The relationship between biology and sexual orientation is a subject of research. While scientists do not know the exact cause of sexual orientation, they theorize that a combination of genetic, hormonal, and social factors determines it. Hypotheses for the impact of the post-natal social environment on sexual orientation, however, are weak, especially for males.
Biological theories for explaining the causes of sexual orientation are favored by scientists and involve a complex interplay of genetic factors, the early uterine environment and brain structure. These factors, which may be related to the development of a heterosexual, homosexual, bisexual, or asexual orientation, include genes, prenatal hormones, and brain structure.
Empirical studies
Twin studies
A number of twin studies
have attempted to compare the relative importance of genetics and
environment in the determination of sexual orientation. In a 1991 study,
Bailey and Pillard conducted a study of male twins recruited from
"homophile publications", and found that 52% of monozygotic (MZ) brothers (of whom 59 were questioned) and 22% of the dizygotic (DZ) twins were concordant for homosexuality.
'MZ' indicates identical twins with the same sets of genes and 'DZ'
indicates fraternal twins where genes are mixed to an extent similar to
that of non-twin siblings. In a study of 61 pairs of twins, researchers
found among their mostly male subjects a concordance rate for
homosexuality of 66% among monozygotic twins and a 30% one among
dizygotic twins.
In 2000 Bailey, Dunne and Martin studied a larger sample of 4,901
Australian twins but reported less than half the level of concordance.
They found 20% concordance in the male identical or MZ twins and 24%
concordance for the female identical or MZ twins. Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. Other researchers support biological causes for both men and women's sexual orientation.
Bearman and Brückner (2002) criticized early studies concentrating on small, select samples and non-representative selection of their subjects.
They studied 289 pairs of identical twins (monozygotic, or from one
fertilized egg) and 495 pairs of fraternal twins (dizygotic, or from two
fertilized eggs) and found concordance rates
for same-sex attraction of only 7.7% for male identical twins and 5.3%
for females, a pattern which they say "does not suggest genetic
influence independent of social context".
A 2010 study of all adult twins in Sweden (more than 7,600 twins)
found that same-sex behavior was explained by both heritable factors
and individual-specific environmental sources (such as prenatal
environment, experience with illness and trauma, as well as peer groups,
and sexual experiences), while influences of shared-environment
variables such as familial environment and social attitudes had a
weaker, but significant effect. Women showed a statistically
non-significant trend to weaker influence of hereditary effects, while
men showed no effect of shared environmental effects. The use of all
adult twins in Sweden was designed to address the criticism of volunteer
studies, in which a potential bias towards participation by gay twins
may influence the results;
Biometric modeling revealed that, in men, genetic effects explained .34–.39 of the variance [of sexual orientation], the shared environment .00, and the individual-specific environment .61–.66 of the variance. Corresponding estimates among women were .18–.19 for genetic factors, .16–.17 for shared environmental, and .64–.66 for unique environmental factors. Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior.
Criticisms
Twin studies have received a number of criticisms including self-selection bias
where homosexuals with gay siblings are more likely to volunteer for
studies. Nonetheless, it is possible to conclude that, given the
difference in sexuality in so many sets of identical twins, sexual
orientation cannot be attributed solely to genetic factors.
Another issue is the finding that even monozygotic twins can be
different and there is a mechanism which might account for monozygotic
twins being discordant for homosexuality. Gringas and Chen (2001)
describe a number of mechanisms which can lead to differences between monozygotic twins, the most relevant here being chorionicity and amniocity. Dichorionic twins
potentially have different hormonal environments because they receive
maternal blood from separate placenta, and this could result in
different levels of brain masculinisation. Monoamniotic twins share a
hormonal environment, but can suffer from the 'twin to twin transfusion
syndrome' in which one twin is "relatively stuffed with blood and the
other exsanguinated".
Chromosome linkage studies
Chromosome
linkage studies of sexual orientation have indicated the presence of
multiple contributing genetic factors throughout the genome. In 1993 Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families.
