The Open Anatomy Journal, 2010, 2, 37-55 37 Open Access Gender Differences in Human Brain: A Review Zeenat F. Zaidi* Department of Anatomy, Faculty of Medicine, King Saud University, Riyadh, Saudi Arabia Abstract: Why do men and women think differently? Why do they behave differently in stressed situations? Why do women act more emotionally as compared to men? Why do men and women excel at different types of tasks? Why do boys like to play with cars and trucks and superman? These are the common questions which arise commonly in minds.The human brain is a highly complex organ. Studies of perception, cognition, memory and neural functions have found apparent gender differences. These differences may be attributed to various genetic, hormonal, and environmental factors and do not reflect any overall superiority advantage to either sex. Both sexes are equal in intelligence, but tend to operate differently. Men and women appear to use different parts of the brain to encode memories, sense emotions, recognize faces, solve certain problems and make decisions. Indeed, when men and women of similar intelligence and aptitude perform equally well, their brains appear to go about it differently, as if nature had separate blueprints. Sex differences in the brain may play a role in learning processes, language development, and progression of neurologically- based diseases. Sex differences need to be considered in studying brain structure and function and may raise the possibility of sex-specific treatments for neurological diseases. In this article it is reviewed that how does the brain of a male look and function differently from a female's brain, and what accounts for these differences? Keywords: Human brain, gender differences, brain aging, neurological disorders, learning. ANATOMICAL DIFFERENCES paralimbic cortices. Men had larger volumes, relative to cerebrum size, in frontomedial cortex, the amygdala and Several postmortem and structural neuroimaging studies hypothalamus. There was greater sexual dimorphism in brain in humans have shown the following morphological areas that are homologous with those identified in animal differences that are likely to reflect an interaction between studies showing greater levels of sex steroid receptors during developmental influences, experience, and hormone actions critical periods of brain development. These findings have on the mature brain. implications for developmental studies that would directly Size & Weight test hypotheses about mechanisms relating sex steroid hormones to sexual dimorphisms in humans [4]. The adult human brain weighs on average about 3lb (1.5 kg) [1] with a size of around 1130 cm3 in women and 1260 Grey Matter vs White Matter cm3 in men although there is substantial individual variation Ratios of grey to white matter also differ significantly [2]. Male brains are about 10% larger than female brains and between the sexes in diverse regions of the human cortex [5]. weigh 11-12% more than that of a woman. Men's heads are Variations in the amount of white and grey matter in the also about 2% bigger than women's. This is due to the larger brain remain significant [6-8]. Men have approximately 6.5 physical stature of men. Male’s larger muscle mass and times more gray matter in the brain than women, and women larger body size requires more neurons to control them. The have about 10 times more white matter than men do [3]. At brain weight is related to the body weight partly because it the age of 20, a man has around 176,000 km and a woman, increases with increasing height [3]. This difference is also about 149,000 km of myelinated axons in their brains [9]. present at birth. A boy’s brain is between 12-20% larger than Men appear to have more gray matter, made up of active that of a girl. The head circumference of boys is also larger neurons, and women more of the white matter responsible (2%) than that of girls. However, when the size of the brain for communication between different areas of the brain [10]. is compared to body weight at this age, there is almost no In women's brains, the neurons are packed in tightly, so that difference between boys and girls. So, a girl baby and a boy they're closer together. Some women even have as many as baby who weigh the same will have similar brain sizes. 