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Intelligence: An overview

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Intelligence: An overview

TimothyC.Bates

DepartmentofPsychology,UniversityofEdinburgh,Edinburgh,UK.

Introduction

Studiesonthegeneticsofhumancognitiveabilityprovideapowerfultoolforunderstandinghumancognition.Thegoalofthisbriefintroductiontointelligenceistoprovideanoverviewoftopicalintelligenceresearchandtheory,intermsthatcanbeintegratedwiththeresultsandtheoryofneuropsychology.

Aconsensusdefinitionofintelligenceamongstover50researchersintheareawasproposedasthe“verygeneralmentalcapabilitythat,amongotherthings,involvestheabilitytoreason,plan,solveproblems,thinkabstractly,comprehendcomplexideas,learnquicklyandlearnfromexperience.Itisnotmerelybooklearning,anarrowacademicskill,ortest-takingsmarts.Rather,itreflectsabroaderanddeepercapabilityforcomprehendingoursurroundings—‘catchingon,’‘makingsense’ofthings,or‘figuringout’whattodo”(Gottfredson,1997).

Acrossallthehundredsoftestsandatleastdozensofseparableabilitiesstudiedinneuropsychology,twofindingshavebeenespeciallysalienttoresearcherssincetheturnofthelastcentury:Alltheseabilitiesarecorrelatedwitheachother(Spearman,1927);andquiteastoundingdoubledissociationsoftenaccompanyacquiredordevelopmentaldisorders(Rapp,2002).Bothoftheseapproaches(ofassociationanddouble-dissociation)reflectkeyconceptualandstatisticalbreakthroughsinpsychologicalmodelling(Shallice,1988)andagoalofthisintroductionistorenderthefindingsofbothintoasingleframeworkofgeneticneuropsychology.

Inthisbriefintroductiontointelligence,Ifirstdiscusstherelationshipbetweeninformationfromassociationsandinformationfromdissociations.Ithensummarisealargeliteratureindicatingthatagenerallatentabilityfactoraccountsforaroundhalfofdifferencesincognitionandthatthisisahighlyheritabletrait.Finally,Inotethebiologicalandgeneticfindingsinintelligence,againwithanemphasisonlinkingtheconceptsofintelligencetoneuropsychology.

AssociationandDissociation:Arapprochementthroughstructuralmodelling

Anaudienceofneuropsychologistsneedsnoreminderthatpeopledifferwidelyintheirabilitiestocreateandmanipulatementalrepresentationsandthatthesedifferencesareprimedataforunderstandingcognition.Theideathatthebrainandneuronalintegrityarecriticaltotheseabilitiesandthatgeneticdifferencesimpactpowerfullyonthedevelopmentofabroadrangeofmentaldifferencesarealsofairlyuniversalamongneuropsychologists.

Thusfar,thegoalsandinterestsofneuropsychologyandofintelligenceresearchersappearidentical:theuseofindividualdifferencesincognition(respondingtoinformation,solving,understanding,manipulating,memorising,etc)tounderstandthestructureofthemindandthebiologicalbasisofthisstructure.Whereintelligenceresearchdiffersfromthetypicalapproachofneurologyandneuropsychologyisinitsfocusontheassociationsamongsttasks,ratherthandissociationsanddouble-dissociationsbetweentasks.

