Whenever the Twain Shall Meet
By: T.J. Bouchard
From: The Sciences, September/October, 1997
J. Geffen
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1.
They arrived at the Minneapolis airport from opposite ends of the United States.
Although they had only spoken on the phone a few times, they hugged, laughed and
cried when they met. As they walked down the airport corridor to the baggage claim, I
followed along, struck by the remarkable similarity in the pace of their speech. As
soon as one stopped talking the other would start, like a pair of jugglers passing
oranges to and fro, and yet the conversation never seemed to stumble or fall out of
sync. My wife and I had been married for well over twenty years, I remember
thinking, and we could not carry off such a highly coordinated exchange. And the
same was true of my conversations with two old, close friends.
2.
I was astounded. It had never occurred to me that the pace and rhythm of speech
might have a genetic basis. And yet the conclusion was inescapable, for these two
people were identical twins, raised in separate homes, who were reuniting for the first
time. “This is amazing,” one of them turned to tell me when we reached the baggage
counter. “It’s as if we’ve known each other all our lives.”
3.
In the past two decades I have heard those words time and again. As the founder
of the Minnesota Center for Twin and Adoption Research at the University of
Minnesota in Minneapolis, I have studied hundreds of pairs of twins, some of them
fraternal, or dizygotic, some identical, or monozygotic, some raised in the same home,
some separated at birth. The most striking of the pairs are identical twins who have
been raised apart. Although identical twins are true human clones, born with exactly
the same genes, their similarities are often ascribed to the fact that they are dressed the
same, treated the same, escorted through childhood side by side. But twins who have
grown up in different families deflate such arguments. They represent a crucial
control in twin studies – an elegant tool for prying apart the relative effects of nature
and nurture on psychological traits.
4.
When we began our work, only twenty-five pairs of such twins had ever been
reunited and studied in the United States. Today more than eighty pairs of identical
twins and almost sixty pairs of fraternal twins reared apart have been studied in our
program alone. They have submitted to dental X-rays, blood tests and psychiatric
interviews, to medical examinations and fingerprinting, and to questionnaires and
ability tests so exhaustive they take nearly fifty hours to complete. The resulting
studies are indisputably important on scientific grounds alone, and they have an
undeniable appeal to the public. But since the recent breakthrough in cloning, they
have taken on an added significance.
Whenever the Twain Shall Meet / 2
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5.
In literature, in films, and in some of the more hysterical accounts in the press,
clones have been portrayed as anything from soulless automatons to evil
doppelgängers. No one really knows how true clones would behave, such accounts
suggest, so the imagination can have free rein. But the truth is, of course, that
biological human clones have existed as long as humanity itself. And though twin
studies have demonstrated the power of genes to shape psychological traits, they have
also demonstrated the limits of that power. Any discussion of future clones, in other
words, ought to begin by revisiting the clones already among us.
6.
The scientific study of twins and their similarities and differences dates back to
the nineteenth century and the work of the English polymath Sir Francis Galton.
Although Galton held no formal position or degree, he was a brilliant explorer and
thinker who invented fingerprinting, among other accomplishments. Galton never
studied identical twins reared apart, but his work on twins reared together, adopted
children and ordinary families, as well as the statistical tools he developed, helped lay
the foundations of behavioral and biometrical genetics and eugenics. Charles Darwin,
in his book The Expression of the Emotions in Man and Animals, cites one of Galton’s
anecdotes:
A gentleman of considerable position was found by his wife to have the
curious trick, when he lay fast asleep on his back in bed, of raising his
right arm slowly in front of his face, up to his forehead, and then dropping
it with a jerk so that the wrist fell heavily on the bridge of his nose…
Many years after his death, his son married a lady who had never heard of
the family incident. She, however, observed precisely the same peculiarity
in her husband; but his nose, from not being particularly prominent, has
never as yet suffered from the blows…. One of his children, a girl, has
inherited the same trick.
7.
Similar stories are sometimes told about siblings or fraternal twins, but much
more frequently about identical twins. In fact, the pair I mentioned earlier (whom we
nicknamed the Talking Twins) are similar in an astonishing range of psychological
traits, as are most other identical twins reared apart. A pair we nicknamed the Giggle
Twins laughed more than anyone else they knew, never engaged in controversial talk
or voted, and had a habit of pushing up their noses with their fingers. Another pair,
alone among the hundreds studied, independently refused to enter an acoustically
shielded room in our psychophysiology laboratory (both acquiesced when the door
was wired open). Still another pair discovered they both used Vitalis hair tonic, Lucky
Strike cigarettes, Vademecum toothpaste and Canoe shaving lotion. We have had two
captains of volunteer fire departments, two gunsmith hobbyists, two women who
would only enter the ocean backward and then just up to their knees, two men who
left love notes around the house for their wives.
