email page print pageAll Topic Reviews
A Companion to GenethicsA Companion to GenethicsA Cooperative SpeciesA Mind So RareA Natural History of RapeAcquiring GenomesAdapting MindsAgeing, Health and CareAlas, Poor DarwinAn Introduction to Evolutionary EthicsAncient Bodies, Modern LivesAnimal ArchitectsAping MankindAre We Hardwired?Bang!BehavingBeyond EvolutionBeyond GeneticsBlood MattersBody BazaarBoneBrain Evolution and CognitionBrain StormBrave New BrainBrave New WorldsChoosing ChildrenCloneCloningConceptual Issues in Evolutionary BiologyConsciousness EvolvingContemporary Debates in Philosophy of BiologyControlling Our DestiniesCooperation and Its EvolutionCreatures of AccidentDarwin Loves YouDarwin's Brave New WorldDarwin's Gift to Science and ReligionDarwin's UniverseDarwin's WormsDarwinian ConservatismDarwinian PsychiatryDarwinism and its DiscontentsDarwinism as ReligionDebating DesignDecoding DarknessDefenders of the TruthDo We Still Need Doctors?Doubting Darwin?Early WarningEngineering the Human GermlineEnhancing EvolutionEnoughEntwined LivesEthical Issues in Human CloningEthical Issues in the New GeneticsEvil GenesEvolutionEvolutionEvolution and Human BehaviorEvolution and Human BehaviorEvolution and Human Sexual BehaviorEvolution and LearningEvolution and ReligionEvolution and the Human MindEvolution in MindEvolution, Gender, and RapeEvolution: The Modern SynthesisEvolutionary Ethics and Contemporary BiologyEvolutionary Origins of MoralityEvolutionary PsychiatryEvolutionary PsychologyEvolutionary Psychology and ViolenceEvolutionary Psychology as Maladapted PsychologyExploding the Gene MythFaces of Huntington'sFlesh of My FleshFrom Chance to ChoiceFrom Darwin to HitlerGenesGenes in ConflictGenes on the CouchGenes, Environment, and PsychopathologyGenes, Environment, and PsychopathologyGenes, Women, EqualityGenetic Nature/CultureGenetic PoliticsGenetic ProspectsGenetic ProspectsGenetic SecretsGenetics of Criminal and Antisocial BehaviourGenetics of Mental DisordersGenetics of Original SinGenetics of Original SinGenomeGenomeGenome: Updated EditionGenomes and What to Make of ThemGlowing GenesHow Women Got Their Curves and Other Just-So StoriesHuman CloningHuman Evolution, Reproduction, and MoralityImproving Nature?In Our Own ImageIn Pursuit of the GeneIn the Name of GodIngenious GenesInheritanceInside the Human GenomeInside the O'BriensIntegrating Evolution and DevelopmentIntelligence, Race, and GeneticsIs Human Nature Obsolete?Language OriginsLess Than HumanLiberal EugenicsLiving with Our GenesMaking Genes, Making WavesMaking Sense of EvolutionMan As The PrayerMean GenesMenMood GenesMoral OriginsMothers and OthersNature Via NurtureNever Let Me GoNot By Genes AloneOf Flies, Mice, and MenOn the Origin of StoriesOrigin of MindOrigins of Human NatureOrigins of PsychopathologyOur Posthuman FuturePhilosophy of BiologyPlaying God?Playing God?Portraits of Huntington'sPrimates and PhilosophersPromiscuityPsychiatric Genetics and GenomicsPsychologyQuality of Life and Human DifferenceRe-creating MedicineRedesigning HumansResearch Advances in Genetics and GenomicsResponsible GeneticsResponsible GeneticsScience, Seeds and CyborgsSex and WarSociological Perspectives on the New GeneticsStrange BedfellowsStrange BehaviorSubjects of the WorldSubordination and DefeatThe Age of EmpathyThe Agile GeneThe Ape and the Sushi MasterThe Biotech CenturyThe Blank SlateThe Book of LifeThe Boy Who Loved Too MuchThe Bridge to HumanityThe Case Against PerfectionThe Case for PerfectionThe Case of the Female OrgasmThe Century of the GeneThe Common ThreadThe Concept of the Gene in Development and EvolutionThe Debated MindThe Double-Edged HelixThe Epidemiology of SchizophreniaThe Ethics of Choosing ChildrenThe Ethics of Human CloningThe Evolution of CooperationThe Evolution of MindThe Evolution of MindThe Evolved ApprenticeThe Evolving WorldThe Extended