Figure1. Reading,LanguageandWorkingMemoryinCognitiveNeuropsychology.Ontherighthandsideareshowncomponentsofthedual-routecascadedsystemforreading(Coltheart,Rastle,Perry,Langdon,Ziegler,2001).Datafrompatientssupportsdissociationsbetweentheabilitytoreadaloudpseudo-wordssuchas“SLINT”andtheabilitytoread-aloudirregularwordssuchas“YACHT”(A.Castles,Bates,Luciano,Martin,Coltheart,2005;AnneCastlesHolmes,1996).Inthecenterweseesemanticsandtheexecutivecomponentsofworkingmemory(Baddeley,2007).Thesearerepresentedasseparatemodulesbecausedissociationsagainsuggestthattherearepatientswithacquiredbraindamagewhoretaintheabilitytoreadbutnolongeraccessmeaningfromwrittenlanguage,andotherpatientswhoretainmeaningdespitelosingtheabilitytoread.Finallyonthelefthandsideofthemodelarecomponentsofmindimplicatedinspecificlanguagedisorder,specificallythesystemsforspeechsoundanalysis,forbriefstorageofphonologicalstrings,independentofmeaning,andconnectionssupportingrehearsalinshort-termmemory(Baddeley,2007).

RepresentedgraphicallyinFigure1andFigure2,thereseemsattimesaloominggulfbetweentherespectiveviewsfromneuropsychology(withitsbox-and-arrowdiagramsofpainstakinglyaccumulatedfrompatientstudiesofdissociationsinthemind:SeeFigure1)andintelligenceresearch,withitshierarchicaltreediagramsbuiltlargelyfromtherelationshipsofperformanceontasksinlargergroupofnormallydevelopingindividuals(SeeFigure2).

Figure2. FactorStructureofabilityasmeasuredintheWechslerAdultIntelligenceScaleIII.

Despiteappearances,bothofthesemodelsarecompatible:thesamemoduleswouldultimatelyappearasboxesinbothdiagramsasthedatadrivingtheircreationisinbothcasestheexistenceofbetween-taskcorrelationssignificantlylessthanone,notexplainablebyscalingfactorssuchasdifficulty.

Wheretheydifferisthatthelatentvariablemodelofintelligencepreservestherawassociationdata(atthecostoflosingthewiringdiagram),whiletheneuropsychologicalbox-and-arrowmodelpreservesconnectivityinformationatthecostoflosinginformationonsharedprocesses.

Surprisingly,recentresearchisrevealingthatnormalvariationintasksrevealsthesamestructuredoubledissociationspatientdatacontributecriticaldataontheseparabilityofdissociationsmodelledasboxesfactthattheabilitiesdonotcorrelate1.0

Figure2showsarepresentativefindingofthestructureofintelligenceresearchfromtheWAIS(Wechsler,1997),atestcommontobothneuropsychologyandintelligenceresearch.Itcanbeseenthatthe13WAISsub-testscorrelate0.49(range.26to.77).Sincethefirstformalmentaltestwasdevised(Binet,1905/1916),structurescloselyrelatedtothatshowninFigure2emergefromallofthemanyhundredsoftestbatteriesdevisedandadministeredtomanymillionsoftest-takers(IanJ.Deary,2001).

Thegeneticandenvironmentalinfluencesonmentalabilitieshavenowbeenexaminedinadiverserangeofgeneticallyinformativestudiestotallyover200,000monozygotic(MZ)anddizygotic(DZ)twin-pairs(SummarizedinTable1).Twokeyfindingsfromthisresearcharethatabilityishighlyheritableandthatgeneticinfluencesincreaseratherthandecreaseovertime,whiletheinfluenceofwithin-familyeffectsdecreasesovertime,sothatbyadulthoodandintooldage,70-80%ofthedifferencesobservedintestperformanceinadulthoodandoldagearegenetic(SeeFigure3).Cruciallyforneuropsychologicalresearch,geneticstudiesofnormaladolescentsandadultsindicateclearlythatthiscorrelationisalmostentirelygeneticinorigin(PlominSpinath,2004).