8.
Some of those similarities are surely coincidental – complete strangers at
cocktail parties routinely discover “astonishing” concurrences in their lives; imagine
Whenever the Twain Shall Meet / 3
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what they might find after fifty hours of filling out questionnaires. But some
similarities are too idiosyncratic, too particular, to be dismissed as mere coincidence.
The Necklace Twins, for instance, had the unconscious habit of slowly rotating the
necklaces they were wearing while responding to interviewers, but not when they
were simply listening. They both also tended to offer interminable answers to even
simple questions.
9.
On average, our questionnaires show that the personality traits of identical twins
have a 50 percent correlation. The traits of fraternal twins, by contrast, have a
correlation of 25 percent, non-twin siblings a correlation of 11 percent and strangers a
correlation of close to zero. When identical twins are reared apart, their personality
correlations must be an effect of genetics. By the same token, since the twins are
genetically identical, their 50 percent dissimilarity must be caused by environmental
influences and measurement errors (psychological tests are less reliable than most
physical measurements). Seen on an electroencephalogram (EEG), the brain waves of
identical twins are even more similar than our studies would suggest. Although EEGs
are usually as distinct as fingerprints, the EEGs of identical twins are about as similar
as two EEGs of the same person plotted at different times.
10. Equally remarkable is the fact that adult identical twins reared in the same
family are no more similar than identical twins reared apart. In other words, if one of
a pair of identical twins were raised in a family of academics, and the other were
brought up in a blue-collar family by parents who only finished high school, the pair
would be as similar at the age of thirty as if they had grown up in the same household.
Such findings fly in the face of the emphasis on the role of the environment in child
development that has pervaded American psychology until very recently.
11. And yet identical twins are much less similar to each other than many people
would suppose. Consider the Talking Twins: one is gay and the other is straight. Like
the rest of us, identical twins are shaped in part by their unique experiences. One of
the Necklace Twins, for instance, was stricken by a severe case of polio as a child,
and today she walks with a distinct limp.
12. Again and again, since the cloning of Dolly was announced, news reports have
raised the specter of human clones who would be identical to their progenitors in
every way. At first glance, the stories from our study may seem to reinforce that idea.
But seen from another perspective, twin studies underscore rather than erase their
subjects’ individuality. In spite of the unconscious urgings of their identical genomes,
in spite of being raised together and schooled together, identical twins still respond
differently to many of the items on our questionnaires. And those differences are only
the tip of the iceberg. Beneath them flow shadowy memories and feelings,
experiences and dreams that no investigator can sound, much less reproduce. Selves,
unlike cells, can never be cloned.
13. If identity were merely a matter of genes, all human beings would be nearreplicas of one another – and so, for that matter, would all primates. Of the 100,000
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genes that make up a person’s genome, nearly three quarters are identical in all
humans, and of those all but a handful are identical to the genes of a gorilla. Known as
monomorphic genes, they describe most of what is human. It is the remaining quarter
of the human genome, made up of so-called polymorphic genes, that defines us as
individuals.
14. At conception the human embryo receives half its genes from the father, half
from the mother. Because monomorphic genes never vary, every child gets the same
standard set, which is why every child looks recognizably human. But each of the
more than 20,000 polymorphic genes can come in between two and more than twenty
varieties. Hence the chance that two people will receive the same combination of
polymorphic genes is virtually nil, with one exception. About once in every 300
births, for reasons not entirely clear, a fertilized egg splits in half, giving rise to
identical twins, each of whom possesses exactly the same genome. (Fraternal twins,
by contrast, are no more genetically similar than any other siblings; they come from
distinct eggs fertilized by distinct sperm.)
15. Just how identical twins differ from other children in terms of their genetic
inheritance is more complicated than it might appear. Some traits, such as blood type,
are determined by one gene. Others, known as additive generic traits, are determined
by the sum of the effects of a number of genes. A child who gets half his stature genes
from a tall father and half from a short mother will end up medium-size. But if the
father happens to contribute all of his genes for tallness, and the mother few of her
genes for shortness, the child will be tall. As one might expect, identical twins are
nearly always the same height (though a twin who is better nourished in or out of the
womb might grow taller), whereas siblings and fraternal twins show, on average, only
about a 50 percent correlation in height.