Hamer et al. found that the gay men had more gay male uncles and
cousins on the maternal side of the family than on the paternal side.
Gay brothers who showed this maternal pedigree were then tested for X
chromosome linkage, using twenty-two markers on the X chromosome to test
for similar alleles. In another finding, thirty-three of the forty
sibling pairs tested were found to have similar alleles in the distal
region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly dubbed the "gay gene"
in the media, causing significant controversy. Sanders et al. in 1998
reported on their similar study, in which they found that 13% of uncles
of gay brothers on the maternal side were homosexual, compared with 6%
on the paternal side.
A later analysis by Hu et al. replicated and refined the earlier
findings. This study revealed that 67% of gay brothers in a new
saturated sample shared a marker on the X chromosome at Xq28.
Two other studies (Bailey et al., 1999; McKnight and Malcolm, 2000)
failed to find a preponderance of gay relatives in the maternal line of
homosexual men. One study by Rice et al. in 1999 failed to replicate the Xq28 linkage results.
Meta-analysis of all available linkage data indicates a significant
link to Xq28, but also indicates that additional genes must be present
to account for the full heritability of sexual orientation.
Mustanski et al. (2005) performed a full-genome scan (instead of
just an X chromosome scan) on individuals and families previously
reported on in Hamer et al. (1993) and Hu et al. (1995), as well as
additional new subjects. In the full sample they did not find linkage to
Xq28.
Results from the first large, comprehensive multi-center genetic
linkage study of male sexual orientation were reported by an independent
group of researchers at the American Society of Human Genetics in 2012. The study population included 409 independent pairs of gay brothers, who were analyzed with over 300,000 single-nucleotide polymorphism
markers. The data strongly replicated Hamer's Xq28 findings as
determined by both two-point and multipoint (MERLIN) LOD score mapping.
Significant linkage was also detected in the pericentromeric region of
chromosome 8, overlapping with one of the regions detected in the Hamer
lab's previous genomewide study. The authors concluded that "our
findings, taken in context with previous work, suggest that genetic
variation in each of these regions contributes to development of the
important psychological trait of male sexual orientation". Female sexual
orientation does not seem to be linked to Xq28, though it does appear moderately heritable.
In addition to sex chromosomal contribution, a potential autosomal
genetic contribution to the development of homosexual orientation has
also been suggested. In a study population composed of more than 7000
participants, Ellis et al. (2008) found a statistically significant
difference in the frequency of blood type A between homosexuals and
heterosexuals. They also found that "unusually high" proportions of
homosexual males and homosexual females were Rh negative in comparison to heterosexuals. As both blood type and Rh factor are genetically inherited traits controlled by alleles
located on chromosome 9 and chromosome 1 respectively, the study
indicates a potential link between genes on autosomes and homosexuality.
The biology of sexual orientation has been studied in detail in several animal model systems. In the common fruit fly Drosophila melanogaster,
the complete pathway of sexual differentiation of the brain and the
behaviors it controls is well established in both males and females,
providing a concise model of biologically controlled courtship.
In mammals, a group of geneticists at the Korea Advanced Institute of
Science and Technology bred a female mice specifically lacking a
particular gene related to sexual behavior. Without the gene, the mice
exhibited masculine sexual behavior and attraction toward urine of other
female mice. Those mice who retained the gene fucose mutarotase (FucM)
were attracted to male mice.
In interviews to the press, researchers have pointed that the
evidence of genetic influences should not be equated with genetic
determinism. According to Dean Hamer and Michael Bailey, genetic aspects
are only one of the multiple causes of homosexuality.
In 2017, Nature published an article with a genome wide association study on male sexual orientation. The research consisted of 1,077 homosexual men and 1,231 heterosexual men. A gene named SLITRK6 on chromosome 13 was identified. The research supports another study which had been done by Simon LeVay. LeVay's research suggested that the hypothalamus of gay men is different from straight men. The SLITRK6 is active in the mid-brain where the hypothalamus is. The researchers found that the thyroid stimulating hormone receptor (TSHR) on chromosome 14 shows sequence differences between gay and straight men. Graves' disease
is associated with TSHR abnormalities, with previous research
indicating that Graves' disease is more common in gay men than in
straight men.
