Articles From Wikipedia

Contents Articles from Wikipedia Prenatal hormones and sexual orientation (2-3) Fraternal birth order and sexual orientation (3-4) Handedness and sexual orientation (4-5) Biology and sexual orientation (5-14) Environment and sexual orientation (14-16)

Simon le Vay’s summary of the studies Non Biological theories (17-18) Biological theories (18-35)

http://en.wikipedia.org/wiki/Prenatal_hormones_and_sexual_orientation

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. Fetal hormones may be seen as the primary determiner of adult sexual orientation, or a co-factor with genes and/or environmental and social conditions. Male homosexuality as hyper-masculine There is evidence of a correlation between sexual orientation and traits that are determined in utero. Williams et al. (2000) found that finger length ratio, a characteristic controlled by prenatal hormones, is different in people of distinct sexual orientations. Another study by McFadden in 1998 found that auditory systems in the brain, another physical trait influenced by prenatal hormones is different in those of differing orientations, likewise the suprachiasmatic nucleus of homosexual men was found by Swaab and Hopffman to be larger in homosexual men than in heterosexual men, the suprachiasmatic nucleus is also known to be larger in men than in women. Gay men have also been shown to have higher levels of circulating androgens and larger penises, on average, than straight men. Gay men have more older brothers on average, a phenomenon known as the fraternal birth order effect. It has been suggested that the greater the number of older male siblings the higher the level of androgen fetuses are exposed to. Male homosexuality as feminine or hypo-masculine In a 1991 study, Simon LeVay demonstrated that a tiny clump of neurons of the anterior hypothalamus—which is believed to control sexual behavior and linked to prenatal hormones—was on average more than twice the size in heterosexual men when contrasted to homosexual men. Initially he could not rule out that this may be due to AIDS since all of his homosexual male subjects had died from it before the autopsies were performed. However in 2003 scientists at Oregon State University announced that it replicated his findings in sheep. Female homosexuality Girls with congenital adrenal hyperplasia (an autosomal recessive condition which results in high androgen levels during fetal development) have more masculinized sex role identities and are more likely to have a homosexual sexual orientation as adults than controls (Dittmann et al. 1990ab, 1992; Zucker et al., 1996; Hines et al., 2004).

An alternative explanation for this effect is that the fact that girls with this condition are born with masculinized external genitalia leads their parents to raise them in a more masculine manner which then influences their sexual orientation as adults. However, the degree to which the girls' genitals are masculinized does not correlate with their sexual orientation, suggesting that prenatal hormones are the causal factor, not parental influence. http://en.wikipedia.org/wiki/Fraternal_birth_order_and_sexual_orientatio n

Fraternal birth order and sexual orientation A correlation between fraternal birth order and sexual orientation has been suggested by recent research. Ray Blanchard identified the association and referred to it as the fraternal birth order effect. The observation is that the more older brothers a man has, the greater the probability is that he will have a homosexual orientation. It has sometimes been called the older brother effect. Empirical basis The fraternal birth order effect is the strongest known predictor of sexual orientation. According to several studies, each older brother increases a man's odds of developing a homosexual sexual orientation by 28%–48%. The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men. There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters. The fraternal birth order effect has also been observed among male-tofemale transsexuals: MtF transsexuals who are sexually interested in men have a greater number of older brothers than MtF transsexuals who are sexually interested in women. This has been reported in samples from Canada, the United Kingdom, the Netherlands, and Polynesia. Criticism Peter Bearman (2002) questions the sampling method of Blanchard and other scientists who report a link between fraternal birth order and sexual orientation. He says that the studies work with nonrepresentative samples, and/or indirect reports on siblings’ sexual orientation. After repeating the experiment done by Blanchard he found "no association between same-sex attraction and number of older siblings, older brothers, or older sisters". Theories of what causes the effect

