| Part of a series on |
| Sex differences in humans |
|---|
 |
| Biology |
| Medicine andhealth |
| Neuroscience andpsychology |
| Sociology and society |
Theneuroscience of sex differences is the study of characteristics that separate brains of different sexes. Psychological sex differences are generally thought to reflect the interaction ofgenes,hormones, and social learning on brain development throughout the lifespan.
A 2021meta-synthesis led by Lise Eliot found that sex accounted for less than 1% of the brain's structure or laterality, finding large group-level differences only in total brain volume.[1] A subsequent 2021 study led by Camille Michèle Williams contradicted Eliot's conclusions, finding that sex differences in total brain volume are not accounted for merely by sex differences in height, and that once global brain size is taken into account, there remain numerous regional sex differences in both directions.[2] In 2022 Alex DeCasien analyzed the studies from both Eliot and Williams, concluding that "The human brain shows highly reproducible sex differences in regional brain anatomy above and beyond sex differences in overall brain size" and that these differences are of a "small-moderate effect size."[3] In 2024 Eliot responded[4] by showing that those small-moderate differences have not reproduced across 6 large recent studies, including Williams et al., and concluding that species-wide regional brain sex differences have not been found to exist in humans.
An earlier review from 2006[5] and meta-analysis from 2014[6] stated that male and female brains cannot always be assumed to be identical from either structural or functional perspective, calling themsexually dimorphic, a term that Williams, DeCasien and Eliot agree does not accurately describe the human brain.
The ideas of differences between the male and female brains have circulated since the time ofAncient Greek philosophers around 850 BC. In 1854, German anatomistEmil Huschke discovered a size difference in thefrontal lobe, where male frontal lobes are 1% larger than those of females.[7] As the 19th century progressed, scientists began researching sexual dimorphisms in the brain significantly more.[8] Until recent decades, scientists knew of several structural sexual dimorphisms of the brain, but they did not think that sex had any impact on how the human brain performs daily tasks. Through molecular, animal, and neuroimaging studies, a great deal of information regarding the differences between male and female brains in regards to both structure and function has been uncovered.[9]
Females show enhanced information recall compared with males. This may be due to the fact that females have a more intricate evaluation of risk–scenario contemplation, based on aprefrontal cortical control of theamygdala. For example, the ability to recall information better than males most likely originated fromsexual selective pressures on females during competition with other females in mate selection. Recognition of social cues was an advantageous characteristic, because it ultimately maximized offspring and was therefore selected for during evolution.[5]
Oxytocin is a hormone that inducescontraction of theuterus andlactation in mammals and is also a characteristic hormone of nursing mothers. Studies have found that oxytocin improvesspatial memory. Through activation of theMAP kinase pathway, oxytocin plays a role in the enhancement of long-termsynaptic plasticity, which is a change in strength between twoneurons over asynapse that lasts for minutes or longer, andlong-term memory. This hormone may have helped mothers remember the location of distant food sources so they could better nurture their offspring.[5]
According to certain studies, men on average have one standard deviation higher spatial cognition than women.[10] This domain is one of the few where clearsex differences in cognition appear. Researchers at theUniversity of Toronto found that differences between men and women on some tasks that require spatial skills are largely eliminated after both groups play a video game for only a few hours.[11] AlthoughHerman Witkin had claimed women are more "visually dependent" than men,[12] this has recently been disputed.[13]
The gender difference in spatial ability was found in one small study to be associated with different morphological findings in male and female brains. The parietal lobe is a part of the brain that is recognized to be involved in spatial ability, especially for mental rotation. Researchers at the University of Iowa found that the thicker grey matter in theparietal lobe of females led to a disadvantage in mental rotations, and that the larger surface areas of theparietal lobe of males led to an advantage in mental rotations. These odd findings were argued to support the notion that gender differences in spatial abilities arose during human evolution, however, they have not been replicated, and the effect of socialization and experience on the difference in spatial ability is likely strong.[14] Eliot et al. reviewed 19 functional imaging studies that collectively showed similar patterns of parietal lobe activation in both men and women during mental rotation tasks, with no reliable sex differences.[1]
