If

Tabinda hasan; Mahmood Fauzi

If " women are from Venus and men are from Mars ", does an answer lie with neuroanatomy?

Tabinda hasan, Mahmood Fauzi

Tabinda hasan^1*, Mahmood Fauzi²

Department of anatomy,Faculty of medicine, Jazan university, Saudi Arabia
Jazan general hospital,Jazan,Saudi Arabia

Corresponding Author: Tabinda hasan, Department of anatomy,Faculty of medicine, Jazan university, Saudi Arabia, E-mail: drtabindahasan@gmail.com mobile 00966532315920

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Abstract

Introduction: - Since times immemorial, the difference of genders has prompted philosophers to divide the civilized society into patriarchic and feminist. Objective: This paper answers an age-old question:"Why do men and women behave differently"?

Method: General and Meta search engines were employed to harvest medical database and infer relevant information from 100 representative articles. (Year 2000 to 2011)

Result: - Published literature suggests that the 'difference of genders' goes deeper than mere psychology. It stems from underlying ultra-structural variations in brain morphology, embryology and bio-molecular physiology.

Conclusion: 'Difference of sexes' is not merely the result of ' post natal cultural socialization' or 'linearly taught mannerism styles'; rather, it is due to an ' innate neural hardware program'. It appears that slowly down the evolutionary chain, nature has designed two completely different sets of blue prints to ensure successful survival. Dimorphism does not impart any overall superiority to either sex; however it could play a pivotal role in social / educational theory implications, cognitive / behavior conditions and natural disease progression.

Keywords

Brain, Anatomy, Behavior

INTRODUCTION

The female is softer in disposition, is more mischievous, less simple, more impulsive and more attentive to the nurture of the young...The fact is, the nature of man is the most rounded off and complete’. Aristotle (Greek philosopher and great thinker)

Human postmortem and structural neuro-imaging studies indicate that morphological differences in the male and female brain reflect an interaction between various developmental processes, experiential influences and hormonal actions on the developing as well as mature brain. Indeed, these structural differences in the human brain majorly govern the behaviors of the two sexes and are responsible for "Why men and women act the way they do". The social stratification of masculine and feminine mannerisms as chauvinistic, altruistic or narcissistic is not merely a result of post natal cultural socialization; rather it springs up from deep within…..from the ingrained neural hardware programmed into us down the millions of years of natural evolution. Indeed, the difference of genders is an innate characteristic that can be observed even in very young children; while boys adore 'cars', girls prefer 'dolls'. It appears that slowly down the evolutionary chain, nature has designed two completely different sets of blue prints to ensure successful survival. This paper answers an age-old question:"What makes us so different from the other sex"?

Material and Method

A systematic meta-analysis of published medical literature with strict inclusion and exclusion criteria was employed to infer relevant information from 100 representative articles ranging from 2000 to 2011. Medical database like Medline, Embase, Cochraine, Directory of Open access journals, Applied Social Sciences Index and Abstracts, PsycINFO (database of psychological abstracts) and Gray literature database were searched through engines like Pubmed, Ovid and Science direct. The search strategies used were MeSh (medical subject heading),Text word searching and Reference list harvesting. Key journals from Jazan Medical Library were hand searched and informative discussions were held with experts of the field form Jazan College of medicine, Saudi Arabia.

Results

Available evidence suggests that ' the difference of genders' goes deeper than mere psychology or social training. It stems from deep rooted ultra-structural variations in brain morphology, embryology and bio-molecular physiology.

