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A
short summary of the neurobiology behind homosexuality and transsexualism:
Homosexuality: The backside (the splenium) of the corpus callosum (a broadbandlike structure of neurons between the two brain-hemispheres) and the structural properties of parts of the hypothalamus in homosexual males are identical with the splenium and the structural property of parts of the hypothalamus in heterosexual women. Homosexual women, on the contrary, have the same splenium and structural property to their hypothalamus as heterosexual males (This can be seen prenatally).
Research scientist Ivanka Savic at Karolinska institutet has found a component in a man’s sweat that activates the hypothalamus (an area in the brain that is of great importance to your sexual desires) in both heterosexual women and homosexual men. The participants in the research were not told in advance if they were given a sample of male or female sweat. Their hypothalamus was still activated with the scent of a male.
In an American study from Monell Chemical Sciences Center, homosexual and heterosexual women and men were given samples of the armpit smell of other homosexual and heterosexual women and men. The samples were presented in pairs and the participants were then asked to choose the one they found the most soothing. The results: Homosexual male participants preferred the smell of other homosexual males, rather than the smell of heterosexual males.
Transsexualism: A discrepancy between the gender identification of the brain and the exterior gender identification of the body (This can be seen prenatally). Approximately 1:30 000 is born with the condition.
A closer look into the neurobiology behind homosexuality and transsexualism:
Excerpt from the book entitled “Könet sitter i hjärnan” – “the gender is in the brain” by Annica Dahlström, 2007, Professor in neurobiology at the university of Göteborg:
Research scientists have identified many anatomical elements in the brain that separate most (but not all) men from women. Men and women are not two separate entities, several individuals have both male and female components to their brain. Below some of the purely anatomical structures, that researchers now regard as statistic differences between the genders, will be shown. (Gender in this case being exterior genitalia.) In the future we will probably discover even more differences.
Largest in males:
- The central parts of "bed nucleus of stria terminalis."
- INAH2
- INAH3, Sex. dimorphic nucl. of
the preoptic area.
- Onuf’s nucleus in the
lumbosacrala spine.
- The weight of the entire brain (>1,2 kg)
Larger in females:
- Commissura anterior.
- Corpus callosum, midsag.arean, the
posterior and its splenium.
-
Isthmus
corpus callosum (as in right handed males – rounder shape in males, slightly
more streatched out in females).
- Massa intermedia.
- The quantity of grey and white substances.
Explanation:
Corpus callosum and partner preference:
For those who are not medical professionals or scientists the previous information might be hard to interpret. The first element to reveal differences between men and women is the corpus callosum, a structure consisting of millions of interconnected neurons between the two brain hemispheres. Already in early studies by DeLacoste-Utamsing and her colleagues, this element was discovered as being thicker, mainly in the backside of the corpus callosum (splenium) when comparing women to men.
When this was presented for the first time the research scientists suggested that these findings could point towards a biological difference between the genders. This was objected against, above all by sociologists, that pointed out that the difference might be caused by environmental factors. One claimed that society always (?) has other demands and expectations to women than to men. It was also pointed out that we treat little boys differently than little girls from birth onwards. (The cause for the latter might be that the newborn child is actually the one setting the terms for the communication with the mother.)
The research party acknowledged that this might be a possible explanation for the physical differences found in the brain. Therefore they decided to form a study on children who were not yet born. This took place with the help of MRI (Magnet Resonance Imaging), which does not damage the tissue. In this study the physical difference between male and female fetuses was discovered at 6-7 months into the pregnancy, a statistical difference had thus been discovered before society had began molding the children.
In the time following Annica Dahlström’s statements in 1996 in the university of Göteborg´s newspaper and in 1999 in the journal Medikament (Dahlström, 1999) the Läkartidningen published a critical synopsis of the results of research done on gender differences in the brain (Hamberg, 2000). The article in the Läkartidningen referred to a study done with the MRI-technique (Bishop & Walsten, 1998) where no definite statistical differences were found regarding how thick the corpus callosum was in men and women. Some studies have found differences, others have not. What could this indicate? Could there be differences in the ways the methods were applied or were there differences in the research participants?
It would seem that there might be more differences in the research participants than what was first assumed. A few years ago reports from abroad came in that showed that homosexual men and heterosexual men are morphologically not the same in the brain. Homosexual men had a thick splenium like most heterosexual women. And the other way around: homosexual women often had a thinner splenium than heterosexual women. It could thus be that the selection of research participants had been different and that one had not taken the masculine development of the brain prenatally into account.
During the development of the fetus, sex-hormones play a big part in gender identity and sexuality. One should note that sexuality is located in the brain and not anywhere else in the body. The conclusion of all of this becomes quite evident: In every comparison between men and women regarding some parameters, any parameters, one should do a complete psycho-social-sexual mapping of the research participants. This had not been done in the first study, nor in the one referred to by Hamberg (2000). A reason for this might be that it is regarded "unethical" inquiring the sexual preference of the research participants?
