Why Can't a Man Be More Like a Woman? Read online

  Understanding the differences between the brains of the two sexes is complex and difficult, and is often further complicated because the differences may be due not to embryonic development but to social experience. Moreover it has been argued that apparent sex differences in the relative size of specific regions of the brain may be accounted for by differences in individual brain volume. During development the brain is influenced both by hormones such as testosterone and oestrogen and by other factors in the environment of the developing brain cells. These can affect which genes are turned on and so which proteins are made, and thus control the development of the brain. The sex chromosomes, particularly the Y, also play a role. Complex regulatory networks that involve hormones and gene activity can promote sex differences in nerve cell connectivity and function. Post-mortem analyses have revealed pronounced sex differences in the distribution of hormone receptors in male and female brains.

  A number of genes, including some from the sex chromosomes, are expressed differently in the male and female brain, with hormones again playing a key role. These genes can affect neuronal connections in the brain in subtle ways. The SRY gene, which initiates male hormone expression via the testes, is expressed also in the brain, and six genes on the X chromosome are expressed at higher levels in the male brain. SRY and another Y chromosome gene have been found to be expressed in the hypothalamus, which links the nervous system to the hormone system, and in the frontal cortex of adult males, but not females. At mid-gestation significant sex differences in gene expression are found for genes encoded on the Y chromosome. A dramatic advance was made by a team of Swedish researchers, Trabzuni and team, who reported that 1,349 genes are expressed differently in the brains of men compared with women. The function of these genes remains to be determined. Ingalhalikar and his group found that male connections in the brain go from front to back while those of females go from side to side. They suggest male brains are structured to facilitate connectivity between perception and co-ordinated action, whereas female brains are designed to facilitate communication between analytical and intuitive processing modes.

  A very different view of the brain is illustrated by two recent books which largely ignore biological influences and focus on social influences. Cordelia Fine, in her book Delusions of Gender: The Real Science Behind Sex Differences, shows an unfortunate misunderstanding of embryonic development and cell biology, arguing that genes do not determine our brains but ‘constrain’ them. That is wrong, as genes do determine the development of the embryo, including the brain with its neural connections that control behaviour. Worse still, Fine does not seem to be aware of biological differences in sexual behaviour, and of how we are a society of cells that determine everything we do. Another book, Rebecca Jordan-Young’s Brainstorm: The Flaws in the Science of Sex Differences, is similar and claims that ‘whatever is written in our genes must be an open-ended story’. She writes that ‘The evidence for hormonal sex determination of the human brain better resembles a hodge-podge pile than a solid structure.’ She believes that any brain differences between the sexes have probably developed as a result of social experience but she ignores studies showing such differences in behaviour even in very young children. Both books also ignore the key insights into genetic differences revealed by an understanding of evolution. By contrast a book which sensibly takes biological factors into account is Diane Halpern’s Sex Differences in Cognitive Abilities.

  The evidence for genetic differences in males and females is, of course, overwhelming; one has simply to look at the question of sexual attraction. The brains of men and women may be mainly alike, but they show consistent and important differences with implications for each sex. The differences can influence specific behaviours and may contribute to susceptibility to specific diseases. The male brain is about ten per cent larger than the female, even when the larger body size of the male is taken into account. Men have on average twenty-three billion neurons in the dorsal region of the cerebral cortex, which is largely responsible for higher brain function, compared with nineteen billion in women. The neurons themselves, whether from men or women, show different properties in cell culture and differ in their responses to toxic chemicals. Male brains contain more neurons in specific areas, while the brains of women show greater complexity in certain areas. One cannot overemphasise the complexity of the brain: for its size, it is the most complex structure in the universe.

  Differences in brain structure have been investigated by a variety of methods, including post-mortem dissection. Recent neuroimaging studies on living subjects have accumulated substantial evidence supporting the notion that sex is associated with differences in brain connectivity. Functional magnetic resonance imaging (fMRI) harnesses the paramagnetic properties of oxygenated and deoxygenated haemoglobin, providing images of neural activity due to changing blood flow in the brain. It takes repeated scans, usually one a second, which track the movement of blood through the brain. From this movement it is possible find out which sections of the brain are active and responding to outside events and activities, and to do so ‘live’. The images created give a visual representation that reflects which brain structures are activated during the performance of various tasks. For example, a subject may be presented with emotional images, have their memory tested or solve a puzzle. Using powerful magnets fMRI can localise changes in brain activity to regions as small as one cubic millimetre, so it is an excellent tool for the investigation of functioning brains. Another MRI technique is diffusion tensor imaging, which can characterise local microstructure based on water diffusion and show connections between different regions.

