The neuroscience of sex differences is the study of characteristics that separate brains of different sexes. Psychological sex differences are generally thought to reflect the interaction of genes, hormones, and social learning on brain development throughout the lifespan. A 2021 meta-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. 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. 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." In 2024 Eliot responded 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 and meta-analysis from 2014 stated that male and female brains cannot always be assumed to be identical from either structural or functional perspective, calling them sexually dimorphic, a term that Williams, DeCasien and Eliot agree does not accurately describe the human brain.
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. As the 19th century progressed, scientists began researching sexual dimorphisms in the brain significantly more. 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.
Evolutionary explanations
Sexual selection
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 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. 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. According to certain studies, men on average have one standard deviation higher spatial cognition than women. This domain is one of the few where clear sex differences in cognition appear. Researchers at the University 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. Although Herman Witkin had claimed women are more "visually dependent" than men, this has recently been disputed. 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 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. 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. 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.
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