Table1.SummaryofworldliteratureonfamilialeffectsonIQ

(adaptedfromBouchardMcGue,1981)

Thebiologyofg

Whilegeneralabilityemergesveryreliablyasaheritabletrait,itisonlymorerecentlythatresearchershavebeguntouncoverbiologicalbasesforgeneralintelligence.Perhapssurprisingly,thekeyfindingofthelast20yearsofresearchhasbeenthefindingthatthestrongestphysicalbraincorrelateofintelligenceisbrainvolumeitself,withameta-analysisofseveralthousandsamplescasessuggestingthatbrainvolumedifferenceexplain10%ofIQvariance,withregionaldifferencecontributingfurthervariance(McDaniel,2005).Twinstudiesindicateboththatbrainvolumeishighlyheritable(Thompsonetal.,2001;Penningtonetal.,2000)andthatgeneralabilityandbrainvolumeshareageneticcorrelation(Posthumaetal.,2003).Thissuggeststhatresearchersongeneticsyndromesshouldexamineregionalandglobalchangesinbrainvolumeaspotentialbiologicalindicators(GrayThompson,2004).

Inadditiontotheglobalvolumeofthebrain,recentstudieshavehighlightedtheimportanceofregionalvariationinbrainactivityandvolumeinintelligence(JungHaier,2007).Thisfurthersupportstheroleofgeneticanalysesofbrainvolumevariationasatooltoidentifybrainsystemswithsharedgeneticinfluencesbutdiverseanatomicalboundaries(TogaThompson,2005).Casestudiesofgeneticabnormalitiescanplayapotentialcrucialroleinthisfield.

Figure3.GeneticandEnvironmentalComponentsofIntelligencefromage3toage82.A(red)istheobservedheritability,C(yellow)istheeffectofsharedorfamilyenvironmentfactorssuchasSESandhome,andE(green)istheremainingeffects,whichareuniquetoeachindividualandincludemeasurementerror.TheFigureshowsthatatveryyoungagesfamilyenvironmenthasalargeeffectoncognitiveability,butthatbyyoungadulthood,thishasbeenreplacedwithlargeeffectsofgenes,whichcontinuetoriseinrelativeimportanceintooldage,possiblydecliningagainaspeoplereachtheir80s.Thedataarecombinedresultsfrom(Bartels,Rietveld,VanBaal,Boomsma,2002;McClearnetal.,1997;DaniellePosthuma,deGeus,Boomsma,2001;Reynoldsetal.,2005;SpinathPlomin,2003).

Moleculargenetics

Whiletheheritabilityofintelligenceimpliestheexistenceofgeneticeffectsonability,thelimitedresearchonintelligencetodatehasbeenlessproductivethnhassimilarresearchonmorespecifictraitssuchasdyslexiaandevennormalvariationinreading,whereasmanyas11regionscontainingreading-relatedgeneshavebeenidentified(Batesetal.,2007).Bycontrast,studiesofgeneralintelligencesuggestthatspecificgenessuchasklotho(IanJDearyetal.,2005)andCOMT(catechol-O-methyltransferase)(WintererGoldman,2003134-163)havesmalleffects,consistentwithapolygenicviewoftheheritabilityofintelligence,i.e.,aviewinwhichthegenesaffectingintelligencenumberinthehundreds,withnonehavingalargeeffectonpopulationvariation.Thisviewthatgenesforintelligencewillhavesmalleffectsisfurtherbuttressedbythefirstgenome-widelinkagestudiesforintelligence,whichsuggestedthatthegeneticeffectsarewidelydistributedwithnoneexplainingmorethan2-3%ofthevarianceinIQ(Lucianoetal.,2006),andbyexploratoryassociationstudies,whichsuggestthatareasonableestimatefortheaveragegeneeffectsizemightbeontheorderof.1%ofthepopulationvariance(Butcheretal.,2005).Theseconclusionsremaintentative,asallthestudiestodatehavebeenmodestinsize,andgenomecoverage,and,importantly,havelackedtheabilitytodetectgeneticeffectsmediatedbymultipleuncommonmutations,aswellasbeingunabletodetectcopy-numbervariations,bothofwhichseemlikelytoharboursignificantcognitiveeffects(Sebatetal.,2007).