Discussion

Brain Size , Weight & Volume

The adult brain weighs about 1.5 kg and the volume ranges between 1260 to 1130 cm3, although individual variations may occur. [1] Male brains are 10-12 % larger than females in size and weight. This is natural, owing to a larger cranium size (2%) in men. Anthropologists attribute this characteristic to a larger male body and muscle mass that requires more neurons to control them. [2] This difference is evident even at birth and a boy’s brain is 12-20% larger than a girl's. In adults, sexual dimorphism is more evident in 'specific' rather than 'general ' brain volumes relative to cerebrum size. Women have larger volumes in frontal and medial paralimbic cortices while men have larger volumes in frontomedial cortex, amygdala and hypothalamus. Studies in animals as well as humans indicate that the degree of dimorphism is proportional to the number of sex steroid receptors in specific brain areas during critical periods of development. [3] There is no clear correlation between intelligence and brain size. Adult humans with brains weighing as little as half the average may have normal or even above average intelligence. [4] In children, however, cortical and subcortical grey mater volume is positively correlated with IQ.(intelligence quotients)

Grey and White Matter in brain

The ratio between gray and white matter in diverse regions of cortex differ significantly between the sexes. Men have 6.5 times more gray matter made of active neurons, while women have 10 times more white matter, particularly in precentral, cingulate, and anterior temporal area. [2] Sex as well as age are the main determinants of total neuronal number in the human neocortex, while body size, per se, has little or no influence. [5] The average number of neocortical neurons is 23 billion in males and 19 billion in females; a difference of about 16%. Researchers have calculated that at 20yrs of age, a man has 176,000 km while a woman has 149,000 km of myelinated axons in their brains. Gray and white matter volumes are reflective of inter and intra hemisphere communication and cognitive performance across the sexes.[6]

Cerebellum and pons

Cerebellum (important for posture and balance), and pons (regulating consciousness), are larger in men than in women.[7] The usually better athletic skills of males may be attributed to this characteristic.

Cerebrum

Male cerebrum is larger than females of the same age and health status; in terms of thickness, density and number of neurons, even if differences in body size are controlled statistically. Men have about 100 grams more brain tissue in their cerebral cortex. These size differences are dimorphic, lateralized, and region specific. The critical thinking, quantitative problem solving and spatial orientation specific right cerebral hemisphere is bigger in males. The enlargement is most prominent in frontal and occipital poles. [8] More developed male occipital lobes allow them to see better in brighter light. These morphological characteristics may account for men being good drivers (specially in finding their way through a route) and fast mathematicians. [9] The aesthetic and communication oriented left hemisphere is more developed in females and explains the characteristic feminine talkativeness. Women usually perform better than men in verbal memory tasks, verbal fluency, articulation speed and artistic expressiveness. [10]

Neurons in female brains are tightly packed, especially in temporal and parietal lobes, sometimes the neuronal density being 12% more than males. Density even varies between layers; the neurons being more crowded in layers responsible for signal transmission. Female cortical neurons are more closely packed in layer 2 and 4 of temporal lobe (a key synthesizer of social and emotional processes and memory) and in layers 3, 5 and 6 of the prefrontal cortex, (which carry outbound signals).This difference may underlie typical male and female behavior trends. Dimorphism exists in cortical volume of Wernicke and Broca areas [11] and frontal and medial paralimbic cortices. [12] Variable patterns are seen in dendritic arbors complexes and dendritic spines density. These differences are apparent from birth itself and maintained throughout life. Men undergo more accelerated cerebral aging than women. This loss of neurons is mirrored in volume losses. Approximately 10% of neocortical neurons are lost with advancing age from 20 to 90 yrs .These anatomical changes appear to be closely tied to a gradual decline in mental performance in men. “There is something going on in the male brain that is not going on in the female brain”. 10Age-related volume decrements occur more in frontal lobe and visual cortex than temporal lobe and in all regions, the loss is greater in men than in women.[13]

Corpus Callosum

Corpus callosum allows free flow of communication between the two hemispheres. It is larger in women, as compared to men.[14] The large sized corpus allows better signal transmission between hemispheres. Thus women can use both hemispheres at the same time, thereby creating more synapses between the two sides of their brains. This decentralized and integrated neuronal connection system allows them to do multiple tasks simultaneously, like cooking with reading recipes; watching television with knitting; talking with writing etc. Shape differences in callosum have also been observed, with the splenium being more bulbous in females and tubular in males.[15]Rightward a- symmetries in callosal thickness, particularly in the anterior third of callosum are common in men, correlating with the predominant functional organization of the right hemisphere in males.[16]

