Differences of the Mind between the Sexes

Overview of the Brain

The inner part of the human brain is notionally divided into three main areas.

The forebrain has evolved from organs concerned with smell and also taste. The mid-brain is concerned with vision, while the hind-brain vibration, sound, and balance.

The aim in this section is not to describe the brain in detail, but to highlight areas that will figure in the discussions that follow. In the diagram the cerebral cortex can be seen almost covering the other component parts. We will return to this in the next section, though it is perhaps an artificial distinction to deal with it separately, when it is part of the brain as an integrated whole. The excuse is that, with the cortex, the sexually dimorphic debate takes a completely new direction.

This comes about because, like most of the body, the brain is divided, more or less, symmetrically about the front-to-back center line. The Corpus Callosum, shown in the diagram is a band of thousands of nerve fibers which connects the two halves of the cortex. Although it is usual to write of the hypothalamus, the amygdala and so on, most of the other parts of the brain are similarly divided, being connected by bundles of fibers called commissures. The cerebellum is concerned with fine motor behavior, the co-ordination of automatic processes like walking and running. After much practice, learned complex behaviors like riding a bicycle are thought to become controlled by the cerebellum.

At the base of the brain is the brain stem, the medulla, previously mentioned, the pons and an area called the reticular activating system. These form the upper end of the spinal column. There are many nerve junctions (synapses) here and it is where many of the nerves cross over, since in general the left side of the brain controls and senses the right side of the body, and vice versa. This is also thought to be an evolutionary hang-over. In a very simple creature, it would make sense for a touch on the left to cause movement to the right, and vice versa, to take it way from possible danger.

The main feature below the cortex is the Limbic System, with a variety of different features. Within the limbic system is the amygdala (not shown in the diagram), the thalamus and the hypothalamus. Just below the last-named is the pituitary gland.

An important reason for relating behavior to the hormone environment is the close coupling of the pituitary gland with the hypothalamus. They are very much the control center for the body, regulating body chemicals, eating, drinking, breathing, body temperature, heart rate and sexual functioning.

In front of the hypothalamus is the optic chiasma, where the nerves from the eyes cross over on their way to the visual cortex at the rear of the brain. There are connections to the suprachiasmatic nucleus in the hypothalamus, to provide information about night and day, working in conjunction with pineal gland, generating circadian rhythms.

In humans the olfactory bulbs have virtually disappeared. They are part of the septum, which connects to the amygdala. Hearing is connected via various nuclei to the auditory cortex.

The Hypothalamus

The hypothalamus is a small organ, but one that affects practically every process in the body. It is difficult therefore, in animal studies, to investigate behavior by surgical intervention.

Adams(17) compared the effect of removing the medial parts and the lateral parts of the hypothalamus in rats. In the first case, what he called defensive aggression, where rats fight on their hind legs, forepaw to forepaw, was reduced. In the second case, what he called territorial behavior was reduced. Clearly, one has to be careful about what one means by aggression.

In another study, Kruk used electrodes placed in the hypothalami of a number of rats. Various parts seemed to be involved in various types of behavior, some had no effect, others produced attack responses, others led to biting without any preliminary skirmishes.

Amygdala and Septum

Towards the rear are two prominences, called the mamilliary bodies. In front of these is a bulge, called the median eminence.

In laboratory animals, removal of the amygdala generally renders them docile and difficult to provoke. Stimulation of different areas, in the rat, produces different behaviors. Either there is no effect, or there is defensive fighting, or there is biting attack without warning. However, stimulation in hamsters removes fighting behavior, reaffirming the difficulty in generalizing across species.

For a time, the experiment was tried of removing the amygdala in violent men. It was generally unsuccessful, and in those where it appeared to be successful, the behavior returned within six months.

The septum is less well studied. Its removal makes rats more easily provoked, indicating a balancing relationship with the other centers. However, this complicated by the fact that, as we seen, having the ability to smell another male's scent affects the tendency to fight.

Testosterone: how it works

During development, then, cells in the CNS might or might not be responsive to testosterone, oestrogen, dihydroxytestosterone or, for that matter, a wide range of other chemicals. Those cells that are responsive might survive rather than die. They might start to divide or differentiate.

Hence the idea that they might become organized in ways which form the basis of future aggressive or sexual behavior. Once organized, structures may be sensitized later on. Thus a given behavior, or a particular response to a situation more likely.

This effect may also be indirect. Without testosterone, a male stickleback cannot make the glue that holds its nest together. Without a nest it has nothing to defend, so it is not likely to be aggressive towards intruders. As was mentioned in the previous chapter, the urine of castrated mice does not have the characteristic odor of male mice, so they are not attacked by other males. Since they also do not smell the odor of intact males they also do not attack other mice.

In passing, a comparison of the relative size of the olfactory centers in mice and humans, suggests that smell plays a much more important role for mice.

Fetal hormones and Social behavior

Evidence has been presented, by various studies, that the hormonal balance in the mother's womb has an effect, not only on the genital system of the foetus, but on the development of the brain.

Given the physiological differences between males and females so far described, it would be surprising if there were no brain differences. To what extent, however, does they affect general social behavior? In particular sexual preference and gender identity?

William Cookson, in The Gene Hunters,(18) recounts the study that involved inserting a piece of genetic material, the SRY gene, into the X chromosome of a mouse, making, not a homosexual or transsexual, but a transgenic mouse. (Why are there no newspaper reports about lesbian fruit- flies?) Then he committed a singular anthropomorphism, saying it "considered itself male . . . and that the female was obviously of the same opinion". Does a mouse consider, or have opinions? Does it need to stop and think "I'm a male, that's a female," or vice versa? All it needs are visual and olfactory cues, and the hypothalamus does the rest.

