Biology and sexual orientation
Encyclopedia
Biology and sexual orientation is the subject of research into the role of biology in the development of human sexual orientation
. No simple, single cause for sexual orientation has been conclusively demonstrated, but research suggests that it is by a combination of genetic, hormonal, and environmental influences, with biological factors involving a complex interplay of genetic factors and the early uterine environment.
Biological factors which may be related to the development of a heterosexual, homosexual, bisexual or asexual orientation include gene
s, prenatal hormones
, and brain
structure.
have attempted to isolate biological factors in sexual orientation. As Bearman and Bruckner (2002) describe it, early studies concentrated on small, select samples, which showed very high genetic influences; however, they were also criticized for non-representative selection of their subjects. Later studies, performed on increasingly representative samples, showed much lesser concordance among monozygotic (MZ, colloquially "identical") twins, although still significantly larger than among dizygotic (DZ) twins.
For example, a meta-study by Hershberger (2001) compares the results of eight different twin studies: among those, all but two showed MZ twins having much higher concordance of sexual orientation than DZ twins, suggesting a non-negligible genetic component. Two additional examples: Bailey and Pillard (1991) in a study of gay twins found that 52% of monozygotic (MZ) brothers and 22% of the dizygotic (DZ) twins were concordant for homosexuality. Also, Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins. Self reported zygosity
, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically
checked when in doubt. MZ twin concordance for homosexuality was found to be 30%.
A recent 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 societal 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 twin may influence the results (see below).
Another issue is the recent 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 and receive maternal blood from separate placenta. 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". If one twin receives less testosterone and the other more, this could result in different levels of brain masculinisation.
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 as 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 to 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. Although 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, a rigorous replication of the maternal loading was reported on samples in Italy in England. 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. A recent study of 894 heterosexual and 694 homosexual men found no evidence of sex linkage.
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. With the larger sample set and complete genome scan, the study found somewhat reduced linkage for Xq28 than reported by Hamer et al. However, they did find other markers with a likelihood score falling just short of significance at 7q36 and likelihood scores approaching significance at 8p12 and 10q26. Interestingly, 10q26 showed highly significant maternal loading, thus further supporting the previous family studies.
. The research showed that when both heterosexual women (lesbians were included in the study, but the results regarding them were "somewhat confused") 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.
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, Swaab and 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
.
Early work of this type was also done by Simon LeVay
. LeVay studied four groups of neuron
s in the hypothalamus
, called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the brain to study, because of evidence that this part of the brain 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 as follows: 19 gay men who had died of AIDS
, 16 presumed heterosexual men (6 of whom had died of AIDS), and 6 presumed heterosexual women (1 of whom had died of AIDS). The AIDS patients in the heterosexual groups were all identified from medical records as intravenous drug abusers or recipients of blood transfusions, though only 2 of the men in this category had specifically denied homosexual activity. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been mostly or all 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 6 AIDS patients were included in the heterosexual group. The size of the INAH3 in the homosexual male brains was similar to that in the heterosexual female brains.
William Byne and colleagues attempted to replicate the differences reported in INAH 1–4 size using a different sample of brains from 14 HIV-positive homosexual males, 34 presumed heterosexual males (10 HIV-positive), and 34 presumed heterosexual females (9 HIV-positive). They found a significant difference in INAH3 size between heterosexual men and 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 stated: "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."
A sort of reverse sexual imprinting has been observed in heterosexual humans; see the section on the "Westermarck effect" in Behavioral imprinting.
Several different triggers for imprinting upon a particular sexual orientation have been proposed, but there is no empirical evidence to support any of them.
, 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 physiological arousal when the child is near members of the gender which it considers as being 'different'. Bem theorizes that this physiological arousal will later be transformed into sexual arousal: children will become sexually attracted to the gender which they see as different ("exotic"). This theory is known as Exotic Becomes Erotic theory.
The theory is based in part on 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. Fourteen studies published since Bailey & Zucker's 1995 also show the same results. In one study by the Kinsey Institute of approximately 1000 gay men and lesbians (and a control group of 500 heterosexual men and women), 63% of both gay men and lesbians reported that they were gender nonconforming in childhood (i.e., did not like activities typical of their sex), compared with only 10–15% of heterosexual men and women. There are also six "prospective" studies—that is longitudinal studies that begin with gender-nonconforming boys at about age 7 and follow them up into adolescence and adulthood. These also show that a majority (63%) of the gender nonconforming boys become gay or bisexual as adults. There are very few prospective studies of gender nonconforming girls. In a group of eighteen behaviorally masculine girls (mean age of assessment: 9 years), all reported a homosexual sexual orientation at adolescence, and eight had requested sex reassignment.
