HORMONES, BIOLOGY

AND GENDER, OH MY!

Courtney Manarky
 Fetal Testosterone and Gender Identity

INTRODUCTION:

Throughout time, males and females have proven to have different brains, both

structurally and hormonally. Variations in their brains are not clear cut, but more of a blurred

line. Consider the brain the ocean of the human body one dark region that has yet to be fully

explored. However that ocean is slowly being uncovered. Structures and hormone levels differ

between males and females. What about other populations as well? This paper will look into the

differences in the brain between autistic males, female-to-male (ftm) and male-to-female (mtf)

transgenders, as well as homosexual males. Research highlighted will display differences in

structure as a result of hormones. The research problem to be studied is the connection between

fetal testosterone levels and gender identity. Fetal testosterone is the amount of testosterone that

is in the amniotic fluid while a child is in the womb. The levels of fetal testosterone have a direct

implication on the brain through growth, development, and behavior. Gender identity is how a

person expresses their preferred gender, through clothing, haircut, or behavior. The goal of this

research is to find a link between the two.

In order to analyze the differences in the brain between these populations, the best way

would be to examine fetal testosterone levels while the fetus is still in the womb and follow the

subjects throughout their life. However that is expensive and may not result in the desired

populations. One way to simplify this research would be by again taking the fetal testosterone

level, but instead recording these levels and comparing this with the childs lifestyle and

behavior. Another way to examine the problem would be to use the ratio of the second finger to

fourth finger, which will be discussed in the paper, to determine the amount of fetal testosterone

that was present and compare that to the lifestyle of the subject. However that involves more

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 Fetal Testosterone and Gender Identity

guesswork as the ratios do not correlate exactly. These experiments will help determine if there

is correlation between fetal testosterone levels, and gender identity.

STATEMENT OF SIGNIFICANCE:

This research has not been tested. Being able to predict a childs gender identity, while

perhaps not accurately at first, will be helpful in parenting and general upbringing of the child.

Also, showing that less rigidity occurs within gender identity based upon the biology of the brain

validates the transgender existence, proving that this is not so much of a cultural phenomenon,

and instead a biological occurrence. Backing up intuition with science is beneficial for anyone

undergoing gender identity issues. Having tangible evidence may increase believability, and

validity from friends and family members.

THESIS STATEMENT:

Fetal testosterone plays an important role in human development, especially in the brain.

This paper will be looking at the connections between hormonal effects, and how fetal

testosterone levels connect to gender identity.

LITERATURE REVIEW:

The original problem in question is simple: what are the differences between male and

female brains? However, the answer to that question is not a simple one, and one that is worth

exploring. When examining differences between male and female brains, the initial difference

considered was brain size. Males have larger brain volumes than females overall, but within the

brain, the main regions1 have the same proportionality. Brain size in turn connects to the amount

1
Frontal, temporal, occipital, parietal, cerebellum.

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 Fetal Testosterone and Gender Identity

of grey matter within the brain.2 Grey matter is a combination of neurons and cell bodies, which

is where messages are transmitted through brain cells3. Larger brains4 are negatively correlated

with grey matter within the brain, with regression analyses explaining between 40-81% of the

variance in the difference in grey matter between the sexes.5 While certain sources6 do not see

any connections between grey matter and sex, other sources note that males have significantly

larger brains than females, so the brain size would play a role in the amount of grey matter

present.7

Grey matter also correlates with the amount of hormones present within the brain. The

administration of testosterone has a negative correlation with grey matter as well.8 When males

hit puberty, their testosterone levels increase drastically9 and the amount of grey matter in their

brains decreases.10 Due to the fact that the subject is a male, which means he has a larger brain,

and undergoing puberty, he will have less amounts of grey matter in his brain. That is a

connection that cannot be ignored, and has been shown through certain studies.11

Within the male and female brains themselves, however, there are many structures that

do differ, the most obvious by size. After puberty, many changes have taken place within the

human body that differentiate males and females. These changes also happen in the brain. By