There are suggestions by some workers, from studies of rats, that sexual dimorphism is a gradual process through pregnancy. They propose three stages: the sex centers, then mating centers, then the gender role centers. This last stage determines personality traits such as aggressiveness, sociability or individualism, adventurousness or timidity. However, since many homosexual men demonstrate a very strong male identity, one might consider is that what is described is a transgendered, rather than a specifically homosexual personality. Clearly, too, these traits can only be assessed in relation to a particular social norm.

There is a study that discovered that a disproportionately large proportion of homosexual boys were born in Germany near the end of the war. Considering the lengths that Hitler had gone to in eliminating homosexuals, along with other 'undesirables' this seems particularly ironic. It was, at one time, theorized that their mothers would be likely to be under great stress and it seems that, under such conditions, the level of oestrogens in women is often high. However, there are all sorts of situational explanations. The boys' fathers would be away fighting, or they were undermined by the grim necessity of survival. They might well have been dead, which might make a mother extremely protective. In any case, the whole social fabric of Germany, particularly Berlin, at that time, was distorted and unnatural.

More men report being homosexual than women, though whether there is an actual difference in the numbers is questionable. It has even been seriously suggested that homosexuality can be predicted by amniocentesis, and prevented by pre-natal injections.

Sexual Dimorphism of the Hypothalamus

Since it is the hypothalamus that, in conjunction with the pituitary, regulates the gonadotrophins, along with other hormones, it seems reasonable look for sex differences in it.

The problem with excising whole areas is that they are often concerned with a wide range of functions. Attention has shifted to groups of nerve cells, called nuclei - not to be confused with the nucleus of a cell.

Gorski and his colleagues found a group of cells, which they called the sexually dimorphic nucleus, that was much larger in male rats than in females.

Subsequently, Dutch workers found an area in humans, that they named the sexually dimorphic nucleus of the preoptic area. Unfortunately, it had already been named the nucleus intermedius but, in any case, no one else was able to replicate the findings.

LeVay(19) claims that various areas in the hypothalamus elicit different behaviors. The medial preoptic area leads to mounting behavior, though not apparently controlling sexual appetite. That is monkeys damaged in this area would still masturbate. The event of ejaculation, however, seems to be triggered by another area, the dorsomedial nucleus. He adds that, in females, the region of note is the ventromedial nucleus, which is also involved in feeding behavior. Some neurons control the likelihood of sexual signals, others control receptive behavior, with others controlling cyclic hormone effect. In female primates removal of ovaries, and hence the supply of hormones, does not reduce sexual appetite. However damage to the adrenal glands which produce androgens in both sexes, does.

However Gorski and Allen discovered three nuclei in the preoptic area that they named the interstitial nuclei of the anterior hypothalamus - INAH 1 through 3. INAH1 was not sexually dimorphic but they claimed that the other two were.

This study was replicated, but I have to return to the point that runs through these essays, that we are dealing with statistical averages. Fausto-Sterling(20) points out that although there was a difference in the average between the sexes, the variation within the sexes was more than ten-fold

LeVay obtained a sample of 41 brains from autopsies of people who had mostly died from AIDS, 19 gay men, 16 heterosexual men and 6 women, and selected INAH3, for study. It turned out to be twice as big in his sample from 'straight' men than from gay men. He took into account that the changes could have been caused by AIDS, but the sizes were similar in his heterosexual men, whether they had died from AIDS or not. He admitted that his sample was small, but within his experiment, the results were statistically sound.

The BST-C - Male/Female differences

Particular areas tend to be selected for study because they have large numbers of receptors for gonadotrophins, at least in the various laboratory animals that are studied.

Allen and Gorski(21) directed their attention to part of a group of cells called the bed nucleus of the stria terminalis.

The study involved taking post mortem sections of human brains, age-matched in pairs. That is, each of 13 pairs of roughly the same age was compared. A table of differences by age was compiled and the average taken, so that the result represented the average over a lifetime. The results claimed were that the nuclei of the females were significantly smaller than those of the males, with a volume difference of 2.47.

The relationship between age and the volume of the BST proved not to be significant. That doesn't mean there was no effect. In fact, there was an increase, especially among the male subjects.

Hormones can go to your head The BST-C - Transsexuals

It was the foregoing study that persuaded Zhou(22) and colleagues to look for sexual dimorphism in the transsexual BST. Male to female transsexuals being still fairly rare, the six specimens had been collected over eleven years.

These were compared with specimens from heterosexual women and men, both homosexual and heterosexual. There was regret that specimens could not be obtained from lesbian women, or female to male transsexuals. The subjects were not age matched. Although it was not stated, all appeared to be middle aged, with the transsexuals around 10 years older. The report went to some length to eliminate possible sources of error, such as the effects of hormones during adulthood, and the fact that some subjects had died of AIDS.

The results obtained confirmed the volume difference between men and women, found in the previous study. Results for the homosexual men were comparable with those for the heterosexual men.

However the transsexual BST volumes were in the range of and even smaller those of the women. They, therefore, were also much smaller than those of the homosexual men. The authors concluded a strong relationship between male to female transsexuals and women, and a clear difference from homosexual men.

Once again there was considerable variability; two of the 16 putatively normal heterosexual men were within the transsexual range.

Breedlove, in the same issue of Nature,(23) comments on the complexity of a condition like transsexuality. He points out that it is becoming clear in neuroscience that experience can alter brain structures. We cannot be sure whether the small BSTc caused the people to be transsexual, or whether it was a result of the complexity of the transsexual life.

In all these studies sexual function is confounded with gender identity. Clearly humans, as much as other species, have an awareness of their genitalia and all it entails. However if transsexuals insist that 'who' they are is in their minds and not their gonads, they can hardly welcome this study as the basis of their identity.

However, transsexuals have seized on the study as medical proof that it gives them a 'male' or 'female' brain. They should not have to rely on a study (that is likely to be brought into serious question by many researchers), in order to obtain fundamental human rights.

