Sexual Orientation: The Genetic Search for an Answer
by Brian Urmanita


Discovering the origins of homosexuality has proved to be an 
elusive task so far.  As of present there has been no evidence found 
that suggests a conclusive link between genetic causes of male and 
female homosexuality.  Any evidence that has been uncovered has not 
given us clues as to the causes but instead has provided researchers a 
clearer path to follow the link between sexual orientation and 
biological structures involved.  The quest to find a genetic substrate 
for homosexuality has yet to be determined.  It is generally agreed 
that the etiology of homosexuality is combination of psychosocial and 
biological components.  
For instance, a researcher in 1991 found that there are 
structural differences of the hypothalamus between heterosexual and 
homosexual men (LeVay, 1991). The hypothalamus is thought to be the 
locus of control for sexual orientation, reproductive behavior and 
gender identity as stated by Swaab and Hofman (1995).  In specific, 
small cluster of neurons in the hypothalamus, called the INAH 2 cells, 
were found to be smaller in gay men and heterosexual women than in 
heterosexual men and homosexual women (1991).  This provided the first 
evidence that brain structures are dimorphic by sexual orientation and 
not just sex (1991).  
In rats, this dimorphism can also be observed, specifically 
called the sexually dimorphic nucleus of the preoptic area (SDN-POA) of 
the hypothalamus as cited by Swaab and Hofman (1995) but exhibits no 
difference between heterosexual and gay men. Swaab and Hofman (1995) 
then go on to dispute LeVay's claim that this particular area of the 
hypothalamus contributes to homosexuality by pointing out that this 
hypothesis has yet to be scrutinized thoroughly.  By attempting to 
identify differing structures in the brains of homosexual and 
heterosexual individuals, researchers then can begin to understand the 
genetic underpinnings that drive these variations.  
What psychologists have discovered repeatedly is that sexual 
orientation does run in families (Bailey et al., 2000).  A recent twins 
study conducted in Australia found that concordance rates for sexual 
orientation of monozygotic (MZ) twins to be 20% for men and 24% for 
women (2000).  This is a much lower finding than what Bailey and 
Pillard found as cited by Bailey (2000) when their research lead them 
to find a concordance rate of 47% and 48% for men and women, 
respectively.  Simply put, it seems that as researchers change their 
criteria of how to rate sexual orientation, the results also vary.  
Yet other studies find a greater relationship between genetic 
expression and sexual orientation.  Miller (2000) cites evidence that 
brothers of gay men were four times as likely to be gay as well.  While 
this does not take into account influential environmental factors, it 
does point out a statistically significant finding hinting at a genetic 
role in sexual orientation.
Miller takes a more theoretical approach to this conundrum of 
sexuality by discussing the role of genes from an evolutionary 
standpoint (2000).  It seems highly unlikely that a genetic mechanism 
for homosexual could survive when its consequences do not lead to 
reproductive success.  Miller then takes the position that it may be a 
certain summation of feminine alleles (variations of genes) that 
heterosexual males inherit, increasing chances of reproductive success 
while male homosexuality is the result of receiving a greater number of 
feminine alleles (2000).  	
Whatever the theoretical discussion may be there has been some 
linkage of sexual orientation to a genetic locus.  Hamer et al. (1993), 
find that the chromosome Xq28 has shown promise in displaying some 
connection to homosexuality.  If this were the case then males would 
receive this exclusively from their mothers meaning that homosexuality 
would be passed maternally to their offspring.  Further research has 
found that the Xq28 chromosome does share an association to 
homosexuality but only within males and under strict experimental 
design (Hu, et al., 1995).  Additionally, these stringent guidelines 
did not provide irrefutable proof that even when all the proper 
criteria were met the Xq28 locus was not necessary expression of a 
homosexual phenotype (1995).
In conclusion, while we see the first hints of a genetic basis to 
appear it is still very tentative.  Even those scientists who are 
involved are critical of their own findings leaving rich opportunities 
to further the understanding of the genetic roots of sexual 
orientation.  What scientists have now are dimly lit leads that for the 
most part give them a means to look further into the physical 
components of this complex characteristic of humanity.

