Twins: And What They Tell Us About Who We Are (22 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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alike than ordinary siblings in intelligence is that they shared the same environment in the womb. Even if this proves to be true, the shared environment they are speaking of ends at birth. We are still left with the question of what in the environment affects us after we are born. Adoption studies have repeatedly demonstrated very little shared environmental influence on cognitive abilityabout thirty percent in studies of children or adolescents, but declining to four percent when adopted children are studied as adults. The battle over these percentage points has been ferocious, and to a disinterested observer it can seem almost comical. The unstated goal of each side of the debate seems to be to lay claim to more than fifty percent of intelligencethus one side wins and the other loses.
In the last twenty years, the argument over nature versus nurture has spread to encompass every aspect of human development. Personality was a late entrant in this contest, largely because no one knew exactly what qualities to measure, or how. That changed with the development of a number of different personality inventories and the recruitment of an enormous population of twins to the many twin registries around the world. Behavioral geneticists divide up personality traits in various ways, but one of the most widely accepted paradigms has been put forward by Hans Eysenck at the Institute of Psychiatry in London. Eysenck proposed that the personality could be described along three axes, which he labeled Extraversion, Neuroticism, and Psychoticism. Each of these dimensions encompasses many common traits. The realm of Extraversion, for instance, sets the outgoing, impulsive, lively personality at one end of the spectrum and the shy introvert at the other. The Neuroticism axis runs from the well-adjusted, stable personality to the anxious, guilty, emotionally un-
 
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stable personality. Psychoticism includes gradients of criminality and mental disorders, but also such traits as creativity. Using twin data, Bouchard and others have placed the overall heritability of personality at about fifty percentsomewhat less than the claims that the geneticists lay to intelligence, but still quite high.
If genes account for half the development of the personality, then environment must account for the remainder. Using increasingly sophisticated models to analyze the data, behavioral geneticists were able to ask a new and highly pertinent question: what, exactly, in the environment shapes personality? Their answer is that the common shared environmentthe family, the neighborhood, the parents' income and level of education, their way of raising childrenhas essentially no effect on the development of personality. Identical twins who have been reared apart are not much different in various personality measurements than twins reared together. This is arguably the most surprising and important discovery of the entire field of behavioral genetics. It is the individual experiences that each person has, such as the education he receives, the friends he makes, birth order, accidentsthe unshared environmentthat account for nearly all the personality difference that can be ascribed to nongenetic factors.
Heritabilities of social attitudes are even higher than for personality: 0.65 for radicalism, for instance, 0.54 for toughmindedness, and 0.59 for religious leisure time interests (the amount of time a person might spend in church or reading religious texts). In fact, identical twins reared apart were actually somewhat more alike in their attitudes than identical twins reared together. Various tests of ability show extremely high levels of heritability: whether raised together or apart, identical twins scored almost as alike as the same person tested twice.
 
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Occupational interests show much lower rates of heritabilityabout 0.36and shared environmental effects are significantabout 0.11especially when compared with the zero effect on personality. The shared environment also plays a significant role in male juvenile delinquency and academic achievement. Taken together, personality, ability, interests, and attitudes make up what geneticists term the behavioral domain.
It could be that some shared environmental effects are invisible to behavioral geneticists because of the people they choose to test. Few twins studied are the products of extreme poverty or highly abusive backgrounds, nor are families with such a history usually allowed to adopt children; therefore, the findings really only apply to the broad middle class. Still, it's confounding to imagine that most families are such neutral environments; any gardener knows that rather small variations of water and fertilizer and soil acidity affect the development of far less complex organismsare humans so indifferent to the environment they are planted in?
Social policy is largely a reflection of what we believe about how intelligence and personality are formed, and why people behave the way they do. It is not surprising, then, that concurrent with the resurgence of behavioral genetics in Western society over the past three decades, there has been a broad shift in political philosophyone that assigns increased responsibility for individual behavior to the individual himself, not to his family, his schooling, his socioeconomic level. This is evident, for example, in the rise of unforgiving criminal sentencing standards and the cutback of innumerable social programs designed to lift people out of deprived circumstances. And yet, both liberals and conservatives remain obsessed with the influence of the environment on society and individual behavior; liberals continue to
 
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fight a rearguard action against racial and gender discrimination, the loss of access to education, the retreat of government from providing for social welfare, and so on; and conservatives still promote "family values" and seek to restrict government control of the economy while expanding legal restraints on certain behaviors, such as abortion and the use of illicit drugs. In the same manner, psychotherapists continue to assume that traumatic childhood experiences create repetitive patterns in people's lives, which can be arrested and changed. Educators continue to impose similar educational standards on children of varying talents. Social workers still work to train mothers how to interact with their children and job-seekers how to gain the skills they need. Employers strive to modify the workplace to accommodate demands for family leave, day care, and educational opportunities for their employees.
In other words, we continue to struggle to control our environments, despite the view of human behavior advanced by behavioral genetics, which suggests that the environment plays a diminished role in creating our personality and intelligence and in determining our behavior. Perhaps we struggle because the alternative is a kind of social and political nihilism that says that there is really very little we can do to change individual lives. Carried to an extreme, this view of human development suggests that the best, and perhaps only, way of improving society is by manipulating the gene pool.
The problem with the portrait of human nature that the behavioral geneticists have drawn for us is that it is at odds with our intuitive understanding of what makes us who we are. Clearly, it makes a difference in a person's identity if he is raised rich or poor, if he is a single child or one of many siblings, if he comes from a rural culture or an urban one, if his parents were happily
 
