The biology of gender is scientific analysis of the physical basis for behavioural differences between men and women. It is more specific than sexual dimorphism, which covers physical and behavioural differences between males and females of any sexually reproducing species, or sexual differentiation, where physical and behavioural differences between men and women are described.
Biological research of gender has explored such areas as: intersex physicalities, gender identity, gender roles and sexual preference. Late twentieth century study focused on hormonal aspects of the biology of gender. With the successful mapping of the human genome, early twenty-first century research started making progress in understanding the effects of gene regulation on the human brain.
Research in this area is generally motivated by the search for causes of diseases in human beings, and ways of treating or preventing those diseases; it is thought that men and women might require different kinds of treatment for certain diseases. The results are relevant to gender issues, but that is not their direct concern.
The late twentieth century saw an explosion in technology capable of aiding sex research. Scientists made great progress towards understanding the formation of gender identity in humans. Extensive advances were also made in understanding sexual dimorphism in other animals. For example, there were studies on the effects of sex hormones on rats. The early twenty first century started producing even more amazing results concerning genetically programmed sexual dimorphism in rat brains, prior even to the influence of hormones on development. "Genes on the sex chromosomes can directly influence sexual dimorphism in cognition and behaviour, independent of the action of sex steroids."
The brains of many animals, including humans, are significantly different for males and females of the species. Both genes and hormones affect the formation of many animal brains before "birth" (or hatching), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. Both kinds of brain difference affect male and female behaviour.
In 2006, Alexandra M. Lopes and others published that:
|“||A sexual dimorphism in levels of expression in brain tissue was observed by quantitative real-time PCR, with females presenting an up to 2-fold excess in the abundance of PCDH11X transcripts. We relate these findings to sexually dimorphic traits in the human brain. Interestingly, PCDH11X/Y gene pair is unique to Homo sapiens, since the X-linked gene was transposed to the Y chromosome after the human–chimpanzee lineages split.||”|
It is commonly accepted that there are many sex-related differences in behavior in the human species. These differences and their neurobiological bases have been sought. Gur et al. suggested that women have a higher percentage of grey matter (GM) in comparison to men, and that men have a higher percentage of white matter (WM) and of cerebrospinal fluid (CSF). The white matter of 20 year men consists of around 176,000 km of myelinated axons compared to 149,000 km in women of the same age. In Gur's study, in men the percentage of GM was higher in the left hemisphere, the percentage of WM was symmetric, and the percentage of CSF was higher in the right hemisphere. In women, no asymmetry was shown. Gur et al. stated that the higher percentage of GM in women could compensate for less space in the cranium, since GM is used for computation more than transfer of information across distant regions. They state that the sex-based differences in percentage and asymmetry of GM and WM in the brain may account for the differences in cognitive functioning of the two sexes. They also stated:
|“||The anatomic results suggest some parallels between sex differences in cognition and differences in GM because both women and the left language hemisphere have higher percentage of GM, and women outperform men on language tasks. [...] Considered separately, the performance data replicated earlier reports of better verbal relative to spatial performance in women as compared with men, against overall similar levels of average performance.||”|
Haier and colleagues used voxel-based morphometry (VBM) to identify brain areas where clusters GM and WM volumes are located in correlation to the FSIQ test. In 2004, Haier et al. claimed that:
|“||These findings support the view that individual differences in grey and white matter volumes, in a relatively small number of areas distributed throughout the brain, account for considerable variance in individual differences in general intelligence.||”|
However, both neuronal cell bodies (grey matter) and axons (white matter) are essential to the function of the nervous system, so the functional implication of having more of one or the other is not clear.
See the sex and intelligence article for more information and research findings on this subject.
A 2001 report by Richard J. Coley of the ETS stated, "A review of the elementary and secondary education achievement data included in this report from NAEP found that females in all racial/ethnic groups scored higher, on average, than males in reading, writing, and civics. There was an advantage found in science for Hispanic and White males. In mathematics, essentially no differences between males and females were found."
Kiefer and Sekaquaptewa proposed that a source of some women's underperformance and lowered perseverance in mathematical fields is these women's underlying "implicit" sex-based stereotypes regarding mathematical ability and association, as well as their identification with their gender.
A number of studies have looked for sex differences in the brain that might relate to sex differences in intelligence or performance on different tasks. These studies have included measures of total brain size, relative amounts of grey and white matter, and a wide variety of measures of brain activity patterns (Sex and Intelligence). However, findings of sex differences in the brain do not answer the Nature versus Nurture controversy raised again by Summers' comments, because studies of neuroplasticity show that the brain can be altered by experience.
In mathematical reasoning, Benbow et al. stated of a 1983 study:
|“||When graphed (Benbow, 1988), the male and female SAT-V [verbal] distributions were found to be essentially equivalent, but the male SAT-M [math] distributions manifested a higher mean and larger variance than was observed for the females. Consequently, an exponential intensification of the male:female ratio occurred in the upper tail of the combined distribution. The ratio was 2:1 for adolescents with SAT-M scores of at least 500, 4:1 for those with scores of at least 600, and 13:1 for those with scores of at least 700.||”|
In 1983, Benbow stated of the study, "The results obtained by both procedures establish that by age 13 a large sex difference in mathematical reasoning ability exists and that it is especially pronounced at the high end of the distribution [...]."
Baron-Cohen states that the male:female ratio of autism is 4:1, and examines autism beginning from a theory of the "male brain type," a hypothetical construct defined as "an individual whose folk physics skills are in advance of his or her social folk psychology skills. That is, they show a folk physics>folk psychology discrepancy. This is regardless of one’s chromosomal sex." Baron-Cohen's theory and findings are controversial and many studies contradict the idea that baby boys and girls differ significantly in the way they learn or reason about objects' mechanical interactions.
Campbell argues that "female competition is more likely to take the form of indirect aggression or low-level direct combat than among males." Campbell goes on to state that "cultural interpretations have 'enhanced' evolutionarily based sex differences by a process of imposition which stigmatises the expression of aggression by females and causes women to offer exculpatory (rather than justificatory) accounts of their own aggression." Research has shown that stimulation of the amygdala induces a delayed and prolonged increase of aggressiveness in male Syrian golden hamsters, and in the hypothalamus of male rats. Many studies have examined the correlation between aggression and certain hormones and neurotransmitters, specifically testosterone. However, the link between testosterone and aggression in humans remains unclear. A more established negative correlation has been discovered between serotonin and aggression, meaning that higher levels of serotonin are correlated with lower levels of aggression and vice versa.