Cosgrove, Kelly P., Carolyn M. Mazure, and Julie K. Staley. "Evolving Knowledge of Sexual Differences in Brain
Structure, Function and Chemistry." Biological Psychology, 15 Oct. 2007, pp. 847-55. National Center for
Biotechnology Information.
2
Luders, E, H Steinmetz, and L Jancke. "Brain Size and Grey Matter Volume in the Healthy Human
Brain." Neuroreport, 3 Dec. 2002. National Center for Biotechnology Information.
3
Ibid.
4
Typically male brains. See Cosgrove.
5
See Luders.
6
Ibid.
7
See Cosgrove.
8
Blakemore, Sarah-Jayne, Stephanie Burnett, and Ronald E. Dahl. "The Role of Puberty in the Developing
Adolescent Brain." Human Brain Mapping, 3 May 2010. National Center for Biotechnology Information.
9
"Testosterone, Total, Bioavailable and Free, Serum." Mayo Medical Laboratories.
10
See Blakemore.
11
Ibid.

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 Fetal Testosterone and Gender Identity

using fMRI, one of the differences viewed in structure between male and female brains is in the

hypothalamus.12 As the hypothalamus is known to have a role in sexual behavior, having the size

comparison for homosexual males shows that there may be a biological component to

homosexuality, as opposed to merely a cultural phenomenon. The hypothalamus is larger in

males than in females, but one study has found that homosexual males have much closer sizing

in their hypothalamus to females, as opposed to other heterosexual males.13

Another structural component in the brain that differs for males and females is the corpus

callosum, specifically a section called the isthmus.14 The isthmus is a small segment of neurons

that connect the left and right hemispheres in the corpus callosum.15 Females have much greater

use of both hemispheres when solving a problem, being able to connect feeling with logic to

create intuition. Males, on the other hand, tend to think within the same hemispheres to solve the

same problem.16 Being able to show that this structure is different not only through an fMRI17,

but by mapping out connectomes18 to show brain pathways supports the bigger size of the

female isthmus, as this part is used much more frequently in females.

In addition to studies of biological differences in the brain, some scholars examine

hormonal influences, which are connected with structural differences. Focusing back on the

hypothalamus, hormone manipulation can result in changes of structure. By taking and castrating

male rats within the first few days of their birth, these rats will not have testosterone exposure to

12
Gibbons, Ann. "The Brain as "Sexual Organ" (Gender Differences)." American Association for the Advancement
of Science, 30 Aug. 1991, p. 957. Gale Psychology Collection.
13
The hypothalamus is known to play a role in sexual behavior. Barinaga, Marcia. "Is Homosexuality
Biological?" American Association for the Advancement of Science, 30 Aug. 1991, p. 956. Gale Psychology
Collection.
14
Latin, meaning small connector between two large pieces of land. See Gibbons.
15
See Gibbons.
16
Connor, Steve. "The Hardwired Difference Between Male and Female Brains Could Explain Why Men are
"Better at Map Reading"." Independent, 3 Dec. 2013.
17
See Gibbons.
18
See Connor.

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 Fetal Testosterone and Gender Identity

their hypothalamus. As a result, the structure will grow as if the brain was actually female, and

behaviorally the rat will act sexually with female typical sex behaviors, even though the rat is

still biologically male from birth.19 Similarly, female rats who were exposed to testosterone

during the first few days of birth exhibited male typical sexual behaviors.20 The testosterone

level affected secondary sex trait growth when administered outside what is known as the critical

period, although sexual behavior was not affected. Due to the testosterone being administered at

an early enough age, behavior was affected.21

One way to measure the effects of different fetal testosterone levels and their effects on

behavior is through play. In one study, typical male and female children were measured, as well

as a group of females who experience the disorder congenital adrenal hyperplasia (CAH).22 As

predicted, the females and males in this study opted to play with objects stereotypical for their

sex. Typical choices for males were trucks and blocks, whereas females chose dolls and pink

colored objects. Females with CAH, however, selected male play choices, such as the blocks and

trucks, at a much higher frequency than they chose the dolls. Although they were raised as

female since birth, the androgen exposure prenatally changes their behavior marker, similarly to

how the sexual behavior was changed in rats.23 There are other examples that show the biological

impact on play as well. Vervet monkeys when given the choice for a typical male toy, such as a

truck or blocks, or a female toy, such as a doll, chose toys that were consistent when their

