Twins: And What They Tell Us About Who We Are (21 page)

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Authors: Lawrence Wright

Tags: #Family & Relationships, #Siblings, #Science, #Life Sciences, #Genetics & Genomics, #test

BOOK: Twins: And What They Tell Us About Who We Are
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of mental development observed between MZ and DZ twins: the two types start out life being almost equally alike but diverge as they pass through childhood, the MZ twins becoming even more similar and the DZ twins going their separate ways.
One lesson from the adopted-adolescent study seemed to be that genetic differences cause individuals to respond differently to similar rearing conditions. Another interesting revelation was that adopted children raised in rural or working-class homes did not differ significantly from adopted children raised by professional parents. From these two findings, Scarr concluded that black and white children were essentially alike in their inherent intelligence or their ability to achieve in school, provided that they were given realistic opportunities to become a part of the culture of the tests and of the schools. Social-class differences among whites, however, were largely attributable to genetic differences. As long as children in a population are reasonably nurtured, Scarr observed, the individual differences between them must be genetic. Therefore, efforts to improve intellectual or academic performance should concentrate on rescuing those who were living on the far margins of society, who were genuinely deprived and unable to gain the skills or knowledge needed to compete in the mainstream culture.
Over the last fifteen years, Scarr has been refining a new theory of development, based largely on her conviction that environments do influence the intellects of young children. At early stages of life, she observed, enriched environments, such as day-care centers with stimulating programs, can boost a deprived child's achievement. So environments can make a difference in the intellects of young children. Even young children, however, are genetically programmed to create certain expe-
 
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riences for themselves. For instance, a smiling, gregarious baby is more likely to be cuddled and petted than a fussy and undemonstrative one. If these two dissimilar infants are siblings, their experiences of living in the same home can be quite different. As children mature, they gain more and more control over their environment and actively select from the superabundance of opportunities those that conform to their genetic disposition. The distinction between genes and environment becomes less and less clear. "The dichotomy of nature and nurture has always been a bad one, not only for the oft-cited reasons that both are required for development, but because a false parallel arises between the two," Scarr wrote (in collaboration with her student Kathleen McCartney) in the 1983 journal
Child Development
. "We propose that development is indeed the result of nature and nurture, but that genes drive experience. Genes are components in a system that organizes the organism to experience the world."
That would explain why MZ twins become more similar over time and DZ twins less so. Identical genes compel MZ twins to experience the world in a similar manner, thus reinforcing the similarities of their natures; whereas the genetic variation of DZ twins awakens different interests and talents, which inevitably pull the twins apart into more distinct individuals. Identical twins who have been reared separately may live in different families, even in different cultures, but they evoke similar responses from their environment and are disposed by their natures to make similar choices and to build similar niches for themselves.
In this school of thought, environment and genes are not separate, countervailing forces. It may not even make sense to parcel out traits, such as IQ or features of personality, into percentages of heritability versus
 
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environment, because after one reaches a certain age the environment is itself a heritable reflection of one's genetic disposition. We make our environments, rather than the other way around; that is, as long as the environment we find ourselves in is not so deprived or abusive that normal development cannot occur. "Good enough" parents, who provide an average environment to support development, will have the same effects on their children as "superparents" who press upon their children every cultural advantage. "In this view," Scarr argues, ''human experience is the construction of reality, not a property of a physical world that imparts the same experience to everyone who encounters it."
Consider the example of major depression in women, which is known to run in families, but which intuitively seems to be a response to personal conflicts, emotional traumas, or past family problems: environmental influences, in other words. A study by Kenneth S. Kendler, a psychiatrist, and his colleagues at the Medical College of Virginia examined more than 1,000 female twins, both fraternal and identical. As expected, identical twins were more alike in their susceptibility to depression than fraternal twins, but the genetic factor was not overwhelming. The scientists calculated the heritability of major depression ranged between thirty-three and forty-five percent: it was strongly influenced by genetic factors, in other words, but not preponderantly so. Similar rates of heritability have been found for heart disease, stroke, and peptic ulcers, and far higher rates for schizophrenia, hypertension, and bipolar illness. In the case of depression, environmental experiences are more clearly controlling than genes.
But what kind of experiences?
Behavioral geneticists break down the vast term "environment" into two parts, shared and non-shared. The
 
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shared environment is what twins or other siblings have in commonspecifically, their family, neighborhood, church, social status, the child-raising techniques of their parentsthe home matrix that they grow up in. The non-shared environment is whatever those siblings do on their own. If one takes maths and the other poetry, those are non-shared experiences, even though they may attend the same school. If one breaks an arm playing baseball and the other falls in love with a cheerleader, those are non-shared experiences, despite the commonalities of childhood accidents and teenage romances. Even certain biological differences, such as might be caused by birth trauma and hormonal imbalances in the womb, would be marked as features of non-shared environments between identical twins. Between other siblings, differences in sex, age, birth order, and treatment by the parents would all be non-shared environments.
Kendler and his colleagues discovered that the kind of shared environments one would intuitively expect to lead to depression in later lifesuch as the early loss of a parent, the social class one was reared in, the parental child-rearing practicesplays at best only a very modest role. This confirmed findings by Bouchard's separated-twins studies that the contribution of the common family environment to the development of personality is virtually nil, and Scarr's adoption studies that the family accounts for about five percent of measurable personality traits. It is the non-shared environment that causes depression. The distinction is important because the non-shared environment is the life we lead on our own. It is what happens to us as individuals. It is the life we choose for ourselves, not the life our parents make for us.
"The statement that parents have few differential effects on children does not mean that not having parents
 
