Behavioral geneticists try to untangle that question, never in a single person but in populations. They arrive at heritability percentages in those groups through statistical analyses of correlations. They measure variation in genetic similarity. The difference in “variability” Summers mentioned in relation to sex, therefore, is a statistical measurement, something an economist would understand well. The gold standard for such measurements are identical (monozygotic) twins as opposed to fraternal (dizygotic) twins because the former have the same genetic material as each other and the latter do not. If twins grow up together, they are believed to share the same “environment” as well as the same (or different) genes. When there are similarities between identical twins who share 100 percent of their genes and who grew up in a shared environment, as opposed to fraternal twins who do not share all the same genes but also grew up together, then the argument goes that the similarity between the identical twins can be attributed to genes. Therefore, through a series of calculations, it is possible to come up with a general number between 0 and 1 of what percentage, to put it crudely, is “nature” and what is “nurture.” Or, to be more exact: these numbers are intended to represent the proportion of phenotypic variance in a given population due to genetic variance. A heritable score of .50 means that half the variation in the distribution of a trait in the population the scientists tested is correlated with “biology” and half with “environmental” factors.
Everyone has read pleasing stories about separated identical twins, who, when reunited years later, turn out to both love the color red, sing Verdi in the shower, and buy the same brand of toothpaste. “Two middle-aged male identical twins, separated for a good portion of their lives, both worked as telephone linemen and both had wirehaired fox terriers named Trixie.”81 This story appears in a book called Born That Way (1998) by the journalist William Wright. Although unable to explain how genes determine job choice and dog naming, Wright is persuaded that they are hard at work. The twins appear to confirm the idea that what really matters in life is your parents’ genes—“the moment of conception”—not how you were brought up.
As with many twin studies that claim to track “separated” twins, these identical twins had spent time together and already knew each other. Had one lived in Tanzania and the other in Sweden, the chances of either of them having terriers named Trixie would arguably have been very low. The unstated time these twins did spend together may have been decisive when it came to jobs and dogs. I don’t know. The rhetorical device used here is important, however. The twins may diverge from each other in many ways, may have different tastes and habits in other matters, but citing two remarkable likenesses achieves its goal: they’re the same and they were raised apart! What I do know is that such coincidences do not tell us much about human development, and the statistics of behavioral genetics tell us nothing about a single pair of twins.
Scientists have to make a blanket assumption that all twins who grow up in the same house with the same parents have the same “environment,” even though that may not be true. Behavioral geneticists would argue that for their purposes it is broadly true. Individual cases do not matter. In a textbook chapter on the subject, the authors articulate the ambiguities involved in measurements of heritability:
In most studies in behavioral genetics the effects of gene-environment correlation and gene-environment interaction cannot be estimated separately and are included within the estimate of heritability. This means that even when a trait is highly heritable, environmental influences may still be important in mediating the effects of the genes on behavior. For example, in the case of gene-environmental correlation, if exposure to friendly company were correlated with a person’s genotype, sociability could be found to be heritable even though it may have arisen as a result of increased exposure to company.82
In other words, there is murkiness about causes. There are also differences in how the unit “gene” is defined. As I have shown, for molecular biologists the gene is a complex, dynamic cell- and tissue-dependent entity—a kind of soft, moving target. In molecular genetics the relation between a gene and a psychological or behavioral trait is extremely difficult to establish. How is the gene used by behavioral geneticists? The gene becomes “a simple calculation unit segregating in the population.”83 It is an abstraction, not a unit connected in a specific way to DNA. The person studying molecules close-up will see them differently from a person who is far away looking at twins in population studies and making statistical calculations. Moreover, each of those persons will employ a different definition of the word “gene.” The “gene” mutates at the level of language as well.
Despite the many stories of uncanny likeness, monozygotic twins can develop very different morphological and psychological traits and many suffer from different illnesses during their lifetimes. A story about identical twin boys that appeared in the popular press demonstrates one such divergence. One of the two boys began his life as Wyatt but is now Nicole.84 The other twin remained a boy. Although the two grew up together in the same house with the same parents, they now have different gender identities. What is interesting about this story is not what it tells us about heredity and environment. I don’t think that is at all clear, but rather that it pushes us to ask whether the twins are still more alike than different or more different than alike. For the twin who now has an altered body, the change aligned her internal reality, idea, or belief with external appearance. Her identical twin, however, had no desire for such a change. The unanswered question is, how does such a desire come about?
An Excursion into the Lives of Johnny and Jimmy and a Scientist Named Myrtle McGraw
Myrtle McGraw (1899–1988) studied infant motor development in the Normal Child Development Study of the Babies’ Hospital, Columbia-Presbyterian Medical Center in New York City. In 1930, well before the discovery of the structure and function of DNA, she began a study that garnered huge press coverage—research on a pair of identical twins, Jimmy and Johnny. The study was conducted when the debate between heredity and environment was at high ebb, a conflict personified in two well-known figures, Arnold Gesell and John Watson. Gesell believed that developmental changes in a child were chiefly due to aging or maturation, not learning or life experience. Watson, a famous behaviorist, took the opposite view. As McGraw tells it, Watson once declared that if he were given a dozen healthy infants, he could make them into whatever he wanted—“doctor, lawyer, artist, merchant, chief, and, yes, even beggar-man and thief.”85 McGraw was in neither camp, which didn’t prevent her twin study from being perceived as a struggle between heredity and environment.