The Cerebral Cortex

When we turn to the structure of the cortex itself, the biological versus social argument takes a new direction. The cerebral cortex is in two halves, connected by the corpus callosum. The controversy arises from the way the two halves are used, particularly by men, as opposed to women. The intricacy of the nervous system comes from the way axons connect to dendrites, between neurons, so that signals are passed from one to another in a complex electro-chemical process occurring at the synapses.

It is thought that some mental conditions occur due to chemical imbalances which affect the action of these synapses, while others appear to be due to problems with the myelin sheath which insulates the axons.

It is the complexity of the cortex that distinguishes humans and provides human cognitive powers. In other mammals, such as mice and rats, it forms a relatively small part of the brain, appearing as a smooth covering sheet of tissue. In more complex species it becomes more complex, its relative area being increased as it becomes more convoluted. Human skulls are not much larger, relatively speaking, than those of the present day great apes, but the area of the cortex, six cells thick and folded into innumerable convolutions, is far larger in area.

The growing Cortex

The foetus is not passive, it is alive and active. From as early as twelve weeks after conception the foetus can be seen moving, slow rhythmic flexing of its muscles. Its hands and feet respond to gentle stroking by clenching its fingers or toes. Even at this stage it can sometimes be seen with its thumb in its mouth, a complicated action for such a tiny organism. By the seventh month the tiny chest is moving.

There have been many studies about how much the foetus can sense of the outside world. It is believed that it responds to bright lights, and it seems certain that it responds to sounds, by the measurement of its heart rate. There has always been a feeling among mothers that their unborn baby responds to events on television programs and, especially, music.

Opinions on this topic range from one extreme to the other, as to how much of this process is dictated by genetic inheritance.

Edelman(1) seems to be going further than anyone, in suggesting that very little of the neural organization occurs directly through the genes, but is mediated by the interaction with the genetically determined foetus of which it is a part. He suggests that left to itself it would be random, but a special kind of randomness bounded by certain constraints, such as the physical body, which is already highly individual, and the things that happen to it and the things it does. He is proposing what is known as a stochastic process which begins at the moment of conception, and continues long after birth.

Geography of the Cortex

The human cortex is not completely featureless in appearance. It is divided from front to back into the left and right cerebral hemispheres. Each hemisphere has four areas, the frontal, parietal, occipital and temporal lobes.

Researchers have, over the years identified a number of physical configurations which are associated with specific functions. It has been known for many years that the patterns of stimuli from light on the retina of the eyes are carried by nerves across the brain, via the optic chiasma, linking into the visual processing area at the rear, the visual cortex.

While there is considerable controversy about how specialized the cortex is, there is no doubt that there are areas are devoted to certain functions. Motor and sensory areas, along with those for hearing, are present in both hemispheres. The three areas for the uniquely human function of language appear only on the left. These are Broca's area, Wernicke's area, to do with producing and understanding language, and the angular gyrus, concerned with matching the visual representation of a word with its auditory form. Although there are these distinct areas, in terms of function, there seems little to distinguish them at the cellular level,

The axons from the various sensory nerves in the body extend from specific points in the spine, and in turn are connected to specific points in the cortex. Extending in a transverse band right over the brain, there is what amounts to a neural map of the body - the sensory cortex. Some areas, like the hands, the lips and the genitals, which have a high density of nerves, take up more room than others.

Next to this is the motor cortex, again a map of the muscles of the body. Again, structures that are involved in precise, fine movements, like the hands, take up larger areas.

Different areas have been found in different animals, according to their lifestyle. For instance animals which make great use of a particular motor function to explore their world have correspondingly large areas of their cortex devoted to that function, depending on whether they use mainly eyesight, say, or smell.

A rat, for instance, has an area of its cortex for each whisker. There is evidence that development of these areas depends on their usage. However, as one writer puts it, there are no muscles specifically devoted to fighting, thus there is no brain structure devoted specifically to aggressive motor output.


The quality of eyesight experienced by humans is probably the most highly organized and complex of functions. We take our eyesight for granted, yet it is very clear that what we see is, literally, what we think we see. A decade ago a review was prepared of all we know about human vision - it ran into sixteen volumes.

The retina of the eyes consists of thousands of light-sensitive cells, interconnected in a myriad ways to each other and to each optic nerve. The two nerve bundles pass through the optic chiasma near the center of the brain to the visual cortex at the rear. All of these have developed in the individual way already described for the brain. The retinal cells do not detect a pattern of dots as in a television picture, but analyze the view in terms of edges and discontinuities, or relative differences in luminance between one point and the next. In the visual cortex, different cells analyze the response according to orientation of the edge stimulus, its length and rate of change in luminance. Other cell groups respond to temporal changes, appearance and disappearance of an edge in successive parts of the retina, in other words, movement.

In order to achieve binocular vision, the stimuli detected by each eye are compared, by combining them within the visual as a three dimensional structure. This is achieved by the way the fibers cross over in the optic chiasma. Briefly the fibers from each eye that sense left half of the view connect to the right side of the cortex and vice versa.

From birth

The development of the brain continues by the growth of axons until at least six years, and perhaps longer. Many people consider that a newborn baby's brain is about three months premature. They suggest this is nature's trade off due to the difficulties of delivering a baby with such a large skull. By comparison with other species, calculations of length of gestation versus brain size, suggests that birth should be at twenty one months.

The first two years are what is what Piaget called the sensorimotor period as the baby develops physical and perceptual abilities that are not mature at birth. Walking and talking are the obvious examples, but even eyesight follows a process of continuous development through early life, as the eyes grow and the number of cells changes. Although the newborn baby has almost its full complement of neurons, its brain roughly doubles in weight during the first three years, almost entirely due to the development of synapses.