Theseresultsdo,however,suggestthattheresultsofstudiesoftheneuropsychologicaleffectsofchromosomalabnormalities,asreportedintherestofthisvolumewillprovetobeacriticalsourceofcandidategenesforunderstandinggeneralcognition(forinstancethecaseofmicrocephalyrelatedgenessuchasASPM(Cox,Jackson,Bond,Woods,2006).Thepaceofthisresearchsuggeststhelikelihoodthatthenextfiveyearswillseenotableprogressinbothintelligenceandneuropsychologicalgeneticsofspecificandgeneralgenesthroughcollaborationandreplicationofeffectsdetectedusingthegapproach,andthosediscoveredthroughfamily-basedsyndromicstudies.

Inconclusion,itishopedthatthisintroductionhasmadetheapproachofintelligenceresearchersmoreaccessibletothoseinneuropsychology,andhighlightedprogressinthisfieldusingphenotypessuchasbrainvolumesandthemethodsofassociationandlinkageaswellasthedifficultiesencounteredinthesearchforgenesaffectingdiversecognitiveandbiologicalfunctions.

References

Baddeley,A.D.(2007)Workingmemory:Multiplemodels,multiplemechanisms.InH.L.Roediger,Y.DudaiS.M.Fitzpatrick(Eds.),ScienceofMemory:Concepts.(pp.151-153).Oxford:OxfordUniversityPress.

Bartels,M.,Rietveld,M.J.,VanBaal,G.C.,Boomsma,D.I.(2002)Geneticandenvironmentalinfluencesonthedevelopmentofintelligence.BehavGenet,32,237-249.

Bates,T.C.,Luciano,M.,Castles,A.,Coltheart,M.,Wright,M.J.,Martin,N.G.(2007)Replicationofreportedlinkagesfordyslexiaandspellingandsuggestiveevidencefornovelregionsonchromosomes4and17.EurJHumGenet,15,194-203.

Binet,A.(1905/1916).NewMethodsfortheDiagnosisoftheIntellectualLevelofSubnormals

L'AnnéePsychologique,12,191-244.(E.S.Kite,Trans.).InThedevelopmentofintelligenceinchildren.Vineland,NJ:PublicationsoftheTrainingSchoolatVineland.

Bouchard,T.J.,Jr.,McGue,M.(1981)Familialstudiesofintelligence:areview.Science,212,1055-1059.

Butcher,L.M.,Meaburn,E.,Knight,J.,Sham,P.C.,Schalkwyk,L.C.,Craig,I.W.,etal.(2005)SNPs,microarraysandpooledDNA:identificationoffourlociassociatedwithmildmentalimpairmentinasampleof6000children.Hum.Mol.Genet.,14,1315-1325.

Castles,A.,Bates,T.C.,Luciano,M.,Martin,N.G.,Coltheart,M.(2005)Twodifferentsetsofgenesforreading(andspelling).AustralianJournalofPsychology,57,47-47.

Castles,A.,Holmes,V.M.(1996).Subtypesofdevelopmentaldyslexiaandlexicalacquisition.AustralianJournalofPsychology,48,130-135.

Coltheart,M.,Rastle,K.,Perry,C.,Langdon,R.,Ziegler,J.(2001)DRC:Adualroutecascadedmodelofvisualwordrecognitionandreadingaloud.PsychologicalReview,108,204-256.

Cox,J.,Jackson,A.P.,Bond,J.,Woods,C.G.(2006)Whatprimarymicrocephalycantellusaboutbraingrowth.TrendsMolMed,12,358-366.

Deary,I.J.(2001)Humanintelligencedifferences:Towardsacombinedexperimental-differentialapproach.TrendsinCognitiveSciences,5,164-170.

Deary,I.J.,Harris,S.E.,Fox,H.C.,Hayward,C.,Wright,A.F.,Starr,J.M.,etal.(2005)KLOTHOgenotypeandcognitiveabilityinchildhoodandoldageinthesameindividuals.NeuroscienceLetters,378,22-27.

