Studies of twins have shown that behaviour is influenced by our genes to a surprising extent. Lawrence Wright examines the social implications
Psychology has been shaken in recent years by studies of separated twins that indicate that the development of an individual's personality is to a surprising extent guided by his genes, with little regard for the family in which he is raised. Even matters that instinctively seem to be a reflection of one's personal experience, such as political orientation or religious commitment, have been shown by various twin studies to be largely under genetic control.
Partly because of the growth of twin studies, and also of adoption studies, which examine unrelated individuals reared together (and which complement the study of twins reared apart), the field of behavioural genetics has been able to study traits such as criminality, alcoholism, smoking, homosexuality, divorce, job satisfaction, hobbies and fear. The results suggest that there are significant genetic contributions in all cases. Even disciplines such as linguistics and economics have seized on twins as a way of understanding language formation (by looking at twins who create a private idiom), or of calculating the additional earning potential of higher education (by comparing twins who go to college versus twins who do not). There is an air of irrefutability about such studies that makes them very appealing.
When Linus Pauling proposed that Vitamin C could cure the common cold, for instance, twin pairs were separated into two groups, one of which received Vitamin C, the other a placebo. Both caught colds, which destroyed Pauling's theory. There are now so many scientists seeking to study twins that the annual festival of twins in Twinsburg, Ohio, allows researchers to set up carnival tents, where browsing twins can stop to take stress tests or fill out questionnaires about their sex lives. Last year 90,000 people - most of them twins - attended.
All this comes after decades of political struggle between those, on the one hand, who believe that people are largely the same and that differences are imposed on them by their environment, and those, on the other, who conclude that people differ mainly because of their genes, and that the environments they find themselves in are largely of their own making. The roots of liberal versus conservative views are buried in such presumptions about human nature.
This argument has been raging for centuries, with science entering evidence on either side and public opinion shifting in response.
Using twins, scientists can now estimate what proportion of our intelligence, our personality, our behaviour, and even seemingly random life events such as bankruptcy or the death of a spouse, might be caused by inherited tendencies. The broad movement from environmentalism to genetic determinism that has occurred in psychology over the past 30 years has foreshadowed the increasingly popular belief that people are genetically programmed to become the way they are, and therefore little can be done, in the way of changing the environment, that will make an appreciable difference, for example, in improving test scores or lowering crime rates.
In the past 20 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 different personality inventories and the recruitment of an enormous population of twins to the many twin registries around the world. Behavioural geneticists divide up personality traits in various ways, but one of the most accepted paradigms was put forward by the late Hans Eysenck at London's Institute of Psychiatry. Eysenck proposed that the personality could be described along three axes, which he labeled Extraversion, Neuroticism, and Psychoticism. The realm of Extraversion sets the outgoing, impulsive 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 unstable personality. Psychoticism includes gradients of criminality and mental disorders, but also such traits as creativity. Using twin data, researchers have estimated the overall heritability of personality at about 50 per cent - somewhat less than the claims that the geneticists lay for intelligence, but still high. Identical twins who have been reared apart are not much different in various personality measurements than twins reared together.
If genes account for half the development of the personality, then environment must account for the rest. Using sophisticated models to analyse the data, behavioural geneticists were able to ask a new and pertinent question: what, exactly, in the environment shapes personality? Their answer is that the common shared environment - the family, the neighbourhood, the parents' income and education, their way of raising children - has no effect on the development of personality. This is arguably the most surprising and important discovery of behavioural genetics. It is the individual experiences people have that tend to make them different from their family, such as the education they receive, the friends they make, birth order, accidents. This the unshared environment accounts for nearly all the difference that can be ascribed to nongenetic factors.
It could be that some shared environmental effects are invisible to behavioural 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 only apply to the broad middle class. Indeed, studies of black twins in Philadelphia, and of lower-class white twins, suggest that the heritability of intelligence is lower in really deprived circumstances. Still, it is confounding to imagine that most middle-class families are such neutral environments; any gardener knows that small variations of water and fertiliser affect the development of far less complex organisms - are humans so indifferent to the environment they are planted in?