Anthony Bogaert's work involving adoptees concludes that the effect is not due to being raised with older brothers, but is hypothesized to have something to do with changes induced in the mother's body when gestating a boy that affects subsequent sons. The effect is present regardless of whether or not the older brothers are raised in the same family environment with the boy. There is no effect when the number of older brothers is increased by adopted brothers or step brothers. An in-utero maternal immune response has been hypothesized for this effect. The fraternal birth order effect appears to have the opposite effect in right-handed individuals than non-right-handed individuals; however, the incidence of homosexuality correlated with an increase in older brothers is seen only in right-handed males. Bogaert (2006) replicated the fraternal birth order effect on male sexual orientation, in a sample including both biological siblings and adopted siblings. Only the older biological brothers influenced sexual orientation; there was no effect of adopted siblings. Bogaert concluded that his finding strongly suggest a prenatal origin to the fraternal birthorder effect. McConaghy (2006) published in a sociological journal that he found no relationship between the strength of the effect and degree of homosexual feelings, leading him to conclude that the influence of birth order on degree of homosexual feelings was not due to a biological, but a social process. http://en.wikipedia.org/wiki/Handedness_and_sexual_orientation

Handedness and sexual orientation A relationship between handedness and sexual orientation has been suggested but not verified by a number of researchers, who report that homosexual individuals are somewhat more likely to be non-righthanded than heterosexual individuals. The relationship between handedness and sexual orientation appears to exist within both sexes and may reflect the biological etiology of homosexuality. Lalumière et al., 2000 study Lalumière and colleagues conducted a meta-analysis using previously acquired data on handedness and sexual orientation, as earlier studies had rendered conflicting results. The meta-analysis included 20 studies involving 6,987 homosexual (6,182 men and 805 women) and 16,423 heterosexual (14,808 men and 1,615 women) participants. Using 20 comparisons between heterosexual and homosexual men and 9 comparisons between heterosexual and homosexual women, it was found that homosexuals were 39% more likely to be non-right-handed

than heterosexuals, with figures of 34% and 91% for homosexual men and homosexual women, respectively. Mustanski et al., 2002a study Mustanski et al. examined sexual orientation and hand preference in a sample of 382 men (205 heterosexual; 177 homosexual) and 354 women (149 heterosexual; 205 homosexual). Although homosexual women were found to be significantly more non-right-handed than heterosexual women (18% vs 10%), no significant differences were found between heterosexual and homosexual men with respect to hand preference.

Lippa, 2003 study Lippa examined sexual orientation and handedness in a sample of 812 men (351 heterosexual; 461 homosexual) and 1189 women (707 heterosexual; 472 homosexual). Homosexual men were 82% more likely to be non-right-handed than heterosexual men, but no significant differences were found between heterosexual and homosexual women in terms of handedness. When combining men and women into one large sample, homosexual individuals were 50% more likely to be nonright-handed than heterosexual individuals.

Blanchard et al., 2006 study Blanchard et al. argued that fraternal birth order effect (the probability that a boy will be homosexual increases with the number of older brothers who have the same biological mother) appears to be limited to right handed males. Moreover, the same study indicates that nonright handed males without older brothers are more likely to be homosexual than non-right handed men who have older brothers. As Blanchard et al. said in their report, "the odds of homosexuality is higher for men who have a non-right hand preference or who have older brothers, relative to men with neither of these features, but the odds for men with both features are similar to the odds for men with neither." http://en.wikipedia.org/wiki/Biology_and_sexual_orientation

Biology and sexual orientation Biology and sexual orientation is research into possible biological influences on the development of human sexual orientation. No simple cause for sexual orientation has been conclusively demonstrated, and there is no scientific consensus as to whether the contributing factors are primarily biological or environmental. Many think both play complex roles. The American Academy of Pediatrics and the American Psychological Association have both stated that sexual orientation

probably has multiple causes. Research has identified several biological factors which may be related to the development of a heterosexual, homosexual or bisexual orientation. These include genes, prenatal hormones, and brain structure. Conclusive proof of a biological cause of sexual orientation would have significant political and cultural implications.