Research indicated that gay people have lower body weight than straight
people. It had been suggested that the overactive TSHR hormone lowered
body weight in gay people, though this remains unproven.
In 2018, Ganna et al. performed another genome wide association study
on sexual orientation of men and women with data from 26,890 people who
had at least one same-sex partner and 450,939 controls. The data in the
study was meta-analyzed and obtained from the UK Biobank study and 23andMe.
The researchers identified four variants more common in people who
reported at least one same-sex experience on chromosomes 7, 11, 12, and
15. The variants on chromosomes 11 and 15 were specific to men, with the
variant on chromosome 11 located in an olfactory gene and the variant
on chromosome 15 having previously been linked to male-pattern baldness.
The four variants were also correlated with mood and mental health
disorders; major depressive disorder and schizophrenia in men and women,
and bipolar disorder in women. However, none of the four variants could
reliably predict sexual orientation.
| Chromosome | Location | Associated Genes | Sex | Study | Origin | Note |
|---|---|---|---|---|---|---|
| X chromosome | Xq28 | male only | Hamer et al. 1993
Sanders et al. 2015
|
genetic |
| |
| Chromosome 1 | 1p36 | both sexes | Ellis et al. 2008 | potential proxy measurement | ||
| Chromosome 7 |
|
|
both sexes | Ganna et al. 2018 | ||
| Chromosome 8 | 8p12 | Unknown | male only | Mustanski et al. 2005
Sanders et al. 2015
|
| |
| Chromosome 9 | 9q34 | ABO | both sexes | Ellis et al. 2008 | potential proxy measurement | |
| Chromosome 11 | 11p15 | OR51A7 (speculative) | male only | Ganna et al. 2018 | Olfactory system in mating preferences | |
| Chromosome 12 |
|
|
both sexes | Ganna et al. 2018 | ||
| Chromosome 13 | 13q31 | SLITRK6 | male only | Sanders et al. 2017 | Diencephalon-associated gene | |
| Chromosome 14 | 14q31 | TSHR | male only | Sanders et al. 2017 |
| |
| Chromosome 15 |
|
|
male only | Ganna et al. 2018 | Male-pattern baldness associated |
Epigenetics studies
A study suggests linkage between a mother's genetic make-up and
homosexuality of her sons. Women have two X chromosomes, one of which is
"switched off". The inactivation of the X chromosome occurs randomly
throughout the embryo, resulting in cells that are mosaic with respect
to which chromosome is active. In some cases though, it appears that
this switching off can occur in a non-random fashion. Bocklandt et al.
(2006) reported that, in mothers of homosexual men, the number of women
with extreme skewing of X chromosome inactivation is significantly
higher than in mothers without gay sons. 13% of mothers with one gay
son, and 23% of mothers with two gay sons, showed extreme skewing,
compared to 4% of mothers without gay sons.
Birth order
Blanchard and Klassen (1997) reported that each additional older brother increases the odds of a man being gay by 33%. This is now "one of the most reliable epidemiological variables ever identified in the study of sexual orientation".
To explain this finding, it has been proposed that male fetuses provoke
a maternal immune reaction that becomes stronger with each successive
male fetus.
This maternal immunization hypothesis (MIH) begins when cells from a
male fetus enter the mother's circulation during pregnancy or while
giving birth.
Male fetuses produce H-Y antigens which are "almost certainly involved
in the sexual differentiation of vertebrates".
These Y-linked proteins would not be recognized in the mother's immune
system because she is female, causing her to develop antibodies which
would travel through the placental barrier into the fetal compartment.
From here, the anti-male bodies would then cross the blood/brain barrier
(BBB) of the developing fetal brain, altering sex-dimorphic brain
structures relative to sexual orientation, increasing the likelihood
that the exposed son will be more attracted to men than women.