Edelman described the learning process of an infant reaching out to grasp an object. A six week-old baby will almost always reach out to try and touch an object in front of it, but at that age it does not know how to co-ordinate its movements. At one time, it was thought that this was somehow genetically programmed in the brain. All that was needed was the necessary muscular development. It turns out that every baby has to learn how to do it. For many weeks its will wave its arms about aimlessly, trying to focus its eyes on the object. Occasionally by luck they will make contact with the toy. Edelman's theory suggests that there are many thousands of possible connections competing with each other. In other words as the brain develops, it lays down a huge diversity of possible firing patterns, and huge range of possible actions, most of which will be of no possible use whatever. Every time that a sequence of movements, and therefore a pattern of synaptic activity, produces a successful outcome, it strengthens the associated connections, especially since the behavior is likely to be repeated, while unsuccessful connections fade away.

Almost nothing is known about the effects of the fetal environment, except in terms of obvious damage or deformity of the resulting infant. Gross effects, due to alcoholism, smoking and other poisons, can be distinguished fairly readily. More subtle effects are a matter of individual interpretation, particularly where behavior is the study, especially if it is the object of social stereotyping.

It may be that differing hormone environments produce different behavior patterns, which in turn, influence neural organization, but it would seem likely that they are even more subject to social environmental influence, especially social ones. Nevertheless, the result is that no two humans have exactly the same neural organization, not even identical twins.

Edelman's theory of 'Neural Darwinism' in its 'strong' form has, not unsurprisingly, received some criticism. His three books are not an easy read for the uninitiated, but they are reviewed in Rose's book The Making of Memory.(2)

There are, for instance, clearly defined maturational stages. What makes the baby reach out for an object in the first place? Crawling and walking are learnt as the muscles and limbs develop. Adults are so much larger and so different to infants, it is by no means clear how the latter recognize that the former are role models to be imitated.

There is also a built-in urge to communicate, later expressed by language. Language develops so quickly that there must be some predetermined disposition, one for which there seems to be a distinct period of maximum ability.(3) Moreover, just as hearing children learn spoken language, deaf children rapidly learn sign language as quickly.

Some writers even suggest the newborn baby has the necessary mental template to enable it to swim but, not being exercized, in the absence of a water environment, it disappears.

The cortex therefore seems to be composed of generally similar general purpose cells, with a subtle innate code which takes the form of guidelines, rather than rules.

What modern biologists are saying is that the idea of nature versus nurture is out of date. An organism must show specificity, that is, it must grow in accordance with its genes, which have developed over thousands of years of natural selection. It becomes a clearly defined species, specialized for a certain lifestyle, developing in a stable way to resist influences in the environment. But it must also show plasticity, the ability to adapt to rapid changes in the environment. The real challenge for biologists is to understand the relationship between specificity and plasticity, and to acknowledge that there is not one archetypical human brain.

Our genes give us a brain which is wired up to become a specifically human brain, but equally they give us the plasticity to enable our memories and our ways of retrieving them to survive whatever befalls us.

Plasticity and Adaptation

Some of the brain's plasticity is thought to last throughout life. However, children show considerable ability to recover from serious brain trauma. There are studies of children who have had a large part of their cortex surgically removed, usually because of a tumor, and of another person who lived a fairly normal life, even though he was born with most of one hemisphere undeveloped.

There is considerable argument between those who suggest that the cortex is largely undifferentiated, and others, such as Fodor(4), who assert the existence of functional modules. The cortex is certainly not like a filing cabinet, with each memory stored in a neat single location. Notwithstanding studies that show strong activity in certain areas for specified tasks, storage and retrieval of memories seems to involve many diffuse areas at once.

As we have seen, there are certain specialized areas, such as the language units and the sensory and motor cortices. They seem to be generally positioned in the same part of the cortex from one person to another. On the other hand, Lashley(5) showed by experiments with rats that loss of parts of the cortex makes learning in general more difficult. It does not inhibit particular kinds of learning and no particular neural circuits can be found for specific problems.

Association Cortex

What applies to the cortex in general, applies especially to the association cortex - simplistically the thinking area. It is safe to say that we know virtually nothing about it.

Different parts, it is true, seem to be used in different ways. The forward areas seem to be involved in problem solving and strategies, whereas towards the rear different areas seem to involved in making inferences from the senses.

A strong clue that biological factors are involved, is if particular specializations are present at birth, not always an easy thing to determine. One writer,(6) for instance, suggests that there is considerable lateralization at birth and it seems that these differences are apparent quite early in life. Another(7) suggests that the two hemispheres are fairly equal up to five years old. Possibly the difference between the two writers is a matter of degree, rather than outright contradiction.

What of a boy who spends his early childhood with books, rather than construction sets? Or the girl who is free to roam the countryside, rather than being closeted within the home? What about a child who grows up in an academic or intellectual family? Would it benefit boys' later language abilities, if fathers, with whom they identify, were more articulate and demonstrated a love of books? What about more subtle social stimuli. Does a child have an innate predisposition to react differently to different sex others? Or is it a learned reaction to different personalities regardless of their sex? Is a toddler, growing up in Northern Ireland forever doomed to an unreasoning hatred of those who don't share its family's religious loyalties?

Left brain, Right brain

The discovery that, between the two hemispheres there have been found marked, and fairly controversial, differences in function, came from a group of operated epileptic patients. Normally, the two halves are in continuous communication through the connecting nerves of the corpus callosum.

In the patients concerned, the very severe seizure they experienced in one hemisphere would cross over and trigger a massive effect in the other. To prevent the crossing over, their corpus callosum was surgically severed, with the result that their symptoms diminished considerably, and they appeared to function perfectly well in daily life.

What had actually happened was fairly subtle. If you gave a such people, say a bunch of keys, they could both name it and undo a lock with it. However, if a word was presented only to their left field of view, and they were asked to select from a group of objects behind a screen, they could do so but not say what the word was.

The two halves of the subject's brains were unable to communicate with each other, and it was suggested that the two halves were specialized for dealing with different kinds of information. It will be noted that naming and recognition are functions of the association cortex, not of the visual system which remained complete. The subjects would be asked to fix their eyes on a spot in the center of their visual field. An image would be flashed briefly on either the left or right.