Twin studies Researchers have traditionally used twin studies to try to isolate genetic influences from environmental or other influences. One common type of twin study compares identical twins (known as monozygotic or "MZ twins") who both have a particular trait to nonidentical or fraternal twins (known as dizygotic or "DZ twins") with that same trait. Since identical twins have the same genetic makeup (genotype) while non-identical twins share only 50% of their genes, a difference between these types of twins provides evidence of a genetic component. For example, if a high percentage of identical twins both have red hair (while a low percentage of non-identical twins both have red hair), that suggests that red hair has a genetic basis. On the other hand, if identical twins share a characteristic just as often as fraternal twins (such as love of music), that suggests that there is not a genetic basis for that trait. A number of twin studies have attempted this kind of isolation. As Bearman and Bruckner (2002) describe it, early studies concentrated on small, select samples, which showed very high genetic influences; however, they were also easily criticized for non-representative selection of their subjects. Later studies, performed on increasingly representative samples, showed much lesser concordance among MZ twins, although still significantly larger than among DZ twins. For example, a recent 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%. As a counter-example, Bearman and Bruckner (2002), analyzed data

from a large longitudinal study of adolescents. They found the data did not support genetic influence: Among [identical] twins, 6.7% are concordant [that is, both express same-sex romantic attraction]. [Fraternal] twin pairs are 7.2% concordant. Full-siblings are 5.5% concordant. Clearly, the observed concordance rates do not correspond to degrees of genetic similarity. None of the comparisons between [identical] twins and others ... are even remotely significant. If same-sex romantic attraction has a genetic component, it is massively overwhelmed by other factors. Their conclusion is that the expression of same-sex attraction requires a social environment: "More plausible is the idea that genetic expression is activated only under strongly circumscribed social structural conditions. In contrast to other theories considered below, we assume that the close connection between gender identity and sexual identity is socially constructed." 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 sharedenvironment variables such as familial environment and societal attitudes had a weaker 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). Overall, the environment shared by twins (including familial and societal attitudes) explained 0-17% of the choice of sexual partner, genetic factors 18-39% and the unique environment 61-66%. The individual's unique environment includes, for example, circumstances during pregnancy and childbirth, physical and psychological trauma (e.g., accidents, violence, and disease), peer groups, and sexual experiences. [...] In men, genetic effects explained .34–.39 of the variance, 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. Criticisms of Twin Studies Twin studies have received a number of criticisms including ascertainment 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

(who are genetically identical, and shared the same fetal environment), sexual orientation cannot be purely biologically caused. 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 Earlier chromosome studies of homosexuality in males have not been replicated, or have had doubt cast on these early suggestions. For example, 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 (but inaccurately) dubbed as the 'gay gene' in the media, causing significant controversy. However, a later analysis by Hu et al. revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28. Sanders et al. (1998) replicated the study, finding 66% Xq28 marker sharing in 54 pairs of gay brothers. On the other hand, 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. Also, a study by Rice et al. in 1999 failed to replicate the Xq28 linkage results. Additionally, 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 much weaker link for Xq28 than reported by Hamer et al. However, they did find other markers with significant likelihood scores at 8p12, 7q36 and 10q26, the latter

two having approximately contributions.

equivalent

maternal

and

paternal

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. Maternal linkage, birth order, and female fertility Blanchard and Klassen (1997) reported that each older brother increases the odds of 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. Bocklandt, Horvath, Vilain and Hamer (2006) reported that some mothers of gay babies have extreme skewing of X chromosome inactivation. Using a sample of 97 mothers of homosexual men and 103 mothers of heterosexual men, the pattern of X inactivation was ascertained from blood assays. 4% of the mothers of straight men showed extreme skewing compared to 13% of the mothers of gay men. Mothers of two or more gay babies had extreme skewing of X inactivation of 23%. This extreme skewing may influence male sexual orientation through the fraternal birth order effect. An alternate theory was proposed by Italian researchers in 2004 supported by 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, however, would explain only 20% of the cases studied, indicating that this might not be the sole genetic factor determining sexual orientation. Homosexuals of either sex are more likely than the general population to be non-right-handed. 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 arousal. 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, that sexual attraction, whether samesex or opposite-sex oriented, operates similarly on a biological level, does not mean that there is necessarily a biological cause for homosexuality. 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 this data with adult sexual orientation. Another form of research was done by Dr. Savic. Dr. Savic uses PET scans to see the brain activity while letting people smell different types of pheromones. Savic used two compounds that resemble the male and female sex hormones. The first is 4,16-androstadien-3-one (AND) which is a derivative of testosterone produced in human axillary secretions in higher concentrations in men than in women. The second compound is oestra-1,3,5(10),16-tetraen-3-ol (EST) which is a substance resembling naturally occurring oestrogenes. These are the pheromones that make men and women attracted to each other. When a man smells a woman's pheromone, EST, there is a degree of brain activity. The same happens to women for the pheromone AND. Savic found that gays had the similar brain activity as women when given the whiff of AND, and vice versa for lesbians. Savic's findings imply that sexual orientation is determined prior to exposure to life’s environmental influences. Also, unlike some of the early researchers, Savic's research is less likely to cater to a gay political agenda or bias, as her field was originally epilepsy research. She inadvertently stumbled onto the pheromone sex differences while studying how smells might trigger temporal lobe epilepsy. 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, 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 neurons 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. However, he also found some contrary results: •