  There have been numerous attempts to find differences in behaviour that correlate with sex differences in the human brain, but the results so far are inconclusive. There are arguments both for and against the influence of sex on the way that men and women’s brains function. It is a controversial area, but it is generally thought that there are differences and that these will be demonstrated eventually. As we shall see, there are significant differences in behaviour from an early age.

  As Melissa Hines has pointed out, experience can change the brain throughout its lifetime, and the development of new connections between neurons continues into adulthood. A brain sex difference may be caused by experience even if it is known to be influenced by early hormone exposure. Some people with early hormone abnormality have been of key significance in studying neural function or structure. Some neural differences in girls and women with congenital adrenal hyperplasia–CAH–have been of great significance in the study of the role of testosterone in human development. The embryos of individuals with CAH are exposed to high testosterone levels before birth, similar to those of healthy male foetuses. Consequently females with CAH are born with partially masculinised external genitalia and show increased male-typical behaviour throughout their lives. Both normal boys and girls with CAH were found to have smaller amygdalas, involved in emotional memory, but no other differences in brain structure were identified. A brain that develops as male in the embryo is far less likely to display female sexual behaviour in adulthood, even if treated with female-typical hormone replacement.

  The cerebrum, including the cerebral cortex, is the largest part of the human brain, and is associated with higher brain function such as thought and the control of movement. The human cerebrum comprises left and right hemispheres, and information is exchanged between them mainly by a bundle of nerve fibres, the corpus callosum. Some small sex-related structural differences have been identified. The two hemispheres look mostly symmetrical, yet there are some controversial studies which have shown that each side functions differently and that in this respect too there are male–female differences.

  Of particular importance in relation to sex differences is the amygdala, which processes and stores memories of emotional events and is also involved in immediate emotional responses. When active it gives rise to fear and anxiety. In the adult human brain, the male amygdala is significantly larger th
an the female, even taking total brain size into account. The right amygdala shows a greater functional connectivity in men than in women, but the left amygdala shows the opposite trend. There is significantly greater activity in the left amygdala of women remembering emotionally arousing pictures. The stria terminalis serves as an output pathway of the amygdala and connects to the hypothalamus. Its central subdivision, the bed nucleus of the stria terminalis (BSTc) is on average twice as large in men as in women. This sexual difference of the BSTc does not develop until adulthood, at about twenty-two years of age.

  Part of the hypothalamus, which links the nervous and endocrine systems, is twice as large in men as in women. The hypothalamus synthesises and secretes a number of neurohormones which in turn stimulate or inhibit the secretion of pituitary hormones, including some related to sex. The sexually dimorphic nucleus (SDN) is a cluster of cells in the hypothalamus that is believed to be related to sexual behaviour. The male SDN is about twice as big as the female because it contains larger cells and more of them. The difference is due to exposure to higher levels of testosterone in the male embryo.

  Only a few neural sex differences in the brain have been linked to behaviour. A well-established link involves the third interstitial nucleus of the anterior hypothalamus (INAH-3). The structure of this part of the hypothalamus has been found to be two and a half times larger in men than in women and to contain twice as many cells. This region is linked to gender identity, as we shall see when considering transsexuals. INAH-3 is smaller, and therefore more like that of females, in homosexual than in heterosexual men.

  Broca’s area is a region with functions linked to speech production and may be larger in women. One region which is certainly larger in women is the hippocampus, which plays a key role in both short-term and long-term memory. Left-hemisphere auditory and language-related regions, compared to overall brain size, are larger in women than in men. This may indicate a structural basis for a female superiority in language processing. Primary visual and visuospatial association areas of the parietal lobes, involved in spatial sense and navigation, are proportionally bigger in men, in line with the reports of superior male visuospatial processing.

  The ratio between the size of the orbitofrontal cortex, a region involved in regulating emotions, and that of the amygdala, involved in emotional reactions, has been found to be significantly larger in women than men. This may relate to behavioural evidence for sex differences in emotions, since women are generally considered to be better at expressing their feelings than men, and to have superior skills in bonding and connecting to others. The straight gyrus, a ridge on the cerebral cortex, is about ten per cent larger in women compared to men after correcting for males’ larger brain size. It is claimed that a large gyrus correlates with a more ‘feminine’ personality in adults, irrespective of biological sex. Unexpectedly, however, the gyrus is actually larger in boys than girls.

