The neuroscience of sex differences is the study of characteristics that separate brains of different sexes. Psychological sex differences are thought by some to reflect the interaction of genes, hormones, and social learning on brain development throughout the lifespan.
A 2021 meta-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 subsequent study contradicted these conclusions, finding that sex differences in total brain volume are not accounted for by sex differences in height and weight, and that once global brain size is taken into account, there remain numerous regional sex differences in both directions.[2] A review from 2006 and a meta-analysis from 2014 found that 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.[3][4]
History
The ideas of differences between the male and female brains have circulated since the time of Ancient Greek philosophers around 850 BC. In 1854, German anatomist Emil Huschke discovered a size difference in the frontal lobe, where male frontal lobes are 1% larger than those of females.[5] As the 19th century progressed, scientists began researching sexual dimorphisms in the brain significantly more.[6] 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 and how much they differ in regards to both structure and function has been uncovered.[7]
Females show enhanced information recall compared to males. This may be due to the fact that females have a more intricate evaluation of risk–scenario contemplation, based on a prefrontal cortical control of the amygdala. For example, the ability to recall information better than males most likely originated from sexual 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.[3]
Oxytocin is a hormone that induces contraction of the uterus and lactation in mammals and is also a characteristic hormone of nursing mothers. Studies have found that oxytocin improves spatial memory. Through activation of the MAP kinase pathway, oxytocin plays a role in the enhancement of long-term synaptic plasticity, which is a change in strength between two neurons over a synapse that lasts for minutes or longer, and long-term memory. This hormone may have helped mothers remember the location of distant food sources so they could better nurture their offspring.[3]
According to certain studies, men on average have one standard deviation higher spatial intelligence quotient than women.[8] This domain is one of the few where clear sex differences in cognition appear. Researchers at the University of Toronto say 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.[9] Although Herman Witkin had claimed women are more "visually dependent" than men,[10] this has recently been disputed.[11]
The gender difference in spatial ability was found to be attributed to morphological differences between male and female brains. The parietal lobe is a part of the brain that is recognized to be involved in spatial ability, especially in 2d- and 3d mental rotation. Researchers at the University of Iowa found that the thicker grey matter in the parietal lobe of females led to a disadvantage in mental rotations, and that the larger surface areas of the parietal lobe of males led to an advantage in mental rotations. The results found by the researches support the notion that gender differences in spatial abilities arose during human evolution such that both sexes cognitively and neurologically developed to behave adaptively. However, the effect of socialization and environment on the difference in spatial ability is still open for debate.[12]
Male and female brain anatomy
A 2021 meta-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] Men were found to have a total myelinated fiber length of 176000km at the age of 20, whereas in women the total length was 149000km (approx. 15% less).[13]
Many similarities and differences in structure, neurotransmitters, and function have been identified,[4][14] but some academics, such as Cordelia Fine[15] and Anelis Kaiser, Sven Haller, Sigrid Schmitz, and Cordula Nitsch[16] 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 alleged neurosexism, methodological flaws and publication bias.[15][16] Clinical psychologist Simon 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."[17]
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),[4] but a relationship between brain volume or density and brain function is not established.[18] Additionally, there are differences in activation patterns that suggest anatomical or developmental differences.
Volume
Structurally, adult male brains are on average 11–12% heavier and 10% bigger than female brains.[19] Though statistically there are sex differences in white matter and gray matter percentage, this ratio is directly related to brain size, and some[20] 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.[14][21]
Researchers also found greater cortical 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.[21] 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.[21]
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 and white 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).[21]
Lateralization may differ between the sexes, with men often being said to have a more lateralized brain. This is often attributed to differences in "left-" and "right-" brained abilities. One factor that contributes support to the idea that there is a sex difference in brain lateralization is that men are more likely to be left-handed. However, it is unclear whether this is due to a difference in lateralization.[22]
A 2014 meta-analysis of grey matter in the brain found sexually dimorphic areas of the brain in both volume and density. When synthesized, these differences show that volume increases for males tend to be on the left side of systems, while females generally see greater volume in the right hemisphere.[4] On the other hand, a previous 2008 meta-analysis found that the difference between male and female brain lateralization was not significant.[22]
Amygdala
The amygdala (red) in a human brain
There are behavioral differences between males and females that may suggest a difference in amygdala 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.[23]
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.[24]
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.[25]
Hippocampus
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.[26]
Grey matter
A 2014 meta-analysis found (where differences were measured) some differences in grey matter levels between the sexes.