Three of the 19 homosexual subjects had a larger group of neurons in the hypothalamus than the average control-group subject.

•

Three of the 16 control-group subjects had a smaller group of neurons in the hypothalamus than the average homosexual subject.

•

One of the subjects classified as gay was actually bisexual. This subject's INAH3 group was about the same size as the heterosexual group.

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. Conclusions LeVay concluded in his 1991 paper that "The discovery that the nucleus differs in size between heterosexual and homosexual men illustrates that sexual orientation in humans is amenable to study at the biological level, and this discovery opens the door to studies of neurotransmitters or receptors that might be involved in regulating this aspect of personality. Further interpretation of the results of this study must be considered speculative. In particular, the results do not allow one to decide if the size of INAH 3 in an individual is the cause or consequence of that individual's sexual orientation, or if the size of INAH 3 and sexual orientation covary under the influence of some third, unidentified variable." He later added, "It's important to stress what I didn't find. I did not prove that homosexuality is genetic, or find a genetic cause for being gay. I didn't show that gay men are born that way, the most common mistake people make in interpreting my work. Nor did I locate a gay center in the brain. ... Since I look at adult brains, we don't know if the differences I found were there at birth or if they appeared later." Early fixation hypothesis The early fixation hypothesis includes research into prenatal development and the environmental factors that control masculinization of the brain. Studies have concluded that there is empirical evidence to support this hypothesis, including 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 block or exaggerate 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. One hypothesis is that something about what young children see in the gender-roles behavior of adults, or some differences (possibly unconscious) in the way adults treat young children, somehow influences or determines a child's eventual 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 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 (EBE) 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. The only cases Reiner found where children born with a X and Y chromosome are attracted to males as adults were those where testosterone receptors were absent, which prevented the male sex hormones from masculinizing the fetus. Pathogenic hypothesis of homosexuality The pathogenic hypothesis of homosexuality, also called the 'gay germ' hypothesis, suggests that homosexuality might be caused by an infectious agent. The speculative hypothesis was suggested by Gregory Cochran and Paul Ewald as part of a larger project advocating a number of pathogenic theories of disease. They argue that because of the supposedly reduced number of offspring produced by gay and lesbian people, evolution would strongly select against it. They also draw an analogy to diseases that alter brain structure and behavior, such as narcolepsy, which are suspected of being triggered by viral infection. Cochran also argues that the prevalence of homosexuality in urban areas suggests that an infectious disease causes homosexuality. They conclude that it is a "feasible hypothesis... no more and no less." After being unable to publish this account in a peer-reviewed journal,

the idea appeared in the popular press. An American Philosophical Association newsletter the following year stated "there is ultimately very little to be said in favor of these contentions", and criticised the press attention gained, given a lack of peer reviewed publication of the theory, and questioned the general ethics of communication of theories about homosexuality by researchers to the public. In an article in Out Magazine, brain researcher William Byne stated "Cochran and Ewald are guilty of pathologizing homosexuality", while in the same article psychology professor J. Michael Bailey posited that a 'germ theory' did not necessarily mean homosexuality was a disease, but recognised the political ammunition such a belief could give to homophobes. 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 maladaptive in an evolutionary context following a simple Darwinian model of Natural Selection—on the assumption that homosexuality would reduce this frequency. Those who believe that homosexuality is purely genetic argue that maladaptive traits will only be removed from a population if the trait is under simple, direct selection, if it derives from a heritable component of a genotype and if the intensity of selection is greater than other evolutionary forces like genetic drift, or inclusive fitness. 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" theory 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 and West Eberhard. This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive. The primary criticism of this theory has to do with the fact that children share on average 25% of their genes with their uncles and aunts, but