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married or divorced. Perhaps these differences aren't measurable on the Neuroticism scale, but if you propose that they don't shape human development, most people react with disbelief. Many studies have shown the connection between high IQ and having been raised in an intellectually stimulating home, for instance, or an abusive childhood and later violent behavior. How can these studiesas well as our intuitive understanding of the environmental influence on human developmentaccord with the persuasive studies showing that twins reared apart turn out to be about as much alike on most personality measurements as twins reared together, or that genetically unrelated siblings who are raised in the same family will turn out to be more like their biological parents than their adopted ones?
It is well known that children of divorce are somewhat more likely to become divorced themselves. They do less well in school and tend to have more discipline problems. The cause and the effect appear to be obvious and entirely environmental: children who have watched their parents' relationship fail haven't learned the skills to create a good marriage of their own. The social stigma associated with divorce, along with the financial hardships that typically follow the disintegration of the family and the stress of single parenting, would seem to explain the behavior problems of the children.
And yet, divorce also looks like a genetic disease. It runs in families. If an identical twin gets divorced, the chances that his co-twin will become divorced is about forty-five percent; for fraternal twins, the chances drop to thirty percent. That difference strongly suggests a genetic component to the risk of divorce. And in fact a large-scale 1992 study of Minnesota twins produced what now seems like predictable results: about fifty percent of the liability for divorce can be attributed to genetic fac-
 
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tors, and zero for shared environmental effects. This is despite the fact that the parents' divorce is a striking feature of the shared environment. ''Children whose parents divorce differ genetically from children whose parents do not divorce," the authors, Matt McGue and David Lykken, concluded. They propose that the genetic risk of divorce is a feature of many inherited personality traits, such as impulsiveness and neurotic behavior. Genes that create havoc in the parents' lives are likely to do the same in the lives of their children.
Divorce rates vary among cultures and over time. Of course, geneticists try to compare individual differences within particular populationsthat's why the rate of divorce among Minnesota twins is compared with the average Minnesota rate, which is significantly lower than the national rate. But when geneticists focus on these individual differences within populations, they miss the fact that the cultural environment itself changes over the generations, often quickly and drastically, and divorce is an interesting example. Obviously divorce is going to be more frequent in a society where the laws permit it; it is also likely to occur more often in a highly sexualized culture where little social stigma attaches to failed marriages and where more couples are choosing not to have children. These extraordinary environmental changes that have taken place over the last several generations obviously affect the rate at which couples choose to separate. Granted that some individuals may be genetically more prone to divorce, this changed environment must act as a powerful reinforcement. But it is we who are remaking the divorce environmentthrough our laws and our behavior. Perhaps this compulsive manipulation of our cultural environment is a larger expression of the phenomenon that Sandra Scarr writes about on the individual level: we
 
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make our environment, and our collective genes drive our national experience. Both individually and culturally, the critical environment is the one that we create for ourselves.
To what extent, then, can we really say that genetic or environmental factors create divorce? We all know that it is easier and more acceptable to be divorced these days in most Western countries, and therefore divorce rates will be higher than in the past or in other cultures. Powerful environmental factors, such as religion, can also hold in check presumed genetic vulnerabilities. Perhaps we make changes in our culture in order to give expression to our genetic longing, whether it be for good or ill. The point is that even at this larger level, genes and the environment interact in a way that makes it difficult, and rather arbitrary, to parcel out percentages based on heritability correlations. Moreover, the entire process of evolution is one of genes adapting to the environment. In the larger sense, we do not make the environment; it makes us.
On the individual level, genes and the environment are similarly fused. The fact that experience itself appears to be partly heritable must be an expression of this complex entanglement. An example of this is that identical twins rate their parents more similarly for warmth than fraternal twins. This might be explained if we assume that identical twins are treated alike. Also, one supposes that parents who rate high in Extraversion and low in Neuroticism will provide warm and loving homes, so that the environment is only a natural expression of the parents' genetic disposition. What confounds this explanation is that identical twins raised apart also rate their experience of parental warmth very similarly, even though they have been raised by different people. Apparently, each twin evokes an experi-
 
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encein this case, parental warmthjust by being who he is. But the experience of being raised by warm and loving parents is a significant environmental influence. So how can it be portioned out on one side of the ledger or the other? A starker example is a small study of adopted children whose biological parents are psychiatric patients. There was a significant association between the psychiatric status of the biological parent and the erratic rearing behavior of the adoptive parents. Presumably the inherited antisociality of the children elicits harsh and capricious parenting on the part of the adoptive parents. This supposedly environmental factor may in turn aggravate the antisocial behavior of the children. A number of factors that are usually described as environmental, such as the level of intellectual stimulation in the home, exposure to stress and trauma, and parental marital discord, also appear to be partly heritable. That is to say, children may elicit these environments whether they are raised in their biological home or an adopted one. But these are all the kinds of circumstances that environmentalists traditionally have pointed to as being powerfully formative in the creation of personality.
Even as different environments may produce (in the case of identical twins raised apart) similar personalities, the same environment may cause different reactions in different individuals. A study of more than four thousand pairs of twins who were veterans of Vietnam showed that those who were subjected to higher levels of combat were more likely to show signs of post-traumatic stress disorderin other words, the environment creates the disorder. However, even among individuals who experienced similar levels of combat, the levels of PTSD varied widely. Identical twins were more alike in their reaction to combat stress than fraternal

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