At an international conference regarding gender differentiation in the brain at Karolinska institute in Stockholm May 2005 new results confirmed the belief that more masculine brains has a thinner corpus callosum, especially the hind part, the splenium, than feminine brains. In numerous new articles regarding other differences the authors point out that one should make sure to note the sexuality of the research participants to be able to perceive the research data correctly.
Hypothalamus and partner preference:
The next difference in the
brain structure in men compared to women one finds in the hypothalamus. This is
no surprise, as the hypothalamus is part of the “old” brain, the reptile brain.
Here we find our automated functions such as eating, surviving and reproducing.
In the hypothalamus we find several groups of neurons (called “nucleuses”) which
are entirely different in most women compared to most men. One of these groups
is the equivalent of the one first found in rats. In humans this neuron nucleus
is named INAH3 (Interstitial Nucleus of the Anterior Hypothalamus 3). In humans
there are more groups of cells than INAH3 in the frontal hypothalamus, namely
INAH1, INAH2 and INAH4.
The first indicator that
the cell nucleus INAH3 was involved with the sexual preference in humans came
from Simon LeVay, a professor in neuron anatomy in Boston. He himself was
homosexual and wanted to find out why he differed from most of his colleagues
and friends. He dissected hundreds of brains from people who had died from AIDS
in the USA. He found that the group of cells, like “the sexually dimorphic
nucleus” in rats, that he suspected was a factor in sexual preference, was twice
the size in drug addicts, who were infected with HIV through needles, compared
to patients, that got infected through their homosexual lifestyle. He even
compared them to a group of women who had died of AIDS, but the sexual
preference of these women was not known. The women, whereof most were probably
heterosexual, had an INAH3 that was the same size as the one found in the
homosexual men, but it had spread much more. The heterosexual men had a nucleus
about twice the size. In conclusion: in the individuals that had a male partner
preference LeVay found a small INAH3-nucelus, whilst a large INAH3 was found in
individuals who preferred female partners. Numerous studies have in the
following years confirmed these findings. The INAH3 remains the same size for
the entire lifespan. The size ratio remains the same even though the amount of
neurons gradually decreases as the age increases in both sexes.
Later it has been discovered that another neuron nucleus in the hypothalamus seems to play a role in partner preference. It’s a part of the so called nucleus suprachiasmaticus (SCN). It’s twice the size in women who prefer men compared to men who prefer women. The difference in size is thereby the opposite of INAH3. Even the SCN-nucleus is probably playing its part in the partner preference. Homosexual men have a larger SCN-nucleus like heterosexual women, while homosexual women have a smaller SCN-nucleus like heterosexual men. None the less, more research is required as yet an another hypothalamus-nucleus seems to play a role in partner preference, namely INAH2, but science still wavers about what role it actually plays.
Gender identity:
An important group of cells that belong to the "bed nucleus of the stria terminalis", or BST for short, is a determining factor for whether we feel like men or women, or a little bit of both. One has found that in the brains of people who were born in the wrong body (transsexual individuals) that this group of cells is related to gender identity. It is located, as the two previously mentioned group of cells, in the hypothalamus.
In men (individuals with a penis) that perceive themselves to be female, this group of cells is about half the size of what it is in most men that do not doubt their gender identity and regardless of them being homosexual or heterosexual. This group of cells is small in both men (individuals with a penis) that mentally perceive themselves as women trapped in a man’s body and in most women who appreciate being women. For women (individuals with a vagina) that mentally perceive themselves to be men trapped in a woman’s body and in most men that appreciate being men, this group of cells is about twice the size.
Now that one is more aware of the biological reasons for transgenderism or transsexualism, doctors and psychologists are trying to help these people who have a “reversed” gender identity. This takes place through hormone therapy and surgery where one tries to correct the exterior gender so that it may be the same as the mental gender identity.
These so far discovered neuron nucleuses (INAH3, SCN and BST), which are of significance to both partner preference and gender identity, are created during the first stages of pregnancy. The three nucleuses do not necessarily have to coincide. Therefore there are probably many variations that affect the individual personality. There are men in women’s bodies who are attracted to men or women. And there are women in men’s bodies who are attracted to men or women.
And while the brain is developed in the child, the neuron nucleuses previously mentioned send their axons and nerve endings in the entire brain and into the different parts of the cerebral cortex. One could say that these neuron shoots “grow along with” the brain during the development prenatally, in children and in adolescents. All the different parts of the cerebral cortex thus get information and communicate, with the earlier parts of the brain. As the group of neurons in the hypothalamus are different in most women compared to most men, this also means that the communication and information from the hypothalamus to our conscious brain is different in most men compared to most women. The composition of neuron endings in the grey substance of the brain is thus different in the different genders. It would be unreasonable to believe that this did not influence the cerebral cortex and therefore our way of thinking.