A 2021meta-synthesis of existing literature found that sex accounted for 1% of the brain's structure or laterality, finding large group-level differences only in total brain volume.[1] A 2022 follow-up study challenged these findings and concluded that "The human brain shows highly reproducible sex differences in regional brain anatomy above and beyond sex differences in overall brain size" and that these differences were of a "small-to-moderate effect size."[15] In another study, men were found to have a totalmyelinated fiber length of 176 000 km at the age of 20, whereas in women the total length was 149 000 km (approx. 15% less).[16]
Many similarities and differences in structure, neurotransmitters, and function have been identified,[6][17] but some academics, such asCordelia Fine[18] andAnelis Kaiser, Sven Haller,Sigrid Schmitz, and Cordula Nitsch[19] dispute the existence of significant sex differences in the brain, arguing that innate differences in the neurobiology of women and men have not been conclusively identified due to factors such as allegedneurosexism, methodological flaws andpublication bias.[18][19] Clinical psychologistSimon Baron-Cohen has defended the neuroscience of sex differences against charges of neurosexism, arguing that "Fine's neurosexism allegation is the mistaken blurring of science with politics," adding that "you can be a scientist interested in the nature of sex differences while being a clear supporter of equal opportunities and a firm opponent of all forms of discrimination in society."[20]
Males and females differ in some aspects of their brains, notably the overall difference in size, with men having larger brains on average (between 8% and 13% larger),[6] but a relationship between brain volume or density and brain function is not established.[21] Additionally, there are differences in activation patterns that suggest functional differences that could arise as a result of experience.
Structurally, adult male brains are on average 11–12% heavier and 10% bigger than female brains.[22] Though statistically there are sex differences inwhite matter andgray matter percentage, this ratio is directly related to brain size, and some[23] argue these sex differences in gray and white matter percentage are caused by the average size difference between men and women. Others argue that these differences partly remain after controlling for brain volume.[17][24]
Researchers also found greatercortical thickness and cortical complexity in females before, and after adjusting for overall brain volume. In contrast, surface area, brain volume and fractional anisotropy was found to be greater in males before, and after adjusting for overall brain volume. Despite attributes remaining greater for both male and female, the overall difference in these attributes decreased after adjusting for overall brain volume, except the cortical thickness in females, which increased.[24] Given that cortical complexity and cortical features have had some evidence of positive correlation with intelligence, researchers postulated that these differences might have evolved for females to compensate for smaller brain size and equalize overall cognitive abilities with males, though the reason for environmental selection of that trait is unknown.[24]
Researchers further analyzed the differences in brain volume, surface area and cortical thickness by testing the men and women on verbal-numerical reasoning and reaction time in separate groups. It was found that the group of men slightly outperformed the women in both the verbal-numerical reasoning and reaction time tests. Subsequently, the researchers tested to what extent the differences in performance was mediated by the varying attributes of the male and female brain (e.g. surface area) using two mixed sample groups. In verbal-numerical reasoning tests, surface area and brain volume mediated performance by >82% in both groups, and cortical thickness mediated performance far less, by 7.1% and 5.4% in each group. In reaction time tests, total brain andwhite matter volumes mediated performance by >27%, but the other attributes all mediated performance by smaller percentages (<15.3%), particularly mean cortical thickness (mediating <3% of performance).[25]