12 percent more neurons than men do [10]. These neurons Brain Volume are densely crowded on certain layers of the cortex, namely the ones responsible for signals coming in and out of the Sexual dimorphisms of adult brain volumes were more brain, and these differences were present from birth [10]. evident in the cortex, with women having larger volumes, When controlling for total cerebral volume, women had a relative to cerebrum size, particularly in frontal and medial higher percentage of grey matter than men, and men had a higher percentage of white matter [6, 8] and both gray and white matter volumes correlated with cognitive performance *Address correspondence to this author at the Department of Anatomy, Faculty of Medicine, King Saud University, P.O. Box 22452, Riyadh 11495, across sex groups. The average number of neocortical Saudi Arabia; Tel: +966-1-4781995; Mobile: +966502151924; Fax: +966-1- neurons was 19 billion in female brains and 23 billion in 4781995; E-mail: [email protected] male brains, a 16% difference. In a study, which covered the 1877-6094/10 2010 Bentham Open 38 The Open Anatomy Journal, 2010, Volume 2 Zeenat F. Zaidi age range from 20 years to 90 years, approximately 10% of males. Among subjects with a massa intermedia, the all neocortical neurons are lost over the life span in both structure was an average of 53.3% or 17.5 mm2 larger in sexes. Sex and age were the main determinants of the total females than in males. Anatomical sex differences in number of neurons in the human neocortex, whereas body structures that connect the two cerebral hemispheres may, in size, per se, had no influence on neuron number [11]. Gender part, underlie functional sex differences in cognitive function differences in precentral, cingulate, and anterior temporal and cerebral lateralization [17]. white matter areas were also found, suggesting that microstructural white matter organization in these regions Cerebellum may have a sexual dimorphism [12]. An area of the brain important for posture and balance, and the pons, a brain structure linked to the cerebellum that Hypothalamus helps control consciousness, are larger in men than in Hypothalamus, where most of the basic functions of life women [18]. are controlled, including hormonal activity via the pituitary gland also shows gender differences. The volume of a Cerebral Hemispheres specific nucleus in the hypothalamus (third cell group of the According to the majority of studies, men possess larger interstitial nuclei of the anterior hypothalamus) is twice as cerebra than women of the same age and health status, even large in heterosexual men as in women and homosexual men if the body size differences are controlled statistically. Male [13]. The preoptic area, involved in mating behavior, is brains were larger than female brains in all locations, though about 2.2 times larger in men than in women and contains 2 male enlargement was most prominent in the frontal and times more cells. This enlargement is dependent on the occipital poles, bilaterally [19]. The male differentiated brain amount of male sex hormones or androgens. Apparently, the has a thicker right hemisphere. This may be the reason males difference in this area is only apparent after a person is 4 tend to be more spatial, and mathematical. The left years old. At 4 years of age, there is a decrease in the number hemisphere, which is important to communication, is thicker of cells in this nucleus in girls. The neuropil of the preoptic in female oriented brains. area is sexually dimorphic [14]. Gender-related differences were found in 2 cell groups in the preoptic-anterior Cerebral Cortex hypothalamic area (PO-AHA) in human brain. Both nuclei Men have 4% more neurons than women, and about 100 were larger in male and appeared to be related in women to grams more of brain tissue. Women have a more developed circulating steroid hormone levels [15]. The suprachiasmatic neuropil, or the space between cell bodies, which contains nucleus of the hypothalamus, involved with circadian synapses, dendrites and axons. This may explain why rhythms and reproduction cycles, is different in shape in women are more prone to dementia (such as Alzheimer's these two sexes. In males, this nucleus is shaped like a disease) than men, because although both may lose the same sphere whereas in females it is more elongated. However, the number of neurons due to the disease, in males, the number of cells and volume of this nucleus are not different functional reserve may be greater as a larger number of in men and women. It is possible that the shape of the nerve cells are present, which could prevent some of the suprachiasmatic nucleus influences the connections that this functional losses [20]. In the temporal neocortex, a key part area makes with other areas of the brain, especially the other which is involved in both social and emotional processes and areas of the hypothalamus. In most hypothalamic areas that memory, men had a one third higher density than women of stain positively for androgen receptor (AR), nuclear staining synapses, and had more brain cells, though the excess was in particular is less intense in young adult women than in slight compared with the excess in the number of synapses. men. The strongest sex difference is found in the lateral and Sexual dimorphism has been reported in the cortical volume medial mamillary nucleus [16].