Through her research and her interdisciplinary knowledge of neurology, embryology, biology, and psychology, as well as the influence of the American Pragmatist philosopher John Dewey, whom she called her “intellectual godfather,” McGraw came to view child development as a holistic, organic, nonlinear process. Children did not progress stage-by-stage or step-by-step, as Gesell argued, but leapt forward and then regressed, only to move ahead once again. She maintained that child development was a continuous, seamless interaction between neural and behavioral growth processes, which could not and should not be reduced to either heredity or environment. She actively resisted dichotomies, but she understood that they were rooted in a lasting, stubborn philosophical hierarchy:
The tendency to classify and dichotomize did not arise with the biological and behavioral sciences. We inherited it from the Greek philosophers—they polarized the “rational” and the sensual as if they were separate and distinct entities. They gave high value to the rational. That value has been dominant for the most part in Western cultures and research down through the centuries. Perhaps that is one reason for the long delay in the study of infancy since it was claimed the infant and the young child were incapable of reasoning.86
McGraw’s view of child development is in tune with Waddington’s embryology. In 1946, she wrote, “Whatever the gene produces is contingent upon the gene co
mbinations and the conditions under which the whole organism develops.”87 But as Donald Dewsbury emphasizes in an essay on McGraw, the scientist also understood the role context played at other levels, including language. She understood that scientific concepts did not exist in nature but are terms created by consensus, and their necessarily fluid character meant they could be refined and improved over time.88
What amazed the press, however, was that Johnny, the twin who received physical training from McGraw and her team, scaled steep inclines when under a year of age, became a highly proficient roller skater at fourteen months, could swim twelve to fifteen feet with his face underwater at the same age, and could arrange towers of large blocks, climb them, and reach a lure (a piece of banana or cracker) hanging from the ceiling. Both boys walked at the same time, and no one bothered to report that although McGraw placed Johnny on a tricycle at an early age (in an attempt to teach him a seemingly less complex action than roller skating) and continued to do so for seven long months, the little guy did not get the hang of it until he hit nineteen months, when, as she puts it, the light “dawned.”
McGraw made a thirteen-minute film of the two boys from two weeks to twenty-two years for a presentation in 1958 that vividly demonstrates baby Johnny’s remarkable accomplishments. The adult twins appear at the film’s end in a series of brief clips. One at a time, Jimmy and Johnny are seen rising from a seated position on the ground to a standing position, walking, jumping, trying to negotiate a tightrope, and climbing a highly unstable ladder to a roof and down again. Johnny, leaner and more muscular than his twin, is also visibly more coordinated. Whether his bodily grace is due to his early training is another question.
Not surprisingly, the press coverage of the twins had little to do with the science and, rather quickly, journalists jumped to defend the untrained Jimmy. The New York Times presented McGraw’s research as a contest between “just plain Jimmy who was brought up in the same manner as any normal boy” and “Scientific Johnny” who had received “university” training. The special boy was pitted against the regular kid, an opposition that played to American egalitarian and anti-intellectual prejudices. When the boys were nine, the Times ran this headline: “Johnny Woods Only a Little Above Average in Studies—Brother Who Had Untutored Infancy Gets Almost Perfect Grades.”89 Although it is not strange that Johnny’s accomplishments attracted attention or that the press lusted after the sensational rather than the thoughtful, the media urge to “dichotomize,” an urge McGraw resisted so strongly, is depressing.
Twins, especially identical twins, are fascinating because it is as if a single self is doubled. A person’s mirror image comes to life and walks around as if by magic, and, as Galton supposed, the pair seems perfectly destined to test the problem of the inborn and the acquired. Sorting available data on many sets of twins and arriving at a heritable score of .50 for a personality trait—shyness, for example—in a population and the long-term scrutiny of a single pair of twins, as in McGraw’s study, employ methods so different, comparing them is effectively useless.
