File Name: difference between male and female brain .zip
On a gray day in mid-January of , Lawrence Summers, then president of Harvard University, suggested that innate differences between the male and female brain might be one factor underlying the relative scarcity of women in science fields. His remarks reignited a debate that has been smoldering for a century, ever since some scientists sizing up the brains of both sexes began using their main finding—that female brains tend to be smaller—to bolster the view that women are intellectually inferior to men. To date, no one has uncovered any evidence that anatomical disparities might render women incapable of achieving academic distinction in math, physics or engineering [ see box on page 11 ].
On a gray day in mid-January of , Lawrence Summers, then president of Harvard University, suggested that innate differences between the male and female brain might be one factor underlying the relative scarcity of women in science fields. His remarks reignited a debate that has been smoldering for a century, ever since some scientists sizing up the brains of both sexes began using their main finding—that female brains tend to be smaller—to bolster the view that women are intellectually inferior to men.
To date, no one has uncovered any evidence that anatomical disparities might render women incapable of achieving academic distinction in math, physics or engineering [ see box on page 11 ]. And the brains of men and women have been shown to be quite clearly similar in many ways.
Nevertheless, over the past decade or so investigators have documented an astonishing array of structural, chemical and functional variations in the brains of males and females. These inequities are not just interesting idiosyncrasies that might explain why more men than women enjoy the Three Stooges.
They raise the possibility that we might need to develop sex-specific treatments for a host of conditions, including depression, addiction, schizophrenia and post-traumatic stress disorder PTSD. Furthermore, the differences imply that researchers exploring the structure and function of the brain must take into account the sex of their subjects when analyzing their data—and include both women and men in future studies or risk obtaining misleading results.
Sculpting the Brain Not so long ago neuroscientists believed that sex differences in the brain were limited mainly to those regions responsible for mating behavior. Levine mentioned only one brain region in his review: the hypothalamus, a small structure at the base of the brain that is involved in regulating hormone production and controlling basic behaviors such as eating, drinking and sex. That view, however, has now been knocked aside by a surge of findings that highlight the influence of sex on many areas of cognition and behavior, including memory, emotion, vision, hearing, the processing of faces and the brain's response to stress hormones.
This progress has been accelerated in the past decade by the growing use of sophisticated noninvasive imaging techniques such as positron-emission tomography PET and functional magnetic resonance imaging fMRI , which can peer into the brains of living subjects. These imaging experiments reveal that anatomical variations occur in an assortment of regions throughout the brain.
Jill M. Goldstein of Harvard Medical School and her colleagues, for example, used MRI to measure the sizes of many cortical and subcortical areas.
Among other things, these investigators found that parts of the frontal cortex, the seat of many higher cognitive functions, are bulkier in women than in men, as are parts of the limbic cortex, which is involved in emotional responses.
In men, on the other hand, parts of the parietal cortex, which is involved in space perception, are bigger than in women, as is the amygdala, an almond-shaped structure that responds to emotionally arousing information—to anything that gets the heart pumping and the adrenaline flowing. These size differences, as well as others mentioned throughout the article, are relative: they refer to the overall volume of the structure relative to the overall volume of the brain.
Differences in the size of brain structures are generally thought to reflect their relative importance to the animal. For example, primates rely more on vision than olfaction; for rats, the opposite is true. As a result, primate brains maintain proportionately larger regions devoted to vision, and rats devote more space to olfaction.
So the existence of widespread anatomical disparities between men and women suggests that sex does influence the way the brain works. Other investigations are finding anatomical sex differences at the cellular level.
For example, Sandra Witelson and her colleagues at McMaster University discovered that women possess a greater density of neurons in parts of the temporal lobe cortex associated with language processing and comprehension.
On counting the neurons in postmortem samples, the researchers found that of the six layers present in the cortex, two show more neurons per unit volume in females than in males. Similar findings were subsequently reported for the frontal lobe. With such information in hand, neuroscientists can now explore whether sex differences in neuron number correlate with differences in cognitive abilities—examining, for example, whether the boost in density in the female auditory cortex relates to women's enhanced performance on tests of verbal fluency.
Such anatomical diversity may be caused in large part by the activity of the sex hormones that bathe the fetal brain. These steroids help to direct the organization and wiring of the brain during development and influence the structure and neuronal density of various regions. Interestingly, the brain areas that Goldstein found to differ between men and women are ones that in animals contain the highest number of sex hormone receptors during development. This correlation between brain region size in adults and sex steroid action in utero suggests that at least some sex differences in cognitive function do not result from cultural influences or the hormonal changes associated with puberty—they are there from birth.