16. There are still other traits, however, that are not passed on proportionally.
Known as nonadditive genetic traits, they depend on combinations of genes as interdependent as telephone numbers: miss one digit and they dial a different person. A
body’s histocompatibility code – the unique set of proteins that marks each of its cells
– is one example: change a single histocompatibility protein on, say, a kidney cell,
and the immune system will reject it as foreign. Beauty is another: take Liam
Neeson’s nose and put it on Uma Thurman’s face, and she probably will not be cast as
Aphrodite again.
17. Every month, it seems, new reports emerge about specific genes identified for
various personality traits – the alcoholism gene, the manic-depression gene, the
novelty-seeking gene. But few have stood up to closer inspection. Clearly, the genetic
components of many complex traits are nonadditive. And yet only ten years ago
nonadditive traits were ignored in the domain of personality, largely because they
were impossible to measure.
18. Extended kinship studies and large twin studies such as ours have changed that.
Because identical twins inherit all of the same genes, they are far more likely to share
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nonadditive traits than other siblings are. (The rule of thumb is that traits more than
twice as common in identical twins than in fraternal twins are likely to be
nonadditive.) As recently as 1989, a review of the genetic literature on behavioral
traits concluded that “religiosity and certain political beliefs … show no genetic
influence.” But our studies show just the opposite: identical twins reared apart are
more than twice as similar in their religious interests and values than fraternal twins
reared apart, suggesting that religiosity may be a nonadditive trait. The same is true of
a penchant for arts and crafts and a wide range of other interests.
19. Any discussion of genetic inheritance, no matter how straightforward, inevitably
hits a sticking point: Psychologists and educators might be willing to concede that an
interest in arts and crafts can be inherited, but ideas about the genetic influence on
intelligence are freighted with too much unsavory history – with eugenics and racism
and ruthless social policy – to pass uncontested. And so educational reports still favor
the so-called average-child hypothesis (the belief that all children are basically
equivalent in their capacity to learn and develop), and some psychologists dismiss
intelligence as an arbitrary category, impossible to define objectively, much less
inherit. In the words of the psychologist Leon J. Kamin of Northeastern University in
Boston, written more than twenty years ago but still cited by textbooks: “There exists
no data which should lead a prudent man to accept the hypothesis that I.Q. test scores
are in any degree heritable.”
20. Studies of genetic influence are, according to some critics, ancestors of an old,
reductionist view known as Galton’s error. Galton apparently borrowed the phrase
“nature and nurture” from Shakespeare’s The Tempest, in which Prospero calls
Caliban “a devil, a born devil, on whose nature/Nurture can never stick; on whom my
pains,/Humanely taken, all, all lost, quite lost.” Findings from twin studies, critics say,
are equally simplistic.
21. The truth is, however, that behavioral genetics theory can explain everything
that socialization can explain, only better – and with statistics and reproducible results
attached. Kamin’s comments notwithstanding, there is a strong consensus among
knowledgeable experts in psychology, sociology, cognitive science and behavioral
genetics that IQ is heritable. In fact, it is the single most heritable trait of all the traits
studied by programs such as ours.
22. How can we be so certain, especially about a subject so prone to
misinterpretation? The answer is that identical twins reared apart are nearly ideal
study subjects – as close to an inbred strain of mice as the human behavioral geneticist
can hope to find. Such twins represent an experiment of nature (twinning) and an
experiment of nurture (adoption). Their psychological traits correlate so highly with
one another that a study of fifty pairs of twins reared apart can often tell as much
about heritability as one conducted on 1,000 pairs of twins raised together.
23. Identical twins raised apart are more similar, in almost every physical and
psychological trait, than fraternal twins raised together. What better proof that our
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genes shape us at least as much as our environment? True, small adoption studies in
France have shown that socioeconomic status can influence a child’s I.Q. But such
influences tend to fade entirely by adulthood – as comparisons of results from studies
of young with studies of older twins have shown. One of our own studies offered a
vivid example of how genetic predisposition comes to the fore. When we asked
identical twins to track a rotating target with a stylus, they often began by performing
at different skill levels. Eventually, though, their scores became quite similar.
Fraternal twins, on the other hand, tended to attain and then maintain different skill
levels, even after three days of practice.
24. The study of intelligence seems to crystallize such similarities. Twin studies of
various kinds consistently find that intelligence is between 60 and 70 percent heritable
– in other words, between 60 and 70 percent of the variance in intelligence is due to
genetic factors. In June the journal Science published a study of 110 pairs of identical
twins reared together and 130 pairs of fraternal twins reared together, all of them
eighty or more years old and living in Sweden. On average, the investigators found,
heredity accounted for 62 percent of the variance in general cognitive ability among
the twins.