Later, goggles were devised that would occlude either the right or the left half of the visual field of each eye. To show that the effect was not a result of incidental brain damage in the patients, similar studies were conducted with people whose corpus callosa were intact.

There are two hypotheses concerning the way this works. One suggests that the information is processed by the hemisphere that first receives it, but there may be a difference in ability. The other view is that the information is transferred to the other hemisphere, incurring a slight delay.

The general hypothesis is that there is a left hemisphere specialization for language functions, and a right hemisphere specialization for visuospatial stimuli. It has been claimed that subjects recognize faces presented to the left visual field, and therefore the right hemisphere, more quickly. Generally, although the processing of words by the left has been supported, it has been more difficult to find material which is preferentially processed by the right.

It should be emphasized that the two halves of the cortex work in conjunction with each other, as an integrated system, by communication through the corpus callosum. However, studies using EEG (electro encephalograph) measurements show differences in activity according to the task. There is proportionally higher activity in the left side for verbal tasks, such as reading, and in the right side for spacial tasks, non verbal tasks, such as making complex designs, or interpreting facial expressions.

There is some evidence from other neurological patients. People with right side damage often lose their sense of direction. Post mortem examinations show that the left hemisphere is usually larger and, while the right hemisphere has many long neural fibers connecting widely separated areas, the left hemisphere has shorter fibers providing rich connections over smaller areas.

There are several studies connecting lateralization with whether one is right or left-handed. Until recently it was thought that only humans were lateralized but it has been shown that other animals show handedness. They will, for instance, use one paw preferentially for handling food or beginning to walk. However the proportion of different individuals is fifty- fifty, whereas around 90% of humans are right-handed. It seems likely that animals begin life using a particular paw, which then acquires more dexterity, so that the habit continues.

Among primates there has been controversy, with arguments that it depended on the tasks observed. It would seem that there is greater handedness on more complex tasks, a handedness that has been magnified in humans. In other words the distinction seems to be between handedness and manual specialization. However, some humans may, for instance, use the right hand exclusively for writing, but either hand for reaching out. Others may be exclusively right handed.

Corpus Callosum

The corpus callosum itself has attracted the attention of biologists searching for sex differences. It will be remembered that it was surgery to sever it that drew attention to the differing organization of the two sides of the cortex.

There is a great deal of dispute about whether there are reliable average differences between the sexes. Originally, it was claimed that it was larger overall in women, relative to brain size. Later the claim was that the posterior portion, the splenium was larger.

Fausto-Stirling(8) is extremely critical of studies in this area. Since 1982 there have been at least seventeen papers published. Since no two approach the problem in the same way, she suggests that none of them corroborate each other. What does appear is that there are changes with age, yet only one of the studies used age-matched subjects. Also, if there any sex differences at all, they show up after birth, possibly not until after adolescence.

Considering the millions of axons which must traverse this region, there is no total picture of their path. Larger nerve bundles can be traced leading to the front and back but, though a reasonable general rule is for them to take the shortest path, this is by no means inflexible..

The result of differences in the corpus callosum are said to result in a greater relative fluency of thought and speech. Reminding ourselves that no-one has actually counted the number of axons, nor traced their connections, we are told that this results in greater communication between the cerebral hemispheres of women. It is suggested that women's greater sensitivity to emotional, non verbal communication, even their intuition, comes from the greater connectivity in their minds. A man is more purpose orientated. Emotions are kept on the right side of his brain, which, being less connected to the left, mean that he can, less easily, express emotions. Clearly, biological effects are not the whole story, for men are expected to be relatively unemotional.

There is another structure that connects between the cerebral hemispheres, the anterior commissure. It communicates visual, olfactory and auditory information and is larger in women than men. Allen has demonstrated that it is also larger in homosexual men.

Size isn't everything

A myth that surfaces, from time to time, is one from the nineteenth century that purported to show that women have smaller brains than men. It had been put forward in the nineteenth century in an effort to prove that women (and black people) were inferior. The authors of that time had not taken account of the fact that women are, or were, in general smaller overall than men. Even then it was pointed out that there was such a wide variation, an enormous sample size would needed to show a significant difference.

Was it, then, true? And why did it matter? Fausto-Sterling answers the first question fairly effectively. "the average male/female difference in brain weight for all ages is 9.8%. when charted as a function of either height or weight, however, the difference in adults virtually disappeared." This from a study of over four thousand subjects.(9)

What matters is the complexity of the cortex. If overall size was all that mattered, elephants would have a considerable intellect. The human cerebral cortex contains some ten to fifteen thousand million neurons, with four times as many glial cells, and one million billion synaptic connections. Spread out, the total surface area would cover about three quarters of a square meter.

Sex and Lateralization

Where the gender debate first arose, was from claims about differences between men and women in the way they use the two halves of the cortex.

The original hypothesis was that men used their logical left side while women used the emotional irrational right side. However, the argument soon arose that, if language was a function of the left side, how was it that women were better at expressing themselves verbally?

This is rather a simplistic view of the controversy, however, the theory was modified to suggest men have greater lateralization, that their abilities are more compartmentalized, while, in women utilization of the two halves is more diffuse.

From the sixties onward, Landsell was working with people who had damage to one side of the cortex or the other. The knowledge of the time indicated that damage to the left hemisphere should lead to deficits in verbal tasks, while right-side damage should produce deficits in visuospatial tasks. This proved particularly true for men, but the prediction was not borne out well for women. It led him to speculate that the abilities of the two hemispheres overlapped to an extent.

Electroencephalogram measurements have also shown a difference. When given abstract problems to work out, men showed a great deal of activity in the right side of their brain, while for women the activity was more generalized to both sides. Similar studies with teenage boys and girls gave similar results.

With women who had Turner's syndrome, which comes about because they have only one X chromosome, XO, and are considered to behave in a very feminine manner, this diffusion of organization was particularly marked. The phenomenon has also been found in men whose exposure to androgens in the womb was reduced.

