acknowledgements

WegratefullyacknowledgetheongoingcontributionoftheparticipantsintheTwinsEarlyDevelopmentStudy(TEDS) and theirfamilies.TEDSis supportedbyaprogramgrant [G0901245;andpreviouslyG0500079]fromtheUKMedical Research Council.R.P.is supported by a researchprofessorship fromthe UKMedical ResearchCouncil [G19/2]and a EuropeanResearchCouncilAdvancedInvestigatorAward[295366].

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