25. Many psychologists maintain that nurture does more than nature does to shape
intelligence. But in that case, unrelated adults who have been reared together (when
one is adopted, for instance) ought to have similar IQ scores. In fact, the opposite is
true: even after growing up in the same home, unrelated adults are no more alike in
intelligence than complete strangers.
26. For all their striking findings, twin studies do not explain how genes influence
personality, intelligence or social attitudes. In any case, few behavioral geneticists
think that genes determine behavior directly. Rather, genes make a given body or
personality more likely to respond to its environment in certain ways. Identical twins
who are strong, sociable and well coordinated, for instance, are both likely to become
interested in team sports. But that hardly equates to a “team sport gene.”
27. It is important to realize that people tend to create their own environments.
Given a sufficiently supportive environment with varied opportunities, individuals
choose and interact with surroundings that match their genotypes. A person
predisposed to religion, in other words, is likely to find her way into the company of
other religious people, even if her parents are confirmed atheists. At the same time,
there are limits to what one can make of one’s surroundings. Just as no cherry tree, no
matter how robust its genes, can grow in the Kalahari, a child raised in crushing
poverty by illiterate parents is unlikely to score well on IQ tests, no matter what his
mental inheritance. (There are exceptions, of course: the late nineteenth-century
Indian mathematician Srinivasa Ramanujan was raised in a one-room adobe hut and
learned mathematics mostly from two books, both in a foreign language.) Twin
studies tend to attract few subjects in such dire straits, so their findings may not
always apply to people exposed to extremes of deprivation or privilege.
Whenever the Twain Shall Meet / 7
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28. All in all, most psychological variability is probably shaped by experience, just
as radical environmentalists have always believed. Those experiences, however, are
largely self-selected, and that selection is guided by the steady pressure of the
genome. As the geneticist Nicholas G. Martin of the Queensland Institute of Medical
Research in Brisbane, Australia, and his colleagues argued in 1986:
[People are] exploring organisms whose innate abilities and
predispositions help them select what is relevant and adaptive from the
range of opportunities and stimuli presented by the environment. The
effects of mobility and learning, therefore, augment rather than eradicate
the effects of the genotype on behavior.
29. That point of view has implications that differ markedly from those of either
biological or environmental determinists. Rather than trying to forcibly shape traits
and abilities – by constantly pushing an introverted child into play groups, for
instance, when she would rather read alone; or by making a child study math when he
wants to study art – parents and policy makers should provide supportive
environments and a wider range of specialized opportunities (art schools, science
schools, theater schools and so forth) at every level of education.
30. The implications of all this for human cloning are quite clear. If clones are
simply identical twins by another name, there is no reason to believe they will be any
more similar than such twins are. In fact, because clones and their adult forebears
would be born into different generations, from different wombs, and have different
arrays of environmental choices, there is every reason to think they will be even more
different from each other than identical twins are. If a clone and his forebear both
inhabit sufficiently rich and varied environments, they will probably make themselves
into somewhat similar people. They will, however, be far from identical.
31. Identical twins, I often say, can be compared to two renditions of a piano
concerto played by musicians with different styles and skill levels. The written piece
of music constitutes a set of specific instructions, yet each version, while clearly
identifiable, carries the unique stamp of its performer. Similarly, twins, while created
from the same genetic instructions, bear the indelible marks of their development
under unique circumstances.
32. Like all analogies, that one is somewhat misleading: the differences between
two performances is a systematic product of the musicians’ training, practice and
preference for a particular style. Many investigators of twins increasingly realize, by
contrast, that a significant amount of the variation between the identical twins in a pair
is caused by neither nature nor nurture, but by developmental noise. Whereas nurture
implies systematic or controlled environmental influences, developmental noise is
chaotic and probably the result of accidental and random environmental effects.
Animal breeders have taken great pains to control the environments in which their
charges are reared, but developmental noise still influences everything from an
animal’s weight to its behavior.
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33. Unfortunately, the nuances and gray areas in that picture – the developmental
contingencies and fuzzy connections between cause and effect – tend to get lost in any
public discussion of cloning. In spite of all the lip service paid to socialization in
recent decades, the news about Dolly touched on an abiding fatalism in the general
public – a suspicion that genes rule our destinies as implacably as the three Fates once
seemed to do.
34. More than 1,500 years ago, the public was entranced by a different kind of
determinism – astrology – that Saint Augustine tried to dismiss with arguments
strikingly similar to my own. “If the distance of time between the births of twins is so
great as to occasion a difference of their constellations … that even from such change
there comes a difference of destiny, how is it possible that this should be so, since
they cannot have been conceived at different times?” he asked in The City of God.
