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Unit8NatureandNurtureTwins,Genes,andEnvironmentHeredityorenvironment:whichisstronger?Thepotentialswhichapersonisbornwithdetermineinsomewaywhathewilldoinlife.Thereforeheredityisfate,akindofpredestination.However,genesdonotworkinavacuum;assoonaswebeginconsideringtherolethattheyplayinthedevelopmentoftheindividual,weseethattherecanbenodevelopmentwithouttheinteractingenvironment.Nocharacteristiciscausedexclusivelybyeitherenvironmentorgenes.Therelativeeffectsofheredityandenvironmentaremostclearlyobservableinidenticaltwins.Mostidenticaltwinsareraisedtogetherandareremarkablyalikeinbothappearanceandbehavior.Thesecasesdemonstratethatindividualswiththesamegenes,whenraisedinthesameenvironment,willrespondtoitinmuchthesameway.Theydonotindicatewhatwouldhappeniftheseidenticalindividualswereraisedseparately.Anumberofstudieshavebeenmadeofidenticaltwinsraisedapart.ThetwinswhowerethesubjectsofthesestudieslivedinAmerica,wereraisedinmuchthesamephysicalenvironments,andexperiencedmuchthesamenutritionalhistories.Therefore,asonemightexpect,theymaintainedtheclosestresemblancetoeachotherinphysicalappearance,height,andweight.Exceptionsoccurredwhenonetwinhaddevelopedarathersevereillnessandtheotherhadnot;butonthewholeeveryoneisimpressedbythegreatpsychologicalandphysicallikenessesthatexistbetweenidenticaltwins,eventhosewhohavebeenseparatedfrominfancy.Inastudyofnineteensetsoftwinswhohadbeenseparatedfrombirth,investigatorsfoundthatinapproximatelytwothirdsofthesetstherewerenomoresignificantdifferencesthanexistedamongunseparatedpairsoftwins.Thisstronglysuggeststhepowerofthegenesandthelimitationoftheeffectofenvironment.However,itmustberememberedthat,althoughtheidenticaltwinswhowerestudiedlivedindifferentfamiliesfarremovedfromeachother,theenvironmentsinthosefamilieswerenot,onthewhole,substantiallydifferent.Usuallyeveryeffortwouldbemadetoputeachchildinahomewithabackgroundsimilartothatofitsownfamily,andthereforeitshouldnotbesurprisingtofindthatthetwinsdevelopedsimilarly.Butinthosecasesinwhichtherehadbeenagreaterdifferenceintheenvironmentsoftheseparatedtwins,thedifferencesbetweenthetwinsweremoresubstantial.Thefollowingcaseillustrateswhathappenstoidenticaltwinswhentheyarebroughtupincontrastingenvironments.GladysandHelenwereborninasmallOhiotownandwereseparatedatabouteighteenmonthsofage.Theydidnotmeetagainuntiltheyweretwenty-eightyearsold.Helenhadbeenadoptedtwice.Herfirstfosterparentshadprovedtobeunstable,andHelenhadbeenreturnedtotheorphanageafteracoupleofyears;afterseveralmonthsshewasagainadopted,byafarmerandhiswifewholivedinsoutheasternMichigan.Thiswasherhomeforthenexttwenty-fiveyears.Hersecondfoster-mother,thoughshehadhadfeweducationaladvantagesherself,wasdeterminedthatHelenshouldreceiveagoodeducation;Heleneventuallygraduatedfromcollege,taughtschoolfortwelveyears,marriedattwenty-six,andhadadaughter.GladyswasadoptedbyaCanadianrailroadconductorandhiswife.Whenshewasinthethirdgrade,thefamilymovedtoaratherisolatedpartoftheCanadianRockies,wheretherewerenoschools,andGladys'formaleducationcametoanend,andwasnotresumeduntilthefamilymovedtoOntario.Shestayedathomeanddidhouseworkuntilshewasseventeen,andthenwenttoworkinaknittingmill.ShewenttoDetroitatnineteen,gotajob,andmarriedwhenshewastwenty-one.HelenhadbeenhealthierthanGladys,inchildhoodandadulthood,butotherthanthat,theirenvironmentshadbeenverysimilarexceptfortheireducations.Theirweight,height,haircolor,andteethwereverysimilar.Thedifferencesthatdistinguishedthemwereobviouslyassociatedwiththedifferentsociallivestheyhadled.Helenwasconfident,graceful,madethemostofherpersonalappearance,andshowedconsiderablepolishandeaseinsocialrelationships.Gladyswasshy,self-conscious,quietandwithoutcharmingorgracefulmanners.Ascientistwhostudiedthemremarked,Asanadvertisementforacollegeeducationthecontrastbetweenthesetwotwinsshouldbequiteeffective.Consideringthenatureoftheirenvironmentalexperiences,thedifferencesinHelenandGladysarenotsurprising.Sincepsychologicaltraitsdependsomuchuponexperience,itistobeexpectedthattheywillreflectit.Ontheotherhand,traitsthatarenotliabletobeinfluencedbytheenvironmentaremorelikelytoexhibitahighdegreeofsimilarityinidenticaltwins.Importantastheyare,genesaloneareneverabsolutelyresponsibleforanytrait.Whatwecandoissetbythegenes,butwhatweactuallydoislargelydeterminedbytheenvironment.基因、环境与双胞胎遗传与环境究竟哪一个影响更大呢？从某种程度上讲，一个人生来具有的潜力将决定他一生的作为。因此遗传即命运，是命中注定的东西。然而，基因并不是在真空中发挥作用的；一旦我们开始认识到基因在个人发展中所起的作用，我们就会明白，没有与其相互作用的环境，就不可能有任何个人的发展。没有一个特点是完全由环境或者完全由基因造成的。遗传和环境的相互影响在同卵双胞胎中最易观察到。大多数的同卵双胞胎是在一起抚养长大的，因而无论在外表还是行为上都惊人的相像。这些实例说明，若在同样的环境中抚养长大，具有相同基因的个体就会以几乎同样的方式对环境做出反应。但这些例子并不能说明如果把这些同卵双胞胎分开抚养会发生什么情况。对分开抚养的同卵双胞胎已进行了许多研究。被研究的这些双胞胎都生活在美国，成长的自然环境几乎相同，并且具有几乎相同的营养史。因而，正如所料，他们在外表、身高和体重上极其相似。但也有例外：一对双胞胎中的一个患了相当严重的疾病而另一个却没生病；但是总体而言，同卵双胞胎，甚至从婴儿时就分离的同卵双胞胎，在心理和身体上会如此相似，给每个人留下了深刻的印象。在对从一出生就分离的19对双胞胎的研究中，研究者发现，其中约三分之二的双胞胎之间的差异与一起长大的双胞胎之间的差异一样不明显。这有力地说明了基因的影响力和环境作用的局限性。然而，必须记住的是，尽管被研究的同卵双胞胎生活在不同家庭里且相距甚远，但总体而言，那些家庭的环境基本上没有什么差异。人们通常都会千方百计地将每个孩子安置在和他自己原来的家庭背景相同的家庭里，因而发现这些双胞胎成长中有不少相似之处就不足为奇了。但是在那些分开抚养的双胞胎的生长环境有较大差异的实例中，双胞胎之间的差异就很大了。下面的例子说明了