William Reiner, a psychiatrist and urologist with the University of Oklahoma
has evaluated more than a hundred cases of children born with sexual differentiation disorders. In the 1960s and 70s, it was common in developed countries for doctors to castrate
boys born with a micropenis
and have them raised as girls. However, this practice has come under attack, because even though these boys were raised as girls, they nearly all report as adults that they are sexually attracted to women. This suggests that their sexual orientation was determined at birth.
in an evolution
ary context following a simple Darwinian model 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 adaptive in a non-obvious way. By way of analogy, the allele
(a particular version of a gene) which causes sickle-cell anemia
when two copies are present may also confer resistance to malaria
with a lesser form of anemia
when one copy is present (this is called heterozygous advantage).
The so-called "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 (food, supervision, defense, shelter, etc.) to the offspring of their closest relatives. This hypothesis is an extension of the theory of kin selection. Kin selection
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.
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." and 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."
Important new evidence on a plausible mechanism for the evolution of "gay genes" has emerged from the work of Camperio-Ciani. They found in two large, independent studies that the female relatives of homosexual men tended to have significantly 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. This indicates that females carrying a putative "gay genes" complex are more fecund than women lacking this complex of genes, and thereby can compensate for any decreased fertility of the males carrying the genes. This is a well known phenomenon in evolution known as "sexual antagonism," and has been widely documented for many traits that are advantageous in one sex but not in the other. This provides solid experimental evidence of how "gay genes" could not only survive but thrive over the course of evolution.
, 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 what groups are considered suspect classes
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.", but did not give any further explanations. On the other hand, some social conservatives such as Reverend Robert Schenck have argued that people can accept the "inevitable... scientific evidence" while still morally opposing homosexuality. As well, 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
writing for The Atlantic Monthly
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." Simon 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."
Sexual orientation
Sexual orientation describes a pattern of emotional, romantic, or sexual attractions to the opposite sex, the same sex, both, or neither, and the genders that accompany them. By the convention of organized researchers, these attractions are subsumed under heterosexuality, homosexuality,...
. No simple, single cause for sexual orientation has been conclusively demonstrated, but research suggests that it is by a combination of genetic, hormonal, and environmental influences, with biological factors involving a complex interplay of genetic factors and the early uterine environment.
Biological factors which may be related to the development of a heterosexual, homosexual, bisexual or asexual orientation include gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s, prenatal hormones
Prenatal hormones and sexual orientation
The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult...
, and brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...
structure.
Twin studies
A number of twin studiesTwin study
Twin studies help disentangle the relative importance of environmental and genetic influences on individual traits and behaviors. Twin research is considered a key tool in behavioral genetics and related fields...
have attempted to isolate biological factors in sexual orientation. As Bearman and Bruckner (2002) describe it, early studies concentrated on small, select samples, which showed very high genetic influences; however, they were also criticized for non-representative selection of their subjects. Later studies, performed on increasingly representative samples, showed much lesser concordance among monozygotic (MZ, colloquially "identical") twins, although still significantly larger than among dizygotic (DZ) twins.
For example, a meta-study by Hershberger (2001) compares the results of eight different twin studies: among those, all but two showed MZ twins having much higher concordance of sexual orientation than DZ twins, suggesting a non-negligible genetic component. Two additional examples: Bailey and Pillard (1991) in a study of gay twins found that 52% of monozygotic (MZ) brothers and 22% of the dizygotic (DZ) twins were concordant for homosexuality. Also, Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins. Self reported zygosity
Zygosity
Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...
, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically
Serology
Serology is the scientific study of blood serum and other bodily fluids. In practice, the term usually refers to the diagnostic identification of antibodies in the serum...
checked when in doubt. MZ twin concordance for homosexuality was found to be 30%.
A recent 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 societal 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 twin may influence the results (see below).
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 purely caused by genetics.Another issue is the recent 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 and receive maternal blood from separate placenta. 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". If one twin receives less testosterone and the other more, this could result in different levels of brain masculinisation.