Selfish GeneThe Fact of EvolutionThe Folly of FoolsThe Future of Human NatureThe God GeneThe Immortal Life of Henrietta LacksThe Impact of the GeneThe Innate MindThe Innate MindThe Innate Mind: Volume 3The Limits and Lies of Human Genetic ResearchThe Lives of the BrainThe Maladapted MindThe Meme MachineThe Misunderstood GeneThe Moral, Social, and Commercial Imperatives of Genetic Testing and ScreeningThe Most Dangerous AnimalThe New Genetic MedicineThe Nurture AssumptionThe Origin and Evolution of CulturesThe Origins of FairnessThe Paradoxical PrimateThe Perfect BabyThe Robot's RebellionThe Selfish GeneThe Shape of ThoughtThe Shattered SelfThe Stem Cell ControversyThe Story WithinThe Stuff of LifeThe Talking ApeThe Temperamental ThreadThe Terrible GiftThe Theory of OptionsThe Top 10 Myths About EvolutionThe Triple HelixThe Triumph of SociobiologyThe Woman Who Walked into the SeaTwinsUnderstanding CloningUnderstanding the GenomeUnnatural SelectionUnto OthersUp From DragonsVoracious Science and Vulnerable AnimalsWar Against the WeakWhat Genes Can't DoWhat It Means to Be 98 Percent ChimpanzeeWho Owns YouWhose View of Life?Why Evolution Is TrueWhy Think? WondergenesWrestling with Behavioral GeneticsYour Genetic Destiny
Who among us has not embarrassed him or herself by gawking at the appearance of identical or monozygotic (MZ) human twins? This visual allure is an apt symbol of the deeper intellectual curiosity that twins hold for the far larger population of "singletons". But as Lawrence Wright indicates in his well-written, Twins And What They Tell Us About Who We Are, the study of MZ twins is not only endlessly fascinating, it also has much to contribute to our understanding of the human.
The most familiar contribution of twin research concerns the ancient question of nature vs. nurture. Plato had mused that each person by nature fell into one of three categories designated by the metals, iron, silver, or gold. His social scheme in The Republic (c. 360 BCE) was constructed to preclude anyone but the golds from ruling. After all, he had seen the damage that democracy, the rule of the irons, had done to his beloved Athenian polis. This Platonic pessimism had receded in the two thousand years before John Locke (c. 1689 CE) confidently declared humans to be tabula rasae, blank slates to be shaped by what experience writes upon them. Lockes view took hold, of course, and became a supporting pillar of western ideology. Liberals rely upon it to trace the etiology of crime, poverty, teen pregnancy, and the like, to class inequalities, while conservatives deny the unfairness of such end-state social differences with claims of equal opportunity, that we all began in the same place. In psychology this tradition became "socialization theory" with the focus upon very early parent-to-child interactions as the prominent contributors of adolescent and adult characteristics. With so much riding upon the tabula rasa assumption we would expect any challenge to arouse the Furies from many sides.
The first important twentieth century challenge to Lockes paradigm in the human sciences came from Noam Chomskys account of language development (for example, Syntactic Structures , 1957). But it has been the new science of behavioral genetics, the formal study of the differential contributions of genotype and environment to human development that has put the socialization theorists on the ropes. The study of identical twins has provided the most dramatic element of this new science. Lawrence Wright is an experienced journalist who tells this part of the story with accuracy and enthusiasm. The ground has been covered earlier and later by Plomins Nature and Nurture (1990), Rowes The Limits of Family Influence (1994), Harris The Nurture Assumption, (1998) [reviewed in Metapsychology March 1999], and journalistically by William Wrights Born that Way (1998).
