1

Corballis, M.C. (2007) The dual-brain myth. In S. Della Sala (Ed.), Tall tales on the
brain (pp. 291-313). Oxford: Oxford University Press

The Dual-Brain Myth

Michael C. Corballis

Right-brained adj: 1. Having the right brain dominant. 2. Of or relating

to the thought processes involved in creativity and imagination, generally

associated with the right brain. 3. Of or relating to a person whose

behavior is dominated by emotion, creativity, intuition, nonverbal

communication, and global reasoning rather than logic and analysis.

―The American Heritage Dictionary of the English Language, 4th

Edition (2000)

INTRODUCTION

Everybody knows about the left brain and the right brain. The left brain is verbal,

rational, linear, computational, scientific. The right brain is spatial, intuitive, emotional,

creative, artistic. The left brain epitomizes the military-industrial establishment of the

West, while the right brain has the glamour and mystery of the East. The left brain is

boring, while the right brain is fun.

These notions came to light in the 1960s, largely as a result of research on people

who had undergone the so-called “split-brain” operation for the relief of intractable
 2

epilepsy. The basic idea behind the operation was that an epileptic disturbance

originating in one side of the brain would be prevented from spreading to the other side,

and so causing a major seizure, if the connections between the two sides of the brain were

cut. This rather drastic operation was largely successful in at least reducing the

frequency and severity of seizures. What was really interesting about these patients,

though, was that the two sides of the brain were effectively separated from one another,

at least with respect to higher mental functions, so it became possible to assess the

capacities of each side without interference from the other.

The leader of this research was Roger W. Sperry, who received the Nobel Prize

for his work in 1981. He and his colleagues were able to show that the only left side of

the brain could actually name objects or words presented to it, while the right remained

speechless1, 2. The right brain of at least some patients could understand language,

though, and could direct the left hand (which it controls) to point to the written names of

objects it had seen, or point to objects whose names it had seen. The right brain’s ability

to comprehend language was clearly below that of the left, but this still came as

something of a surprise, since a century of research on the effects of damage to the left

side of the brain had suggested that the intact right brain had little ability to either

understand or produce language2. It is still a matter of controversy as to whether studies

of the split brain have painted an accurate picture of right-brain verbal capacities in

normal people.

But what was more interesting was the idea that the right brain might have special

abilities of its own, abilities not shared by the left. Until the 1960s, the right brain (or

right hemisphere), had generally been considered subordinate to the left, and was
 3

generally known as the “minor” or “nondominant” hemisphere—a view that was further

encouraged by the fact that the left brain controls the right hand, which in most of us is

the dominant hand. The split-brain experiments, though, began to reveal a few activities

in which the right brain outperformed the left. These were largely spatial, as in matching

parts of shapes to wholes, or in imagining shapes in different orientations, or detecting

emotion in faces or in speech. The right brain also seemed to be better at identifying

melodies, although the left seems to be the more specialized for rhythm, and there is

some evidence that professional musicians are more generally left-brain dominant for

music. Although the compendium of suggested right-brain functions is quite broad, the

advantages are usually slight, and the functions themselves simple perceptual ones3.

The most obvious and extreme dominance of the right brain has to do with the control of

spatial attention. People with right brain damage often show a striking neglect of the left

side of space, in extreme cases failing to dress the left side of the body, or eating from

only the right side of the plate, or leaving the left flank ridiculously exposed when

playing chess. People with left brain damage seldom show a complementary neglect of

the right side of space, and if they do it is usually transitory. These phenomena are

usually taken to mean that the left brain can direct attention only to the right side of

space, while the right brain can attend to both sides. Although this difference is fairly

striking, it scarcely represents a profoundly different style of thought.

These right-brain advantages may have come about simply because of the left-

brain’s specialization for language, and for related skills such as mathematics. Language

occupies a lot of brain space, so the left brain may have forfeited some of its capacity for

more elementary functions. True language is almost certainly uniquely human, so the
 4

right brain may simply represent the cerebral capacities of both sides of the brain before

language came along to destroy the symmetry. This is perhaps a slight

oversimplification, since there is evidence for a right-brain advantage for spatial and

emotional processing in some nonhuman species, but even these might be secondary to

communicatory functions lodged largely in the left. In any event, Michael Gazzaniga,

one of Sperry’s original collaborators and a long-time researcher on the split brain, was

once moved to remark that “it could well be argued that the cognitive skills of a normal

disconnected right hemisphere without language are vastly inferior to the cognitive skills

of a chimpanzee”4.

To be fair, it should be said that it is not easy to test the right brain because of its

limited verbal comprehension. It may well possess abilities untapped by researchers.

Moreover, Gazzaniga’s extreme view provoked strong rebuttals from other split-brain

researchers5, 6, 7, including one of the surgeons (Bogen) who carried out the split-brain

operations in the 1960. Nevertheless Gazzaniga was not repentant, and four years later

still insisted “the vast majority of the cases from all [split-brain] surgical cases reveal

little cognitive capacity in their right hemispheres”8. In a still more recent survey of the

now extensive literature on the split brain, he writes:

While the right hemisphere remains superior to the isolated left hemisphere

for some perceptual and attentional skills, and perhaps also for emotions, it is

poor at problem solving and many other mental activities. A brain system

(the right hemisphere) with roughly the same number of neurons as one that
 5

easily cogitates (the left hemisphere) is incapable of higher-order cognition;9

(italics added).