According to theneuroscience journal review seriesProgress in Brain Research, it has been found that males have larger and longer planum temporale andSylvian fissure while females have significantly larger proportionate volumes to total brain volume in thesuperior temporal cortex,Broca's area, the hippocampus and thecaudate.[24] Themidsagittal and fiber numbers in theanterior commissure that connect the temporal poles and mass intermedia that connects the thalami is also larger in women.[24]
Lateralization may differ between the sexes, with men often being said to have a more lateralized brain, because they tend to use one hemisphere for a behavior more consistently than females. One factor supporting this idea is the higher rate of left-handedness among males. Another example is that language is typically more strongly left-lateralized in males than in females. Additionally, males show stronger right-lateralization in emotional-face processing tasks, suggesting sex differences in brain lateralization go beyond just language. These differences may be linked to factors like how the two brain hemispheres interact and differences in the corpus callosum, rather than directly to sex hormones. Other biological factors, such as cortisol levels and dopamine asymmetries, may also play a role in these differences, particularly in decision-making and emotional responses.[26]
Further evidence comes from studies using turning preferences as a way to measure brain lateralization. While no major sex differences were found in the direction or strength of laterality, males showed more consistent lateralization across different situations. This suggests that males may have stronger and more stable brain lateralization. However, the relationship between brain lateralization and cognitive abilities remains unclear and is likely influenced by many biological factors.[27]
Overall, the evidence for sex difference in human brain lateralization remains weak. A 2008 meta-analysis found no significant difference between males and females in either planum temporale asymmetry or functional language lateralization.[28] Moreover, a 2019 review of 40 years of research concluded that sex differences in laterality are extremely small (Cohen's d < 0.10) and "are certainly not the driving force behind sex differences in cognitive functioning."[29]
There are behavioral differences between males and females that may suggest a difference inamygdala size or function. A 2017 review of amygdala volume studies found that there was a raw size difference, with males having a 10% larger amygdala, however, because male brains are larger, this finding was found to be misleading. After normalizing for brain size, there was no significant difference in size of the amygdala across sex.[30]
In terms of activation, there is no difference in amygdala activation across sex. Differences in behavioral tests may be due to potential anatomical and physiological differences in the amygdala across sexes rather than activation differences.[31]
Emotional expression, understanding, and behavior appears to vary between males and females. A 2012 review concluded that males and females have differences in the processing of emotions. Males tend to have stronger reactions to threatening stimuli and that males react with more physical violence.[32]
Hippocampus atrophy is associated with a variety of psychiatric disorders that have higher prevalence in females. Additionally, there are differences in memory skills between males and females which may suggest a difference in the hippocampal volume (HCV). A 2016 meta-analysis of volume differences found a higher HCV in males without correcting for total brain size. However, after adjusting for individual differences and total brain volume, they found no significant sex difference, despite the expectation that women may have larger hippocampus volume.[33]
A 2014 meta-analysis found (where differences were measured) some differences ingrey matter levels between the sexes.
The findings included females having more grey matter volume in the rightfrontal pole,inferior andmiddlefrontal gyrus,pars triangularis,planum temporale/parietal operculum,anterior cingulate gyrus,insular cortex, andHeschl's gyrus; boththalami and precuneus; the left parahippocampal gyrus and lateraloccipital cortex (superior division).[6] Larger volumes in females were most pronounced in areas in the right hemisphere related to language in addition to severallimbic structures such as the right insular cortex and anterior cingulate gyrus.[6]
Males had more grey matter volume in both amygdalae,hippocampi, anteriorparahippocampal gyri, posteriorcingulate gyri,precuneus,putamen andtemporal poles, areas in the left posterior andanterior cingulate gyri, and areas in thecerebellum bilateral VIIb, VIIIa and Crus I lobes, left VI and right Crus II lobes.[6]
In terms of density, there were also differences between the sexes. Males tended to have a denser left amygdala, hippocampus, insula,pallidum, putamen,claustrum, and areas of the right VIlobule of thecerebellum, among other areas.[6] Females tended to have denser left frontal pole.[6]
