page_title: Sex differences in human physiology
section_title: Brain structure
hierarchical_section_title: Sex differences in human physiology / Brain and nervous system / Brain / Brain structure
caption_attribution_description: Human brain right dissected lateral view description.JPG Lateral Portion of Frontal, Parietal, Occipital, and Superior Portion of Temporal Lobe Resected. The anterior horn of the lateral ventricle is located in the frontal lobe. The body of the lateral ventricle continues posteriorly into the parietal lobe, the posterior horn into the occipital lobe, and the inferior horn down into the temporal lobe. Some structures produce elevations or bumps in the walls of the posterior and/or inferior horns of the lateral ventricles. Ventriculus lateralis, Cornu frontale Ventriculus lateralis, Pars centralis Calcar avis Ventriculus lateralis, Cornu occipitale Trigonum collaterale Eminentia collateralis Hippocampus Ventriculus lateralis, Cornu temporale Capsula interna Nucleus caudatus (font: arial black, size: 10)
context_page_description: Sex differences in human physiology are distinctions of physiological characteristics associated with either male or female humans. These can be of several types, including direct and indirect. Direct being the direct result of differences prescribed by the Y-chromosome, and indirect being a characteristic influenced indirectly by the Y-chromosome. Sexual dimorphism is a term for the phenotypic difference between males and females of the same species.
  Direct sex differences follow a bimodal distribution. Through the process of meiosis and fertilization, each individual is created with zero or one Y-chromosome. The complementary result for the X-chromosome follows, either a double or a single X. Therefore, direct sex differences are usually binary in expression. These include, most conspicuously, male gonads.
  Indirect sex differences are general differences as quantified by empirical data and statistical analysis. Most differing characteristics will conform to a bell-curve distribution which can be broadly described by the mean and standard deviation. Often only the mean or mean difference between sexes is given. This may or may not preclude overlap in distributions.
context_section_description: Structural brain differences usually correspond to sexually dimorphic attributes that bring about functional brain differences.
  On average, female brains have a larger ratio of grey matter to the white matter than males (particularly in the dorsolateral prefrontal cortex and superior temporal gyrus), even when sex-differences in the total intracranial volume are taken into consideration. Most notably, males have a larger amount of white matter in the frontal and temporal perisylvian region, and in the temporal stem and optic radiation, of the left hemisphere, whereas females have a larger amount of gray matter in the superior temporal gyrus, planum temporale, Heschl gyrus, cingulate gyrus, inferior frontal, and central sulci margins, of the left hemisphere.
  The degree of hemispheric asymmetry in males corresponds to the relative size of corpus callosum; however, this is not true in females. An increase in hemispheric asymmetry in male brains causes a male sex-dependent decrease in inter-hemispheric connectivity. Numerous studies suggest that, on average, female brains have more commissural tracts involved in inter-hemispheric connectivity than males. More specifically, it suggests that: the anterior commissure is larger in females than males; massa intermedia is more abundant in females than males; females have a larger ratio of the cross-sectional area of the corpus callosum to cerebral volume and to forebrain size than males. Although, fewer studies have alternatively found otherwise.
  Typically, male brains are more asymmetric than female brains. Females have less asymmetry than males between left and right hemispheric cortical thickness. Males have a larger intra-hemispheric long-range interconnectivity than females, whereas females have larger inter-hemispheric connectivity. Males have larger left-hemispheric asymmetries than females in various brain areas, including the superior temporal gyrus, Heschl gyrus, deeper central sulcus, overall temporal and parietal, and inferior parietal lobule, thalamus and posterior cingulate.  Although, few studies have found otherwise.
  There are also differences in the structure of specific areas of the brain. On average, the SDN has been repeatedly found to be considerably larger in males than in females. The volume of the SDN was 2.2 times as large in males as in females. On average, the BSTc is twice as large in men as in women. On average, the INAH-3 is significantly larger in males than in females regardless of age. Two studies found that men have larger parietal lobes, an area responsible for sensory input including spatial sense and navigation; though, another study failed to find any statistically significant difference. At the same time, females have larger Wernicke's and Broca's areas, areas responsible for language processing. Studies using MRI scanning have shown that the auditory and language-related regions in the left hemisphere are proportionally expanded in females versus in males. Conversely, the primary visual, and visuo-spatial association areas of the parietal lobes are proportionally larger in males. The corpus callous is located at the sagittal divide and is the primary commissure in the human brain. It connects the left and right hemispheres of the cerebral cortex, which allows them to communicate with each other.  With respect to language, males predominantly use their left hemisphere but females use both their right and left hemispheres. The right hemisphere controls emotion, so using the right hemisphere adds more prosody to speech. In males, the corpus callosum is larger than females.  However, the splenium and the isthmus subregions of the corpus callosum are larger in females. The genu subregion is larger in males. These subregions may serve as the basis for sex differences in language. However, a 1997 meta-study concluded that there is no relative size difference, and that the larger corpus callosum in males is due to generally larger brains in males on average.
  In total and on average, females have a higher percentage of grey matter in comparison to ma
caption: The male and female brains show some differences in internal structure. One difference is the proportions of white matter relative to grey matter.
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