It is this antigen which maternal H-Y antibodies are proposed to both
react to and 'remember'. Successive male fetuses are then attacked by
H-Y antibodies which somehow decrease the ability of H-Y antigens to
perform their usual function in brain masculinization.
However, the maternal immune hypothesis has been criticized
because the prevalence of the type of immune attack proposed is rare
compared with the prevalence of homosexuality.
The "fraternal birth order effect" however, cannot account for between 71-85% of male homosexual preference. Additionally, it does not explain instances where a firstborn child displays male homosexual preference (MHP).
In 2017, researchers discovered a biological mechanism of gay people who tend to have older brothers. They think Neuroligin 4 Y-linked
protein is responsible for a later son being gay. They found that women
had significantly higher anti-NLGN4Y levels than men. The result also
indicates that number of pregnancies, mothers of gay sons, particularly
those with older brothers, had significantly higher anti-NLGN4Y levels
than did the control samples of women, including mothers of heterosexual
sons.
Female fertility
In
2004, Italian researchers conducted a study of about 4,600 people who
were the relatives of 98 homosexual and 100 heterosexual men. Female
relatives of the homosexual men tended to have more offspring than those
of the heterosexual men. Female relatives of the homosexual men on
their mother's side tended to have more offspring than those on the
father's side. The researchers concluded that there was genetic material
being passed down on the X chromosome which both promotes fertility in
the mother and homosexuality in her male offspring. The connections
discovered would explain about 20% of the cases studied, indicating that
this is a highly significant but not the sole genetic factor
determining sexual orientation.
Pheromone studies
Research conducted in Sweden has suggested that gay and straight men respond differently to two odors that are believed to be involved in sexual arousal.
The research showed that when both heterosexual women and gay men are
exposed to a testosterone derivative found in men's sweat, a region in
the hypothalamus is activated. Heterosexual men, on the other hand, have
a similar response to an estrogen-like compound found in women's urine.
The conclusion is that sexual attraction, whether same-sex or
opposite-sex oriented, operates similarly on a biological level.
Researchers have suggested that this possibility could be further
explored by studying young subjects to see if similar responses in the
hypothalamus are found and then correlating these data with adult sexual
orientation.
Studies of brain structure
A number of sections of the brain
have been reported to be sexually dimorphic; that is, they vary between
men and women. There have also been reports of variations in brain
structure corresponding to sexual orientation. In 1990, Dick Swaab and Michel A. Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men. In 1992, Allen and Gorski reported a difference related to sexual orientation in the size of the anterior commissure,
but this research was refuted by numerous studies, one of which found
that the entirety of the variation was caused by a single outlier.
Research on the physiologic differences between male and female
brains are based on the idea that people have male or a female brain,
and this mirrors the behavioral differences between the two sexes. Some
researchers state that solid scientific support for this is lacking.
Although consistent differences have been identified, including the size
of the brain and of specific brain regions, male and female brains are
very similar.
Sexually dimorphic nuclei in the anterior hypothalamus
Simon LeVay, too, conducted some of these early researches. He studied four groups of neurons in the hypothalamus
called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the
brain to study, because of evidence that it played a role in the
regulation of sexual behaviour in animals, and because INAH2 and INAH3 had previously been reported to differ in size between men and women.
He obtained brains from 41 deceased hospital patients. The
subjects were classified into three groups. The first group comprised 19
gay men who had died of AIDS-related
illnesses. The second group comprised 16 men whose sexual orientation
was unknown, but whom the researchers presumed to be heterosexual. Six
of these men had died of AIDS-related illnesses. The third group was of
six women whom the researchers presumed to be heterosexual. One of the
women had died of an AIDS-related illness.
The HIV-positive people in the presumably heterosexual patient groups were all identified from medical records as either intravenous drug abusers or recipients of blood transfusions.
Two of the men who identified as heterosexual specifically denied ever
engaging in a homosexual sex act. The records of the remaining
heterosexual subjects contained no information about their sexual
orientation; they were assumed to have been primarily or exclusively
heterosexual "on the basis of the numerical preponderance of
heterosexual men in the population".
