Biological factors during fetal development may influence sexual orientation. The maternal immune hypothesis suggests that a mother's immune response to male antigens could affect brain development and increase the likelihood of homosexuality. The hormone wash theory proposes that stress-induced maternal hormones could disrupt the process that masculinizes the male fetus's brain. Studies have also found structural differences in brain regions involved in sexual behavior and attraction between homosexual and heterosexual individuals, such as larger hypothalamic regions in gay men. While both nature and nurture may play a role, biological theories focus on prenatal influences on the formation of the "homosexual brain."

1. Biological Causes for the
Homosexual Brain
BRYAN CURRIE
BRAIN DEVELOPMENT OF YOUTH
HDFS 892
MICHIGAN STATE UNIVERSITY

2. Why Study Homosexual Brain Development?
Puberty – the amazing time when
adolescents realize their sexual attractions
and abilities – is both exciting and terrifying.
During puberty, heterosexual youth may be
confused by their newfound attractions, but
are assured by their culture that they are
“normal.”
Homosexual youth, however, are not as
fortunate. From a very early age, lesbian,
gay, bisexual, and transgender (LGBT) youth
are often keenly aware that they are
“different” (Flowers & Buston, 2001). In
addition to figuring out the “normal”
struggles of sexual attraction, LGBT youth It is important for youth development
often question whether they are broken, professionals to understand theories
strange, or immoral because their attractions surrounding the development of the homosexual
aren’t like everyone else’s. Some of these brain so that they can assure frightened LGBT
youth even pursue dangerous therapies to try youth that their newly discovered desires are
to “fix” their sexual orientation. simply a part of their beautiful design.

3. The Homosexual Brain: Premise and Thesis
Premise: Various in-utero factors may affect the development of the homosexual
brain so that both its form (i.e. physical structure) and function (i.e. sexual
attraction) are slightly different from the heterosexual brain.
Thesis: Although both nature (genetics and/or biology) and nurture
(environment, etc.) may play a role in the development of sexual orientation, this
presentation will focus on biological theories for homosexual brain development.
Biological theories include the ways hormones affect the fetus in-utero and should
not be confused with genetic theories, which include the search for the “gay gene.”
In addition to discussing the formation of the homosexual brain, this presentation
will also explore whether the brains of homosexual youth are physically different
from those of their heterosexual peers.
Disclaimer: It is important to understand that “masculine” and “male” are not synonyms. Gay men
are neither inherently less “masculine” nor “male” simply because of their sexual orientation.
Although many of the studies referenced in this report refer to the “masculine” or “feminine” brain, it
should be understood that these words refer to biological differentiation of the brain (ie. whether it is
structured more like a male brain or a female brain). These words are not intended to comment on
the masculinity/femininity (i.e. culturally defined behavior) of homosexual people.

4. Biological Theories of
Development
THE HOMOSEXUAL BRAIN

5. Major Biological Theories
There are two major theories for biological (in-utero)
causes of homosexuality:
The mother may produce an immune reaction
that prevents the male brain from developing in a
typically “masculine” pattern.
The hormone wash which “defeminizes” the
brain in-utero is disturbed due to maternal
stress.

6. Maternal Immune Reaction
IT HAS BEEN SUGGESTED THAT THE MALE
FETUS MAY SOMETIMES PRODUCE AN
IMMUNE RESPONSE IN THE MOTHER WHICH
TRIGGERS THE RELEASE OF ANTIBODIES.
THESE ANTIBODIES CHANGE THE FETUS’S
BRAIN DEVELOPMENT AND CAUSE A
HOMOSEXUAL ORIENTATION.

7. Maternal Immune Reaction: Antigens
Antigen: any substance foreign to the body that evokes an immune response
from the host.
Because of a male child’s XY gene structure, HY antigens are
present on the surface of his developing cells. Because the
expectant mother is used to having only XX cells in her body,
Blanchard and Bogaert (1996) propose that when the male fetus’s
HY antigens are released into the mother’s bloodstream, her body
may trigger the immune system to release HY antibodies. These
antibodies cross the placenta and enter into the male fetus’s
developing brain.
HY antigens help the male fetus develop sex-typical traits.
Exposure to HY antibodies (which attack and potentially weaken the
HY antigens) may therefore affect subsequent sexual behavior in
men, increasing the likelihood that the male child will be more
attracted to men than women (Blanchard & Klassen, 1997).