References
Bailey, M.J., et al., (2000). Genetic and environmental 
influences on sexual orientation and its correlates in an Australian 
twins sample. Journal of Personality and Social Psychology, 78. 524-
536.
Hamer, et al.,(1993). A linkage between DNA markers and the x 
chromosome and male sexual orientation. Science, 261. 321-327.
Hu, S., et al.,(1995). Linkage between sexual orientation and 
chromosome xq28 in males but not females. Nature Genetics, 11. 248-256.
Miller, E.M. (2000). Homosexuality, birth order and evolution: 
Toward an equilibrium reproductive economics of homosexuality. Archive 
of Sexual Behavior, 29. 1-14.
LeVay, S., (1991). A difference in hypothalamic structure between 
heterosexual and homosexual men. Science, 25. 1034-1036.
Swaab, D.F., & Hofman, M.A., (1995). Sexual differentiation of 
the human hypothalamus in relation to gender and sexual orientation. 
Trends in Neuroscience, 18. 264-270.


							Jennifer Beauharnois
							May 05, 2001			
							Psychobiology	

Theoretical Arguments Concerning the Origins of Sexual Orientation 

	
		The causes of sexual orientation (heterosexual and 
homosexual) have managed to provoke intense scientific interest, 
inspiring both empirical studies and various theories over the years. 
Setting aside the social and ethical reasons for interest in the cause 
of sexual orientation leads us to examine the legitimate and scientific 
reasons for interest in the issue. Sexual orientation is a fundamental 
aspect of human sexuality, and furthermore it is empirically linked to 
some aspects of gender roles, for example childhood play behavior and 
gender identity. 	
	Empirical research about the origins of sexual orientation has 
been rooted, for the most part, around the "nature-nurture 
controversy". Two main approaches are motivated by this dichotomy. The 
first, is often called the "neurohormonal" theory (Elis & Ames, 1987), 
and studies the possibility that homosexual people have been subject to 
atypical levels of hormones during critical periods of brain 
development (perhaps the second month of pregnancy until the fifth 
month).	In support of this theory, LeVay (1991) found that for one 
hypothalamic nucleus, homosexual men are more similar to heterosexual 
women than to heterosexual men.
	The problem with the above theory is in the means of measurement 
to support it. Animal studies have been done on the hormonal 
manipulation of pregnant subjects. Males that were exposed to much-
decreased levels of testosterone early in life have as adults shown 
sexual interest in other males. Females exposed to extra testosterone 
during that period show an increased probability of attempting to mount 
sexual partners in the way that males typically would do. The problem 
is that as a result of the hormonal manipulation abnormalities in 
genital development were detected (Adkins-Regan, 1988). In addition, 
what is true of rats may not be true of humans. Hormonal influences on 
sexual behavior vary even between one primate species and another. 
LeVay's discovery of similarities between brain structure of homosexual 
men and heterosexual females may be the most important finding 
motivated by this perspective.
	The second theoretical approach has focused on whether sexual 
orientation is familial, and if so, whether familial aggregation is 
attributable to genetic or shared environmental factors. Both male and 
female homosexuality appears to run in families; studies of unseparated 
twins have suggested that this is primarily due to genetic rather than 
familial environmental influences (Bailey & Bell, 1993). 
Furthermore, there is some evidence that a gene on the X 
chromosome (Hamer, Hu, Magnuson, Hu, & Pattatucci, 1993) influences 
male sexual orientation. Miller (2000) notes that there's a large body 
of literature trying to explain how genes for homosexuality could have 
survived. One of the arguments against genetically inherited homosexual 
orientation is that a gene controlling such inheritance would be lost 
from the population because of the lack of reproduction resulting from 
homosexual acts (Ruse 1981). While offspring are not produced in same-
sex encounters, homosexual men and women can and do have children of 
their own.
	So then the argument turns to reasons how a gene(s) for 
homosexuality would survive and how it would be expressed as the 
phenotype. Miller (2000) states that attraction to the same sex is hard 
to imagine evolving from scratch. Yet since homosexuals do find 
themselves attracted to males, but not to females, it follows that 
brains do somehow reliably separate males from females. It is likely, 
that the mechanism for attraction to males is present in the human 
genotype, but it is normally turned on only in females and in 
homosexuals.
	Miller adds that sexual orientation is just one of a number of 
traits that normally separate females from males and that it is likely 
that all or most of the sex-specific traits are activated (or turned 
off) by a single hormonal mechanism, which is occasionally partially 
activated in males producing homosexuality. Miller states that "for a 
variation of a gene (an allele) which frequently produces an effect in 
some individuals (homosexuals) that's opposed to reproductive success 
to survive over the long run, the allele must also contribute to 
reproductive success when in other individuals (heterosexuals)"(p. 
323). Miller presumes that two alleles at the same location produce 
homosexuality, but one allele produces a desirable effect in 
heterosexuals.
	One popular sexual orientation theorist, Daryl Bem (2000), 
examines the specific correlates of homosexuality to help explain how 
genetic influences effect our behavior. Bem recognizes that 
developmental and etiological theories of sexual orientation must try 
to account for the strong association that has been studied between the 
two traits of sexual orientation and childhood gender nonconformity. 
Bem's theory "exotic becomes erotic"(EBE theory), specifies that 
biological factors may cause childhood gender nonconformity and that 
gender-nonconforming children tend to feel different from other 
children of their sex, and as a result (through a rather complicated 
pathway), eroticize those feelings. 
	The EBE theory notes that traits, which are evidently genetic 
(like temperaments and aggression), predispose children to enjoy some 
activities more than others. Children will prefer to play with peers 
who share their activity preferences. Those children who prefer sex-
atypical activities and opposed-sex playmates are referred to as gender 
nonconforming. It is these children who eventually eroticize the 
strange feelings for the same sex. Bem emphasizes that his model isn't 
intended to describe and inevitable, universal path to sexual 
orientation but a modal path followed by most men and women in a 
"gender-polarizing culture" like ours (p.535).
	The data now available suggest effects of genes, prenatal 
hormones, and differences in brain organization, but in no case are we 
absolutely certain how these factors contribute, how much of the 
variance they control, or how they interact with the environment. 
Because it is unethical to do specific experiments on humans dealing 
with environmental variables, hormonal manipulation, and certain gene-
knockout techniques, we are still far away from understanding the 
relationship and influence of the "nature-nurture" dichotomy on sexual 
orientation.