Chromosome linkage studies
Chromosome linkage studies of sexual orientation have indicated the presence of multiple contributing genetic factors throughout the genome. In 1993, Dean HamerDean Hamer
Dr. Dean Hamer is an American geneticist, author, and filmmaker. He is known for his contributions to biotechnology and AIDS prevention, his research on the genetics of human behavior including sexual orientation and spirituality, and his popular books and documentaries on a wide range of...
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
Xq28
Xq28 is a genetic marker on the X chromosome found by Dean Hamer and others in 1993. Hamer's study led to his belief that they found a link between the Xq28 marker and male homosexuality, but the original study's results have been disputed.-1993 study:...
, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly dubbed as 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 to 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. Although 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, a rigorous replication of the maternal loading was reported on samples in Italy in England. 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. A recent study of 894 heterosexual and 694 homosexual men found no evidence of sex linkage.
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. With the larger sample set and complete genome scan, the study found somewhat reduced linkage for Xq28 than reported by Hamer et al. However, they did find other markers with a likelihood score falling just short of significance at 7q36 and likelihood scores approaching significance at 8p12 and 10q26. Interestingly, 10q26 showed highly significant maternal loading, thus further supporting the previous family studies.
Epigenetics studies
A recent 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. Thirteen percent of mothers with one gay son, and 23% of mothers with two gay sons showed extreme skewing, compared to 4% percent of mothers without gay sons.Birth order
Blanchard and Klassen (1997) reported that each 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. Male fetuses produce HY antigens which are "almost certainly involved in the sexual differentiation of vertebrates." 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 masculinisation. However the theory has been criticized because symptoms which would be typical of such effects are rare compared with prevalence of homosexualityFemale 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
Recent research conducted in Sweden has suggested that gay and straight men respond differently to two odors that are believed to be involved in sexual arousalSexual arousal
Sexual arousal, or sexual excitement, is the arousal of sexual desire, during or in anticipation of sexual activity. Things that precipitate human sexual arousal are called erotic stimuli, or colloquially known as turn-ons. There are many potential stimuli, both physical or mental, which can cause...
. The research showed that when both heterosexual women (lesbians were included in the study, but the results regarding them were "somewhat confused") 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 brainHuman brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...
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, Swaab and Hofman reported a difference in the size of the suprachiasmatic nucleus
Suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...
between homosexual and heterosexual men. In 1992, Allen and Gorski reported a difference related to sexual orientation in the size of the anterior commissure
Anterior commissure
The anterior commissure is a bundle of nerve fibers , connecting the two cerebral hemispheres across the midline, and placed in front of the columns of the fornix...
.
Early work of this type was also done by Simon LeVay
Simon LeVay
Simon LeVay is a British-American neuroscientist. He is known for his studies about brain structures and sexual orientation.-Personal life:LeVay was born on August 28, 1943 in Oxford, England...
. LeVay studied four groups of neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s in the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the brain to study, because of evidence that this part of the brain 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 as follows: 19 gay men who had died of AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...
, 16 presumed heterosexual men (6 of whom had died of AIDS), and 6 presumed heterosexual women (1 of whom had died of AIDS). The AIDS patients in the heterosexual groups were all identified from medical records as intravenous drug abusers or recipients of blood transfusions, though only 2 of the men in this category had specifically denied homosexual activity. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been mostly or all 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 6 AIDS patients were included in the heterosexual group. The size of the INAH3 in the homosexual male brains was similar to that in the heterosexual female brains.
William Byne and colleagues attempted to replicate the differences reported in INAH 1–4 size using a different sample of brains from 14 HIV-positive homosexual males, 34 presumed heterosexual males (10 HIV-positive), and 34 presumed heterosexual females (9 HIV-positive). They found a significant difference in INAH3 size between heterosexual men and 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 stated: "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."
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 also connected to the fraternal birth order research.Imprinting/critical period
This type of theory holds that the formation of gender identity occurs in the first few years of life after birth. It argues that individuals can be predisposed to homosexual orientation by biological factors but are triggered in some cases by upbringing. Part of adopting a gender identity involves establishing the gender(s) of sexual attraction. This process is analogous to the "imprinting" process observed in animals. A baby duckling may be genetically programmed to "imprint" on a mother, but what entity it actually imprints upon depends on what objects it sees immediately after hatching. Most importantly, once this process has occurred, it cannot be reversed, any more than the duckling can hatch twice.A sort of reverse sexual imprinting has been observed in heterosexual humans; see the section on the "Westermarck effect" in Behavioral imprinting.