LeVay found no evidence for a difference between the groups in
the size of INAH1, INAH2 or INAH4. However, the INAH3 group appeared to
be twice as big in the heterosexual male group as in the gay male group;
the difference was highly significant, and remained significant when
only the six AIDS patients were included in the heterosexual group. The
size of INAH3 in the homosexual men's brains was comparable to the size
of INAH3 in the heterosexual women's brains.
However, other studies have shown that the sexually dimorphic
nucleus of the preoptic area, which include the INAH3, are of similar
size in homosexual males who died of AIDS to heterosexual males, and
therefore larger than female. This clearly contradicts the hypothesis
that homosexual males have a female hypothalamus. Furthermore, the SCN
of homosexual males is extremely large (both the volume and the number
of neurons are twice as many as in heterosexual males). These areas of
the hypothalamus have not yet been explored in homosexual females nor
bisexual males nor females. Although the functional implications of such
findings still haven't been examined in detail, they cast serious doubt
over the widely accepted Dörner hypothesis that homosexual males have a
"female hypothalamus" and that the key mechanism of differentiating the
"male brain from originally female brain" is the epigenetic influence
of testosterone during prenatal development.
William Byne and colleagues attempted to identify the size
differences reported in INAH 1–4 by replicating the experiment using
brain sample from other subjects: 14 HIV-positive homosexual males, 34
presumed heterosexual males (10 HIV-positive), and 34 presumed
heterosexual females (9 HIV-positive). The researchers found a
significant difference in INAH3 size between heterosexual men and
heterosexual women. The INAH3 size of the homosexual men was apparently
smaller than that of the heterosexual men, and larger than that of the
heterosexual women, though neither difference quite reached statistical
significance.
Byne and colleagues also weighed and counted numbers of neurons
in INAH3 tests not carried out by LeVay. The results for INAH3 weight
were similar to those for INAH3 size; that is, the INAH3 weight for the
heterosexual male brains was significantly larger than for the
heterosexual female brains, while the results for the gay male group
were between those of the other two groups but not quite significantly
different from either. The neuron count also found a male-female
difference in INAH3, but found no trend related to sexual orientation.
A 2010 study, Garcia-Falgueras and Swaab asserted that "the fetal
brain develops during the intrauterine period in the male direction
through a direct action of testosterone on the developing nerve cells,
or in the female direction through the absence of this hormone surge. In
this way, our gender identity (the conviction of belonging to the male
or female gender) and sexual orientation are programmed or organized
into our brain structures when we are still in the womb. There is no
indication that social environment after birth has an effect on gender
identity or sexual orientation."
Ovine model
The domestic ram
is used as an experimental model to study early programming of the
neural mechanisms which underlie homosexuality, developing from the
observation that approximately 8% of domestic rams are sexually
attracted to other rams (male-oriented) when compared to the majority of
rams which are female-oriented. In many species, a prominent feature of
sexual differentiation is the presence of a sexually dimorphic nucleus
(SDN) in the preoptic hypothalamus, which is larger in males than in
females.
Roselli et al. discovered an ovine SDN (oSDN) in the preoptic
hypothalamus that is smaller in male-oriented rams than in
female-oriented rams, but similar in size to the oSDN of females.
Neurons of the oSDN show aromatase
expression which is also smaller in male-oriented rams versus
female-oriented rams, suggesting that sexual orientation is
neurologically hard-wired and may be influenced by hormones. However,
results failed to associate the role of neural aromatase in the sexual
differentiation of brain and behavior in the sheep, due to the lack of
defeminization of adult sexual partner preference or oSDN volume as a
result of aromatase activity in the brain of the fetuses during the
critical period. Having said this, it is more likely that oSDN
morphology and homosexuality may be programmed through an androgen
receptor that does not involve aromatisation.