19
See Gibbons.
20
Gibbons, see also Kimura, Doreen. "Sex Differences in the Brain." Scientific American, 1 Sept. 1992.
21
See Kimura.
22
CAH is the administration of excess androgen levels in the womb, resulting in higher levels of male behavior,
male play choices, and a male dominated career path. See Kimura.
Kukreti, Prerna, Manish Kandpal, and R Jiloha. "Mistaken Gender Identity in Non-Classical Congenital Adrenal
Hyperplasia." Indian Journal of Psychiatry, Apr. 2014, p. 182. Gale Psychology Collection.
23
See Kimura.

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 Fetal Testosterone and Gender Identity

gender.24 This demonstrates that even across species males biologically choose trucks, and

females biologically choose dolls. This biological impact is greater than the socialization impact,

showing that socialization in actuality may have little to no actual affect.

A study examining infants on their first day of birth further illustrates how socialization

may have little impact. The infants were measured on whether they spent more time looking at a

mechanical mobile, or a human face. Without any environmental factors, as this was their first

day alive, males spent significantly more time looking at the mobile, and females spent much

more time looking at the face.25 This directly connects with the Empathizing-Systemizing (ES)

idea that is used to study autism. Before looking at how this connects to autism, males tend to

have higher scores for systemizing, supporting their superior spatial abilities and logic. Females

on the other hand have greater empathizing abilities, and are seen as better nurturers.26

Autistic children have been known to have extreme male brains27 as they were exposed to

more testosterone in the womb. Male children, as mentioned earlier, have larger brains than

female children. Males with autism, however, have even larger heads and brains than non-

autistic males.28 In those extra-large brains, there is an even more so decreased amount of grey

matter in the brain, which supports the theory that autistic children, males in particular, exhibit

the extreme male brain29. A final structural comparison is seen within the amygdala, which is

the fear center of the brain. The amygdala is significantly larger in autistic males than non-

24
Baron-Cohen, Simon, Rebecca C. Knickmeyer, and Matthew K. Belmonte. "Sex Differences in the Brain:
Implications for Explaining Autism." American Association for the Advancement of Science, 4 Nov. 2005, p.
819. Gale Psychology Collection.
25
See Baron-Cohen et al. Sex Differences
26
Ibid.
27
Bejerot, Susanne, Jonna M. Eriksson, Sabina Bonde, Kjell Carlstrom, Mats B. Humble and Elias Eriksson. "The
Extreme Male Brain Revisited: Gender Coherence in Adults with Autism Spectrum Disorder." The British Journal
of Psychiatry, Aug. 2012, pp. 116-23.
28
See Baron-Cohen.
29
Ibid. Baron-Cohen coined the term extreme male brain.

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 Fetal Testosterone and Gender Identity

autistic males, which in turn is larger than in females.30 With a larger fear center, autistic

children tend to be more impulsive and act before they think. Females, on the other hand,

typically think before acting, and allow emotion and empathy to play into their decisions.

A typical test that measures empathy is the Reading the Mind in the Eyes test. This test

took 16 females and measured their ability to pick up emotions based on viewing another

persons eyes in predetermined pictures.31 Before the treatment of testosterone administered

under their tongues, the females were measured on their ability to decipher emotions. After the

administration, there was a significant decrease in picking up emotion, showing that increased

levels of testosterone reduce the ability to empathize.32 This supports Baron-Cohens extreme

male brain theory, as autistic children display noticeably less emotion and eye contact, which is

a sign of empathy.33 Excess testosterone shows reduced empathy, so having the high amount in

autistic individuals shows an extremely low capacity to connect on emotional levels.