Gottfredson,L.S.(1997)Mainstreamscienceonintelligence:aneditorialwith52signatories,history,andbibliography.Intelligence,224,13-23.

Gray,J.R.,Thompson,P.M.(2004)Neurobiologyofintelligence:Scienceandethics.NatureReviewsNeuroscience,5,471-482.

Jung,R.E.,Haier,R.J.(2007)TheParieto-FrontalIntegrationTheory(P-FIT)ofintelligence:convergingneuroimagingevidence.BehavBrainSci,30,135-154;discussion154-187.

Luciano,M.,Wright,M.J.,Duffy,D.L.,Wainwright,M.A.,Zhu,G.,Evans,D.M.,etal.(2006).Genome-widescanofIQfindssignificantlinkagetoaquantitativetraitlocuson2q.BehaviorGenetics,36,45-55.

McClearn,G.E.,Johansson,B.,Berg,S.,Pedersen,N.L.,Ahern,F.,Petrill,S.A.,etal.(1997)Substantialgeneticinfluenceoncognitiveabilitiesintwins80ormoreyearsold.Science,276,1560-1563.

McDaniel,M.A.(2005)Big-brainedpeoplearesmarter:Ameta-analysisoftherelationshipbetweeninvivobrainvolumeandintelligence.Intelligence,33,337-346.

Plomin,R.,Spinath,F.M.(2004)Intelligence:genetics,genes,andgenomics.JPersSocPsychol,86,112-129.

Posthuma,D.,Baare,W.F.C.,Pol,H.E.H.,Kahn,R.S.,Boomsma,D.I.,DeGeus,E.J.C.(2003)GeneticcorrelationsbetweenbrainvolumesandtheWAIS-IIIdimensionsofverbalcomprehension,workingmemory,perceptualorganization,andprocessingspeed.TwinResearch,6,131-139.

Posthuma,D.,deGeus,E.J.,Boomsma,D.I.(2001)PerceptualspeedandIQareassociatedthroughcommongeneticfactors.BehavGenet,31,593-602.

Rapp,B.(Ed.).(2002)TheHandbookofCognitiveNeuropsychology:WhatDeficitsRevealAbouttheHumanMind.NewYork:PsychologyPress.

Reynolds,C.A.,Finkel,D.,McArdle,J.J.,Gatz,M.,Berg,S.,Pedersen,N.L.(2005)Quantitativegeneticanalysisoflatentgrowthcurvemodelsofcognitiveabilitiesinadulthood.DevPsychol,41,3-16.

Sebat,J.,Lakshmi,B.,Malhotra,D.,Troge,J.,Lese-Martin,C.,Walsh,T.,etal.(2007)Strongassociationofdenovocopynumbermutationswithautism.Science,316,445-449.

Shallice,T.(1988)FromNeuropsychologytoMentalStructure.Cambridge,UK:CambridgeUniversityPress.

Spearman,C.(1927)Theabilitiesofman.NewYorkNY:Macmillan.

Spinath,F.M.,Plomin,R.(2003)Amplificationofgeneticinfluenceongfromearlychildhoodtotheearlyschoolyears.PaperpresentedattheIVthMeetingoftheInternationalSocietyfortheStudyofIntelligence,Irvine,USA,4-6December.

Toga,A.W.,Thompson,P.M.(2005)Geneticsofbrainstructureandintelligence.AnnuRevNeurosci,28,1-23.

Wechsler,D.(1997)WechslerAdultIntelligenceScaleIII.SanAntonio:PsychologicalCorporation.

Winterer,G.,Goldman,D.(2003)Geneticsofhumanprefrontalfunction.BrainResBrainResRev,43,134-163.

Correspondenceconcerningthisarticleshouldbeaddressedto: ProfessorTimothyC.Bates, DepartmentofPsychology,UniversityofEdinburgh,7GeorgeSquare, EdinburghEH89JZScotland,UK.