Workers following hormonal hypotheses have found that in rats given testosterone at birth, the females developed a larger corpus callosum. Others have found that male rats showed a thicker right hemisphere, except when they were very old. One developmental theory is that high levels of prenatal testosterone slow neuron growth in left hemisphere.

However, Shute(10) analyzed blood samples from groups of males and females whose hormones were within the normal range. For spatial tests, females with high androgen levels performed better than their lower androgen counterparts. However, low testosterone men performed better than high testosterone men, leading the researchers to conclude that high androgens may inhibit the acquisition of spatial skills, and that there may a low optimum level.

Other tests have claimed that females are superior in language, verbal fluency, speed of articulation and grammar, also arithmetic calculation. Their perceptual speed, for instance in matching items is better, and so is their manual precision. Males are reckoned to be better at tasks that are spatial in nature, such as maze performance and mental rotation tasks. Also mechanical skills, mathematical reasoning and finding their way through a route. Certainly, among brain injury patients, after damage to the left hemisphere, long term speech difficulties occur three times more often in males.

Some critics asked why, after a hundred years of research, these findings have only just appeared. One reason may be that most of the subjects studied originally were male war veterans. But, in any case, nobody had looked for sex differences. What we are discussing are average differences which are statistically significant but their effect is very small within a very wide range of individual variation. The investigator must be specifically looking for them, using a large number of subjects.


Differences in brain anatomy have included the length of the left temporal plane, which is usually longer than the right. Of those showing a reversal, which was assumed to reflect a lesser degree of lateralization, most were female. However, as Springer and Deutsch(11) warn us: "the link between anatomical asymmetries and functional hemispheres is an untested assumption."

Cerebral blood flow is used as a measure of cerebral activation and, in a mental rotation task, women scored significantly lower. Both men and women showed greater right hemisphere activity, though with men it was greater in the right frontal lobe, and with women it was greater in the temporal-parietal region. Other differences have been found in other tasks, but there is no way of telling whether they are due to a difference in structural organization, or simply the use of different strategies.

Some of the results are difficult to compare with others. For instance in recognizing melodies and familiar sounds, women have had a left ear advantage, while in men, the difference was very small. Some workers have suggested that lateralization for certain nonverbal auditory stimuli may be greater in women, rather than less.

Another problem is that the degree of lateralization for auditory and visual tests do not always correlate for one individual. It may be that different individuals have different organization for different tasks, or they are bringing in strategies that the experimenter didn't intend, thus confounding the results. Repeating the tests at a later date, with the same subject, does not always produce the same result, as though on each occasion the problem has been approached in a slightly different way.

Unlearning learning

We have seen how plastic cortical development is. Even with laboratory rats, it has been shown that those reared in a stimulating environment develop a much more intricate cerebral organization than those reared in nothing more than a bare cage. Development is not either predicted by biology or learning.

Brain development goes on for many years after birth. It clearly must be influenced as much by the environment after birth as it was before. Exactly how and why, and by how much, is something that psychologists and biologists generally are very reticent to explore. They continue to work on independently following their separate paradigms, and do not cross the boundary. Psychologists use the general assumption that memory is composed of patterns of neuron firing. Biologists tend to work with permanent structures. It is thought that if a particular synapse is active often enough, it becomes more permanent, operating in preference to other possible synapses.

Others(12) have made suggestions based on the assumption that the degree of myelinization of a particular area of the nervous system is a measure of its maturity - or, conversely, its loss of plasticity.

Clearly the social experience of a young baby is limited, but even then it is interacting, soaking up experience like a sponge. In an astonishingly short time it becomes proficient in a complicated, illogical language. Even before an infant begins to talk, it understands sentences containing quite complex logical sequences.

Socialization begins when it meets other children. In the days of the tribal group, this may have been from its first steps. In recent England, school began at five, and its primary experience would have been its parents, its siblings, relatives and visitors, perhaps next-door's children.

The author has, from time to time, met counselors, and other, who claim that transvestites can be cured. Gender reassignment is seen by a prejudiced National Health Service as elective cosmetic surgery. Gay people choose their way of life. Can anyone become other than who they really are? Something that is learned can be unlearned surely? Perhaps it is in reaction to such attitudes that certain groups of TV's and others are so insistent about the biological model - otherwise they could 'help' being who they are.

It is assumed that much of one's personality is learned, with an Eysenckian biological substrate, yet it is also assumed that any extensive personality change means trouble. It's a question that psychology has not really addressed, perhaps developmental neurobiology will, one day, provide some answers, if it can, once and for all, free itself from political gender bias.


Many critics have complained of the prevalence of what psychologists call the type 1 error in a number of these studies. That is, the differences are real when the results are actually due to chance. The problem is in extracting common features in a area where individual people vary greatly.

On balance, Springer and Deutsch(13) accept that there is a very small but consistent greater degree of lateralization in male humans. They conclude "Our review of the lateralization literature in general has given us a healthy respect for the type 1 error . . . . the consistency of reports of sex differences . . . . lead us to accept their reality, at least as a working hypothesis . . . . . there are true differences that are small in magnitude and easily masked by individual variability or other factors that are not controlled."

Such differences as have been found have been labeled by most writers as differences in cognitive style. Given the difference in socialization between girls and boys, it is hardly surprising that this occurs.

Witleson concluded that people use their 'preferred cognitive strategy' based on the faculties they have. It is suggested that men and women may tend to think in different ways, but every individual thinks in his, or her, individual way - each of us uses our preferred mental strategy. Let us not come to believe that all women think in one way and all men in the other.

Certainly, a study of adult male-to-female transsexuals found that they were better in verbal memory, and worse in mental rotation tasks than a control group of men. Groups of both male and female transsexuals groups also did not show a clear degree of lateralization. Apart from the fact that, once again, they were possibly extreme cases, it does not necessarily show that their minds were 'opposite sexed' for biological reasons. It could just as well be argued that they acquired transsexual minds because of their conflict with the cultural criteria demanded of them.