Studies have also been done connecting the autistic brain with gender coherence. 50

adults with autism were matched with 50 controls. Body and facial photographs, voice

recordings, hormone levels, and finger ratios between the 2nd and 4th finger were measured.

Autistic males were found to have less masculinized bodies and voices, and had a more

feminized finger length ratio. Females with autism had higher testosterone levels, larger heads,

and reportedly less feminine features.34 Overall this suggests that instead of both males and

females becoming hyper masculinized, that there is a sense of biological gender defiance as a

result of their condition.

30
Ibid.
31
Extra Testosterone Reduces Your Empathy. Annals of Psychotherapy and Integrative Health, 10 Feb. 2011, p.
9. Gale Psychology Collection. Accessed 28 Nov. 2016.
32
Dinsdale, Paul. Fetal Testosterone Levels Linked to Autistic Traits. Learning Disability Practice, Oct. 2007, p.
5 Gale Psychology Collection. Accessed 28 Nov. 2016.
33
See Baron-Cohen
34
See Berejot.

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 Fetal Testosterone and Gender Identity

Autism has been shown to be linked to fetal testosterone. Based off of a study done by

Baron-Cohen, researchers gathered the amniotic fluid from over 200 children. Once the children

turned eight, they were measured for where they fell on the autistic spectrum quotient. The

children also were instructed to take a computer exam that favored the autistic brain. The fetal

testosterone levels, autism spectrum quotient scores, and the computer scores all had a strong

correlation.35 Using a longitudinal test is crucial in showing exact fetal testosterone levels. Fetal

testosterone levels have huge impacts on the brain.

Fetal testosterone is the amount of testosterone present within the brain before a baby is

born.36 Testosterone levels vary greatly while the child is in the womb and after birth up through

puberty and adulthood.37 A certain study has found that fetal testosterone has been linked to

playtime behavior in children from 6-10 years old. 212 pregnant mothers were tested for their

fetal testosterone levels. Through a longitudinal study, when the child reached between 6 and 10,

the mother was asked to rate their child on the Pre School Activities Inventory, which is able to

give parents a general idea of where their child falls for masculine or feminine play type. 38

Amniotic testosterone levels were linked to play type, with higher levels resulting in more male

typical play, and lower levels resulting in more female typical play.39

The Pre School Activities Inventory can be the first documented example a parent has to

show that their child may not be as masculine or feminine as expected for a child their age. This

may be the first sign that their child has Gender Identity Disorder, which is when a persons self-

35
See Dinsdale.
36
Auyeung, Bonnie, Simon Baron-Cohen, Emma Ashwin, Rebecca Knickmeyer, Kevin Taylor, Gerald Hackett and
Melissa Hines. Fetal Testosterone Predicts Sexually Differentiated Childhood Behavior in Girls and Boys.
Psychological Science, 17 Nov. 2009. National Center for Biotechnology Information. Accessed 15 Nov. 2016.
37
Testosterone, Total, Bioavailable and Free, Serum. Mayo Medical Laboratories. Accessed 30 Nov. 2016.
38
See Auyenug.
39
Ibid.

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 Fetal Testosterone and Gender Identity

described gender does not match up with their external features.40 Explorations done involving

the transgender brain display differences between cisgender and transgender brains. The

University of Barcelona examined both female-to-male and male-to-female transgenders before

they began hormone treatment. Female-to-male brains had thinner subcortical areas, which is

more prevalent in males. Male-to-female subjects had thinner cortical regions in their right

hemispheres, which is seen commonly in females. After hormone therapy, these changes were

exaggerated.41

Another study supports the differences in transgender brains as well. Researchers in

Amsterdam took adolescents with gender dysphoria and exposed them to a pheromone-like odor,

and recorded their responses. This odor was chosen as males and females elicit much different

responses to the smell. The adolescents responded with the response typical to their preferred

gender, which is worth noting.42 The reaction to a smell that is atypical cannot be taught.