Hypothalamus

The hypothalamus, regulating basic life functions shows gender differences as well. The volume of interstitial hypothalamic nucleus (third cell group) is twice as large in heterosexual men as in women and homosexual men. [17] The pre-optic nucleus, involved in mating behavior, is 2.2 times larger in men than women and contains two times more cells. This enlargement is related to the amount of circulating male androgens. The difference manifests at 4 years of age in children, when a decrease in number of cells is observed in girls. Gender-related differences are also found between the two cell groups (acidophils and basophils) in the preoptic-anterior hypothalamic area. (PO-AHA) Cells and their nuclei are larger in size and more numerous in males. The suprachiasmatic nucleus, responsible for circadian rhythms and reproduction cycles, differs in shape between the two sexes. Males exhibit a sphere shape while females exhibit an elongated shape. However, the number of cells and nuclear volume do not differ significantly. It has been postulated that the shape of suprachiasmatic nucleus influences the connections framework in this area, thereby altering signal transmission trends and information relay patterns. Androgen receptor (AR) staining varies in different hypothalamic areas in males and females. Overall, AR staining is less intense in young women than men. The strongest staining dimorphism is observed in lateral and medial mamillary nucleus.[18] This is physiologically significant, since the mamillary complex receives input from hippocampus through fornix and is involved in cognition. Additionaly, AR staining variation is present in the preoptic area, parventricular nucleus, supraoptic nucleus, ventromedial nucleus and infundibular nucleus.

Limbic System

Females have a larger and more developed deep limbic system (including hippocampus and anterior commissure) than males. [19] This gives women an acute sense of smell and the psychic ability to be 'more in touch with their feelings' along with an increased ability to bond and remain connected with others. However, the flip side of a larger limbic system makes females more susceptible to depression, especially at times of significant hormonal changes such as puberty, menses, child birth and menopause. Women also attempt suicide three times more often than men.[20]

Massa Intermedia

It crosses the third ventricle between the two thalami (forming intra-thalmic connections), and is present in 78% females and 68% males. Also, the size of mass intermedia is 53.3% or 17.5 mm2 larger in females than in males. Since thalamus is the center for relaying and processing information, anatomical differences among structures connecting the two thalmii may, in part, underlie functional differences in mental processes among the sexes.

Anterior Commissure

The anterior commisure connects several regions of the frontal and temporal lobes. It is 12 %, or 1.17mm2 larger in women than in men.19 This increased connectivity may be responsible for a greater integration of signals across hemispheres in females.

Orbitofrontal cortex to Amygdala Ratio (OAR)

Orbitofrontal cortex regulates emotions while the amygdale is involved in generating emotional reactions. The ratio between these two regions is significantly larger in females (with women having larger orbital grey than men). This typically explains why women are more capable than men in displaying as well as controlling and modifying emotional reactions.[21]

Inferior-Parietal Lobule (IPL)

In terms of size, the ratio of right and left IPL is reversed for men and women. The left IPL is significantly larger in men.[22] Incidentally, this is also the same area that was shown to be larger in the brains of Albert Einstein and other known physicists and mathematicians .It seems that the left IPL confers mathematical ability, spatial understanding, logical thinking and the ability to rotate 3-D figures in the brain. Through a more extensive bilateral activation, men outperform women in mental rotation and visio-spatial tasks such as aiming at stationary or moving targets, throwing and intercepting projectiles etc. Women, being confined to only right sided activity, are usually inferior to their male counterparts for similar tasks. The right IPL is larger than left IPL in females. This area enables the brain to selectively process input from senses and aids in specific attention and directed perception. This may explain why mothers are more attuned to specific sensory inputs than fathers; like the sound of baby crying at night.