He goes on to argue that the left brain is the seat of what he calls the “interpreter,” which

may be likened to the chief executive officer of the mind. The right brain is essentially

relegated to routine jobs.

THE MYTH-MAKERS

The paucity of evidence did not stop the myth-makers, who greeted the right brain as

though it were some long-lost but exotic uncle. First off the mark was the surgeon,

Joseph Bogen. In a discursive but scholarly review, he suggested that the right brain

might be considered to harbor an “appositional mind”, complementary to the

“propositional mind” seated in the left brain10. Besides drawing on the neurological

evidence, Bogen referred to long-standing notions about the dual nature of the mind, such

as the Chinese concepts of yang and yin, the Hindu distinction between intellect (buddhi)

and mind (manas), Hobbes’ notions of directed versus unordered thinking, and the

everyday distinction between reason and intuition. Bogen and his colleagues undertook a

study comparing different ethnic and racial groups on a battery purported to be sensitive

to the different specialized capacities of the two sides of the brain. Among the groups

they tested, Hopi Indians were the most “right-brained”, followed by urban Afro-

American women, urban Afro-American men, rural whites, and urban whites11. The idea

that primitive peoples might be more right-brained than those from industrialized cultures

is a common one, smacking somewhat of 18th-century Romanticism and the concept of

 6

the Noble Savage, and is probably not without a touch of condescension. The

“pernicious myth of the right-brained Indian” is easily dismantled12.

The dual brain was enthusiastically pursued by Robert E. Ornstein in his 1972

best seller The Psychology of Consciousness13. Part of Ornstein’s message was that

society in general, and educationalists in particular, had placed too much emphasis on

left-brain thinking, and that there was a need to liberate the creative powers of the right

brain. So quickly did this idea grow that in 1977 the editor of Psychology Today called it

“the fad of the year”, and went on to say that it would soon peter out14. A decade later,

Lauren J. Harris15 noted that it was still going strong, and it shows no signs of abating

even now. The authors of Superlearning 2000, published in 1994, had the audacity to

write as follows:

Yes, it’s the left brain/right brain, and you’ve heard it before.

Superlearning helped popularize the idea in the early 1980s16.

In the early 1980s, it had been swilling around in the popular press at least since the

publication of Ornstein’s book in 1972.

Betty Edward’s 1979 book Drawing on the Right Side of the Brain, purporting to

teach people how to draw by exploiting the spatial and creative powers of the right brain,

has been even more successful than Ornstein’s book. It has sold over 2.5 million copies,

and a second revision, The New Drawing on the Right Side of the Brain, was published in

1999. Picking up on Bogen’s theme, some anthropologists have argued that differences

between the two sides of the brain might explain cultural differences17. Even science
 7

itself was not immune. In his 1977 book The Dragons of Eden, the noted cosmologist

and popularizer of science, Carl Sagan, portrays the right hemisphere as the creative but

paranoid instigator of scientific ideas, often seeing patterns and conspiracies where they

do not exist18. The role of the rational left hemisphere is to submit these ideas to critical

scrutiny.

The duality of the brain was enthusiastically received in educational circles. In

1977 an art teacher, anticipating Betty Edwards, was quoted in the Los Angeles Times as

saying that the essence of her method was to teach people “to gain access to the right

hemisphere and be able to put it to use for education in general.” Another author chimed

in by deploring the emphasis on left-hemispheric values in US schools, and “the tragic

lack of effort to develop our children’s right-brain strengths. That potential—a source of

equally essential creative, artistic, and intellectual capacity—is at present largely

unawakened in our schools”19. Suggested ways to enhance right-brain participation in

the classroom included being more tolerant of children’s wrong answers and of their

excursions into dreams and fantasy20, and greater use of such meditation devices as

transcendental meditation, yoga, Sufi, biofeedback, biorhythms, and hypnosis21.

Zdenek22 interviewed a number of creative artists and writers, and informed a rather

bemused Charles Schulz, the cartoonist, of the ways in which he has been putting his

right brain to work. But at least she managed to cheer him up. “Well, I’m glad you came

all the way up here,” he said at the end of the interview, “You helped the sadness go

away”22.

Perhaps the real reason why these ideas about the two sides of the brain have

persisted is that they are good for business. In 1976 a professor in the Faculty of
 8

Management at McGill University was moved to write in the Harvard Business Review

as follows:

The important policy processes of managing an organization rely to a

considerable extent on the faculties identified with the brain’s right

hemisphere. Effective managers seem to revel in ambiguity; in complex,

mysterious systems with no order23.

This kind of thinking has persisted, although whether as a genuine belief or as a plot to

sabotage competing business is not clear. It is manifest in such books as Harry Alder’s

Right Brain Manager: How to Harness the Powers of your Mind to Achieve Personal and

Business Success24. As of 1986, a scheme known as “whole brain learning” was

available on tape for $195, and provided “mind-brain expression” through subliminal

messages,” three hearable by the left brain and four by the right brain. Those interested

in the scheme were invited to pay $1400 to attend seminars so that they might become

“certified in accelerated teaching and learning”15. Nowadays, such programs are beamed

at us through the Internet—a Google search in late 2004 unearths 3,350,000 references to

“whole-brain learning.” You might also try a website called gocreate.com, which offers

the help of right-brain people to get your business going. The Memphis Business Journal

of 21 October, 2002, describes the success of a market research group that dubs itself The

Right Brain People, who can persuade people to buy things by appealing to the right

brain. Programmes like Superlearning and Neuro-Linguistic Programming25, 26, which

 9

pay extensive homage to the two sides of the brain, continue to attract fee-paying

converts and are big business world-wide.

