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Studying Gender and Sexuality in the Brain

aamen's picture

The individually differing traits of gender and sexuality are some of our clearest biological variations, but the biology behind these factors has only begun to be studied in the past several decades.  Social stigma generally implies that men and women, heterosexuals and homosexuals, etc., tend to have differing personality traits, behavioral traits, and specific skills.  Because of this, current research efforts have begun to investigate these differences in terms of how they may be mapped in the brain.  For example, men are stereotypically supposed to be better in math and science than women – perhaps we can discover a region of the brain that causes this.  We find, however, as more scientific research is done to pinpoint specific differences in the brain associated with gender and sexuality, that such studies may have both benefits (in terms of how it can be used to help people, through medicine, education, or otherwise) and drawbacks (in terms of perpetuating potentially meaningless stigma).  The question, then, is overall whether or not research into brain differences in gender and sexuality should be done, and if so, for what purpose.

Studies of gender and sexuality both have objectionable beginnings.  The study of women in psychology can be dated back to Freud and Sir Frances Galton, whose theories tended to state that women were inferior to and envious of men (Buss, 2006).  Freud’s famous theory of “penis envy” proposed the idea that female children were angry at their mothers for not causing them (the child) to be male, and this led to a stronger relationship with the father (Wagner, 2005).  In modern day research, however, these ideas are generally discounted.  Current women’s psychological studies have been in place for almost 30 years, and involve (in part) observing female versus male actions in the real world using scientific methods, and attempting to change the stigmatized language that is associated with differences in gender (Shields, 1982).  In terms of sexuality, early studies were equally biased and unfair towards non-heterosexuals.  It was originally thought that homosexuals could be “taught” to be heterosexual, but beginning with research by Freund (1963) examining sexual arousal as a result of blood vessel dilation, this was found to be impossible.  After non-heterosexual behaviors were determined generally not to be a matter of learning, more effort was put in to determining what other underlying biological factors could be coming into play.

Between males and females, various structural asymmetries in the brain have been observed, though asymmetries are often controversial.  Most notably, these include differences in the corpus collosum (connecting the halves of the brain), planum temporale (involved in language), and sexually dimorphic nucleus (relating to sexual behaviors).  The brain also has been said to be less lateralized (i.e. less specialized between the two hemispheres) than in men (Kimura, 1987).  Most structural differences in the brain are considered to be a product of hormonal differences during development – that is, the effect of androgens in males and estrogens in females (Rainbow, Parsons, & McEwen, 1982).  These structural differences (and generally differing hormones) have been discussed as having implications in terms of males’ and females’ cognitive abilities.

Cognitive differences between males and females are also rather controversial.  Highly stereotypical differences, such as math ability, are not observed in scientific study.  There also appears to be no overall difference in intelligence between men and women (Dean, 2006).  Cognitive differences that do typically occur include factors such as better spatial orientation in men and better verbal skills in women, and female interest in people versus male interest in objects (Kimura, 2002).  These cognitive differences are most often considered to be a result of both cultural and biological variances in gender, and gender differences in the brain are used to support the idea of biology affecting cognition.  Studying cognitive differences and the possible biology behind them may have some helpful purposes.  For example, scientists are in the process of attempting to map biological and cognitive differences between males and females to differences in learning style, which could lead to a more effective education system (Dean, 2006).

Another field of research related to differences in the male versus female brain has to do with the treatment of mental disorders.  Because studies in this area are so new, theories have not developed to the point where gender-specific treatments are applied.  However, it is clear that there are overall sex differences in prevalence, symptom expression, and disease course in many major mental disorders such as depression, bipolar disorder, addiction, and schizophrenia (Nauert, 2008).  Obviously psychological and cultural factors influence these factors as well.  However, much of the current evidence suggests a link between sex differences in hormone levels and differing responses to mental disorders.  For example, in terms of addiction, females tend to show a faster release and reuptake of dopamine, and higher dopamine receptor density in reward pathways.  Females also appear to become addicted more easily than males, and to be more at risk for relapse (Carroll et al, 2004).  This is hypothesized to be because of the effects of estrogen on dopamine (facilitating its release), which then leads to more dopamine in the reward pathways (potentially causing sexual dimorphisms in dopamine levels/receptor densities), which in turn may lead to a greater effect of addictive drugs.  Research such as this has implications for potentially differing treatments of conditions like addiction between males and females, and also more generally in terms of overall treatment – for example, estrogen regulation could help in treating addiction (Blanchard et al, 1993).

Just as differences in the brain have been studied between genders, so have structural asymmetries and hormones been studied in terms of sexuality.  Gender identity disorder refers to a condition in which a person shows physical development typical to one sex, but feels that they are in fact the other (Nangeroni, 1996).  The idea of transsexuality has been the focus of some research in recent years, yielding relatively interesting results.  For example, males typically have about two times as many neurons as do females.  However, in cases of transsexuals, their neuron density occurs in the range of the opposite sex (Byne et al, 2000).  That is, a person who appears physically to be male but feels female shows a neuron density comparable to that of a typical female.  There is also evidence indicating that variations in fetal and neonatal gonadal hormones (specifically, lack of testosterone in males and presence of testosterone in females) can lead to the development of gender identity disorder (Swaab, 2004).

Brain differences between homosexuals and heterosexuals are more controversial than brain differences in conditions such as gender identity disorder.  In terms of homosexuality versus heterosexuality, Simon LeVay’s famous experiment (1991) led to the conclusion that the interstitial nucleus of the anterior hypothalamus was larger in heterosexual men than in both heterosexual women and homosexual men.  These results were controversial given the participant sample (homosexual men with AIDS) and the fact that these differences could be a result of social learning – causation cannot be inferred (Rahman & Wilson, 2003).  However, more research has shown that identical twins are about twice as likely to share sexual preference than fraternal twins (Bailey & Pillard, 1991), which implies that biology does play a role in sexuality.  It also shows, however, that biology is not the only factor, given that if that were the case 100% of identical twins should share sexual preference.  In terms of hormonal factors, homosexuals are sometimes (but not always) shown to have differ from heterosexuals in androgen levels, such that homosexual women have higher androgen levels than heterosexual women, and homosexual men have lower androgen levels than heterosexual men (McCormick & Witelson, 1991; Meyer-Bahlburg, 1979).