Several different triggers for imprinting upon a particular sexual orientation have been proposed, but there is no empirical evidence to support any of them.
Exotic becomes erotic
Daryl BemDaryl Bem
Daryl J. Bem is a social psychologist and professor emeritus at Cornell University. He is the originator of the self-perception theory of attitude change, and has carried out research on psi phenomena , group decision making, handwriting analysis, sexual orientation and personality theory and...
, a social psychologist at Cornell University
Cornell University
Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...
, 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 physiological arousal when the child is near members of the gender which it considers as being 'different'. Bem theorizes that this physiological arousal will later be transformed into sexual arousal: children will become sexually attracted to the gender which they see as different ("exotic"). This theory is known as Exotic Becomes Erotic theory.
The theory is based in part on 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
Childhood Gender Nonconformity
Childhood gender nonconformity is a phenomenon in which pre-pubescent children do not conform to expected gender-related sociological or psychological patterns, and/or identify with the opposite gender...
to be the strongest predictor of a homosexual orientation for both men and women. Fourteen studies published since Bailey & Zucker's 1995 also show the same results. In one study by the Kinsey Institute of approximately 1000 gay men and lesbians (and a control group of 500 heterosexual men and women), 63% of both gay men and lesbians reported that they were gender nonconforming in childhood (i.e., did not like activities typical of their sex), compared with only 10–15% of heterosexual men and women. There are also six "prospective" studies—that is longitudinal studies that begin with gender-nonconforming boys at about age 7 and follow them up into adolescence and adulthood. These also show that a majority (63%) of the gender nonconforming boys become gay or bisexual as adults. There are very few prospective studies of gender nonconforming girls. In a group of eighteen behaviorally masculine girls (mean age of assessment: 9 years), all reported a homosexual sexual orientation at adolescence, and eight had requested sex reassignment.
William Reiner, a psychiatrist and urologist with the University of Oklahoma
University of Oklahoma
The University of Oklahoma is a coeducational public research university located in Norman, Oklahoma. Founded in 1890, it existed in Oklahoma Territory near Indian Territory for 17 years before the two became the state of Oklahoma. the university had 29,931 students enrolled, most located at its...
has evaluated more than a hundred cases of children born with sexual differentiation disorders. In the 1960s and 70s, it was common in developed countries for doctors to castrate
Castration
Castration is any action, surgical, chemical, or otherwise, by which a male loses the functions of the testicles or a female loses the functions of the ovaries.-Humans:...
boys born with a micropenis
Micropenis
Micropenis is an unusually small penis. A common criterion is a dorsal erect penile length of at least 2.5 standard deviations smaller than the mean human penis size. The condition is usually recognized shortly after birth...
and have them raised as girls. However, this practice has come under attack, because even though these boys were raised as girls, they nearly all report as adults that they are sexually attracted to women. This suggests that their sexual orientation was determined at birth.
Sexual orientation and evolution
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 maladaptiveAdaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
in an evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary context following a simple Darwinian model 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 adaptive in a non-obvious way. By way of analogy, the allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
(a particular version of a gene) which causes sickle-cell anemia
Sickle-cell disease
Sickle-cell disease , or sickle-cell anaemia or drepanocytosis, is an autosomal recessive genetic blood disorder with overdominance, characterized by red blood cells that assume an abnormal, rigid, sickle shape. Sickling decreases the cells' flexibility and results in a risk of various...
when two copies are present may also confer resistance to malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
with a lesser form of anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...
when one copy is present (this is called heterozygous advantage).
The so-called "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 (food, supervision, defense, shelter, etc.) to the offspring of their closest relatives. This hypothesis is an extension of the theory of kin selection. Kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...
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
John Maynard Smith
John Maynard Smith,His surname was Maynard Smith, not Smith, nor was it hyphenated. F.R.S. was a British theoretical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics under the well-known biologist J.B.S....
, W. D. Hamilton
W. D. Hamilton
William Donald Hamilton FRS was a British evolutionary biologist, widely recognised as one of the greatest evolutionary theorists of the 20th century....
and Mary Jane West-Eberhard
Mary Jane West-Eberhard
Mary Jane West-Eberhard is an American theoretical biologist noted for arguing that phenotypic and developmental plasticity played a key role in shaping animal evolution and speciation...
. This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive.
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." and 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."