The right brain also managed to infiltrate English literature, a remarkable

achievement when it is considered that it has little, if any, language capacity. In 1983 a

professor of English published a book entitled Writing the Natural Way: Using Right-

Brain Techniques to Release Your Expressive Powers27. Perhaps that was just the

beginning; a Google search for “right brain and literature” in late 2004 turns up 2,930,000

hits. In New Zealand, the syllabus for the teaching of English in schools divides

languages into three categories: written language, spoken language, and something called

visual language. This last category seems to include film, television, and theatre, as well

as posters, computer-generated text, and fax machines. It is no doubt in part a concession

to political correctness, as well as to the right brain, so that children with little ability in

spoken or written language might nevertheless hope to find expression for other talents.

They might be better off sticking to rugby. Karl Stead, a distinguished New Zealand

novelist, poet, and critic, has been watching these developments and foresees dire

consequences for the literacy of New Zealand children28.

For the ultimate in dual-brain rhetoric applied to literature, though, we can

perhaps do no better than turn to the late Poet Laureate, Ted Hughes:

An explanation for some aspects of the poetic effects of Shakespeare’s

device is suggested by what is now known of the cooperative inter-activity

of the left and right hemispheres of the brain. We are told that, in general,

the left side processes verbal language, abstract concepts, linear argument,
 10

while the right side is virtually wordless, and processes sensuous imagery,

intuitive ideas, spacial [sic] patterns of wholeness and simultaneity … By

nature the two sides presumably live in a kind of happy marriage. A

noisily chattering society is supercharged with right-hemisphere right-side

participation: music, song, dance, colour, imagery—and a vernacular

tending naturally to imagery and musicality … But, as history

demonstrates, the onset of rationality institutes proceedings for a kind of

divorce29.

Hughes goes on to convince us that Shakespeare had two sides to his brain. There might

be something to the idea, since over a century ago Rudyard Kipling wrote the following

poem, which appeared in 1901 in Kim:30

Something I owe to the soil that grew -

More to the life that fed –

But most to Allah who gave me two

Separate sides to my head.

I would go without shirt or shoes

Friends, tobacco or bread

Sooner than for an instant lose

Either side of my head.

 11

HISTORY REPEATS ITSELF

If that seemed to be remarkable prescience on Kipling’s part, I should explain that there

was an obsession with the left and right sides of the brain in the latter part of the 19th

century that eerily foreshadowed that which occurred a century later. In the 1860s, the

French physician Paul Broca reported observations from brain-injured patients indicating

that the loss of speech (which he called aphemia) was associated with damage

exclusively to part of the left side of the brain31. Shortly afterwards, the German

neurologist Karl Wernicke32 associated the loss of language comprehension with another

part of the left brain. These two regions of the left brain, Broca’s area in the frontal lobe

and Wernicke’s area around the junction of the temporal, occipital and parietal areas, are

regarded as the two major language areas in the brain.

Even though the left brain was thereafter widely considered the “major” or

“dominant” hemisphere, there were some who found odd jobs for the right brain to do.

The British neurologist Hughlings Jackson33 speculated that if “expression” resided in the

left brain, then maybe “perception” occupied the right, an idea that was echoed

independently by the French neurologist De Fleury34 and the Austrian physiologist

Exner35. Speculation began to mount, however, when the French neuroanatomist Luys 36

noted differences in personality between those with left- and right-brain damage, and

suggested that the “emotion” center was in the right brain and the “intellectual” centers

were in the left. It had also been observed that hysterical disorders tended to show

predominantly left-sided symptoms, implicating the right brain. Although this was first

observed by Briquet37, even before Broca’s observations were made known, it was
 12

attributed to Jean-Martin Charcot, well known for his work on hypnosis and hysteria, and

was even dubbed “Charcot’s rule.”

By then the game was on. Brown-Sequard38 argued that the left brain

represented “the life of relations” and the right brain “the organic life,” and pronounced

that right-brain damage was likely to lead to “troubles of nutrition,” such as bedsores,

oedema, pulmonary congestion, and involuntary evacuation of feces and urine. He went

so far as to believe that each side of the brain was a complete brain, each with bilateral

control over the whole body, and continued to expand on this theme until well into his

old age39. Luys40 maintained that the left brain was the repository of civilization, with the

right brain representing the primitive, prehuman side of our nature. Madness was the

result of an imbalance, with the right brain assuming dominance. Another influential

theorist, Delaunay associated the left and right sides of the brain with male and female

characteristics, respectively, and in 1898 another French physician declared: “The terms

‘male hemisphere’ and ‘female hemisphere’ should render rather well the differences in

nature of the two brains, of which one, more intellectual, is more stable, and of which the

other, more excitable, is also more rapidly exhausted “41.