Both Jimmy and Johnny were “normal,” reasonably contented children, who were well treated by their parents at home and by McGraw at the clinic. A psychiatrist who knew and followed Jimmy and Johnny from their infancies recorded insights about their personalities. McGraw includes a fascinating excerpt from Dr. Langford’s report in a 1939 paper. He noticed that both boys had a lisping style of speech, but otherwise the children were very different. The untrained twin, Jimmy, was the dominant brother of the two at home. Langford uses the word “leader.” He writes that Jimmy is the twin who “bosses” his brother around and tattles on him, despite the fact that his parents discouraged this behavior. McGraw is typically more blunt. She refers to Jimmy as “rather the bully,” while acknowledging that the relationship between the two boys is warm nevertheless. A chatterbox prone to tantrums as a toddler, Jimmy was closer to his mother than Johnny, liked helping her with chores, and frequently left his own bed at night to crawl in with her. Johnny was the quieter boy, restless at home, not helpful to his mother, sucked his thumb, and wet his bed. None of these traits was observed in him while he was at the clinic. Everyone involved with the twins acknowledged that Johnny’s position at the clinic as the active, trained boy led to compensations for Jimmy at home. Langford writes,
Jimmy is the outgoing, helter-skelter type of child who lives for the moment. Johnny is the more serious, thoughtful and contemplative youngster who looks to the consequences before he acts. Jimmy reacts mostly to external stimuli, Johnny, as a result of his more active inner life, reacts in a less direct manner to external stimuli and frequently gives the impression of preoccupation . . . The differences in their personalities may well be largely constitutionally determined and not entirely the result of their diverse earlier experience. However, one feels that these experiences are of importance. I should feel that the home attitudes were of great importance, as well as Johnny’s “conditioning,” and they do seem a little easier to evaluate.90
Langford is nothing if not cautious. He observes, records, then suggests but does not conclude anything certain about the effect of Johnny’s training on his psyche. Johnny was the weaker twin at birth. Was Johnny’s rich inner life in comparison to his brother’s related to his experiences at the clinic or to his somewhat more difficult home life? How did mastering skating and swimming and moving big block pedestals around a room and climbing them alter his mode of thinking, his cognitive abilities, and his personality? McGraw’s honesty on this point is exemplary: “To evaluate the extent to which these personality differences are constitutional or determined by their early experimental experiences or their later home environment, is beyond achievement at the present time and will probably remain beyond the scope of actual determination for all time.”91 How exactly would one untangle one from the other when faced with two living, breathing human beings?
In the film of the twenty-two-year-old men, I was struck by Jimmy’s hesitancy and awkwardness and by Johnny’s pleasure and confidence as the two young men were put through their paces in the demonstration. I doubt these brief images give us a deep view of either twin’s personality as grown-ups. I wondered instead if, when asked to perform again for his once dear, indeed beloved ally, Myrtle McGraw, Johnny relived what must have been his mostly forgotten conquests (he was under three for most of them), which nevertheless had made him a famous baby and were recorded on film. And I wondered if Jimmy found himself suddenly returned to his lower status as the untrained twin at the clinic and betrayed this in his body language. It is impossible to know for sure.
It is obvious that behavioral geneticists would have to acknowledge that although the two twins had the same parents and same home, they had different early environments. They became different men, but then, so do many twins without any clinical intervention. The problem in the statistical research involves questions of sameness and difference and how they are defined and measured. What should we take away from these stories and statistics, and to what degree are both particular cases and population numbers meaningful? Statisticians are not interested in development, in charting the changes of a person or persons in time. The nuanced narrative of human growth must be left out of the calculation. Twins become a list of factors broken down into fixed categories, which are finally assigned a number value.
Number Certainties?
What is certain is that the percentage numbers in behavioral genetics do not accurately represent genetic causes. According to the Stanford Encyclopedia of Philosophy: “The current consensus among philosophers of biology is that heritability analyses are misleading about the genetic causes of human traits.”92 In a review of How the Mind Works, the biologist Jeremy Ahouse and the computational linguist Robert Berwick note Steven Pinker’s substitution of the word genetic “cause” for “correlation” as inaccurate: “Pinker’s assertion is simply the authority of modern science pressed into the service of speculative fictions—truly biology as ideology.�
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Ahouse and Berwick may be viewed as two in a chorus of critical voices. Their review of Pinker is openly hostile, and I suspect their anger is in part “caused” by the rhetorical style of How the Mind Works. Since no one actually knows how the mind works, the title itself may be read as a case of overconfidence. Like the philosopher Daniel Dennett’s Consciousness Explained, a book title that suggests the author has penetrated the mystery of mysteries, what has been called “the hard problem” in contemporary analytical philosophy and neuroscience, How the Mind Works announces the writer’s assurance, warranted or not. In striking opposition to the careful tone of McGraw’s studies, which avoid overstatement at every turn, these are works with titles that swagger. A glimpse at covers emblazoned with such bold declarations tells the reader that the books’ authors are pretty darned certain about whatever it is they have to tell us, and although irritating to some readers, the pugnacious confidence represented by these titles is attractive to many others. The heroic male figure ready to fight his adversaries has considerable charm. Can style be separated from argument, form from content? Is there anything wrong with a sweeping declaration that is, after all, simply asserting one point of view among many? Is there anything wrong with making your case with all the force you can muster, even if you clearly state you are describing a “consensus” among behavioral geneticists, as Pinker does, and therefore substitute the word “cause” for “correlation,” perhaps because you are so convinced that genes are causing a good part, say half, of our personalities?
A Woman Looking at Men Looking at Women Page 23