The significance of these differences lies both in the lateralization (males having more volume in the left hemisphere and females having more volume in the right hemisphere) and the possible uses of these findings to explore differences in neurological and psychiatric conditions.[34]
Early postmortem studies oftransgender neurological differentiation were focused on thehypothalamic and amygdala regions of the brain. Usingmagnetic resonance imaging (MRI), sometrans women were found to have female-typicalputamina that were larger in size than those ofcisgender males.[35] Some trans women have also shown a female-typical central part of thebed nucleus of the stria terminalis (BSTc) andinterstitial nucleus of the anterior hypothalamus number 3 (INAH-3), looking at the number of neurons found within each.[36]
Both males and females have consistent activeworking memory networks composed of both middle frontal gyri, the leftcingulate gyrus, the rightprecuneus, the leftinferior andsuperior parietal lobes, the rightclaustrum, and the leftmiddle temporal gyrus.[37] Although the same brain networks are used for working memory, specific regions are sex-specific. Sex differences were evident in other networks, as women also tend to have higher activity in the prefrontal andlimbic regions, such as the anterior cingulate, bilateral amygdala, and right hippocampus, while men tend to have a distributed network spread out among thecerebellum, portions of the superiorparietal lobe, the leftinsula, and bilateralthalamus.[37]
A 2017 review from the perspective oflarge-scale brain networks hypothesized that women's higher susceptibility to stress-prone disorders such aspost-traumatic stress disorder andmajor depressive disorder, in which thesalience network is theorized to be overactive and to interfere with theexecutive control network, may be due in part, along with societal exposure to stressors and the coping strategies that are available to women, to underlying sex-based brain differences.[38]
Gonadal hormones, or sex hormones, includeandrogens (such astestosterone) andestrogens (such asestradiol), which aresteroid hormones synthesized primarily in thetestes andovaries, respectively. Sex hormone production is regulated by thegonadotropic hormonesluteinizing hormone (LH) andfollicle-stimulating hormone (FSH), whose release from theanterior pituitary is stimulated bygonadotropin-releasing hormone (GnRH) from the hypothalamus.[39]
Steroid hormones have several effects on brain development as well as maintenance ofhomeostasis throughout adulthood. Estrogen receptors have been found in the hypothalamus,pituitary gland, hippocampus, and frontal cortex, indicating the estrogen plays a role in brain development. Gonadal hormone receptors have also been found in the basal fore-brain nuclei.[40]
Estradiol influences cognitive function, specifically by enhancing learning and memory in a dose-sensitive manner. Too much estrogen can have negative effects by weakening performance of learned tasks as well as hindering performance of memory tasks; this can result in females exhibiting poorer performance of such tasks when compared to males.[41]
Ovariectomies, surgeries inducingmenopause, or natural menopause cause fluctuating and decreased estrogen levels in women. This in turn can "attenuate the effects" of endogenousopioid peptides. Opioid peptides are known to play a role in emotion and motivation. The content ofβ-endorphin (β-EP), an endogenous opioid peptide, has been found to decrease (in varying amounts/brain region) post ovariectomy in female rats within the hypothalamus, hippocampus, and pituitary gland. Such a change in β-EP levels could be the cause of mood swings, behavioral disturbances, andhot flashes in post menopausal women.[40]
Progesterone is a steroid hormone synthesized in both male and female brains. It contains characteristics found in the chemical nucleus of both estrogen and androgen hormones.[42] As a female sex hormone, progesterone is more significant in females than in males. During themenstrual cycle, progesterone increases just after theovulatory phase to inhibit luteinizing hormones, such as oxytocin absorption.[43] In men, increased progesterone has been linked to adolescents with suicidal ideation.[44]
The gonadal hormone testosterone is an androgenic, or masculinizing, hormone that is synthesized in both the male testes and female ovaries,[45] at a rate of about 14,000 μg/day and 600 μg/day, respectively.[39] Testosterone exerts organizational effects on the developing brain, many of which are mediated throughestrogen receptors following its conversion to estrogen by the enzymearomatase within the brain.[39]
{{cite book}}:|journal= ignored (help)