The dual brain was also used to account for cases of dual personality, with the left

brain representing the educated, civilized Dr Jekyll and the right brain the crude,

passionate Mr Hyde. One extraordinary case, known as Louis V, apparently suffered

paralyses that could be transferred from one side of the body to the other. This transfer

was accompanied by changes in personality: “Louis V, directed by the right hemisphere

is a different individual than the Louis V directed by the left hemisphere. The right-sided
 13

paralysis [implying right-brain control] only allows the violent and brutal aspects of his

character to appear; the left-sided paralysis transforms him into a peaceful boy”42.

These claims soon led to therapies directed to one or other side of the brain. A

technique known as “metallotherapy” involved the application of metal discs, and later

magnets, to one or other side of the body in order to transfer symptoms from one side to

the other, and it was soon claimed that these transfers produced changes in personality

and intellect. This was known as “psychic transfer”43. Hypnotic techniques were also

developed, especially in France, to hypnotize each side of the brain separately. In one

case, the hypnotized person was said to simultaneously express horror on one side of the

face and calm contentment on the other through having one side of the brain induced to

hallucinate an attack by dogs and the other a pleasant country fete44. There were visions

of a brave new world in which magnets and hemihypnotic techniques might produce a

doubling of mental faculties. John Jackson, one of the founders of the British

Ambidextral Cuture Society in 1903, wrote of a new age in which “each shall be

absolutely independent of the other in the production of ANY KIND OF WORK

whatever; … if required, one hand shall be writing an original letter, and the other shall

be playing the piano, with no diminution of concentration whatever”45.

Needless to say, there were spoilsports. From about 1885, Bernheim46 began a

campaign to discredit metallotherapy, claiming that the effects were due entirely to

“suggestion.” Hemihypnosis soon lost credibility because its proponents had naively

assumed that one could gain access to a single half of the brain by having the patient

cover one eye and directing their attention to the other. In the 1880s there was some

confusion as to how information from the eye reached the brain, but it was widely
 14

thought that each eye projected wholly to the opposite side of the brain. It was later

established that there was partial decussation of the optic tract, so that each eye actually

projects to both sides of the brain. By the time this was made clear, the whole fanciful

façade of hemihypnosis, metallotherapy, and assorted dual-brain techniques had

collapsed. The historian Harrington47 states that she could find almost nothing written on

the dual brain from 1920 to 1960, when the cycle was destined to repeat itself.

It is entirely to Harrington’s credit that this earlier episode is recalled at all. The

great majority of researchers on laterality from the 1960s on were oblivious to the fact

that they were repeating history, until Harrington revealed all39, 47. And if history is truly

to repeat itself, the present-day phase is likely to soon burn itself out, notwithstanding its

resilience over the past 35 years. It is to be hoped that when the next round begins in the

2060s, there is another astute historian who can remind our great-grand-children of the

excesses of the previous two centuries.

What can we learn from history? We have seen that the laterality myths of the

late 19th century were very similar to those of the late 20th, but there was at least one

important difference. In the earlier version, the right brain was clearly inferior to the left,

and stood for the primitive, uncivilized, and dare I say feminine side of our nature. In

these respects, of course, it echoed the prejudices of a dominant, “civilized”, male-

oriented Europe—“dead white males,” to borrow a feminist expression. Small wonder,

then, that the right brain, despite the array of functions attributed to it, was referred to as

the minor or nondominant hemisphere—nondominant, that is, unless some disease caused

it to assume dominance and induce madness in the hapless victim. Notwithstanding the

unflattering depiction of the right brain as a retarded chimpanzee8, modern myth makers
 15

are much more respectful of the right brain, even elevating it to a creative genius

struggling for release. This again owes more to the prejudices of the time than to the

neurological facts. The 1960s and 1970s were a time of protest, against the Vietnam

War, against the military-industrial establishment, and against sexism. The right brain

became the symbol for the creative, peaceful, exploited people of the East against the

brutal Western juggernaut. In the protest slogan “Make love, not war,” the right brain

was love, the left brain war.

In homage to the feminist movement, too, the creative, intuitive, feminine side of

human nature is associated with the right brain, to be released from the slavery imposed

by the bullying, masculine left brain. In neuropsychological terms, though, the evidence

on sex differences is muddled, to say the least. Women are widely regarded as more

intuitive and better able to express emotion than men are, suggesting greater right-brain

involvement. But women are also regarded as more verbal than men, and men more

spatially adept than women, yet verbal ability is associated with the left brain and spatial

ability with the right. Indeed, in the 1980s the neurologist Norman Geschwind saw fit to

reverse the polarity by arguing that the left brain in males, far from being dominant, was

likely to be deficient. He suggested that the male sex hormone testosterone inhibits the

early development of the left brain, which might explain why men are more likely than

women to be left-handed and to suffer from language disorders such as reading disability

and stuttering. The influence of testosterone was also said to increase the likelihood of

autoimmune disorders in hapless males and in left-handers48, 49. In a way, though,

Geschwind’s theory couples the 19th-century idea of the brutish, primitive right brain

with the more contemporary feminist depiction of men as violent—according to a

 16

feminist slogan of the 1980s, “All men are rapists.” Again we see that the popularity of a

theory may owe more to the culture of the age than to neurological evidence.

Geschwind’s theory has not held up well in the face of research50, 51, and is already

confined to the dustbin of history.