The theory must be able to accommodate itself to allow for general differences, not stigmatizing or clinicizing those who do not conform. Men and women, perhaps, follow careers that utilize their individual abilities in the most satisfying and successful way. In spite of the predictions of biological determinism, there are female artists, designers, even mathematicians, and we are not short of male communicators.

As Sayers(14) says: "If boys are more able in Mathematics and girls have a greater verbal ability, it is hard to see how men can be better fitted for political life and their dominant role there." What we have discovered should not be a prohibition against a man or a woman from entering a career normally viewed as being the province of the other gender, because of the way we suggest he, or she, 'ought' to think.


Throughout this chapter the difference between the cerebral hemispheres has been described as being between verbal versus spatial abilities, with a qualitative difference between women and men. Most workers believe this to be far too simple an idea. It may be that we are labelling the mental organization in terms of the rather limited tests we are applying - we look for something, so we find it.

Considering the whole range of thought processes to which humans bring a whole range of strategies, it is possible that each problem that an individual's brain attends to is unique, happening for the first time in human history.

What else can be said about the features of brain lateralization? A more realistic way of describing the situation may be to suggest that each hemisphere approaches a task in a different way. Thus the left side may analyze the problem while the right considers it as a whole. This division has created a whole raft of hypotheses, such as rational vs. intuitive, and western versus eastern thinking.

In turn there has been a rash of claims like "Unleash the power of your right brain. Send $50 for our five-day course." Another is quizzes in popular journals which claim to test whether readers think like a man or a women. Naturally those completing the questionnaire already know how they ought to think, as men or women, and even know the 'correct' answers to the questions.

As one group of writers(15) suggest "hemispheric specialization has become a sort of trash can for all sorts of mystical speculation."

Nevertheless some insights have come from some more reputable sources. One needs to describe first the difference between conscious and automatic behaviors. Once we have learned to walk or ride a bicycle, we never forget. Current thinking is that such knowledge is transferred to the cerebellum. Probably, the automatic actions in manipulating the controls of a motor car are stored there also.

However, in our daily round we develop what are called action scripts, habitual procedures like making a cup of tea. If one goes to one's bedroom to change for an outing and, instead, puts on one's nightclothes and get into bed, it is the confusion of two action scripts. So, some workers believe that the right hemisphere handles processes for which there is an established routine, while the left side deals with novel situations. Perhaps the right brain handles more familiar tasks for which an action script is already available, while the left analytical side is better equipped to handle new situations.

This leads to an interesting speculation. We have all been cursed with the driver on the motorway, hogging the middle lane, operating on right side 'autopilot' mode, while his attentional left hemisphere is chatting to his passenger. If women have better communication between the hemispheres, perhaps they can switch control more easily, and they really are better drivers than men. Perhaps insurance companies should calculate premiums on the basis of brain scans taken while the person is performing a series of standardized tasks. Crazy, perhaps, but no more outlandish than the claims in some 'pop' psychology books.

Another hypothesis includes the function of the corpus callosum, which connects each side of the brain topographically - that is each fibre from a neuron in one side connects to its equivalent in the opposite side. This is described more fully in Springer and Deutsch,(16) but the idea is that an image in the left half, say a cow, inhibits the image in the right half, which allows it to conjure up associated images, like milk or a field.

Psychology students will be familiar with the words "Top down, bottom up," but other speculations have included distinctions between analysis and insight, while another compares the right hemisphere to Freud's seat of the unconscious

It has been suggested that, not only is the human brain more complex than we think, it is more complex than we can comprehend.

No doubt the debate about sex differences in general will continue ad nauseum. One study will suggest "the difference in size between the sexes has not escaped the notice of sociobiologists." Another will point out that the size dimorphism in humans is less than for any other primate. It all depends on which side of the bread you like to spread your butter.


  1. Edelman. G.M, (1987-8)
    Neural Darwinism.
    The theory of neuronal group selection: The Remembered Present,
    New York: Basic Books Inc.
  2. Rose. S, (1993) The Making of Memory: From molecules to mind, Ealing: Bantam Books.
  3. Pinker, S., (1994) The Language Instinct: How the mind creates language, New York: William Morrow and Company
  4. Fodor, J., (1996),The Modularity of Mind, Cambridge, Massachusetts: MIT Press.
  5. Lashley, (1929) uncited reference in Gross.R.D., (1987) Psychology: The Science of Mind and Behavior (p391), Hodder and Stoughton.
  6. Springer, S.P., Deutsch, G., (1993) Left Brain Right Brain (Fourth ed. pp 233-252), New York: W.H.Freeman.
  7. Zaidel (1978), uncited reference in Gross.R.D., (1987) Psychology: The Science of Mind and Behavior, Hodder and Stoughton.
  8. Fausto Sterling, A., (1992) Myths of Gender, Biological Theories about Women and Men, New York: Basic Books
  9. Dekaban, A., (1978) Changes in Brain Weights during the span of Human Lives: relation of brain weights to body heights to body weights, Annals of Neurology 4(1978):345-56 in Fausto Sterling, A., (1992) Myths of Gender, Biological Theories about Women and Men, (p227) New York: Basic Books
  10. Springer, S.P., Deutsch, G., (p215)
  11. Springer, S.P., Deutsch, G.,
  12. Gibson, K.R., (1985) Myelinization and Behavioral Development: A Comparative Perspective on Questions of Neoteny, Altricity and Intelligence, in Gibson, K.R., Petersen, A.C., Brain Maturation and Cognitive Development, New York: Aldine De Gruyter.
  13. Springer, S.P., Deutsch, G.,(p212),
  14. Sayers, J., (1982) Biological Politics, London: Tavistock
  15. Rose.S, Lewontin.R.C, Kamin.L.J, (1990) Not In Our Genes: Biology, Idealogy and Human Nature. (p146) Harmondsworth: Penguin Books.
  16. Springer, S.P., Deutsch, G., (p299)
  17. Adams, D., (1971) Defence and territorial Behavior dissocaiated by hypothalamic lesions in the rat, Nature, 232, (573-574) also Kruk, M.R., van der Poel, A.M., Meelis, W., Hermans, J., Mostert, P.G., Mos, J., Lohman, A.H.M., (1983) Discriminant analysis of the localization of aggression inducing electrode placements in the hypothalamus of male rats, Brain Research, 260, (61-79) in Toates, F., (ed) (1992) Biology, Brain and Behavior: Control of Behavior, Milton Keynes: Open University Press.
  18. Cookson, W., (1994) The Gene Hunters: Adventures in the Genome Jungle London: Aurum Press
  19. LeVay,S., (1994) The Sexual Brain, (Page 72) Massachusetts Institute of Technology.
  20. Fausto Sterling, A., (1992) Myths of Gender, Biological Theories about Women and Men, (p244) New York: Basic Books
  21. Allen, L.S., Gorski, R.A., (1990) Sex Difference in the Bed Nucleus of the Stria Terminalis of the Human Brain, Journal of Comparative Neurology, 303:697-706
  22. Zhou, J.N., Hofman, M.A., Gooren, L.J.G., Swaab, D., (1995) A Sex Difference in the Human Brain and its Relation to Transsexuality, Nature 378, 68-70,
  23. Breedlove, M., in the same issue Another Important Organ, pp 15-16.