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is just as good as having parents. It may not matter much that children have
different
parents, but it does matter that they have parent(s) or some supportive, affectionate person who is willing to be parent-like," Scarr said, in a 1991 presidential address to the Society for Research in Child Development. "To see the effects of having no parents (or parent surrogates), one would have to return to the orphanages of long ago . . . or see children trapped in crack houses of inner cities in the United States, locked in basements and attics by vengeful, crazy relatives. Really deprived, abusive, and neglectful environments do not support normal development for any child." Despite these distinctions, Scarr's speech was attacked by some developmental psychologists and others who fought her election and bitterly oppose her views, believing that they discourage efforts to improve the welfare of childrenespecially black childrenand fail to hold parents accountable for their children's behavior. In fairness, Scarr concedes that parents can have effects on children's motivation and self-esteem, but she insists that, beyond a minimum level of nurturing, they have little measurable impact on intelligence, interests, and personality.
 
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10
Beyond Nature versus Nurture
Twins threaten us because they undermine our notion of identity. We think we are who we are because of the life we have lived. We think we shape the character and values of our children by the way we raise them. We think that we are born with the potential to be many things, and to behave in an infinite variety of ways, and that we consciously navigate a path through the obstacles and opportunities that life presents us with, through a faculty we call free will. But when we read about twins who have been separated at birth and reunited in middle age only to discover that in many respects they have become the same person, it suggests that life is a charade, that the experiences that we presume have shaped us are little more than ornaments or curiosities we have picked up along the way, and that the injunctions of our parents or the traumas of our youth that we believed to have been the lodestones of our character may have had little more effect on us than a book we may have read or a show we may have seen on television. The science of behavioral genetics, largely through twin studies, has made a persuasive case that much of our identity is stamped on us from conception; to that extent our lives seem to be pre-chosenall we
 
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have to do is live out the script that is written in our genes.
But this view, for most people, seems stark and frightening and full of dire political and philosophical consequences. If we are only living out our lives like actors reading our lines, then the nobility of life is cheapened. Our accomplishments are not really earned, they are simply arrived at. Our failures are just as expectable. We are like genetic rockets, programmed to travel in a set direction with a given amount of fuel. Barring some accident of fate, our trajectory is predeterminedwe're just along for the ride.
Moreover, if some of us are more inclined toward sinhowever it may be definedbecause we are genetically more violent, more lustful, more greedy, or less religious, then theology has new questions to ask itself. Are some people "chosen" because they are born meek and devout? What does free will mean if nature has already shortened the spectrum of opportunity within ourselves?
Since Galton's first twin study more than a hundred years ago, there has been a nonstop debate over the relative contributions of genes and environment to the creation of intelligence. The question of what constitutes intelligence has dogged the debate from the start. Galton himself developed a primitive test of mental abilities, based on the speed and sensitivity of sensory perceptions. In the early part of the twentieth century, Alfred Binet, a psychologist at the Sorbonne, constructed a series of tests designed to measure a person's ability to identify patterns, draw analogies, and solve problems. He assigned each of the various tasks involved in the test a "mental age," which was the youngest age a child of normal intelligence could be expected to accomplish the task successfully. When the
 
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mental age was divided by the chronological age, the concept of the "intelligence quotient" was born.
By testing twins and other family members, it was possible to demonstrate that IQ levels were heritable.
How
heritable became one of the great debates of the new century. The eugenics movement, which was to some extent the child of IQ testing, made the assumption that genes were almost entirely responsible for intelligence and that environmental influences were negligible. By the 1960s it was clear that such an assumption was unsubstantiated by the data; moreover, the world had suffered the hideous consequences of the unbridled hereditarian viewpoint. The study of human genetics was colored by American racism, British classism, and Nazi ethnic madness. As eugenics fell into disrepute, extremists in the environmental school began to trumpet their own bias, which was that nearly all of our intelligence is the result of conditioning and that genes play little, if any, role.
This argument also failed, in part because of the emergence of more rigorous and intensive studies of identical twins raised apart. These studies generally placed the heritability of intelligence at about 0.75; however, other kinship and adoption studies, as well as different models of evaluating the data, arrive at lower figures, between 0.40 and 0.60. A 1990 analysis of various kinship studies calculated the heritability of IQ to be 0.51. However, in other studies the heritability of IQ appeared to increase as the twins grew older, which would make the figure derived from studies of twins reared apart more plausible. Shared environmental effects, on the other hand, appear to account for twenty to thirty percent of the variance in IQ, with unshared environment accounting for a larger portion. Some prominent critics of the hereditarian view have proposed that the reason twins are more

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