History shows us, then, that the two sides of the brain serve, at least in part, as

pegs upon which to hang some of our cultural preconceptions. But this is a trend that

goes much further back than the 19th century, except that in the beginning it was not the

sides of the brain, it was the hands.

READING THE HANDS

Throughout history, people of diverse cultures have associated different values with the

two hands, or more generally with the two sides of the body. In general, these values are

the reverse of those associated with the two sides of the brain, presumably because each

side of the body is mapped to the opposite side of the brain. Positive attributes tend to be

associated with the right hand and negative attributes with the left, although in some

cases the attributes are complementary without being obviously value-laden.

In the Pythagorean table of opposites, recorded by Aristotle, the right was

associated with the limited, the one, the odd numbers, the light, the straight, the good, and

the male, while the left was associated with the unlimited, the many, the even numbers,

the dark, the curved, the evil, and the female. Many similar examples can be drawn up

from quite unrelated cultures52. For example, to the New Zealand Maori, the right is the

sacred side, the side of the gods, the side of strength and life, while the left is the side of

profanity, demons, weakness, and death53. And we should not overlook the Bible:
 17

And He will set the sheep upon His right hand and the goats upon His left.

Then shall the King say to those upon His right, “Come, ye blessed of my

father, and inherit the Kingdom prepared for you from the beginning of the

world.” … Then shall He also say to those on the left, “Depart from me, ye

accursed, into everlasting fire prepared for the Devil and his Angels.”

(Matthew 25: 33-34; 41)

According to Barsley54, there are over 100 favorable references to the right hand in the

Bible, and about 25 unfavorable references to the left hand.

The universality of left-right symbolism, with the right nearly always associated

with positive attributes and the left with negative ones, no doubt reflects the fact that the

majority of people in all human societies are right-handed. It has sometimes been argued

that some nation or race of people was predominantly left-handed. In a popular article

written in 1956, Trevor Holloway asserted that “The Antanalas of Madagascar are unique

among the races of the world for almost every member of this tribe of 100,000 is left-

handed55. I have been able to find no basis for this extraordinary claim. It has also been

suggested that the ancient Hebrews must have been left-handed because Hebrew is

written from right to left56, but up until about 1500 AD there were about as many right-

to-left scripts as left-to-right ones, and the gradual predominance of left-to-right writing

is almost certainly due to historical events unrelated to handedness57 . It was also thought

for a time that the ancient Hebrews were mostly left-handed because they usually

depicted humans and animals in right profile, whereas it is more natural for right-handers
 18

to draw left profiles. But this was probably simply a reflection of the widespread cultural

belief that the right side is sacred and the left side profane, and that the left side should be

hidden from view. Dennis58 pointed out that if one considers how the use of the hands

themselves is depicted, the evidence for right-handedness in ancient Egypt is comparable

to that in modern societies. As long ago as the 1860s, Andrew Buchanan wrote boldly as

follows:

The use of the right hand in preference to the left must be regarded as a

general characteristic of the family of man. There is no nation, race, or tribe

of men on earth at the present day, among whom the preference does not

obtain; while in former times, it is shown to have existed, both by historical

documents and by the still more authentic testimony of certain words,

phrases, and modes of speaking, which are, I believe, to be found in every

spoken language59, 60.

We might qualify this in the light of recent evidence that the great apes may show a

population-level preference for the right hand61, but this bias is not nearly so extreme as

that in the human population, where it seems to be universal across cultures.

A near-universal aspect of left-right symbolism is the association of the right with

male and the left with female. To the Maori, tama tane refers to the right side, but

literally means “male side,” while tama wahine, literally “female side,” refers to the

left53. Hertz also quotes a Maori proverb: “All evils, misery, and death come from the

female element”62. Empedocles, the Sicilian argued in the 5th century BC that males
 19

were hotter than females and the right hotter than the left, so that sex was determined by

relative placement in the womb. Perhaps he was right: Mittwoch63 reported that in

hermaphrodites testes are more likely to be found on the right and ovaries on the left, and

she went on to suggest that the same opposing tendencies are present in normal males and

females but are overridden by the influence of the sex chromosomes. The association of

the female with the left has not always implied disrespect or inferiority, though. In the

matriarchal Isis cult of ancient Egypt, honor was accorded to Isis over Osiris, to mother

over son, and to night over day, and the Isis procession was led by a priest carrying an

image of the left hand.

THE SYMBOLIC POTENCY OF LEFT AND RIGHT

We may now ask what it is about left and right that inspires such symbolic potency.

Perhaps it is in part the sense of paradox that the two hands seem to present: “What

resemblance more perfect than that between the hands,” exclaimed Hertz62, “and yet what

a striking difference there is!” (p. 89). Part of the paradox is that the two hands are

mirror images, and the mirror-image relation has the paradoxical property that every

point on one surface can be mapped uniquely onto a point on the surface of its mirror

image, yet the two cannot occupy exactly the same space as previously occupied by the

other—except in the trivial case of shapes that are themselves symmetrical. Lewis

Carroll, who was obsessed by mirrors, makes the point in The White Knight’s Song:

And now, if e’er by chance I put

My fingers into glue,

 20

Or madly squeeze a right-hand foot

Into a left-hand shoe …

The left-hand foot, to use Carroll’s quaint terminology, easily slips into the left shoe, but

the seemingly identical right-hand foot does not.