Most of the data suggests that homosexual rams, like female-oriented
rams, are masculinized and defeminized with respect to mounting,
receptivity, and gonadotrophin secretion, but are not defeminized for
sexual partner preferences, also suggesting that such behaviors may be
programmed differently. Although the exact function of the oSDN is not
fully known, its volume, length, and cell number seem to correlate with
sexual orientation, and a dimorphism in its volume and of cells could
bias the processing cues involved in partner selection. More research is
needed in order to understand the requirements and timing of the
development of the oSDN and how prenatal programming effects the
expression of mate choice in adulthood.
Biological theories of cause of sexual orientation
Early fixation hypothesis
The early fixation hypothesis includes research into prenatal
development and the environmental factors that control masculinization
of the brain. Some studies have seen pre-natal hormone exposures as the
primary factor involved in determining sexual orientation.
This hypothesis is supported by both the observed differences in brain
structure and cognitive processing between homosexual and heterosexual
men. One explanation for these differences is the idea that differential
exposure to hormone levels in the womb during fetal development may
change the masculinization of the brain in homosexual men. The
concentrations of these chemicals is thought to be influenced by fetal
and maternal immune systems, maternal consumption of certain drugs,
maternal stress, and direct injection. This hypothesis is connected to
the well-measured effect of fraternal birth order on sexual orientation.
Exotic becomes erotic
Daryl Bem, a social psychologist at Cornell University,
has theorized that the influence of biological factors on sexual
orientation may be mediated by experiences in childhood. A child's
temperament predisposes the child to prefer certain activities over
others. Because of their temperament, which is influenced by biological
variables such as genetic factors, some children will be attracted to
activities that are commonly enjoyed by other children of the same
gender. Others will prefer activities that are typical of another
gender. This will make a gender-conforming child feel different from
opposite-gender children, while gender-nonconforming children will feel
different from children of their own gender. According to Bem, this
feeling of difference will evoke psychological arousal when the child is
near members of the gender which it considers as being 'different'. Bem
theorizes that this psychological arousal will later be transformed
into sexual arousal: children will become sexually attracted to the
gender which they see as different ("exotic"). This proposal is known as
the "exotic becomes erotic" theory.
Bem sought support from published literature but did not present new data testing his theory. Research cited by him as evidence of the "exotic becomes erotic" theory includes the study Sexual Preference by Bell et al. (1981) and studies showing the frequent finding that a majority of gay men and lesbians report being gender-nonconforming during their childhood
years. A meta-analysis of 48 studies showed childhood gender
nonconformity to be the strongest predictor of a homosexual orientation
for both men and women.
In six "prospective" studies—that is, longitudinal studies that began
with gender-nonconforming boys at about age 7 and followed them up into
adolescence and adulthood— 63% of the gender nonconforming boys had a
homosexual or bisexual orientation as adults.
Sexual orientation and evolution
General
Sexual
practices that significantly reduce the frequency of heterosexual
intercourse also significantly decrease the chances of successful
reproduction, and for this reason, they would appear to be maladaptive in an evolutionary
context following a simple Darwinian model (competition amongst
individuals) of natural selection—on the assumption that homosexuality
would reduce this frequency. Several theories have been advanced to
explain this contradiction, and new experimental evidence has
demonstrated their feasibility.
Some scholars
have suggested that homosexuality is indirectly adaptive, by conferring
a reproductive advantage in a non-obvious way on heterosexual siblings
or their children. By way of analogy, the allele (a particular version of a gene) which causes sickle-cell anemia when two copies are present, also confers resistance to malaria with a lesser form of anemia when one copy is present (this is called heterozygous advantage).
Scholars have also pointed out that Darwin himself described kin selection in The Origin of Species, so under a Darwinian model of evolution, not only individuals, but family groups (bloodlines) can compete for selection.
Brendan Zietsch of the Queensland Institute of Medical Research
proposes the alternative theory that men exhibiting female traits become
more attractive to females and are thus more likely to mate, provided
the genes involved do not drive them to complete rejection of
heterosexuality.