Lawrence Wrights chapter 4, "The Minnesota Experience", chronicles the reared-apart MZ twin research of Thomas Bouchard, who had been lured from the socialization ranks by Arthur Jensens famous Harvard Education Review article (1969) on the genetics of IQ. As with Jensen, Bouchard had been the subject of political pressures from leftist students and, as Wright notes, his funding from the ultra right Pioneer Fund did not make matters better. In the sixteen years between 1979 95 Bouchard studied 132 identical twins and 102 fraternal twins. Wright highlights some of the quirky cases that Bouchard studied such as the "two Jims", the "giggle twins", and the "Nazi and the Jew". He also summarizes the rather startling results, some of which go to the heart of socialization theorys emphasis upon the causal efficacy of shared (home) environments, "Moreover, there was not a single one of those personality traits in which fraternal twins reared together were more alike than identical twins reared apart."(62) Wrights chapter 5, "The Critics Respond", is a fair representation of the storys other side.
The behavioral genetic study of twin research has vastly enriched and complicated the terms of the nature vs. nurture dispute, and as such has made important contributions to social science methodology. Wrights treatment of the increasing complexity of the debate in his Chapters 9 and 10, "The Environment We Make" and "Beyond Nature versus Nurture", is more sketchy than the topic deserves . Some of these complicating factors are as follows. First, the term "nurture" should no longer be equated with "environment". The former term holds too much association with parenting and with the very young, while very good research seems to indicate that in all but extreme cases such early parenting has little effect. Second, the term "environment" cannot be taken to mean experiences after birth since, as Wright aptly describes, a great deal more than previously thought goes on in utero. Third, the core of parenting research is the measurement of "parenting effects", for example, of "what works and what doesnt work". But the idea of "parenting effects" should not be equated, as the "nature" literature does, only with the direct effects upon the cognitive capacities and personality traits of parent-to-child behavior. A more fruitful concept of "parenting effects" would include the direct effects of parent-to-child crisis intervention (e.g., dealing with childhood depression, anorexia, drug use, teen pregnancy, etc.) which has little to do with affecting a childs cognitive capacities or personality traits. Parenting effects must measure as well the teaching of information and skills (as opposed to the shaping of traits) that a parent can accomplish which significantly broaden the childs range of adult choices. Finally, there are "indirect effects" of parenting behaviors that result from the parents control of a childs social environments. This is a "nurture" factor that even the "nature" side agrees is substantial, but is reluctant to acknowledge as "parenting effects". If the concept of "parenting effects" was broadened in this way the research would of course become more complex, but the slice of an adolescent or adults life that is attributable to parental actions would become far greater.
The final complication involves the idea of "genetic effects". One need not limit the idea of "genetic effects" only to direct genotype-to-trait interactions as one would find with genotype-to-mild retardation. There are also environmental effects that are "induced" by genetic effects. Children whose genes supply musical giftedness are likely to have musical parents who create reinforcing musical environments (passive effects). Musically gifted children are likely to receive strong reinforcements from peers and others, creating environments that enhance their musical accomplishments (evocative effects). Such children are likely to seek out musical peers, thus creating environments that contribute to their musicality (active effects). Behavioral geneticists refer to these as "indirect genetic effects", effectively reassigning a slice of the "nurture" pie to the "nature" camp (Plomin 1990). Wrights treatment of this issue is cursory. Social and behavioral researchers of all stripes and interests can learn a great deal from these contributions to the added complexity of nature vs. nurture as explanatory devices.
There is much more of interest in this volume including new estimates made possible by advanced ultrasound technologies of the frequency of twin conception, the "mystery of lost twins" in utero, conjectures that left-handed singletons are survivors of vanished twin pairs, and more. While preparing this review I had the pleasure of sharing dinner with parents of adult MZ twin girls. The material of this work made for interesting conversation testing its contents against their experiences. The book is recommended to general audiences.
John Mullen is Professor of Philosophy at Dowling College, Long Island. He is author of Hard Thinking: The Reintroduction of Logic into Everyday Life and Kierkegaard's Philosophy: Self-Deception and Cowardice in the Present Age.He is currently working on a book on ethical issues in the family.
To discuss this book or the review you have just read, join the Metapsychology Discussion E-Mail Group by going to this URL: http://www.onelist.com/subscribe/metapsy-discussion