But what Hertz probably had in mind was not the mirror-image relation per se,

but rather the fact that the difference in the functional capacities of the hands seems to

belie their identical structure. You cannot easily tell a person’s handedness by

inspecting the structure of their two hands, but their handedness is at once apparent if you

ask them to write or throw with each hand in turn. Parity, as physicists are wont to say, is

not conserved.

I have suggested that this apparent mismatch between function and structure may

have encouraged a sort of Cartesian wish-fulfilment64. Descartes65 argued that humans

were distinguished from other animals by virtue of a non-material influence that could

stimulate the material brain through the pineal gland. The superiority of the right hand

over its seemingly identical twin might therefore be taken as a manifestation of this non-

material power that sets us apart, and endows us with consciousness and free will.

Something of the sort seems implicit in Gazzaniga’s9 notion of the left brain—

driver of the right hand—as the interpreter, and source of executive consciousness. This

again seems to imply a mismatch between structure and function. One can also detect a

whiff of Cartesian wish-fulfilment in the views of Sir John Eccles, who argued that only

the left brain of humans is capable of consciousness66, and in particular self-

consciousness67, while the right brain is a mere “computer” comparable to the brains of
 21

other animals. Zangwill68 dismissed these ideas as “little more than a rearguard action to

save the existence and indivisibility of the soul” (p. 304). Eccles69 later conceded

“limited self-consciousness to the minor [sic] hemisphere” (p. 105).

Of course, one function that we can attribute to the left brain is language itself,

but even this is not untainted by Cartesian wish-fulfillment. Descartes considered

language to be one of the objective signs of the non-material influence that uniquely

endows humans with freedom from mechanical control. This idea was picked up by

Noam Chomsky, a neo-Cartesian and the foremost linguist of the late 20th century:

… a chimpanzee is very smart and has all kinds of sensorimotor constructions

(causality, representational functions, and so forth), but one thing is missing:

that little part of the left hemisphere that is responsible for the very specific

functions of human language70.

There is nevertheless increasing evidence for systematic asymmetries in the brains of

other animals, including left-brained control of vocalization in the lowly frog71! Recent

reviews suggest, in fact, that cerebral asymmetries in other vertebrates closely parallel

those in humans72, 73, and the lateralization for language is uniquely human only because

true language is unique, not because lateralization is unique. There are some hardy souls

who even challenge the view that language is uniquely human74, although the consensus

probably remains as articulated by Pinker75—namely, that language is an instinct, but a

uniquely human one. That said, the point is that Cartesian wish-fulfillment may lead to

exaggerated claims, especially of human uniqueness.

 22

The discrepancy between functional asymmetry and structural asymmetry is in

any event not an absolute one, since the two sides of the brain are not in fact perfect

anatomical mirror images. There are some fairly systematic asymmetries that are at least

weakly correlated with left-cerebral dominance for language. For example, Geschwind

and Levitsky76 reported that an area of the brain called the temporal planum, which is

involved in language comprehension, is larger on the left than on the right in the majority

of people, and this asymmetry is even evident in newborns77. These and other anatomical

asymmetries of the brain78 are present in only about two-thirds of human brains, whereas

the proportion of people with left-brained dominance for language is probably over 90%3.

Ironically, it has been reported that the leftward bias in the temporal planum was present

in 17 out of 18 chimpanzees, which is a proportion significantly greater than that

observed in humans79.

ON SYMMETRY

In the frenzied attempt to discover asymmetries of the brain, now extended to nonhuman

animals, we are apt to forget that there is a very striking characteristic that we share with

most other species—bilateral symmetry. We belong, in fact, to an ancient phylum known

as the Bilateria, which established bilateral symmetry as the default condition80. This is

itself an evolutionary adaptation to a world that is essentially indifferent with respect to

left and right. Limbs are symmetrically placed so that movement, whether we walk, run,

fly, or swim, can proceed in a straight line. Our eyes, ears, and skin receptors are

symmetrically placed because the events that matter to us are as likely to occur on one

side as on the other. Predators or prey may lurk on either side, and an animal with sense
 23

organs on one side would be easy meat for an attacker on the other side. Since the brain

is largely concerned with inputs and outputs, the symmetry of the limbs and sense organs

dictated a symmetrical plan for much of the brain. The psychological consequences of

bilateral symmetry are discussed at length elsewhere81.

Even in the face of the evidence for the asymmetrical representation of language

in the brain, the French physician Pierre Marie was so impressed with the brain’s

symmetry that he thought that each hemisphere must have at least the potential for

language82. There is in fact good evidence that if the left brain is incapacitated or

removed early in life, the right brain can take over language, with little or no loss of

efficiency83. At one time it was claimed that people who had undergone total removal of

the left brain in early childhood later showed deficits in syntax, supposedly the essence of

language and the exclusive preserve of the left brain84, 85, but this has been disputed on

methodological grounds86. There is clear evidence that syntax is preserved in at least

some cases of people whose right brains have taken over language following

incapacitation of the left brain in early childhood87, 88.