Even more controversial have been twin studies of behaviour. A Virginia study of 1,000 female twin pairs concluded that genetic factors account for about half of the risk for developing problems with alcohol. Behaviours as diverse as smoking, homosexuality, insomnia, choice of careers, use of contraceptives, consumption of coffee (but not, oddly, consumption of tea), menstrual symptoms, and suicide have all been shown to have far higher rates of concordance for identical than for fraternal twins (twins from separate ova), suggesting that such behaviours are more influenced by genes than previously suspected. German studies during the Nazi era implicated criminality as a heritable trait, but the reared-apart studies have shown little evidence to support this thesis, and much to suggest that the environment is largely to blame.
A 1986 survey of Australian twins posed 50 questions about social attitudes and found a significant genetic component for 47 of them, including such diverse items as socialism, the authority of the church, the death penalty, chastity and birth control. Only three motley subjects - co-education, the use of straitjackets, and pyjama parties - showed no meaningful genetic influence. This seemed particularly strange because one would expect the family environment to play an almost overwhelming role in determining social attitudes; and yet, that simply was not found. An especially interesting Swedish study of elderly twins, led by Robert Plomin, a behavioural genticist at London's Institute of Psychiatry and at Pennsylvania State University, looked at life events such as divorce, retirement, death of a child, mental illness of a spouse, and financial reverses - many of which might seem, almost by definition, to be accidents of the environment. The researchers concluded that 40 per cent of the variance of their total life-events score was due to genetic effects. Adding to the puzzle was the fact that twins reared apart were even more alike in terms of their major life events than twins reared together.
Underlying these momentous assertions is an insistent unanswered question: How? Is there a gene for neurosis or alpine skiing or traditional values? Nothing in molecular biology indicates anything of the sort. The assumption of the twin model is that if one controls for the genes, by comparing identical versus fraternal twins, then the differences must be environmental; and further, if one controls for the environment, by comparing reared-apart identicals to those reared-together, then what is the same must be genetic. The logic seems unassailable, but it leads to unanswerable riddles. "Events have no DNA," the authors of the Swedish study conceded; "therefore, genetic factors cannot affect events per se. However, life events are defined as events that happen to people - their experiences. Genetic influence on experiences must be due to genetically influenced characteristics of individuals, not of the environments".
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 behavioural genetics in the West, there has been a broad shift in political philosophy - one that assigns increased responsibility for individual behaviour to the person, not to their family, schooling, or socioeconomic level. This is evident, for example, in the rise of unforgiving criminal sentencing standards and the cutback of social programmes designed to lift people out of deprivation. And yet, both liberals and conservatives remain obsessed with the influence of the environment on behaviour; liberals continue to fight a rearguard action against racial and gender discrimination, the loss of access to education, the retreat of government from providing for social welfare, etc; and conservatives still promote "family values" and seek to restrict government control of the economy. 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.
In other words, we continue to struggle to control our environments, despite the view of human behaviour advanced by behavioural genetics, which suggests that the environment plays a diminished role in creating our personality and intelligence and in determining our behaviour. Perhaps we struggle because the alternative is a kind of social and political nihilism which says that there is little we can do to change our 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 hallmark of liberalism is that changes in the social environment produce corresponding changes in human development. But if people's destinies are written in their genes, why waste money on social programmes? Such thinking has led to a profound conservative shift in the past 30 years. These changes have taken place not only in the West but in many other countries. The widespread retreat of communism in world politics is doubtless linked to the collapse of faith in social engineering, caused by the failure of communism to create the positive changes expected of it.
The genetic idea has had a tumultuous passage through the 20th century, but the prevailing view of human nature at the end of the century resembles in many ways the view we had at the beginning. That is that people are largely responsible for their station in life, and that circumstances do not so much dictate the outcome of a person's life as reflect the inner nature of the person living it. Twins have been used to prove a point, and the point is that we do not become. We are.
Lawrence Wright is a journalist at The New Yorker. Parts of this article appear in his book Twins, Weidenfield and Nicolson, Pounds 14.99.
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