Therefore there is a biological aspect to gender identity and brain structure. This is supported by

MRI scans, which show that pre-hormonal female-to-male transgenders having similar amounts

of grey matter to that of a cisgender male, showing masculinization within their brains. 43 A

similar study was run with male-to-female transgenders, this time the amount of grey matter was

balanced between males and females, suggesting that male-to-females brains are not fully

feminized, but not completely masculinized.44

40
Kraemer, Bernd, Aba Delsignore, Ronnie Gundelfinger, Ulrich Schnyder, and Urs Hepp. Comorbidity of
Asperger Syndrome and Gender Identity Disorder. Early Childhood Adolescent Psychology, 2005, pp. 292-96.
Springer Link. Accessed 30 Nov. 2016.
41
Russo, Francine. Is There Something Unique about the Transgender Brain? Scientific American, 1 Jan. 2016.
Accessed 27 Nov. 2016.
42
Ibid.
43
Hamzelou, Jessica. Transsexual Differences Caught on Brain Scan. New Scientist, 26 Jan 2011. Accessed 27
Nov. 2016.
44
Ibid.

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 Fetal Testosterone and Gender Identity

A final group that has a different brain structure than predicted is the population of

homosexual males, particularly when compared to heterosexual males. Simon LeVay studies the

anterior hypothalamus, which contributes to sexual behavior. When dissecting the brains of

AIDS patients, LeVay found that the size of the hypothalamus in homosexual males brains was

closely related in size to that of a heterosexual females brain. This, in turn, is roughly half the

size of the anterior hypothalamus in a heterosexual males brain.45 The suprachiasmatic nucleus,

on the contrary, is twice as large in homosexual males as in heterosexual males, displaying

multiple differences in their structures.46 Brain regions in Swedish studies show that the amount

of nerves in the brain between homosexual males and heterosexual females are roughly the same,

proposing that homosexual male brains are more feminized.47 This suggests that homosexuality

has a biological influence, rather than perhaps a cultural and peer influence.

Upon exploring the research in the field, all of these different connections between

testosterone, autism, and gender identity, changed the original question that was being proposed.

Instead of trying to find a brain mapping, the goal now is to connect the research. The next step

is taking research involving fetal testosterone and seeing if there is any connection with gender

identity.

ORGANIZATION OF PAPER:

After looking into the background, and research in the field, further steps will be taken to

improve upon and connect the research. A longitudinal study proposing taking the testosterone

levels of 1000 newborns to follow over time will be assessed over a 20 year period. Although

45
Barinaga, Marcia. Is Homosexuality Biological? American Association for the Advancement of Science, 30
Aug. 1991, p. 956. Gale Psychology Collection. Accessed 28 Nov. 2016.
46
Ibid.
47
Park, Alice. What the Gay Brain Looks Line. Time, 17 June 2008. Accessed 30 Nov. 2016.

Manarky 10
 Fetal Testosterone and Gender Identity

this is a time consuming study, and some subjects will be lost in the process, this study

guarantees exact measurements of fetal testosterone levels. Self-described measures of gender

identity will be taken into account to correlate with measures of fetal testosterone. The studys

results will be documented, and show connections with previous research. This will make

connections within the field and show whether or not fetal testosterone levels have any bearing

on a persons gender identity. Levels correlating between cisgender and transgender males and

females will also be presented, if available, trying to document rough boundaries for which a

person would identify a certain way. The paper would then conclude with possible flaws in the

research or ways to improve the design.

BIBLIOGRAPHY:

Auyeung, Bonnie, Simon Baron-Cohen, Emma Ashwin, Rebecca Knickmeyer, Kevin Taylor,
Gerald Hackett and Melissa Hines. "Fetal Testosterone Predicts Sexually Differentiated
Childhood Behavior in Girls and in Boys." Psychological Science, 17 Nov.
2009. National Center for Biotechnology Information. Accessed 15 Nov. 2016.

Barinaga, Marcia. "Is Homosexuality Biological?" American Association for the Advancement of
Science, 30 Aug. 1991, p. 956. Gale Psychology Collection. Accessed 28 Nov. 2016.