In a 2008 study, its authors stated that "There is considerable
evidence that human sexual orientation is genetically influenced, so it
is not known how homosexuality, which tends to lower reproductive
success, is maintained in the population at a relatively high
frequency." They hypothesized that "while genes predisposing to
homosexuality reduce homosexuals' reproductive success, they may confer
some advantage in heterosexuals who carry them". Their results suggested
that "genes predisposing to homosexuality may confer a mating advantage
in heterosexuals, which could help explain the evolution and
maintenance of homosexuality in the population".
However, in the same study, the authors noted that "nongenetic
alternative explanations cannot be ruled out" as a reason for the
heterosexual in the homosexual-heterosexual twin pair having more
partners, specifically citing "social pressure on the other twin to act
in a more heterosexual way" (and thus seek out a greater number of
sexual partners) as an example of one alternative explanation. Also, the
authors of the study acknowledge that a large number of sexual partners
may not lead to greater reproductive success, specifically noting there
is an "absence of evidence relating the number of sexual partners and
actual reproductive success, either in the present or in our
evolutionary past".
The heterosexual advantage hypothesis was given strong support by the 2004 Italian study demonstrating increased fecundity in the female matrilineal relatives of gay men. As originally pointed out by Hamer,
even a modest increase in reproductive capacity in females carrying a
"gay gene" could easily account for its maintenance at high levels in
the population.
Gay uncle hypothesis
The
"gay uncle hypothesis" posits that people who themselves do not have
children may nonetheless increase the prevalence of their family's genes
in future generations by providing resources (e.g., food, supervision,
defense, shelter) to the offspring of their closest relatives.
This hypothesis is an extension of the theory of kin selection,
which was originally developed to explain apparent altruistic acts
which seemed to be maladaptive. The initial concept was suggested by J. B. S. Haldane in 1932 and later elaborated by many others including John Maynard Smith, W. D. Hamilton and Mary Jane West-Eberhard. This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive.
Vasey and VanderLaan (2010) tested the theory on the Pacific island of Samoa, where they studied women, straight men, and the fa'afafine,
men who prefer other men as sexual partners and are accepted within the
culture as a distinct third gender category. Vasey and VanderLaan found
that the fa'afafine said they were significantly more willing to help
kin, yet much less interested in helping children who aren't family,
providing the first evidence to support the kin selection hypothesis.
The hypothesis is consistent with other studies on homosexuality,
which show that it is more prevalent amongst both siblings and twins.
Since both twins and non-twin siblings share genes and therefore have a
higher factor of genetic redundancy, there is less genetic familial
risk if the strategy is expressed. It is speculated that environmental
and hormonal stress factors (linked to resource feedbacks) may act as
triggers.
Since the hypothesis solves the problem of why homosexuality has
not been selected out over thousands of years, despite it being
antithetical to reproduction, many scientists consider it the best
explanatory model for non-heterosexual behaviour such as homosexuality
and bisexuality. The natural bell curve variation that occurs in biology
and sociology everywhere, explains the variable spectrum of expression.
Vasal and VanderLaan (2011) provides evidence that if an
adaptively designed avuncular male androphilic phenotype exists and its
development is contingent on a particular social environment, then a
collectivistic cultural context is insufficient, in and of itself, for
the expression of such a phenotype.
Biological differences in gay men and lesbian women
Physiological
Some
studies have found correlations between physiology of people and their
sexuality; these studies provide evidence which suggests that:
- Gay men and straight women have, on average, equally proportioned brain hemispheres. Lesbian women and straight men have, on average, slightly larger right brain hemispheres.
- The suprachiasmatic nucleus of the hypothalamus was found by Swaab and Hopffman to be larger in gay men than in non-gay men, the suprachiasmatic nucleus is also known to be larger in men than in women.
- Gay men report, on an average, slightly longer and thicker penises than non-gay men.
- The average size of the INAH 3 in the brains of gay men is approximately the same size as INAH 3 in women, which is significantly smaller, and the cells more densely packed, than in heterosexual men's brains.
- The anterior commissure is larger in women than men and was reported to be larger in gay men than in non-gay men, but a subsequent study found no such difference.