References:

	Adkins, S., & Regan, E., (1998). Sex Hormones and sexual 
orientation in animals. Psychobiology, 16, 335-347.

	Bailey, M., & Bell, P., (1993). Familiality of female and male 
homosexuality. Behavior Genetics, 23, 313-322.

	Bem, D.,(2000). Exotic Becomes Erotic: Interpreting the 
biological correlates of sexual orientation. Archives of Sexual 
Behavior, 29, 531-547.

	Elis, L., & Ames, A., (1987) Neurohormonal functioning and sexual 
orientation: A theory of homosexuality-heterosexuality. Psychological 
Bullitin, 10, 233-258.

	Hammer, H., Hu, S., Magnuson L., Hu, N., & Pattatucci, L., (1993, 
July) A linkage between DNA markers on the X- chromosome and male 
sexual orientation. Science, 261, 321-327.

	LeVay, S., (1991) A difference in hypothalamic structure between 
heterosexual and homosexual men. Science, 253, 1034-1037.

	Miller, E., (2000) Homosexuality, birth order, and evolution: 
Toward an equilibrium reproductive economics of homosexuality. Archives 
of Sexual Behavior, 29,1-31.

	Ruse, M., (1981) Are there gay genes? Sociobiology and 
homosexuality. Journal of Homosexuality, 6(4), 5-33.