Recent Findings

Sexual dimorphism of human brain anatomy has not been well- studied between 4 and 18 years of age, a time of emerging sex differences in behavior and the sexually specific hormonal changes of adrenarche (the predominantly androgenic augmentation of adrenal cortex function occurring at approximately age 8) and puberty. 2. To assess sex differences in brain structures during this developmental period volumes of the cerebrum, lateral ventricles, caudate, putamen, globus pallidus temporal lobe, amygdala, and hippocampus, and midsagittal area measurements of the corpus callosum were quantified from brain magnetic resonance images of 121 healthy children and adolescent and examined in relation to age and sex. 3. Males had a 9% larger cerebral volume. When adjusted for cerebral volume by ANCOVA only the basal ganglia demonstrated sex differences in mean volume with the caudate being relatively larger in females and the globus pallidus being relatively larger in males. The lateral ventricles demonstrated a prominent sex difference in brain maturation with robust increases in size in males only. A piecewise-linear model revealed a significant change in the linear regression slope of lateral ventricular volume in males after age 11 that was not shared by females at that or other ages. 4. Amygdala and hippocampal volume increased for both sexes but with the amygdala increasing significantly more in males than females and hippocampal volume increasing more in females. 5. These sexually dimorphic patterns of brain development may be related to the observed sex differences in age of onset, prevalence, and symptomatology seen in nearly all neuropsychiatric disorders of childhood.
Giedd, J. N.; Castellanos, F. X.; Rajapakse, J. C.; Vaituzis, A. C.; Rapoport, J. L.
1997 Nov; 21(8): 1185-201; ISSN: 0278-5846.

The developmental effects of androgen play a central role in sexual differentiation of the mammalian central nervous system. The cellular mechanisms responsible for mediating these effects remain incompletely understood. A considerable amount of evidence has accumulated indicating that one of the earliest detectable events in the mechanism of sexual differentiation is a selective and permanent reduction in estrogen receptor concentrations in specific regions of the brain. Using quantitative autoradiographic methods, it has been possible to precisely map the regional distribution of estrogen receptors in the brains of male and female rats, as well as to study the development of sexual dimorphisms in receptor distribution. Despite previous data suggesting that the left and right sides of the brain may be differentially responsive to early androgen exposure, there is no significant right-left asymmetry in estrogen receptor distribution, in either sex. Significant sex differences in receptor density are, however, observed in several regions of the preoptic area, the bed nucleus of the stria terminalis and the ventromedial nucleus of the hypothalamus, particularly in its most rostral and caudal aspects. In the periventricular preoptic area of the female, highest estrogen receptor density occurs in the anteroventral periventricular region: binding in this region is reduced by approximately 50% in the male, as compared to the female. These data are consistent with the hypothesis that androgen-induced defeminization of feminine behavioral and neuroendocrine responses to estrogen may involve selective reductions in the estrogen sensitivity of critical components of the neural circuitry regulating these responses, mediated in part through a reduction in estrogen receptor biosynthesis.
MacLusky, N. J.; Bowlby, D. A.; Brown, T. J.; Peterson, R. E.; Hochberg, R. B.
1997 Nov; 22(11): 1395-414; ISSN: 0364-3190.

The ganglioside composition of the cerebral hemispheres of young and adult rats of either sex has been herein assessed for the first time. In females, the total ganglioside content at any age, the content of GM1, GD1a, and GD1b at 8 days, and the content of GM1, GD1b, GT1b, and GQ1b at 60 days were higher in the right than in the left hemisphere. In males, no difference was observed. Concerning the ceramide moiety, a difference was displayed by C18:1 long-chain base in GD1a, whose proportion was higher in the left than in the right hemisphere of females aged 8 days. The comparison between homolateral hemispheres of rats of different sex revealed several differences. On average, in 8- day-old animals, the content of gangliosides was higher in females than in males. At 60 days the amount of gangliosides was on average lower in females than in males, even if with some exception. The data obtained with the current investigation show the existence of a ganglioside lateralization in rat brain, exclusively in females, and almost entirely at charge of the oligosaccharide portion. Moreover, age-dependent changes of ganglioside pattern and content show a dependence on brain lateralization.
Palestini, P.; Toppi, N.; Ferraretto, A.; Pitto, M.; Masserini, M.
1997 Nov 15; 50(4): 643-8; ISSN: 0360-4012.

Need web page hosting, Design or codework in HTML, CGI, or Java?
Email us at:

Copyright 1999 by MINDFART