Important new evidence on a plausible mechanism for the evolution of "gay genes" has emerged from the work of Camperio-Ciani. They found in two large, independent studies that the female relatives of homosexual men tended to have significantly 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. This indicates that females carrying a putative "gay genes" complex are more fecund than women lacking this complex of genes, and thereby can compensate for any decreased fertility of the males carrying the genes. This is a well known phenomenon in evolution known as "sexual antagonism," and has been widely documented for many traits that are advantageous in one sex but not in the other. This provides solid experimental evidence of how "gay genes" could not only survive but thrive over the course of evolution.
Physiological
Some studies have found correlations between physiology of people and their sexuality. These studies provide evidence which they claim suggests that:- Gay men report, on an average, slightly longer and thicker penises than non-gay men.
- 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 VIP SCN nucleus of the hypothalamus is larger in men than in women, and larger in gay men than in heterosexual men.
- The average size of the INAH-3INAH 3The INAH 3 is the third interstitial nucleus of the anterior hypothalamus. Part of the sexually dimorphic nucleus, the INAH 3 is significantly larger in males than in females irrespective of age...
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 commissureAnterior commissureThe anterior commissure is a bundle of nerve fibers , connecting the two cerebral hemispheres across the midline, and placed in front of the columns of the fornix...
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. - Gay men's brains respond differently to fluoxetineFluoxetineFluoxetine is an antidepressant of the selective serotonin reuptake inhibitor class. It is manufactured and marketed by Eli Lilly and Company...
, a selective serotonin reuptake inhibitorSelective serotonin reuptake inhibitorSelective serotonin re-uptake inhibitors or serotonin-specific reuptake inhibitor are a class of compounds typically used as antidepressants in the treatment of depression, anxiety disorders, and some personality disorders. The efficacy of SSRIs is disputed...
. - 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 orientationPrenatal hormones and sexual orientationThe hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult...
). - The suprachiasmatic nucleusSuprachiasmatic nucleusThe suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...
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. - 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).
- One region of the brain (amygdalaAmygdalaThe ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
) is more active in gay men than non-gay men when exposed to sexually arousing material. - Finger length ratiosDigit ratioThe digit ratio is the ratio of the lengths of different digits or fingers typically measured from the bottom crease where the finger joins the hand to the tip of the finger. It has been suggested by some scientists that the ratio of two digits in particular, the 2nd and 4th , is affected by...
between the index and ring fingers may be different between non-gay and lesbian women. - Gay men and lesbians are significantly more likely to be left-handed or ambidextrousHandedness and sexual orientationA relationship between handedness and sexual orientation has been suggested but not verified by a number of researchers, who report that heterosexual individuals are somewhat more likely to be right-handed than homosexual individuals...
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 50 gay men found 23% had counterclockwise hair whirl, 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 pinkies.
- Length of limbs and hands of gay men is smaller compared to height than the general population, but only among white men.
Cognitive
Recent studies suggest the presence of subtle differences in the way gay people and non-gay people process certain kinds of information. Researchers have found that:- Gay men and lesbians are more verbally fluentFluencyFluency is the property of a person or of a system that delivers information quickly and with expertise.-Speech:...
than heterosexuals of the same sex (but two studies did not find this result). - Gay men may receive higher scores than non-gay men on tests of object location memory (no difference was found between lesbians and non-gay women).
Political aspects
Whether genetic or other physiological determinants form the basis of sexual orientation is a highly politicized issue. The AdvocateThe Advocate
The Advocate is an American LGBT-interest magazine, printed monthly and available by subscription. The Advocate brand also includes a web site. Both magazine and web site have an editorial focus on news, politics, opinion, and arts and entertainment of interest to LGBT people...
, 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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, and Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
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
Equal Protection Clause
The Equal Protection Clause, part of the Fourteenth Amendment to the United States Constitution, provides that "no state shall ... deny to any person within its jurisdiction the equal protection of the laws"...
analysis in U.S. law determines what groups are considered suspect classes
Suspect classification
In American jurisprudence, a suspect classification is any classification of groups meeting a series of criteria suggesting they are likely the subject of discrimination...
and therefore eligible for heightened scrutiny
Strict scrutiny
Strict scrutiny is the most stringent standard of judicial review used by United States courts. It is part of the hierarchy of standards that courts use to weigh the government's interest against a constitutional right or principle. The lesser standards are rational basis review and exacting or...
based on several factors, one of which is immutability
Immutable characteristic
An immutable characteristic is any sort of physical attribute which is perceived as being unchangeable, entrenched and innate. The term is often used to describe segments of the population which share such attributes and are contrasted from others by those attributes, and is used in human rights...