Given all of these ideas involving differences in brain structure and hormone levels in relation to gender and sexuality, the obvious next step is to determine what we can do with such research, and what it really means.  On one hand, the idea that we can use variations in health disorders between men and women to better understand how hormones might be playing a role in the disorder is a fascinating one, and could lead to improvements in treatment options in the future.  Though such ideas have been studied in relation to males versus females, considering that other research implies possible differences in hormone levels relating to sexuality, perhaps hormonal differences could be shown to cause differences in disease symptoms or progression in conjunction with sexuality as well.  Knowing, understanding, and accepting differences in the ways in which health disorders affect people based on gender (and possibly sexuality) would seem highly beneficial.

On the other hand, providing differences in gender and sexuality with a biological basis may serve to perpetuate negative stigma and stereotypes, and thus have overall negative consequences.  In terms of studying cognitive differences between males and females, as mentioned this may have some interesting implications in terms of the educational system.  However, cognitive differences can never be said to be purely biological – it is impossible to separate the effects of biology from the effects of culture and society.  In addition, if females can be said to be less good at spatial orientation tasks than males, and this can be shown to map to a brain region, people will be more inclined to accept that females are “hardwired” not to succeed in tasks or professions involving spatial orientation.  On the subject of sexuality, while finding differing brain regions and determining a biological basis for homosexuality, transsexuality, etc., may help in terms of dispelling the notion that people can always choose their sexual preferences, it may also give rise to the idea that non-heterosexuality is some sort of “condition” or “disorder”, affecting the brain in the same way that something such as OCD or bipolar disorder would.  This could then lead to a perpetuation of negative stigma surrounding non-heterosexuals.

Another major issue in studying brain differences in gender and sexuality is the fact that as of yet, none of the asymmetries in the brain that have been discovered are directly related to differences between men and women, homosexuals and heterosexuals, etc. in terms of behaviors and cognitive abilities.  Men may be better at spatial tasks than women, but there is no “spatial task” area of the brain that is clearly more developed in men.  Instead, it seems that the research on brain differences is stretched to attempt to explain behaviors and cognitions, when really there is no clear connection.  This raises the question of the applicability of such research.  Studying male versus female differences in the brain may be interesting, but if such differences do not help us to understand any of the possible concrete differences between the sexes, then what is the point?

Taking a step back, one of the largest problems surrounding the research described here is that it attempts to place people into discrete categories of gender and sexuality.  Sexuality is often conceived as a continuum, but in order to perform studies in which the brains of “heterosexuals” are compared to the brains of “homosexuals”, it is necessary to classify subjects as one or the other.  However, some heterosexuals may feel more sexual attraction towards the same sex than others, while some homosexuals may feel more attraction towards the opposite sex than others.  Gender, too, does not necessary exist in two discrete categories.  Even putting issues of transsexuality, androgen insensitivity, etc. aside, females exist in a spectrum of widely varying traits, as do males.  These may be related to cultural factors, but additionally may be related to differences in hormone levels.  That is, not all females experience the same levels of estrogen during development, nor do all males experience the same levels of testosterone (Jolly, 2004).  In addition, it is clear that while studies may show differences between males and females in terms of brain structures and cognitive abilities, there is more variation within genders than there is between genders (Kimura, 2002).  This makes the idea of a biological differences being used to explain cognitive differences even more controversial.  For example, men may overall be better at spatial tasks than women, but certainly there are some women who are better at spatial tasks than some men.  Acknowledging such differences in cognition will likely result in the assumption that one group is universally better than the other, which is simply untrue.

Overall, studying gender and sexuality differences in the brain does have strong merits – for example, in treating health conditions, and maybe even in terms of education.  However, there are many clear drawbacks to the research that is currently being done.  Given its potentially positive uses, it seems clear that such research should continue.  At the same time, researchers need to be more careful, and to give clear disclaimers as to the generalizability of their research.  It seems that more effort should be put into understand hormonal variations in gender and sexuality since these differences seem to be what most directly cause differences in cognition, mental disorders, etc. as opposed to specific structural asymmetries in the brain.  In addition, in terms of both hormonal and structural variations in the brain, perhaps future studies should have participants rate themselves on a continuum in terms of their gender and/or sexuality.  Brain dimorphisms could then be correlated with this continuum.  Alternately, such a continuum could be administered to select for subjects who rate themselves on the far right or left sides of the continuum only – thus determining the general characteristics of (for example) an extreme male or an extreme female.  Researchers should be sure, then, when publishing results, to discuss that the population in general will fall somewhere between these two extremes.  Clearly there are issues with these sorts of research designs as well, but it would be a step in the right direction for research that may have the potential to truly benefit us as a society.


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Paul Grobstein's picture

Gender/sex, controversy

Perhaps its useful to distinguish between several kinds of "controversity" ...

  1. Observations that may or may not be replicable
  2. Interpretations that may or may not be adequately supported by replicable observations
  3. Interpretations that are adequately supported by replicable observations but are disliked for other reasons

The third kind of controversy seems most significant here. And perhaps it could be eliminated if everyone agreed that difference can be acknowledged without passing value judgements? ie that people can be different without being "better" or "worse"? It seems to me that is a legitimate and important general intepretation that can be drawn form lots of replicable observations.