8. Maternal Immune Reaction: Evidence
In most cases, a mother’s immune system becomes
stronger with every pregnancy. Therefore, each time
a mother carries a male fetus, the chances increase
that her body will develop an immune response to
his HY Antigens.
This may explain why male homosexuality correlates
with birth order. Each additional older brother
increases the likelihood a male child will be
homosexual by 33% (Blanchard & Klassen, 1997;
Cantor, Blanchard, Paterson, & Bogaert, 2002).

9. Maternal Immune Reaction: Evidence
New research has, however, begun to question whether having multiple male children
is a cause of homosexuality (due to an immune reaction) or an evolutionary product of
the “gay gene.” According to research that will be published in an upcoming issue of
the Journal of Sexual Medicine:
“the same genetic factors that induce gayness in males also promote
fecundity (high reproductive success) in those males’ female maternal
relatives” (Wolchover, 2012,).
If there is a “gay gene” it likely resides on the X chromosome.
Evolutionary biologists suggest that the presence of this gene in
mothers may “increase androphilia, or attraction to men, thereby
making the males who possess the gene homosexual and the females
who possess it more promiscuous” (Wolchover, 2012).
If this theory holds true, larger family sizes may not be the cause of
homosexual male children, but rather an evolutionary result of the
“gay gene” as it is carried by women on the X chromosome.

10. Hormone Wash
ALL FETUSES BEGIN BIOLOGICALLY
FEMALE. A WASH OF TESTOSTERONE
D U R I N G T H E 1 2 - 1 4 TH W E E K S O F P R E G N A N C Y
PRODUCES A MASCULINE BRAIN.
DISRUPTIONS IN THIS PROCESS MAY
PREVENT THE BRAIN FROM BECOMING
FULLY MASCULINE.

11. Hormone Wash: Androgens
Androgen: A steroid, such as testosterone, that controls the development
and maintenance of masculine characteristics.
If a fetus carries the XY chromosome, testosterone is needed to activate the
newly forming hypothalamus into a male brain. This process is called
“defeminization” (Kula & Sowikowska-Hilczer, 2000).
Different levels of testosterone exposure during this process influence “the
structure and function of brain regions that control the direction of sexual
attraction” (Wilson & Rahman, 2005, p. 70).
Because the genitals are developed during a different period of gestation
and through a different process than the defeminization of the brain, the
presence of a masculine body does not necessarily indicate the presence of
a masculine brain. This may partially explain why the brain structure of
homosexual males may more closely resemble that of heterosexual females.

12. Hormone Wash & Maternal Stress
During the 12th to 14th week of pregnancy, a developing male fetus will receive a wash
of androgens (testosterone) over its brain. This hormone wash defeminizes it and
differentiates it into the male form (Gooren & Kruijver, 2002).
If a mother is stressed during the early stages of pregnancy, she will release an
adrenaline related hormone. This hormone (androstendione) is structurally similar to
testosterone, but affects the fetus differently.
Because the stress hormone seems to mimic testosterone, the wash of testosterone is
less effective, causing a disturbance in the "defeminization" of the hypothalamus
(Swab, Chung, Kruijiver, Hofman, & Ishunia, 2002) and preventing “normal” sex
differences in the brain to be acquired (Looy, 1995).

13. Hormone Wash: Evidence
Ingebog Ward (1972) tested this theory in studies of pregnant
rats. Ward divided pregnant rats into three groups. She
subjected the first group to stress during the first ten days of
gestation by irritating the mothers with bright lights, noise
and vibrations. Ten days in a rat's pregnancy corresponds to
the first trimester (3 months) of human pregnancy. The
second group was subjected to stress during the end of their
pregnancy, just before birth. The third group was comprised
of male offspring from both prenatal stressed mothers and
unstressed mothers.
Dr. Ward then allowed all the males to grow to adulthood
without further interference. The sexually mature male rats
were placed in cages with healthy females to observe their
ability and desire to mate with normal adult females.