Straight Gyrus (SG)

This is a narrow band at the base of the frontal lobe and is involved in social cognition and interpersonal judgment. It is 10% bigger in women than in men. [23] The larger SG size in women correlates with their higher scores in social recognition and interpersonal awareness tests. Interestingly, there does seem to be a relation between SG size and femininity. Higher degrees of femininity and social perception were seen with greater SG volumes and surface areas. Studies document that even in early childhood, females are more advanced than males in social development.[24]

Hippocampus

Representative studies demonstrate that male and female hippocampi differ significantly in their anatomical structure and neuroanatomic make-up. This explains differential reactivity of males and females to stressful situations. Imaging shows that the hippocampus is larger in women than in men when adjusted for total brain size.[3] Differences also exist within the hippocampal neurotransmitter systems, including adrenergic, serotonergic, cholinergic, corticosterone, benzodiazepine and cholecystokinin systems .Serotonergic and dopamine mechanisms regulate gender specific behaviors and psychiatric conditions like aggression, eating disorders, depression, impulsivity or suicide.[25] The mean rate of serotonin synthesis and serotonin receptor binding capacity in normal males is 52% higher than in females.[26]

Amygdala

Rapidly growing evidence documents the sexually dimorphic nature of human amygdale.[27] In particular, the left amygdala, shows marked diffrrences .When adjusted for total brain size, it is significantly larger in men than in women, particularly in areas regulating sexual and social behavior. Studies in lower mammals have demonstrated that androgen hormonal treatment or withdrawal can reverse the amygdale size in male and female animals, therefore indicating the correlation of male hormones with amygdale size. Differences also exist in amygdala's spatial relationship with the rest of the brain. When a subject is at rest, the brain regions communicating with amygdala are different in men and women. In males, the right amygdala is more active and shows increased connectivity with visual cortex and striatum. Conversely, in women, the left amygdala is more active and extensively connected to insular cortex and hypothalamus. Based on the connection patterns, it can be inferred that under normal conditions of living, the male amygdale is more responsive to external stimuli (vision, motor actions) while the female amygdale is more attuned to internal body environment. Amygdala has an important role in storing and modulating emotional memory, particularly of certain stressful events. The amygdale-stress hormone mechanism is an evolutionary adaptation that provides a natural defense against repetition of harmful or unpleasant episodes. Females exhibit a preferential activation of left amygdale during processing of emotional memory (generally visual images) while males exhibit right amygdale activation for the same memory.[28] Fundamental differences in the functional mechanisms of amygdala during resting conditions and the different ways of processing emotionally arousing memories in males and females explain characteristic 'masculine and feminine' type responses to stressful situations.

The anatomic substrate of language and memory:

Brain regions playing a key role in language processing differ between the sexes. When compared to male brains, the frontal and temporal areas are more developed and precisely organized in women.[29] Broca's, and Wernicke's areas are larger in women by 13-23%. Fewer synapses in female temporal neo-cortex represent an increasing evolutionary sophistication for language processing and may be responsible for their better performance on language tasks when compared to men of similar aptitudes. The part of the left hemisphere responsible for coordinated speech and hand movements also differs. In women, the frontal region is more important and speech problems are more likely to occur if the front part of the left hemisphere is damaged. Conversely, in men, the parietal region is more important. Imaging studies demonstrate that men and women process single words similarly, but in the listening and interpretation of whole sentences, women use both hemispheres while men use only the left side.[30,31] This accounts for females outperforming males in phonological and verbal fluency tasks. Several interesting f-MRI (fluorescence magnetic resonance imaging) studies demonstrated that while brain areas associated with language worked harder in girls than in boys during language tasks; both boys and girls sounded out the words equally well, indicating that there are more ways than one for the brain to arrive at a particular result.