  A distinction is drawn between grey and white matter in the brain. Grey areas are those which contain more cell bodies, while white matter is mainly made up of axons, the long extensions whose colour arises from the whiteness of their insulating myelin. In the past, studies of white matter used invasive techniques such as dissection and histological staining, and so could only be done post mortem. Modern neuroimaging techniques are non-invasive, and several MRI techniques can be used to investigate white and grey matter. Males have more grey matter in their brain than women, but there are certain regions in the brain where women have more grey matter than men. Grey matter represents information processing centres in the brain, and white matter represents the connections between such centres. The significance of these differences is not fully understood, and the different sizes of the brain complicates matters, but the differences may help to explain why some men are better at tasks requiring local processing like maths, while women are better at integrating and assimilating information, such as that required for language.

  Yet although these structural differences in the brain may be related to certain skills, they have no effect on intelligence. Since women have proportionately less white matter but more grey matter, it seems possible that they might make more efficient use of the available white matter. Diffusion magnetic resonance imaging has in fact shown more efficient organisation in the female brain, finding that women have higher local functional connectivity–the degree to which activity in each part of the brain is correlated to activity in every other part. According to Tomasi and Volkow, men have lower brain connectivity compared to women and this might affect functions that require specialised processing, such as spatial orienting, whereas women’s higher connectivity may be related to optimising language skills. Indeed, they say, brain imaging studies show that women have higher brain activation and better performance during difficult verbal tasks than men. Women access both sides of their brain more efficiently and can therefore make more use of the right side of the brain. This may explain why they can focus with greater facility on more than one task at a time. They are reported to often preferring to solve problems through multiple activities, and this may be related to claims that they are better at multitasking.

  There is evidence that activities which require high levels of training or which make intensive cognitive and motor demands, such as preparing for exams or even juggling, can lead to changes in the amount of grey matter in particular regions. Learning involves changes in the connections between neurons and two things may happen: new connections may form and the structure of existing junctions may change. For example, London taxi drivers are reported to have larger hippocampuses than London bus drivers, because the hippocampus is specialised for acquiring and using complex spatial information such as that needed to navigate efficiently. Taxi drivers have to remember hundreds of routes whereas bus drivers follow a fixed route. Small changes due to genes could also have significant effects on grey and white matter.

  Having described some of the differences in brain structure between the sexes, in later chapters I will examine how these might be related to differences in behaviour.

  6

  Children

  Women are nothing but machines for producing children.

  Napoleon Bonaparte

  Girls and boys are different from birth, not only physically but also psychologically. Clear sex differences with a biological basis can be seen in the behaviour of children from a very early age, implying that these differences were programmed during development and have a genetic basis. A few hours after birth girls are more sensitive than boys to touch, and some forty hours after birth girls look longer at a face than boys, and boys look longer at a suspended mechanical mobile.

  Brain maturation in male infants is delayed, and at two to four weeks old they are more easily aroused from quiet sleep, possibly because of this. That female infants should appear to sleep more soundly is consistent with reports of increased sleep disruption and problematic crying in male infants throughout the first six months. The development of sleeping rhythms is slower in boys than in girls. Their motor systems also develop differently. When newborn, boys are less able to control their movements, though later they take part in more active and co-ordinated play.

  Girls as young as twelve months make more eye contact than boys of the same age and respond more empathetically to the distress of others, with sad looks and sympathetic vocalisations. Foetal testosterone levels are related to frequency of eye contact at the age of twelve months: the lower the concentration, the more eye contact. This is a nice example of how testosterone can affect brain function. The male preference for focusing on systems, as will be discussed later, is evident very early. At one year research showed boys to have preferred to watch a film of cars than a film of a face expressing strong emotion. Little girls showed the opposite. Female babies are more sensitive to smells, to sweet tastes and to touch, and they engage more in making sounds.

  Boys have on average a stronger grasp reflex at birth, and they have greater leg strength from three months and greater
arm strength from nine months old. Little boys are more physical than little girls, and two-year-old boys are better at throwing. Six- to nine-month-old boys are more likely than girls to imitate rapid movements such as hitting a balloon. There is thus an early male advantage in some motor skills.

  It has been found that when babies are frightened in a strange room at four months old, twice as many girls as boys cry, and at twelve months old in the same situation girls move toward their mothers, while boys look around. The young of monkeys or baboons behave in a similar way which supports the possibility that these psychological differences between girls and boys are the product of biological differences. This makes sense as females are much less physically able to resist attacks of any sort.

  Conversations can have a strong positive effect on infants’ development, and Cheslack-Postava and Jordan-Young found mothers may offer more verbal stimulation to girls than to boys. Mothers were found in one study to make more interpretations and have more conversation with their daughters, whereas they made more comments and gave more instructions to their sons. This may be because girls have a wider social environment and more opportunities for contact with others, not just with their mothers, either through play, speech or merely eye contact. These interactions are important in helping to develop empathy. Girls begin to gesture when they speak a month or two earlier than boys, at around two years, and social factors like mothers speaking more to girls may be the cause of this.