on average 50% with their parents. This means that to be adaptive, a "gay uncle" would need to somehow assist an extra two nieces or nephews, on average, to reach adulthood for every one of their own offspring they give up. Critics of the theory find this trade-off to be unlikely to produce a net reproductive gain. http://en.wikipedia.org/wiki/Environment_and_sexual_orientation

Environment and sexual orientation Environment 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 environmental influences. No simple cause for sexual orientation has been conclusively demonstrated, and there is no scientific consensus as to whether the contributing factors are primarily biological or environmental. Many think both play complex roles. The American Academy of Pediatrics and the American Psychological Association have both stated that environment is probably one of several causes of sexual orientation. Results from a 2008 twin study were consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior. Childhood gender non-conformity Researchers have found childhood gender nonconformity to be the largest predictor of homosexuality in adulthood. Daryl Bem's Exotic Becomes Erotic theory suggests that some children will prefer activities that are typical of the other sex and that this will make a genderconforming child feel different from opposite-sex children, while gender-nonconforming children will feel different from children of their own sex, which may evoke physiological arousal when the child is near members of the sex which it considers as being "different", which will later be transformed into sexual arousal. Researchers have suggested that this nonconformity may be a result of genetics, prenatal hormones, personality, parental care or other environmental factors. Peter Bearman showed that males with a female twin are twice as likely to report same-sex attractions, unless there was an older brother. He says that his findings support the hypothesis that less gendered socialization in early childhood and preadolescence shapes subsequent same-sex romantic preferences. He suggests that parents of oppositesex twins are more likely to give them unisex treatment, but that an older brother establishes gendersocializing mechanisms for the

younger brother to follow. Family influences Researchers have provided evidence that gay men report having had less loving and more rejecting fathers, and closer relationships with their mothers, than non-gay men. Some researchers think this may indicate that childhood family experiences are important determinants to homosexuality, or that parents behave this way in response to gender-variant traits in a child. Michael Ruse suggests that both possibilities might be true in different cases. From their research on 275 men in the Taiwanese military, Shu and Lung concluded that "paternal protection and maternal care were determined to be the main vulnerability factors in the development of homosexual males." Key factors in the development of homosexuals were "paternal attachment, introversion, and neurotic characteristics." One study reported that homosexual males reported more positive early relationships with mothers than did homosexual females. A 2000 American twin study showed that familial factors, which may be at least partly genetic, influence sexual orientation. Research also indicates that homosexual men have significantly more siblings than the homosexual women, who, in turn, have significantly more siblings than the nonhomosexual men. A 2006 Danish study compared people who had a heterosexual marriage versus people who had a same-sex marriage. Heterosexual marriage was significantly linked to having young parents, small age differences between parents, stable parental relationships, large sibships, and late birth order. Children who experience parental divorce are less likely to marry heterosexually than those growing up in intact families. For men, homosexual marriage was associated with having older mothers, divorced parents, absent fathers, and being the youngest child. For women, maternal death during adolescence and being the only or youngest child or the only girl in the family increased the likelihood of homosexual marriage. Fraternal birth order According to several studies, each older brother increases a man's odds of developing a homosexual orientation by 28%–48%. Most researchers attribute this to prenatal environmental factors, such as prenatal hormones. McConaghy (2006) found no relationship between the strength of the effect and degree of homosexual feelings, suggesting the influence of fraternal birth order was not due to a biological, but a social process. City of origin In the United States, the Social Organization of Sexuality found that homosexuality was positively correlated with urbanization of the place

