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Plomin,1997).Oneofthemostsignificantattemptstoprovideinformationaboutheredi-
taryprocessesandtheenvironmentwastheTexasAdoptionProject(Loehlin,Horn,
&Willerman,1997).Onthebasisoftheirresearch,theauthorsdeclarethatthemajor
contributortofamilialresemblanceisinthegenes.TheheritabilityofIQisapproximate-
ly70%.Parentalrearingpractices,whicharethefocusofSocialisationTheory,account
fornomorethan10%ofthevariationinintelligence.Otherfactors,suchassocio-
-economicstatus,culturalbackground,parents’educationandoccupationorthenumber
ofbooksinthehomeaccountforlittleornovariationintheIQofadolescentsandadults
(Jensen,1997).Sharedfamilyenvironmenthasasignificanteffect,butonlyatanearly
age;aschildrengrowup,theeffectofthefamilyenvironmentbecomesofminorim-
portance.Astheresearchersconclude,geneticeffectsincreasewithageratherthande-
crease.Bouchard’sreviewofliteratureoncognitiveabilityprovidedabasisforthefol-
lowingconclusion:"NoplausiblealternativetogeneticinfluenceexiststoexplaintheIQ
similarityinmonozygotictwinsrearedapart”and"geneticfactorsarethepredominant
sourceofvariationinadultmeasuredintelligenceinmodernWesternsocieties”(1997,
p.153).Anassumptionunderlyingthisreasoningisthattheprocessofupbringingtakes
placeunderanormalrangeofcircumstances.Acumulativebodyofevidencesuggests
thatgcannotbetrainedandimproved.Asitreflectsindividualdifferencesintheneural
mechanismsofinformationprocessing,itseemstobesusceptibletobiological,notto
pedagogicalinfluences(Jensen,1997).
Jensendirectsattentiontothesecondpossiblesourceofvariation,thatisprenatal
stageofdevelopment,whensmall,random(microenvironmental)eventsoccurinfluenc-
ingneuraldevelopmentandresultingindifferencesinabilities,evenbetweenmonozy-
gotic(identical)twins.Ifsuchsmalleventsbegintosnowball,itcanleadtolowered
abilities(Jensen,1997).Randomnessorluckmightbeconsideredthethirdsourceof
variation.Anotherimportantassumptionisthatpeopleseekenvironmentsfittingtheir
genotype,whichshapestheirabilities.AsJensenpointsout:"Peopleseekoutdifferent
environments,includingfriendsandactivitiesthatarecongenialtotheirnature[…].It
accountsformoreofthetotalvariance(i.e.individualdifferences)thanwasformerly
thought”(1997,p.42).
Quiteoftenthereareremarkabledifferencesinabilitiesobservedbetweenonefamily
memberandtheothers.Anexceptionaltalentcannotbeexplainedbytheadditiveeffects
ofgenesordifferencesinupbringing.Jensenpresentsexamplesofgeniusessuchas
Beethoven,Ramanujanamathematicalgenius,orthegreatconductorToscanini,
whoseparents,siblingsandchildrenrevealednotalentatall.Theirgeniusseemsto
‘comeoutoftheblue’.AsJensenargues,suchexceptionaltalentsarearesultofanunu-
sualandrarecombinationofmanygenes(poligenes),whichsimultaneouslyinfluence
anumberofdifferentabilitiesandtraits.Suchacombinationiscalledemergenesis(Jen-
sen,1997,pp.43-44).Asbothparentsdeliveronlyhalfoftheirgeneseachtotheirchild,
itisveryunlikelythatsucharandomhalfwillincludetherarecombinationofgenesto
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