- The functioning of the inner ear and the central auditory system in lesbians and bisexual women are more like the functional properties found in men than in non-gay women (the researchers argued this finding was consistent with the prenatal hormonal theory of sexual orientation).
- The startle response (eyeblink following a loud sound) is similarly masculinized in lesbians and bisexual women.
- Gay and non-gay people's brains respond differently to two putative sex pheromones (AND, found in male armpit secretions, and EST, found in female urine).
- The amygdala, a region of the brain, is more active in gay men than non-gay men when exposed to sexually arousing material.
- Finger length ratios between the index and ring fingers have been reported to differ, on average, between non-gay and lesbian women.
- Gay men and lesbians are significantly more likely to be left-handed or ambidextrous than non-gay men and women; Simon LeVay argues that because "[h]and preference is observable before birth... [t]he observation of increased non-right-handness in gay people is therefore consistent with the idea that sexual orientation is influenced by prenatal processes," perhaps heredity.
- A study of over 50 gay men found that about 23% had counterclockwise hair whorl, as opposed to 8% in the general population. This may correlate with left-handedness.
- Gay men have increased ridge density in the fingerprints on their left thumbs and little fingers.
- Length of limbs and hands of gay men is smaller compared to height than the general population, but only among white men.
Political aspects
Whether genetic or other physiological determinants form the basis of sexual orientation is a highly politicized issue. The Advocate,
a U.S. gay and lesbian newsmagazine, reported in 1996 that 61% of its
readers believed that "it would mostly help gay and lesbian rights if
homosexuality were found to be biologically determined". A cross-national study in the United States, the Philippines, and Sweden
found that those who believed that "homosexuals are born that way" held
significantly more positive attitudes toward homosexuality than those
who believed that "homosexuals choose to be that way" or "learn to be
that way".
Equal protection
analysis in U.S. law determines when government requirements create a
“suspect classification" of groups and therefore eligible for heightened
scrutiny based on several factors, one of which is immutability.
Evidence that sexual orientation is biologically determined (and
therefore perhaps immutable in the legal sense) would strengthen the
legal case for heightened scrutiny of laws discriminating on that basis.
The perceived causes of sexual orientation have a significant
bearing on the status of sexual minorities in the eyes of social
conservatives. The Family Research Council, a conservative Christian think tank in Washington, D.C., argues in the book Getting It Straight
that finding people are born gay "would advance the idea that sexual
orientation is an innate characteristic, like race; that homosexuals,
like African-Americans, should be legally protected against
'discrimination;' and that disapproval of homosexuality should be as
socially stigmatized as racism. However, it is not true." On the other
hand, some social conservatives
such as Reverend Robert Schenck have argued that people can accept any
scientific evidence while still morally opposing homosexuality. National Organization for Marriage board member and fiction writer Orson Scott Card
has supported biological research on homosexuality, writing that "our
scientific efforts in regard to homosexuality should be to identify
genetic and uterine causes... so that the incidence of this dysfunction
can be minimized.... [However, this should not be seen] as an attack on
homosexuals, a desire to 'commit genocide' against the homosexual
community.... There is no 'cure' for homosexuality because it is not a
disease. There are, however, different ways of living with homosexual
desires."
Some advocates for the rights of sexual minorities resist linking
that cause with the concept that sexuality is biologically determined
or fixed at birth. They argue that sexual orientation can shift over the
course of a person's life.
At the same time, others resist any attempts to pathologise or
medicalise 'deviant' sexuality, and choose to fight for acceptance in a
moral or social realm. Chandler Burr
has stated that "[s]ome, recalling earlier psychiatric "treatments" for
homosexuality, discern in the biological quest the seeds of genocide.
They conjure up the specter of the surgical or chemical "rewiring" of
gay people, or of abortions of fetal homosexuals who have been hunted
down in the womb."
LeVay has said in response to letters from gays and lesbians making
such criticisms that the research "has contributed to the status of gay
people in society".
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