Baron-Cohen, Simon, Rebecca C. Knickmeyer, and Matthew K. Belmonte. "Sex Differences in

Bejerot, Susanne, and Jonna M. Eriksson. "Sexuality and Gender Role in Autism Spectrum
Disorder: A Case Control Study." Public Library of Science, 31 Jan. 2014. Accessed 30
Nov. 2016.

Bejerot, Susanne, Jonna M. Eriksson, Sabina Bonde, Kjell Carlstrom, Mats B. Humble and Elias
Eriksson. "The Extreme Male Brain Revisited: Gender Coherence in Adults with Autism
Spectrum Disorder." The British Journal of Psychiatry, Aug. 2012, pp. 116-23. Accessed
30 Nov. 2016.

Blakemore, Sarah-Jayne, Stephanie Burnett, and Ronald E. Dahl. "The Role of Puberty in the
Developing Adolescent Brain." Human Brain Mapping, 3 May 2010. National Center for
Biotechnology Information. Accessed 3 Oct. 2016.

Manarky 11
 Fetal Testosterone and Gender Identity

Connor, Steve. "The Hardwired Difference Between Male and Female Brains Could Explain
Why Men are "Better at Map Reading"." Independent, 3 Dec. 2013. Accessed 1 Dec.
2016.

Cosgrove, Kelly P., Carolyn M. Mazure, and Julie K. Staley. "Evolving Knowledge of Sexual
Differences in Brain Structure, Function and Chemistry." Biological Psychology, 15 Oct.
2007, pp. 847-55. National Center for Biotechnology Information. Accessed 29 Nov.
2016

Dinsdale, Paul. "Fetal Testosterone Levels Linked to Autistic Traits." Learning Disability
Practice, Oct. 2007, p. 5. Gale Psychology Collection. Accessed 28 Nov. 2016.

"Extra Testosterone Reduces Your Empathy." Annals of Psychotherapy and Integrative Health,
10 Feb. 2011, p. 9. Gale Psychology Collection. Accessed 28 Nov. 2016.

Gibbons, Ann. "The Brain as "Sexual Organ" (Gender Differences)." American Association for
the Advancement of Science, 30 Aug. 1991, p. 957. Gale Psychology Collection.
Accessed 13 Oct. 2016.

Hamzelou, Jessica. "Transsexual Differences Caught on Brain Scan." New Scientist, 26 Jan.
2011. Accessed 27 Nov. 2016.

Kimura, Doreen. "Sex Differences in the Brain." Scientific American, 1 Sept. 1992. Accessed 20
Oct. 2016

Kraemer, Bernd, Aba Delsignore, Ronnie Gundelfinger, Ulrich Schnyder, and Urs Hepp.
"Comorbidity of Asperger Syndrome and Gender Identity Disorder." Early Childhood
Adolescent Psychiatry, 2005, pp. 292-96. Springer Link. Accessed 30 Nov. 2016.

Kukreti, Prerna, Manish Kandpal, and R Jiloha. "Mistaken Gender Identity in Non-Classical
Congenital Adrenal Hyperplasia." Indian Journal of Psychiatry, Apr. 2014, p. 182. Gale
Psychology Collection. Accessed 30 Nov. 2016.

Luders, E, H Steinmetz, and L Jancke. "Brain Size and Grey Matter Volume in the Healthy
Human Brain." Neuroreport, 3 Dec. 2002. National Center for Biotechnology
Information. Accessed 30 Nov. 2016.

McCarthy, Margaret M. "Bias was Built into Research from the Beginning." Nature, 3 Aug.
2006, p. 510. Accessed 30 Nov. 2016.

Park, Alice. "What the Gay Brain Looks Like." Time, 17 June 2008. Accessed 30 Nov. 2016.

Russo, Francine. "Is There Something Unique about the Transgender Brain?" Scientific
American, 1 Jan. 2016. Accessed 27 Nov. 2016.

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 Fetal Testosterone and Gender Identity

"Testosterone, Total, Bioavailable and Free, Serum." Mayo Medical Laboratories. Accessed 30
Nov. 2016.

Manarky 13