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 VI lobule of the cerebellum, among other areas.[4] Females tended to have denser left frontal pole.[4]
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.[citation needed]
Both males and females have consistent active working memory networks composed of both middle frontal gyri, the left cingulate gyrus, the right precuneus, the left inferior and superior parietal lobes, the right claustrum, and the left middle temporal gyrus.[29] 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 and limbic regions, such as the anterior cingulate, bilateral amygdala, and right hippocampus, while men tend to have a distributed network spread out among the cerebellum, portions of the superior parietal lobe, the left insula, and bilateral thalamus.[29]
Steroid hormones have several effects on brain development as well as maintenance of homeostasis 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.[32]
Estrogen and the female brain
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.[33]
Ovariectomies, surgeries inducing menopause, or natural menopause cause fluctuating and decreased estrogen levels in women. This in turn can "attenuate the effects" of endogenous opioid peptides. Opioid peptides are known to play a role in emotion and motivation. The content of β-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, and hot flashes in post menopausal women.[32]
Progesterone and the male and female brain
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.[34] As a female sex hormone, progesterone is more significant in females than in males. During the menstrual cycle, progesterone increases just after the ovulatory phase to inhibit luteinizing hormones, such as oxytocin absorption.[35] In men, increased progesterone has been linked to adolescents with suicidal ideation.[36]
Testosterone and the male brain
The gonadal hormone testosterone is an androgenic, or masculinizing, hormone that is synthesized in both the male testes and female ovaries,[37] at a rate of about 14,000 μg/day and 600 μg/day, respectively.[31] Testosterone exerts organizational effects on the developing brain, many of which are mediated through estrogen receptors following its conversion to estrogen by the enzyme aromatase within the brain.[31]
The hippocampus is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, and plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation. The hippocampus is located in the allocortex, with neural projections into the neocortex, in humans as well as other primates. The hippocampus, as the medial pallium, is a structure found in all vertebrates. In humans, it contains two main interlocking parts: the hippocampus proper, and the dentate gyrus.
The hypothalamus is a small part of the brain that contains a number of nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamus is located below the thalamus and is part of the limbic system. It forms the ventral part of the diencephalon. All vertebrate brains contain a hypothalamus. In humans, it is the size of an almond.
The amygdala is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex vertebrates, including humans. Shown to perform a primary role in the processing of memory, decision making, and emotional responses, the amygdalae are considered part of the limbic system. The term "amygdala" was first introduced by Karl Friedrich Burdach in 1822.
The limbic system, also known as the paleomammalian cortex, is a set of brain structures located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.
An androgen is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes the embryological development of the primary male sex organs, and the development of male secondary sex characteristics at puberty. Androgens are synthesized in the testes, the ovaries, and the adrenal glands.
The cingulate cortex is a part of the brain situated in the medial aspect of the cerebral cortex. The cingulate cortex includes the entire cingulate gyrus, which lies immediately above the corpus callosum, and the continuation of this in the cingulate sulcus. The cingulate cortex is usually considered part of the limbic lobe.
The piriform cortex, or pyriform cortex, is a region in the brain, part of the rhinencephalon situated in the cerebrum. The function of the piriform cortex relates to the sense of smell.
The planum temporale is the cortical area just posterior to the auditory cortex within the Sylvian fissure. It is a triangular region which forms the heart of Wernicke's area, one of the most important functional areas for language. Original studies on this area found that the planum temporale was one of the most asymmetric regions in the brain, with this area being up to ten times larger in the left cerebral hemisphere than the right.
Sex differences in human physiology are distinctions of physiological characteristics associated with either male or female humans. These differences are caused by the effects of the different sex chromosome complement in males and females, and differential exposure to gonadal sex hormones during development. Sexual dimorphism is a term for the phenotypic difference between males and females of the same species.
The septal area, consisting of the lateral septum and medial septum, is an area in the lower, posterior part of the medial surface of the frontal lobe, and refers to the nearby septum pellucidum.
The inferior temporal gyrus is one of three gyri of the temporal lobe and is located below the middle temporal gyrus, connected behind with the inferior occipital gyrus; it also extends around the infero-lateral border on to the inferior surface of the temporal lobe, where it is limited by the inferior sulcus. This region is one of the higher levels of the ventral stream of visual processing, associated with the representation of objects, places, faces, and colors. It may also be involved in face perception, and in the recognition of numbers and words.
Gender incongruence is the state of having a gender identity that does not correspond to one's sex assigned at birth. This is experienced by people who identify as transgender or transsexual, and often results in gender dysphoria. The causes of gender incongruence have been studied for decades.
In human neuroanatomy, brain asymmetry can refer to at least two quite distinct findings:
Sexual orientation is an enduring pattern of romantic or sexual attraction to persons of the opposite sex or gender, the same sex or gender, or to both sexes or more than one gender, or none of the aforementioned at all. The ultimate causes and mechanisms of sexual orientation development in humans remain unclear and many theories are speculative and controversial. However, advances in neuroscience explain and illustrate characteristics linked to sexual orientation. Studies have explored structural neural-correlates, functional and/or cognitive relationships, and developmental theories relating to sexual orientation in humans.
The biology of obsessive–compulsive disorder (OCD) refers biologically based theories about the mechanism of OCD. Cognitive models generally fall into the category of executive dysfunction or modulatory control. Neuroanatomically, functional and structural neuroimaging studies implicate the prefrontal cortex (PFC), basal ganglia (BG), insula, and posterior cingulate cortex (PCC). Genetic and neurochemical studies implicate glutamate and monoamine neurotransmitters, especially serotonin and dopamine.
