Read Twins: And What They Tell Us About Who We Are Online

Authors: Lawrence Wright

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

Twins: And What They Tell Us About Who We Are (5 page)

BOOK: Twins: And What They Tell Us About Who We Are
4.1Mb size Format: txt, pdf, ePub
ads
 
Page 21
Aryan race; however, his attempts to change the hair and eye color of the Jews and Gypsies at his disposal suggest that he was also trying to engineer genetic mutations that were scientifically absurd (perhaps his own dark coloring was a spur in this pursuit). One of the prison doctors, Miklos Nyiszli, wrote in his account of Auschwitz that Mengele hoped to discover the secret of multiple births in order to repopulate his depleted nation. There was (and still is) an ongoing debate about the causes of twinning, which were of enormous interest to Verschuer and presumably to his protégé. And yet Mengele never systematically examined the parents of twins, nor did he have the time or the focus to conduct breeding experiments, which in any case would have required several generations of healthy twins who were not submitted to the extraordinary biological stresses of the camp. Despite the testimony of Mengele's colleagues, as well as some of the prisoners, that Mengele was a highly competent researcher, one cannot help but be struck by his scientific naïveté, by the sadism of his experiments, and the wantonness and waste of his precious resourcesthe twins themselves. One can only suppose that the madness of the entire Nazi enterprise was so total that it overwhelmed scientific discipline and crushed ordinary human reason. If his records ever were to be found, they would be unlikely to be of much interest to twin researchers, except as a cautionary example of the perversion of the scientific impulse by political fanaticism.
It is an interesting question to pose, given the Nazi attachment to genetic predeterminism, whether Mengele could have been a good scientist and a decent human being in another political environment. Was he born to be a monster? Or did he simply adapt to a monstrous situation?
 
Page 22
One consequence of Nazism was to discredit genetic theories, and twin studies in particular, for a generation. And in proportion to the fall of genetics was the rise of behaviorism. The behavioristic movement began in the lectures of John B. Watson at Columbia University in 1912, and it quickly produced its own tempestuous controversies. "We have been accused of being propagandists, of heralding our conclusions in the public press rather than in the more dignified scientific journals, of writing as though no one else had ever contributed to the field of psychology, of being bolshevists," Watson recalled in 1930. Watson was doing his work in an age that was convinced of the inheritance of talent, ability, and temperament, that believed in families of genius and families of criminality, that accepted racial differences without question. Watson agreed that physical traits, such as hair color or the length of one's fingers, passed through bloodlines, but the hereditary structure was only waiting to be shaped by the environment; and it might be shaped in a million different ways, depending on the training the child experienced. "Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might selectdoctor, lawyer, artist, merchant-chief and, yes, even beggar-man and thief regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors," Watson wrote, in one of the most famous boasts in psychology. Of course, the social implications of Watson's workespecially concerning race and class differenceswere shattering. If people were merely creatures of their environment, and not of their genes, then society imposed these differences, rather than simply reflecting them. Twentieth-century liberalism was born in the crusades
 
Page 23
for social reform that were spawned in part by these behaviorist ideals. The parenting guides of Benjamin Spock carried the behaviorist philosophy into the family. Generations of parents assumed responsibility for their children's talents and defects. Charles Fries did the same for education, instilling the ideal of universal equality as a goal of standardized schooling. Psychotherapy and self-help books became entrenched features of American culture. All these defining trends of modern society arose from the behaviorist doctrine that environment created individual differences.
Watson confronted the question of identical twins by drawing from the very slim literature on twins reared apart available to him in the 1920s. He cited three cases in which twins who had been separated in childhood and lived in different environments were later tested and found to differ significantly in certain respects: one set of twins showed a marked difference in motor skills, another set differed in personality, the third in intelligence. "Suppose we were to take individual twins into the laboratory and begin rigidly to condition them from birth to the twentieth year along utterly different lines," Watson remarked. "We might even condition one of the children to grow up without language. Those of us who have spent years in the conditioning of children and animals cannot help but realize that the two end products would be as different as day is from night."
Watson found an enthusiastic disciple in a young man named B. F. Skinner. During the war, while Mengele was trying to torture the secrets of genetic behavior out of his captive twins, Skinner was at the University of Minnesota teaching economics to rats (who could "buy" food with marbles), and training pigeons to pilot missiles, hoping they could be used instead of humans to conduct warfare. Skinner had refined Watson's
 
Page 24
principles of behavior to a level of considerable sophistication, as his animal experiments proved, and he hoped to extend his experimental insights into what he called a technology of behavior. He believed that all behavior is genetically based, because we are nothing more than the product of natural selection, but he disputed the notion that there are separate genes for altruism or criminality or any other character trait. What our genes do give us, Skinner reasoned, is the capacity to adapt to our environment. People are not innately good or bad; like any other organism they are determined by their environment. He attacked the notion of individual responsibility. It makes little sense, he believed, to hold people to account for their actions. If one wants to change behavior, then design a different environment. Skinner began the movement toward programmed instruction in public schools, which he expected would cut in half the amount of time required to learn a specified body of material, leading some enthusiasts to predict that eleven-year-olds would soon be earning their Ph.D.'s.
Today, few on either side would argue that we are exclusively the creation of nature or the reflection of nurture. The discussion has evolved into a statistical war over percentages:
how much
of our personality or behavior or intelligence or susceptibility to disease is attributable to our genes as compared to environmental factors, such as the family we are born into or the neighborhood we live in or the years of school we attend. The fulcrum upon which one side rises while the other falls is the concept of heritability, which was first defined by the biologist J. L. Lush in 1940. Heritability, he said, is the fraction of the observed variation in a population that is caused by differences in heredity. Lush was working with farm animals, and he had the
 