The two sexes process and store language memories in different sides of the brain. Studies have proved this through using a particular drug, propranolol, that blocks language memory differently in men and women.[32] Piefke et al, in their study on neural correlates, examined the retrieval of emotional and autobiographical memories in men and women.[33] They found that memory performance did not differ, nor did the degree of emotion induced by its retrieval. However, the brain regions associated with retrieval differed significantly between the sexes.

Neurobehavioral mechanisms

Neurobehavioral mechanisms of men and women are very characteristic and unlike each other. The differences in emotion processing, perception, experience and expression are most notably reflected in greater male aggression. Young boys engage in rough-tough play like mock fighting, gun/sword battles etc. while girls of the same age are more nurturing and prefer somber toys like dolls, cooking sets etc . The sexual difference in aggression level also extends into the ways of aggression manifestation. Men show a far more 'direct' aggression policy such as pushing, hitting and punching while females exhibit 'indirect or covert' aggression.[34] Women increasingly use language skills to their advantage when they compete. They usually gossip, orate or manipulate information to direct relationships and modify situations.[35] Men hardly express their feelings or cry as frequently as women. This has more to do with their neuro-anatomic make-up than their personal inner strengths. Male brains can separate language in the left and emotions in the right, while female emotions and language coexist in both hemispheres, accounting for their sentimental eloquence and explicit outbursts. According to the empathizing-systemizing theory,[36] the female brain is hard-wired for empathy (identifying another person's emotions and thoughts and responding to these with an appropriate emotional expression). Women intuitively figure out how people are feeling and how to treat people with care and sensitivity. On the other hand, the male brain is predominantly conditioned for understanding and building systems. The typical male mind is an engineer that intuitively figures out how things work or what are the underlying rules for constructing and controlling systems. Most women value the development of altruistic, reciprocal relationships, which basically require empathizing. In contrast, men are more inclined towards power play, ranks, independence, dominance, politics, and competition.[37]

The thought process

Men and women think in different ways, but every person tends to think in his or her own unique way that is unrelated to differences in the general level of intelligence of individuals. In the form of Man and Woman, human evolution has created two unique brains types designed for equally intelligent behavior. This phenomenon was well described by Witleson in his work on 'preferred cognitive strategy'.[38]

Neuro- imaging reveals that men think predominantly with their grey matter, which is full of active neurons while women think with their white matter, which consists more of connections between neurons. Broadly stating, a woman's brain is a bit more 'complicated' in setup, but these connections also allow a woman's mind to work faster than a man's in certain situations. Females think more observantly and men think more rationally. When negotiating a virtual reality maze, women navigate by seeking out landmarks and things that can be seen or heard. Men navigate by using abstract concepts like north and south directions or distances. Both men and women use the right hippocampus to figure out how to exit mazes. However, men also use their left hippocampus for this task, which women do not, and women also use their right prefrontal cortex, which men do not.[39]

While the brain allows us to think, it also drives our ability to identify and control emotions. The problem solving and decision making areas of the frontal lobe and the emotion regulating limbic cortex are larger in women. This makes women more adept than men at manual precision and carrying out detailed and pre-planned tasks, identifying emotions, encoding facial expressions and determining subtly changing vocal intonations.[40]

Metabolism characteristics

Functional brain imaging studies indicate sex differences in brain metabolism, particularly global and regional cerebral blood flow and glucose consumption patterns during different cognitive activities.[41] The direction and degree of hemispheric flow asymmetry is mainly influenced by sex and handedness; females and left-handers have higher blood flow rate per unit weight of brain. Men have higher glucose metabolism than women in temporal-limbic regions and cerebellum and lower glucose metabolism in cingulate regions. In both sexes, metabolism is higher in left association cortices, cingulate region, right ventro-temporal limbic regions and their projections.[42] Male and female brains work differently under stress. Men respond with an increased blood flow to the right prefrontal cortex (responsible for “fight or flight” and higher levels of cortisol) while women direct an increased blood flow to their limbic system which produces a more nurturing and composed response. The brain changes during stress response also last longer in females than males.