Inborn Inclinations Several intriguing behavioral studies add to the evidence that some sex differences in the brain arise before a baby draws its first breath. Through the years many researchers have demonstrated that when selecting toys, young boys and girls part ways. Boys tend to gravitate toward balls or toy cars, whereas girls more typically reach for a doll.
But no one could really say whether those preferences are dictated by culture or by innate brain biology. The researchers presented a group of vervet monkeys with a selection of toys, including rag dolls, trucks and some gender-neutral items such as picture books.
Both sexes spent equal time monkeying with the picture books and other gender-neutral toys. Because vervet monkeys are unlikely to be swayed by the social pressures of human culture, the results imply that toy preferences in children result at least in part from innate biological differences.
This divergence, and indeed all the anatomical sex differences in the brain, presumably arose as a result of selective pressures during evolution. In the case of the toy study, males—both human and primate—prefer toys that can be propelled through space and that promote rough-and-tumble play. These qualities, it seems reasonable to speculate, might relate to the behaviors useful for hunting and for securing a mate.
Similarly, one might also hypothesize that females, on the other hand, select toys that allow them to hone the skills they will one day need to nurture their young. Simon Baron-Cohen and his associates at the University of Cambridge took a different but equally creative approach to addressing the influence of nature versus nurture regarding sex differences.
For example, Baron-Cohen and his then student Svetlana Lutchmaya found that one-year-old girls spend more time looking at their mothers than boys of the same age do. And when these babies are presented with a choice of films to watch, the girls look longer at a film of a face, whereas boys lean toward a film featuring cars. Of course, these preferences might be attributable to differences in the way adults handle or play with boys and girls.
To eliminate this possibility, Baron-Cohen and his students went a step further. They took their video camera to a maternity ward to examine the preferences of babies that were only one day old.
The infants saw either the friendly face of a live female student or a mobile that matched the color, size and shape of the student's face and included a scrambled mix of her facial features. To avoid any bias, the experimenters were unaware of each baby's sex during testing. When they watched the tapes, they found that the girls spent more time looking at the student, whereas the boys spent more time looking at the mechanical object. This difference in social interest was evident on day one of life—implying again that we come out of the womb with some cognitive sex differences built in.
Under Stress In many cases, sex differences in the brain's chemistry and construction influence how males and females respond to the environment or react to, and remember, stressful events. Take, for example, the amygdala. Goldstein and others have reported that the amygdala is larger in men than in women. And in rats, the neurons in this region make many more interconnections in males than in females.
To assess whether male and female amygdalae respond differently to stress, Katharina Braun and her co-workers at Otto von Guericke University in Magdeburg, Germany, briefly removed a litter of degu pups from their mother.
For these social South American rodents, which live in large colonies like prairie dogs do, even temporary separation can be quite upsetting. The researchers then measured the concentration of serotonin receptors in various brain regions.
Serotonin is a neurotransmitter, or signal-carrying molecule, that is key for mediating emotional behavior. Prozac, for example, acts by increasing serotonin function. The workers allowed the pups to hear their mother's call during the period of separation and found that this auditory input increased the serotonin receptor concentration in the males' amygdala yet decreased the concentration of these same receptors in females. Although it is difficult to extrapolate from this study to human behavior, the results hint that if something similar occurs in children, separation anxiety might differentially affect the emotional well-being of male and female infants.
Experiments such as these are necessary if we are to understand why, for instance, anxiety disorders are far more prevalent in girls than in boys. Another brain region now known to diverge in the sexes anatomically and in its response to stress is the hippocampus, a structure crucial for memory storage and for spatial mapping of the physical environment. Imaging consistently demonstrates that the hippocampus is larger in women than in men.
These anatomical differences might well relate somehow to differences in the way males and females navigate. Interestingly, a similar sex difference exists in rats. Male rats are more likely to navigate mazes using directional and positional information, whereas female rats are more likely to navigate the same mazes using available landmarks. Even the neurons in the hippocampus behave differently in males and females, at least in how they react to learning experiences.
For example, Janice M. This change presumably reflects an increase in neuronal connections, which in turn is thought to be involved with the laying down of memories. In males, however, the complex environment either had no effect on the dendritic trees or pruned them slightly. Benefits of Stress?
But male rats sometimes learn better in the face of stress. Tracey J. Shors of Rutgers University and her collaborators have found that a brief exposure to a series of one-second tail shocks enhanced performance of a learned task and increased the density of dendritic connections to other neurons in male rats yet impaired performance and decreased connection density in female rats.
Findings such as these have interesting social implications. The more we discover about how brain mechanisms of learning differ between the sexes, the more we may need to consider how optimal learning environments potentially differ for boys and girls.