Page 25
luxury of doing breeding experiments. It is easy to establish the transmission of traits in plant and animal populations; in fact, it is the basis of selective breeding. In humans, however, matters are more complex. The most common way of measuring heritability in humans is through twin studies. For any trait, the greater the difference in concordance between identical twins and fraternal twins, the greater the heritability. An example is tuberculosis, which is caused by an infectious organism; however, people have differing degrees of susceptibility to the disease. An identical twin has a fifty-six percent chance of getting the disease if his sibling catches it, but a fraternal twin has only a twenty-two percent chance. The impressive difference between these rates demonstrates a genetic factor at work. If a trait is completely heritablesuch as blood type or eye colorthen it will be one hundred percent concordant in identical twins and about fifty percent in fraternal twins and other siblings.
*
The heritability correlation would be a perfect 1.0. If all the differences between siblings are environmental, the heritability would be 0.0.
But environmental factors can also affect traits that are genetically transmitted. Height, for instance, is a highly heritable trait, and in well-nourished Western populations most of the variation in stature is an expression of the genes. The heritability for height among white European and North American populations is about 0.90. This does not mean that if a man is six feet tall, seven inches are due to his environment and the remaining ninety percent to his genes. If an individual is ten inches taller than the average for his population,
*
Heritability depends also on frequencies within a population. For instance, if nearly everyone in a population has brown eyes, then fraternal twins will be nearly as concordant for this trait as identical twins.
 
Page 26
however, one could estimate that one of those inches is probably accounted for by environment and the other nine inches by his genes. Moreover, heritability can vary significantly in different populations, because the genes require a supportive environment in order to be expressed in the first place. A population that exists on a starvation diet would have little variation in height because growth would be arrested; there would be no way of telling who had tall genes and who had short ones. If one group within the population enjoyed an abundant diet while the rest were starving, the variation of height would be largely environmental. Children of Japanese immigrants who are born and raised in North America tend to be taller than their parents but shorter than the North American average, a difference that is attributed to changes in nutrition (more meat in the diet, for example). The North American grandchildren of those immigrants are taller still, which must mean that environmental effects gain an even stronger hold in the second generation.
Nowhere is the argument about heritability more heated or more consequential in its implications than over the question of intelligence. For much of this century, the debate was grounded in the work of Sir Cyril Burt, who was in his lifetime Britain's most honored and acclaimed psychologist. His name has become associated with a bizarre controversy that scandalized the academic world and is still furiously contested more than a quarter of a century after his death in 1971. Burt was a crusty, opinionated, dominating figure, whose political views were slightly left of center, and whose personality was characterized even by his friends as neurotic and occasionally paranoid. He began his career in 1913 as a research psychologist in the London County Council, which put him in the position of being in
 
Page 27
charge of all mental and scholastic testing for the London school system. Over the next several decades, Burt founded child guidance clinics and a special school for the handicapped; he developed important new tests and surveys; he wrote a series of books that became landmarks in the rather new fields of juvenile delinquency (
The Young Delinquent
) and mental retardation (
The Subnormal Mind
and
The Backward Child
), which established his reputation as one of the world's leading educational psychologists. This was despite the fact that Burt showed little caution in betraying his class prejudices (describing, for instance, a lower-class white delinquent as ''a typical slum monkey with the muzzle of a pale-faced chimpanzee").
Early in his tenure, while working with the London schools, Burt began comparing IQ and scholastic achievement scores on twins and other kinship groups, eventually compiling an unrivaled collection of data on heredity and intelligence. In 1966, at the height of his eminence, he published an epochal paper on fifty-three pairs of identical twins who had been reared apart, a surprisingly large sample of such a rare population. He had accumulated the twins over a period of forty-three years. Separated twins are at once an experiment of nature and an experiment of society. Until that time, there had been only three substantial studies in the literature (Neubauer and his colleagues at the Psychoanalytic Institute had just begun their study). In theory, the average differences between MZ twins reared apart and MZ twins reared together should provide an unassailable measure of environmental effects; however, the environments in which the separated twins are reared may be similar in important ways. Burt's study was particularly stunning because he claimed to have analyzed the socioeconomic status of
BOOK: Twins: And What They Tell Us About Who We Are
4.1Mb size Format: txt, pdf, ePub
ads

Other books

An Ideal Duchess by Evangeline Holland
Virginia Lovers by Michael Parker
Figgs & Phantoms by Ellen Raskin
Thrust & Parry: Z Day by Luke Ashton
Ruthless: Mob Boss Book One by Michelle St. James
The Plot by Evelyn Piper
Discover Me by Thereon, Cara