Susceptibility to Neurological Disorders

Boys have a higher incidence of developmental and learning disabilities. This can be explained embryologially; male fetuses require the maintenance of a higher number of nerve cells in their cerebral cortex than female fetuses. Early damage to the developing male brain could result in a proportionally higher functional loss.[43] Low birth-weight and premature infants, especially males, are at a greater risk of prenatal brain injury and subsequent cognitive impairment and learning difficulties. Arnold et al. postulated that these differences have a developmental basis.[44] Their origin reflects differences in brain lateralization and maturation during prenatal exposure to gonadal steroids.

Major depressive disorder, anxiety and eating disorders like anorexia and bulimia are more prevalent in women, while schizophrenia, autism and attention deficit disorders are more common in men .Women have a more developed interstitial space between cell bodies, which contains synapses, dendrites and axons. This may explain the disproportionate female proneness to dementia (such as Alzheimer's) because although both sexes may lose the same number of neurons during similar disease states; the neuronal reserve is greater in males as compared to females, which could be functionally protective against some degree of loss .

Understanding our own evolution

Sex hormones, chromosomes, chemical transmitters, neuro anatomy and genes; all have played together in the homosapien evolutionary chain to bring us to our present being. In ancient times, both sexes had defined roles to ensure survival. Cave-men hunted and gathered food while Cave-women cooked and reared children. Brain areas may have evolved differently to enable each sex to carry out specific jobs in a best possible manner. Superior navigation skills and aggressiveness may have enabled men to become better hunters. Women's developed verbal skills would have enabled them to use language to gain social advantage by argumentation and persuasion. Polygamous nature of man is a way to ensure genetic immortality whereas the characteristic feminine softness and empathy ensures perfect motherhood for future generations. While the difference in our natures is 'hard wired' into our inherent neuronal framework; culture, lifestyle and social training widen this gap even more during the advancing years from infancy to adulthood.

Future scope

Since male and female brains fire differently when they are planning to achieve something, rehabilitation after brain damage may need to be tailored to the sex of the person for better results. Varied functional and topographical organization of male and female brains for specialized tasks could cause any regional neuronal damage to translate differently in the sexes. This could play a prominent role in gender specific prediction of birth defects, prenatal diagnostics and personalized therapeutics.

Quantitative and qualitative knowledge of the developing human brain will play an increasingly significant role in improving sensitivity and specificity of interpretation of brain abnormalities in patients within the clinical environment, as well as in groups of children with suspected brain dysfunction. Recognizing the impact of these sex differences in the development of medical conditions could elevate epidemiology to a whole new level of understanding and intervention. Intensive researches on male and female brain function promise significant implications for pedagogy. Boys tend to lose focus with too much verbalism while remaining more engaged with visual and 'hands on' learning techniques. Alternatively, girls can be more engrossed with verbose and expressive discussions. Also, females are language and observation oriented while males are more spatial and mechanical. Thus, employing gender specific instructional strategies could help to bring out the best in learners of both sexes.

Conclusion

Men and women not only differ in their physical attributes and reproductive functions but also in their cognitive and emotional development. The anatomical peculiarities of male and female brains underlie strategic differences in functional organization like learning processes, behavior development, intellect, problem solving, encoding memories, sensing emotions and decision making. Although this diversity does not impart any overall superiority or advantage to either sex, it could help in developing guidelines for creating best suited vocational tasks and personalized performance environments for the two sexes.

There is a lot of evidence that we can build up our brain's representation of skills and even our neurons and synaptic connections with gradual practice. Despite all its inherent anatomic advantages and limitations, effectively nurturing one's brain can significantly enhance the gifts that nature has endowed.

Conflict of Interest: None.

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