35. According to Augustine, the mathematician Nigidius answered with a
compelling logic of his own. Given the great speed with which the celestial spheres
revolve, Nigidius said, even minute differences in the time of birth can create large
differences in people. But Augustine countered with what today would be called a
psychometric argument. If such small differences have such a large effect on the
outcome, he said, then no one measures ordinary birth times with sufficient precision
– ipso facto, all astrological predictions must be seriously in error.
36. Today Augustine seems the clear winner of the debate, not least because
astrology is widely acknowledged as superstition. But the fact is that Augustine made
many biological errors (he did not realize, for instance, that only identical twins, and
not fraternal twins, are conceived simultaneously), and he was arguing on behalf of
what many might call another superstition: Christian doctrine. In the same way,
today’s arguments about cloning are more cultural than scientific. Biologists still do
not fully understand the etiology of natural twinning, so tomorrow’s discoveries may
undermine today’s theories (a common event in empirical science and one that
distinguishes it from ideology and superstition). But that will not stop people from
arguing for and against the creation of human clones, or from stressing or deemphasizing the clones’ relative individuality.
37. Consider a few cultural attitudes towards twins. In some societies it was once
common to kill one twin; in others both twins were killed; in others only male-female
pairs were killed; and in still others twins were welcomed and seen as a blessing. How
much each tribe knew about twins probably had little to do with how it decided to
treat them.
38. The results of twin studies, I believe, refute both biological and environmental
determinism. They do not negate the effect of the environment on behavior, nor do
they over-glorify the role of genes. They account for the uniqueness of each of us and
remind us that we are an integral part of a complex biological world and not apart
from it. whether we will heed that knowledge is another matter.
Whenever the Twain Shall Meet / 9
Answer in your own words.
1.
2.
3.
4.
5.
Answer the following question in English.
What did the author find most striking – paragraphs 1-2 – about the behaviour
of the two people he had met?
Answer: _____________________________________________________________
Answer the following question in English.
What scientific conclusion was the author driven to – paragraph 2 – by his
experience with the identical twins?
Answer: _____________________________________________________________
Answer the following question in English.
What thesis appears to be disproved by the experience with the identical twins
described in paragraphs 1-3?
Answer: _____________________________________________________________
Answer the following question in English.
What do Galton’s anecdotes – paragraph 6 – suggest?
Answer: _____________________________________________________________
Answer the following question in English.
In paragraph 10 the author keeps coming back to the similarity of identical
twins whether reared together or apart; does this fact necessarily suggest that if
they were given an intelligence test they would achieve the same results?
Elaborate and substantiate your answer.
Answer: _____________________________________________________________
Whenever the Twain Shall Meet / 10
Answer the following question in Hebrew.
6.
What does the fact that one of the twins – paragraph 11 – is gay and the other is
straight tell us about the factors shaping our sexual preferences?
Answer: _____________________________________________________________
7.
Answer the following question in English.
In what sense could one consider the information provided in paragraphs 11-12
as rather comforting?
Answer: _____________________________________________________________
Answer the following question in Hebrew.
8.
What thesis is reinforced by the fact – paragraph 13 – that though genetically
overwhelmingly alike, humans are yet so different from gorillas?
Answer: _____________________________________________________________
9.
10.
Answer the following question in English.
Compare the author’s view on intelligence heritability with that of Leon J.
Kamin – paragraphs 19-20.
Answer: _____________________________________________________________
Answer the following question in English.
What makes intelligence testing – paragraph 19 – such a delicate issue?
Answer: _____________________________________________________________
Answer the following question in Hebrew.
11. What makes the claim, that the influence of socioeconomic status upon I.Q. –
paragraph 23 – tends to fade entirely by adulthood, not entirely convincing?
Answer: _____________________________________________________________
Whenever the Twain Shall Meet / 11
12.
Answer the following question in English.
What could make the statement that unrelated adults who have been reared
together (when one is adopted, for instance) ought to have similar IQ scores –
paragraph 25 – appear rather disingenuous?
Answer: _____________________________________________________________
Answer the following question in Hebrew.
13. A. What does the fact that clones – paragraph 30 – are unlikely to be exact
replicas of their forebears suggest? B. How is it relevant to the whole issue of
education?
Answer:
A.
B.
Choose the best answer.
14. What does the alarm caused by the first act of cloning suggest?
Answer : It suggests that we
a. all fear that cloning is inevitable.
b. suspect too many evil characters are likely to be replicated.
c. suspect one’s fate is indeed determined by one’s genetic equipment.
Answer the following question in English.
15. In what sense can one say that the article ends on a hopeful note?
Answer: _____________________________________________________________