These facts do not suggest that nature has endowed us with a left brain uniquely

endowed to perform the special functions of language, and a right brain wired for quite

different—some have suggested, complementary―functions. Cerebral asymmetry is

altogether more fluid than this static picture allows. It seems much more likely that we

are endowed with two half-brains that are ready for almost anything, and that some

switch operates early in development to tip the balance toward the left-brained

representation of language. That switch may well depend on genetically controlled

growth gradients89. There is evidence that, between the ages of 2 and 4, the left brain
 24

undergoes a growth spurt that may be instrumental in ensuring the syntax is firmly lodged

in that side of the brain90. But if something goes wrong with the switching mechanism,

or if the left brain is incapacitated, the faithful right brain is following along behind,

ready to oblige.

Another point to note is that, when the right brain does take over language, it does

so at the expense of the spatial functions usually associated with that side91, 88. What this

seems to suggest is not that the right brain is intrinsically specialized for spatial function,

or for intuition or creativity or any of the other transcendent properties attributed to it, but

rather that whichever side of the brain gets burdened with language loses some of its

capacity for everything else92, 89, 9, 93.

THE VEXED PROBLEM OF LEFT-HANDERS

This brings us to that much-maligned minority, the left-handed. Through most of history

and in most cultures, the negative associations with the left have meant that left-

handedness has been generally discouraged, and there is still many a leftie who was

forced to switch to the right hand for writing and/or eating. The American psychiatrist

Blau56 dismissed left-handedness as “infantile negativism.” Even the sexual identity of

left-handers was called into question by a colleague94 and close friend of Freud:

Where left-handedness is present, the character pertaining to the opposite sex

seems more pronounced. This sentence is not only invariably correct, but its

converse is also true; where a woman resembles a man, or a man resembles a

woman, we find the emphasis is on the left side of the body. Once we know
 25

this, we have the diviner’s rod for the discovery of left-handedness. This

diagnosis is always correct95.

Sir Cyril Burt, the British educational psychologist, anticipated Blau in describing left-

handers as willful or “just cussed”, and echoed Fliess by noting that “Even left-handed

girls … often possess a strong, self-willed and almost masculine disposition.” He went

on to complete the demolition of left-handers as follows:

They squint, they stammer, they shuffle and shamble, they flounder about like

seals out of water. Awkward in the house, and clumsy in their games, they

are fumblers and bunglers at whatever they do96.

Among the fumblers and bunglers are Alexander the great, George Bush (41st US

President), Julius Caesar, Charlie Chaplin, Charlemagne, Winston Churchill, Cicero, Bill

Clinton, Gerald Ford, Benjamin Franklin, Rock Hudson, Goran Ivanisevich, Paul Klee,

Rod Laver, Harpo Marx, Michelangelo, Leonard da Vinci, Paul McCartney, John

McEnroe, Martina Navratilova, Ronald Reagan, Peter-Paul Rubens, Babe Ruth, Monica

Seles, Ringo Starr, Emperor Tiberius, and Harry Truman.

Much of the prejudice against left-handers has dissipated, especially in Western

countries. But left-handers are a little awkward in the house of dual-brain theory.

Following his discovery of left-brain dominance for speech, Broca31 conjectured that in

left-handers this would be reversed, and the right brain would be dominant for speech.

This became known as Broca’s rule. One might then have simply supposed that the left
 26

brain would contain the functions attributed to the right brain in right-handers. But

Broca’s rule turned out to be wrong, and studies have shown that the majority of left-

handers, perhaps as many as 70% of them, are left-dominant for speech. To be sure, a

higher proportion of left- than right-handers are right-brain dominant for speech, but

there is a substantial majority who have speech represented bilaterally97. The best

explanation for these finding is that most left-handers belong to a minority who do not

exhibit the strong lateralizing influence that controls handedness and cerebral asymmetry

in most right-handers. In this minority, which includes some right-handers as well,

handedness and cerebral asymmetries are determined at random. It has been suggested,

but remains unproven, that whether or not this lateralizing influence is expressed depends

on a single genetic locus98, 99.

This poses a problem already for dual-brain theory, since it implies that left-

handers do not have brains in which different functions are neatly divided between the

two sides of the brain. A glance at the list of left-handers suggests that they are not

deficient in creativity or artistic ability. Indeed there are some reasons to believe that

left-handers may be slightly superior to right-handers in mathematical and artistic skills,

and perhaps slightly inferior in musical and verbal talents100. There is a long but

contentious history of claims that left-handers may be slightly more at risk for reading

disability and stuttering3, while a more bilaterally symmetrical brain may provide a slight

advantage in spatial skills98.

Any advantage of possessing an asymmetrical brain probably has to do with the

programming of complex motor skills, such as speech, rather than with the division of

mental capacities into packages of opposites. If something as complex as speech

 27

involved neural circuits in both sides of the brain, it might be prone to interhemispheric

delays and interferences that could potentially create dysfluencies92, 101. But there is also

evidence that the dominance of one or other side of the brain is achieved by the pruning

of the nondominant side. Annett98, 102 has proposed that this is under genetic control.

The hypothesized gene has two alleles (alternative forms), one that prunes and one that

does not. In homozygotes with a double dose of the pruning allele, the right brain may be

so diminished as to create spatial difficulties to offset any verbal advantage. Conversely,

those who lack the pruning allele may have full right-brain spatial function, and are

equally likely to be left- as right-handed, but may run the risk of language disorders. The

ideal, if you can arrange it, is to be heterozygotic, with one copy of each allele, which

minimizes the chances of either spatial or verbal disorder. It may be this so-called

heterozygotic advantage that has maintained both alleles in the population, and held the

proportion of left-handers roughly constant for at least the last 5,000 years103.