Although the hippocampus of the female rat can show a decrement in response to acute stress, it appears to be more resilient than its male counterpart in the face of chronic stress. Cheryl D. Conrad and her co-workers at Arizona State University restrained rats in a mesh cage for six hours—a situation that the rodents found disturbing. The researchers then assessed how vulnerable the rodents' hippocampal neurons were to killing by a neurotoxin—a standard measure of the effect of stress on these cells.
They noted that chronic restraint rendered the males' hippocampal cells more susceptible to the toxin but had no effect on the females' vulnerability. These findings and others suggest that in terms of brain damage, females may be better equipped to tolerate chronic stress than males are.
Extending the work on how the brain handles and remembers stressful events, my colleagues and I have found contrasts in the way men and women lay down memories of emotionally arousing incidents—a process known from animal research to involve activation of the amygdala. In one of our first experiments with human subjects, we showed volunteers a series of graphically violent films while we measured their brain activity using PET.
A few weeks later we gave them a quiz to see what they remembered. We discovered that the number of disturbing films they could recall correlated with how active their amygdala had been during the viewing. Subsequent work from our laboratory and others confirmed this general finding. But then I noticed something strange. The amygdala activation in some studies involved only the right hemisphere, and in others only the left hemisphere.
It was then I realized that the experiments in which the right amygdala lit up involved only men; those in which the left amygdala was fired up involved women. Since then, three subsequent studies—two from our group and one from John Gabrieli and Turhan Canli, both then at Stanford, and their collaborators—have confirmed this difference in how the brains of men and women handle emotional memories.
The realization that male and female brains were processing the same emotionally arousing material into memory differently led us to wonder what this disparity might mean. To address this question, we turned to a century-old theory stating that the right hemisphere is biased toward processing the central aspects of a situation, whereas the left hemisphere tends to be involved in the finer details.
If that conception is true, we reasoned, a drug that dampens the activity of the amygdala should impair a man's ability to recall the gist of an emotional story by hampering the right amygdala but should hinder a woman's ability to come up with the precise details by hampering the left amygdala. Propranolol is such a drug. This so-called beta blocker quiets the activity of adrenaline and its cousin noradrenaline and, in so doing, dampens the activation of the amygdala and weakens recall of emotionally arousing memories.
Expanding previous studies of human cerebral cortical sexual dimorphism showing higher neuronal densities in males, we investigated whether gender differences also exist in the extent of neuropil, size of neuronal somata, and volumes of astrocytes. This histo-morphometric study includes select autopsy brains of 6 males and 5 females, 12 to 24 yr old. In each brain, 86 defined loci were analyzed for cortical thickness, neuronal and astrocytic 8 loci density stereological counts , and neuronal and astrocytic 8 loci soma size, enabling calculations of neuropil and astrocytic volumes. The female group showed significantly larger neuropil volumes than males, whereas neuronal soma size and astrocytic volumes did not differ. The expanded data confirmed higher neuronal densities in males than in females without a gender difference in cortical thickness.
Male brains are about 10% larger than female brains and weigh % more than that of a woman. Men's heads are also about 2% bigger than women's. A boy's brain is between % larger than that of a girl. The head circumference of boys is also larger (2%) than that of girls.
New study examines thousands of brains from two decades of research to reveal differences between male and female brain structure.
But he did have a good reason. So, he zeroed in on sex-associated behavioral differences in mating, parenting and aggression. At the time, this was not a universally popular idea.
Box , CY Nicosia, Cyprus. The prevalence, age of onset, and symptomatology of many neuropsychiatric conditions differ between males and females. To understand the causes and consequences of sex differences it is important to establish where they occur in the human brain. We report the first meta-analysis of typical sex differences on global brain volume, a descriptive account of the breakdown of studies of each compartmental volume by six age categories, and whole-brain voxel-wise meta-analyses on brain volume and density. Gaussian-process regression coordinate-based meta-analysis was used to examine sex differences in voxel-based regional volume and density. On average, males have larger total brain volumes than females. Examination of the breakdown of studies providing total volumes by age categories indicated a bias towards the 18—59 year-old category.
The neuroscience of sex differences is the study of characteristics that separate the male and female brain. Psychological sex differences are thought by some to reflect the interaction of genes , hormones , and social learning on brain development throughout the lifespan. Some evidence from brain morphology and function studies indicates that male and female brains cannot always be assumed to be identical from either a structural or functional perspective, and some brain structures are sexually dimorphic.
Publicly available datasets were analyzed in this study. Do men and women have different brains? Previous neuroimage studies sought to answer this question based on morphological difference between specific brain regions, reporting unfortunately conflicting results.
By Dr. Saul McLeod , published People often get confused between the terms sex and gender. Sex refers to biological differences between males and females. For example, chromosomes female XX, male XY , reproductive organs ovaries, testes , hormones oestrogen, testosterone.
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