The causes of schizophrenia that underlie the development of schizophrenia, a psychiatric disorder, are complex and not clearly understood. A number of hypotheses including the dopamine hypothesis, and the glutamate hypothesis have been put forward in an attempt to explain the link between altered brain function and the symptoms and development of schizophrenia.
Parental experience, as well as changing hormone levels during pregnancy and postpartum, cause changes in the parental brain. Displaying maternal sensitivity towards infant cues, processing those cues and being motivated to engage socially with her infant and attend to the infant's needs in any context could be described as mothering behavior and is regulated by many systems in the maternal brain. Research has shown that hormones such as oxytocin, prolactin, estradiol and progesterone are essential for the onset and the maintenance of maternal behavior in rats, and other mammals as well. Mothering behavior has also been classified within the basic drives.
Even though intimacy has been broadly defined in terms of romantic love and sexual desire, the neuroanatomy of intimacy needs further explanation in order to fully understand their neurological functions in different components within intimate relationships, which are romantic love, lust, attachment, and rejection in love. Also, known functions of the neuroanatomy involved can be applied to observations seen in people who are experiencing any of the stages in intimacy. Research analysis of these systems provide insight on the biological basis of intimacy, but the neurological aspect must be considered as well in areas that require special attention to mitigate issues in intimacy, such as violence against a beloved partner or problems with social bonding.
Catherine S. Woolley is an American neuroendocrinologist. Woolley holds the William Deering Chair in Biological Sciences in the Department of Neurobiology, Weinberg College of Arts & Sciences, at Northwestern University. She is also a member of the Women's Health Research Institute in the Feinberg School of Medicine at Northwestern University.
Neurosexism is an alleged bias in the neuroscience of sex differences towards reinforcing harmful gender stereotypes. The term was coined by feminist scholar Cordelia Fine in a 2008 article and popularised by her 2010 book Delusions of Gender. The concept is now widely used by critics of the neuroscience of sex differences in neuroscience, neuroethics and philosophy.
↑ Swaab DF, Hofman MA (1984). "Sexual Differentiation of the Human Brain a Historical Perspective". Sex Differences in the Brain - the Relation Between Structure and Function. Progress in Brain Research. Vol.61. pp.361–74. doi:10.1016/S0079-6123(08)64447-7. ISBN9780444805324. PMID6396708.
↑ Witkin, H. A.; Lewis, H. B.; Hertzman, M.; Machover, K.; Meissner, P. B.; Wapner, S. (1954). Personality through perception: An experimental and clinical study. New York: Harper. OCLC2660853.
↑ Marner L, Nyengaard JR, Tang Y, Pakkenberg B (July 2003). "Marked loss of myelinated nerve fibers in the human brain with age". The Journal of Comparative Neurology. 462 (2): 144–52. doi:10.1002/cne.10714. PMID12794739. S2CID35293796.
1 2 Sommer IE, Aleman A, Somers M, Boks MP, Kahn RS (April 2008). "Sex differences in handedness, asymmetry of the planum temporale and functional language lateralization". Brain Research. 1206: 76–88. doi:10.1016/j.brainres.2008.01.003. PMID18359009. S2CID7371496.
↑ Sergerie K, Chochol C, Armony JL (2008). "The role of the amygdala in emotional processing: a quantitative meta-analysis of functional neuroimaging studies". Neuroscience and Biobehavioral Reviews. 32 (4): 811–30. doi:10.1016/j.neubiorev.2007.12.002. PMID18316124. S2CID10980762.
↑ Tan A, Ma W, Vira A, Marwha D, Eliot L (January 2016). "The human hippocampus is not sexually-dimorphic: Meta-analysis of structural MRI volumes". NeuroImage. 124 (Pt A): 350–366. doi:10.1016/j.neuroimage.2015.08.050. PMID26334947. S2CID26316768.
↑ Homberg JR, Kozicz T, Fernandez G (April 2017). "Large-scale network balances in the transition from adaptive to maladaptive stress responses". Current Opinion in Behavioral Sciences. 14: 27–32. doi:10.1016/j.cobeha.2016.11.003. S2CID53161342.
↑ Funk and Wagnalls (2018). Progesterone. World Almanac Education Group.
↑ Ulshöfer, Gotlind; Karafyllis, Nicole (2008). Sexualized Brains: Scientific Modeling of Emotional Intelligence from a Cultural Perspective. Cambridge, Massachusetts: The MIT Press. p.213.
↑ Lester, David; Gunn, John F. III; Quinnett, Paul, eds. (2014). Suicide in Men: How Men Differ from Women in Expressing their Distress. Springfield, Illinois: Charles C Thomas. p.61. ISBN978-0-398-08794-4.
↑ Hadley ME, Levine JE (2007). Endocrinology. Levine, Jon E. (6th, Newed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN978-0131876064. OCLC70929277.
Further reading
Rippon, Gina (28 Feb 2019). The gendered brain: The new neuroscience that shatters the myth of the female brain. Bodley Head. ISBN978-1847924759.
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