There are studies done with so called fMRI (functional Magnet Resonance Imaging) that show which part of the cerebral cortex is activated when we are performing a specific task. These studies have shown the actual difference in how we use our brains and how we communicate. For instance in the use of language, it is clear that the main part of the studies done on female brains work with both hemispheres, while male brains work only with the left hemisphere (if he, like most, is right-handed).
Many studies also show
that we use language differently, so often men and women in relationships have a
hard time understanding each other. This will naturally, if the partners are not
aware of this built-in difference in communication, create conflicts in the
relationship.
When the brain activity in research participants who were asked to solve a mathematical task was measured, one could see that even if the task was solved in the same time range, the best girls used significantly less of their brain volume than the corresponding best boys. The scientists responsible for this study wondered if this might suggest that these young female students used their brain in a more efficient way than the young male students. This was meanwhile only apparent when the task was of the more practical nature. If it was about creating abstract formulas, the men had certain advantages. Men are even statistically a little better at handling three-dimensional structures and mental rotation. When it comes to the use of geographical maps, men and women apply different strategies. Women often turn the map until it is facing the same way as they are, in fact just like homosexual men. Men generally don’t need to do this, they turn the map mentally. And in the same way men and women apply different strategies to give road directions. While men usually mention the number of kilometers to go and then to turn north, and then to continue for a few hundred meters, the women give directions in relation to buildings, things, post-offices etc. in their environment. These differences are well documented by many scientists, amongst them Doreen Kimura.
That male and female
brains work differently even if it regards unconscious stimuli is also apparent in a new study done by Ivanka Savic
and colleagues (2005) at Karolinska Institutet in Stockholm. The brains of
homosexual men and heterosexual women perceived the male pheromones in the same
area of the hypothalamus. The heterosexual men were to the contrary stimulated
by female pheromones and reacted with a great increase in activity in the
hypothalamus. Homosexual men reacted to the scent of male pheromones, but
reacted neutrally to female pheromones.
Read more about homosexuality and pheromones on the following website:
http://www.pnas.org/cgi/search?fulltext=Ivanka+savic&submit.x=7&submit.y=7
To read more about the neurobiology of transsexualism go to following websites:
http://www.princehenrys.org/news/media_releases/genetic_link_to_gender_identity.htm
http://www.journals.elsevierhealth.com/periodicals/bps/article/S0006-3223(08)01087-1/abstract
http://www.symposion.com/ijt/ijtc0106.htm
http://jcem.endojournals.org/cgi/content/full/85/5/2034
The
following article is published at
www.pubmed.com
Henningsson S,
Westberg L,
Nilsson S,
Lundstrom B,
Ekselius L,
Bodlund O,
Lindstrom E,
Hellstrand M,
Rosmond R,
Eriksson E,
Landen MDepartment of
Pharmacology, Institute of Physiology and Pharmacology, Goteborg University,
Goteborg, Sweden.
Transsexualism is
characterised by lifelong discomfort with the assigned sex and a strong
identification with the opposite sex. The cause of transsexualism is unknown,
but it has been suggested that an aberration in the early sexual differentiation
of various brain structures may be involved. Animal experiments have revealed
that the sexual differentiation of the brain is mainly due to an influence of
testosterone, acting both via androgen receptors (ARs) and--after
aromatase-catalyzed conversion to estradiol--via estrogen receptors (ERs). The
present study examined the possible importance of three polymorphisms and their
pairwise interactions for the development of male-to-female transsexualism: a
CAG repeat sequence in the first exon of the AR gene, a tetra nucleotide repeat
polymorphism in intron 4 of the aromatase gene, and a CA repeat polymorphism in
intron 5 of the ERbeta gene. Subjects were 29 Caucasian male-to-female
transsexuals and 229 healthy male controls. Transsexuals differed from controls
with respect to the mean length of the ERbeta repeat polymorphism, but not with
respect to the length of the other two studied polymorphisms. However, binary
logistic regression analysis revealed significant partial effects for all three
polymorphisms, as well as for the interaction between the AR and aromatase gene
polymorphisms, on the risk of developing transsexualism. Given the small number
of transsexuals in the study, the results should be interpreted with the utmost
caution. Further study of the putative role of these and other sex
steroid-related genes for the development of transsexualism may, however, be
worthwhile.
Norwegian organisation for people with transsexualism:
Swedish organisation for people with transsexualism:
The Swedish Federation for Lesbian, Gay, Bisexual and Transgender Rights
Swedish Amnesty International group working for the human rights of homosexuals, bisexuals, transvestites, transsexuals and intersexuals:
http://www2.amnesty.se/hbt.nsf/allad2/2386F2C8A167D3F2C125723B0040C777?opendocument
Questions? Send an e-mail to support@brain-info.eu
Roger Gustavsson. Last updated 20.01.2009