Greg Rickel
Psych 325
Project 2
Dr. Morgan

Research Methods Used In Determining Genetic Influence On 
Sexual Orientation


	For many years scientific research has tried to 
determine the specific influences that cause a variety of 
personality traits and human behaviors.  What is the level 
of influence genetic endowment plays in comparison to the 
social environment?  This question has been toiled over 
through a broad range of research, but as technological 
advances in biological testing methods increase the ability 
to understand this question becomes clearer and clearer.  
One specific trait or behavior that has recently been given 
a relatively large amount of attention is that of sexual 
orientation.  For researchers exploring this topic, various 
methods are being used to discriminate between the age-old 
questions of nature vs. nurture.  These methods include 
genetic screening and testing on self reported homo- and 
heterosexuals, extensive brain autopsies of deceased 
individuals known to be gay compared with heterosexual 
females and males, as well as twin and adoption studies.  
This paper will serve as a brief overview of a variety of 
methods used as well a statement about an advantage or 
disadvantage of the method.
	An important aspect of sexual development occurs 
prenate, in which certain hormones must be present or 
absent in order for the individual to develop 'normally'.  
To some, these 'critical periods' are the basis for sexual 
orientation.  The influence of sex appropriate hormones at 
specific times is thought to determine the masculinity or 
femininity of an individual.  Therefore researches measure 
the different hormone levels of babies at known critical 
periods to determine baseline levels and aberrant levels.  
One problem with this type of research is that it must be 
correlated with information about the individuals sexual 
functioning, which will not become apparent for many years.  
Within that time social influence may play a big role.
	Another method used in a vast amount of research based 
in determining genetic influence of behavior is the use of 
twin and adoption studies.  The theory is that since 
monozygotic twins share one hundred percent of their 
genotype, then their behaviors, actions, and personalities 
should be strikingly similar.  Usually these types of 
studies are combined with similar studies on dizygotic 
twins, who share fifty percent of their genes, and regular 
siblings, who also share fifty percent of their 
genotype.  When correlated with studies that try to rule 
out environmental influences, such as twin adoption 
studies, a high concordance can be seen among individuals 
who grow up apart yet present the same personality traits 
and behaviors.  Thus it is possible to speculate on the 
influence of environmental influence by looking at 
concordance rate.  This method is currently the most widely 
used and accepted form of determining the influences on 
behavior, though the specific disadvantages to this type of 
design lie within the particular research ventures biases.
	More advanced, and still skeptically new, are seen in 
research adventures employing biotechnical methods.  
Various gene research is on the rise due to the discovery 
of the human genome, and much of it is fairly 
unsubstantiated as of yet.  Though a lot of attention has 
been given to the study of the genes on the X chromosome.  
An early 1993 research venture by Hamer et al. used self-
reported pedigree analysis combined with what is known as 
PCR amplification.  This process involves retrieving DNA 
samples from self-reported homosexuals and their homosexual 
siblings.  The DNA samples are "amplified", more or less 
stretched out, and matched onto each other.  Theoretically 
homosexual siblings should carry the same DNA sequence on 
the same gene.  The advantage to this is that is seems very 
effective at ruling out environmental factors, except if 
the particular gene one is looking at has no contribution 
to sexual orientation.
	A variety of other biotechnical manipulations are 
used, but the current ethical paradigm controlling research 
does not allow this to occur on human subjects.  So in 
place various higher order organisms, such as monkeys, 
apes, or rats, are used and correlations with human 
behavior are made based on the relative genetic closeness 
that the particular organism shares with us.  The methods 
include "blocking" genes during development so they do not 
appear in the phenotype or genotype as a way of pinpointing 
which genes cause which behaviors, introducing chemicals 
into pinpointed areas during critical developmental periods 
to try and produce homosexual behavior, as well as 
recording neural activity from internal electrodes cemented 
to the animals skull in order to understand which areas are 
active during certain behaviors.  The main disadvantage to 
these methods is that the correlation between animal 
behavior and human behavior does not seem to be a plausible 
one to be making.  As humans our genetic code and brain 
structures may be similar to animals, but in reality they 
are still slightly different, thus accounting for the 
behaviors observed in humans that other organisms do not 
seem carry out. 
	The most interesting research method used is to 
autopsy the brains of individuals who are known to gay and 
comparing the autopsies of heterosexual individuals.  In 
theory, and seen in some research reports, specific 
structures within the brain will be a different size 
allowing one to speculate as to that structures 
contribution to behavior.  For instance feminized males 
have been shown to have enlarged corpus callosums, much 
similar to heterosexual females yet quite different from 
heterosexual males.  Other areas have been shown to have 
differences, but the causes of these size differences are 
highly interpretive.  Since the individuals are dead there 
is not direct way of telling whether these changes are 
specifically biological.  
	In general, research on behaviors such as sexual 
orientation has carried various political and social 
opinions as to specific contributions.  Personal agendas of 
those performing the research will ultimately determine how 
a design will turn out.  The current biotechnical designs 
are highly speculative and unsubstantiated.  There is quite 
a bit of room for growth in fields employing these designs.  
"The proof for the involvement of genes in a human 
behavioral trait must ultimately consist of chromosomal 
mapping of the loci and isolation of the relevant DNA 
sequence" [1].


[1] Hamer DH, Hu S, Magnuson VL, Hu N, Pattatuci AML. A 
linkage between DNA markers on the X chromosome and male 
sexual orientation. Science 1993;261:321-327.

[2] Lewis R. Human Genetics: Concepts and Applications(3rd 
edition). Mcgraw-Hill; Boston, MA: 1999.

[3] Carlson NR. Physiology of Behavior(6th edition). Allyn 
and Bacon; Boston, MA: 1998.

Copyright © 2001, Dr. John M. Morgan, All rights reserved - This page last edited 8 May, 2001
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