14. Hormone Wash: Evidence
Instead of trying to “mount”
Ward concluded that
female rats, the stressed males
“exposure of pregnant
allowed themselves to be
rats to environmental Stress during
mounted. This was attributed
stressors modifies the pregnancy
to stress caused reactions
normal process of seems to
during critical stages of sexual
sexual behavior prevent
differentiation. “Specifically, it
differentiation in male testosterone
appears that stress causes an
fetuses by decreasing from
increase in the weak adrenal
functional developing a
androgen, androstendione, from
testosterone and brain that is
the maternal fetal adrenal
elevating sexually
cortices, or both, and a
androstenedione attracted to
concurrent decrease in the
levels during prenatal females.
potent gonadal androgen,
development”
testosterone” (Ward, 1972, p.
(Ward, 1972, p. 83).
82).

15. Neurological Differences
THE HOMOSEXUAL BRAIN

16. Brain Differences
It has been shown that the
brains of heterosexual
males, homosexual males,
heterosexual females, and
homosexual females
function differently
and are
differently structured.
* It is not clear if the differences
between the homosexual and
heterosexual brain are due to biological
factors, genetic factors, or conditioning.

18. Structural Differences
Many of the structural differences between the homosexual
and heterosexual brain are found in the hypothalamus, the
region of the brain that controls how humans experience
sexual attraction (Dorner, 1979).
These differences may be contained
in any of three regions:
Sexually Dimorphic Nucleus (SDN)
Suprachuiasmatic Nucleus (SCN)
Anterior Commissure (AC)

19. Structural Differences: SDN
The sexually dimorphic nucleus (SDN) is a cluster of cells in the preoptic area of the
hypothalamus. The SDN is believed to help regulate sexual behavior
(Allen, Hines, Shyne, & Gorski, 1989; Anderson, Fleming, Rhees, & Kinghorn, 1986).
The SDN is commonly 2.5x larger in males than females, containing 2.2x
more cells (Swaab & Fliers, 1985).
Researchers hypothesize that brains attracted to women (heterosexual men
and homosexual women) should have a lager SDN.
Sections of the SDN, known as INAH 3, have been shown to be up to 3x
larger in heterosexual men than heterosexual men (LeVay, 1991).*
These findings (that both heterosexual women and homosexual men have
a smaller SDN) seem to indicate that male homosexual brain structure
influences/causes a sexual attraction to men.
*The study referenced (LeVay, 1991) was performed on post-mortem AIDS patents.
Some researchers question the study’s findings, claiming that the enlarged INAH 3 region
found in homosexual men may be a result of the AIDS virus.

20. Structural Differences: SCN
The suprachiasmatic nucleus (SCN) is a small region in the brain’s midline.
It is responsible for regulating circadian rhythms (i.e. body functions in a 24
hour cycle) and has been associated with reproductive processes (Swaab &
Hofman, 1995).
The SCN has been shown to be 1.7x larger in homosexual males than
in heterosexual males, containing 2.1x more cells (Swaab & Hoffman,
1990).
Swaab produced a similar difference in rats by disturbing the
interaction between testosterone and the developing brain. The
experiment yielded bisexual rats that had a significantly larger SCN.
Swaab (2008) suggests that his tests on rats indicate that the larger
SCN in homosexual males is not caused by their behavior (nurture),
but rather “by an atypical interaction between sex hormones and the
developing brain” (p. 10273).

21. Structural Differences: Anterior Commissure
The Anterior Commissure (AC) is a fiber tract that is responsible for
transmitting sensory information between the brain’s temporal lobes
(Harrison, 1994).
The AC is typically larger in women than in men (Allen &
Gorski, 1991).
The AC of homosexual men has been shown to be 18% larger
than heterosexual women and 34% larger than heterosexual
men (Allen & Gorski, 1992).
While the AC is not associated with reproductive functions, it
is related to cognitive abilities and language (Swaab, 2008).
The presence of a larger AC may explain why homosexual men
seem to display many “feminine” brain functions (see
“functional differences” above).

23. Bibliography
Bibliographical references can be found in the notes
section below.