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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.
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