Another perspective is provided by suggestions that lack of consistent

handedness, perhaps due to lack of the pruning allele, is associated with more general

deficits in academic ability104, and also with a tendency to magical ideation105. There is

also evidence that schizophrenia is associated with the lack of consistent handedness106.

Horrobin107 has argued that in earlier times individuals with schizophrenia were regarded

as exceptionally creative and charismatic, but the condition became less adaptive with the

rise of animal husbandry and a switch from a fish diet to a diet of red meat. Therefore the

two alleles may express, not only the tension between lateralization and symmetry, but

the age-old conflict between reason and religion, with its roots in spirituality. According

to the dual-brain, of course, this is essentially the polarity associated with the left and
 28

right brains, respectively, and the suggestion here is that it might be more appropriate to

remap the polarity onto lateralized and unlateralized brains, respectively. But to go too

far down this path might be to encourage a new mythology, and I would not wish to do

that (unless there was money to be made from it).

CONCLUSIONS

We do have asymmetrical brains, and this is a fact of considerable interest and

importance. But our brains are also highly symmetrical, the result of hundreds of

millions of years of evolution in a world where the difference between left and right is of

virtually no consequence. It is perhaps in the world constructed by humans that the left-

right polarity matters most, as in reading and writing, shaking hands in greeting, driving,

and so forth, but this is in turn a consequence of our own asymmetry. The most likely

explanation or our asymmetrical brains is that certain complex computations are

inefficient if constrained by symmetrical circuits, and are better accomplished within a

hemisphere than by circuits straddling the hemispheres. The advantages of asymmetrical

representation would apply particularly to computations that are not constrained by the

forces that led to bilateral symmetry in the first place—namely, linear movement and the

ability to detect and react to spatial events in the environment. Spoken language fits this

criterion, since it is internally generated and manifest in time, not space.

Even so, the representation of language in the brain is established against a

background of structural symmetry, and we have seen that each hemisphere has at least

the potential to accommodate it. The asymmetrical representation of language does

confer some disadvantages, such as a slight bias toward processing words in the right ear
 29

or on the right side of space, and a corresponding bias of spatial attention and spatial

processing toward the left side of space. The fact that these biases are slight suggests that

bilateral symmetry is still of overriding adaptive significance. If the left brain were to be

totally occupied with language, then we might be easy prey to monsters lurking on the

right (politically as well as spatially, perhaps). Shortly after the developments of the

1960s that led to the modern left brain-right brain cult, Brenda Milner, one of the

pioneers of modern neuropsychology and a careful researcher of cerebral asymmetry,

warned against overemphasizing the asymmetries of the brain at the expense of the

considerable overlap in function between the two sides108, but her warning has been little

heeded.

Given the nature of evolution, it is likely that cerebral asymmetry has been

achieved by tinkering with what was already there, rather than by a rewiring of cerebral

circuits. The kick needed to give the left brain first option for language may have been as

simple as a growth spurt favoring that side at a critical period in the development of

syntax, or it may have been a pruning mechanism that slightly retarded growth in the

right brain—or both. It is extremely unlikely that the incremental processes of natural

selection somehow managed a rewiring of the cerebral hemispheres so that one became

specialized for the complex temporal sequence required for language, while the other was

adapted to complementary spatial, intuitive, and emotional functions. This is not to say

that there are no asymmetries in the way these different functions are represented in the

brain; the problem lies in the simplistic notion that the two half-brains somehow embody

opposite ways of thinking, and that the right brain’s talents have been subjugated. To

understand how the mind works, we need to consider how the brain works as a whole,
 30

and it will not do to simply throw our different mental capacities into those convenient

bins, the left and right brains.

Does the dual-brain myth do any harm? It is perfectly acceptable to contrast

intuition with reason, or holistic thinking with analytic thinking, or emotion with logic,

and it might be argued that there is no harm in linking these polarities with the left and

right brains. The main difficulty is that reference to the brain can be seen as a

legitimizing force to give scientific credence to dubious practices. The idea that there

may be hidden talents lying dormant in a subjugated right brain is a powerful and

reassuring one—almost as reassuring, perhaps, as the idea of life after death, and ripe for

exploitation in much the same way. Unscrupulous therapists, healers, and self-

proclaimed educators, as well as some who are simply naïve, offer ways to release that

hidden potential, and so discover the “stranger within”109, whether through music or

meditation or electrodes—or breathing through the left nostril for a while110. There is

always a market for those who would exploit our fears and disappointments.

My Chambers Concise Dictionary (1989 paperback edition) defines a myth as “an

ancient traditional story of gods or heroes, esp. one offering an explanation of some fact

or phenomenon.” Except for the word “ancient”, this is not a bad definition of science,

where our modern gods are genes and muons and black holes. We do, of course, go

beyond the evidence in constructing theories, and the view of cerebral asymmetry I have

presented in this chapter no doubt contains its share of myth. The problems arise when

we allow the myth to escape from scientific scrutiny and become dogma, and when that

dogma creates financial opportunities from charlatans and false prophets. That is what I

think has happened to the left and right brains.

 31

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