. 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
Family Research Council
The Family Research Council is a conservative or right-wing Christian group and lobbying organization formed in the United States in 1981 by James Dobson. It was fully incorporated in 1983...
, a conservative Christian think tank
Think tank
A think tank is an organization that conducts research and engages in advocacy in areas such as social policy, political strategy, economics, military, and technology issues. Most think tanks are non-profit organizations, which some countries such as the United States and Canada provide with tax...
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.", but did not give any further explanations. On the other hand, some social conservatives such as Reverend Robert Schenck have argued that people can accept the "inevitable... scientific evidence" while still morally opposing homosexuality. As well, National Organization for Marriage
National Organization for Marriage
The National Organization for Marriage is a nonprofit political association established in 2007 to work against legalization of same-sex marriage in the United States, specifically to pass California Proposition 8 banning same-sex marriage in California...
board member and fiction writer Orson Scott Card
Orson Scott Card
Orson Scott Card is an American author, critic, public speaker, essayist, columnist, and political activist. He writes in several genres, but is primarily known for his science fiction. His novel Ender's Game and its sequel Speaker for the Dead both won Hugo and Nebula Awards, making Card the...
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
Chandler Burr
Chandler Burr is an American journalist, author, and museum curator. Since December 2010 he has been curator of olfactory art at the Museum of Art and Design in New York City....
writing for The Atlantic Monthly
The Atlantic Monthly
The Atlantic is an American magazine founded in Boston, Massachusetts, in 1857. It was created as a literary and cultural commentary magazine. It quickly achieved a national reputation, which it held for more than a century. It was important for recognizing and publishing new writers and poets,...
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." Simon LeVay
Simon LeVay
Simon LeVay is a British-American neuroscientist. He is known for his studies about brain structures and sexual orientation.-Personal life:LeVay was born on August 28, 1943 in Oxford, England...
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."
See also
- Against Nature?Against Nature?Against Nature? is an exhibition on homosexuality in animals made by the Natural History Museum, University of Oslo, Norway. The exhibition focuses on occurrence and function of homosexuality in animals, and is the first of its kind....
- Alfred KinseyAlfred KinseyAlfred Charles Kinsey was an American biologist and professor of entomology and zoology, who in 1947 founded the Institute for Sex Research at Indiana University, now known as the Kinsey Institute for Research in Sex, Gender, and Reproduction, as well as producing the Kinsey Reports and the Kinsey...
- Arnold AletrinoArnold AletrinoArnold Aletrino was a Dutch criminal anthropologist and writer, who published works on homosexuality in Dutch and French...
- Norms of reactionNorms of reactionIn ecology and genetics, a norm of reaction describes the pattern of phenotypic expression of a single genotype across a range of environments. One use of norms of reaction is in describing how different species—especially related species—respond to varying environments...
- Xq28Xq28Xq28 is a genetic marker on the X chromosome found by Dean Hamer and others in 1993. Hamer's study led to his belief that they found a link between the Xq28 marker and male homosexuality, but the original study's results have been disputed.-1993 study:...
- Environment and sexual orientationEnvironment and sexual orientationEnvironment and sexual orientation is research into possible environmental influences on the development of human sexual orientation. Some researchers distinguish environmental influences from hormonal influences while others include biological influences such as prenatal hormones as part of...
- Gay bombGay bombThe "halitosis bomb" and "gay bomb" are informal names for two theoretical non-lethal chemical weapons, which a United States Air Force research laboratory speculated about producing, which involved discharging female sex pheromones over enemy forces in order to make them sexually attracted to each...
- Neuroscience and sexual orientationNeuroscience and sexual orientationSexual orientation refers to an individual’s personal and social identity involving behaviors, ideas, and/or emotions concerning sexuality. The ultimate causes and mechanisms of sexual orientation development in humans remain unclear and many theories are speculative and controversial. However,...
- Mental roots of sexual orientationMental roots of sexual orientationA theory about the development of sexual orientation and sexual arousal suggests that these behavior patterns are learned unconsciously from asexual experiences during childhood. The learning begins when the child distinguishes between men and women by the pitch of their voice...
- Homosexual behavior in animals
External links
- Molecular Genetic Study of Sexual Orientation A genetic study of gay brothers at Northwestern University.