of residence at age 14. The correlation was more substantial among men than women. The authors hypothesize that "Large cities may provide a congenial environment for the development and expression of same-gender interest." In Denmark, people born in the capital area were significantly less likely to marry heterosexually, but more likely to marry homosexually, than their rural-born peers. Historical observations Anthropologists had observed that relatively uncompetitive primitive cultures such as those that do not distinguish or reward the best hunters in distinction to the other men in the tribe have virtually no homosexuality. Miron Baron commented "Some cultures - for example, the Assyrian and Graeco-Roman - were more tolerant of homosexuality. The behavior was practiced openly and was highly prevalent. Sexual patterns are to some extent a product of society's expectations, but it would be difficult to envisage a change in the prevalence of the genetic trait merely in response to changing cultural norms."

http://www.simonlevay.com/the-biology-of-sexual-orientation

Non-biological theories Psychoanalytic theories Early in the 20th century, Sigmund Freud postulated that family dynamics influence a child’s ultimate sexual orientation. For example, a dominant, close binding mother, or an absent or distant father, might steer a boy toward homosexuality by disrupting his exit from the postulated “Oedipal phase” of psychosexual development (Freud 1957). Girls might become lesbian because of unconscious hatred of their mothers, envy of a brother’s penis, and the like (Freud 1920/1955). Retrospective studies confirm that gay men tend to describe their relationships with their mothers as unusually close and with their fathers as distant or hostile (Bell, Weinberg et al. 1981; Freund and Blanchard 1983). Comment: These retrospective findings don’t necessarily mean that parental attitudes influence the child’s sexual orientation in the way Freud envisaged, however. A contemporary American analyst has suggested that parental attitudes to pre-gay children, such as a father’s withdrawal or hostility, may actually

be a response to gender-variant traits in the child rather than a cause of them (Isay 1989; Isay 1996). Behaviorism/socialization Learning theorists have suggested that gendered traits, including sexual orientation, emerge from a conscious or unconscious “training regimen” imposed by parents, teachers, peers, and society in general (Money and Ehrhardt 1971). Most feminist thinkers have also attributed the development of gendered traits to socialization. Comment: The main difficulty with these ideas is that heterosexual parents don’t seem to inculcate homosexuality or gender-nonconformity, in fact they often attempt to prevent these traits in children who nevertheless become gay. Parents who happen to be gay themselves might tolerate or even foster gender variance and homosexuality in their children, but in fact the children of gay parents usually become heterosexual (Stacey and Biblarz 2001). One much-publicized attempt to change a child’s gender and future sexual orientation by parental socialization (after his penis was accidentally destroyed during circumcision) ended in failure (Colapinto 2000). Effect of sexual experiences It has been proposed that early sexual experiences (pleasant or traumatic) influence sexual orientation—that a girl who is raped by a man at an early age may be “turned off” men and thus become lesbian, for example, while a boy who is seduced by a man (or molested by an older brother) and who derives sexual pleasure from the experience may become gay (Churchill 1967; Cameron and Cameron 1995). Comment: Such ideas fail to explain how it is that many people whose initial sexual experiences are heterosexual and consensual nevertheless become gay, or how it is that children who attend single-sex boarding schools, where consensual homosexual encounters are common, are no more likely to become homosexual adults that are children who do not attend such schools (Wellings, Field et al. 1994). Social constructionism This school of thought proposes that a person’s identity as gay, straight, or bisexual is a label imposed by society and

internalized by the individual, rather than arising from within (Foucault 1978; Halperin 1990). Comment: Social constructionism has contributed valuable insights to our understanding of human sexuality in its cultural context, but it has had relatively little to say about the question that interests us here, which is why specific individuals become gay, straight, or bisexual.

Biological theories General comments A contrasting view is that sexual orientation is determined or influenced by biological factors such as genes and hormones. Of course, there doesn’t have to be a sharp distinction between biological and life-experience theories. It’s conceivable, for example, that a close-binding mother might induce hormonal changes in the young child that in turn lead to adult homosexuality. Conversely, a biological trait such as facial beauty might influence parents to treat a son in such a way as to steer him toward homosexuality. At the very least, though, testing biological and life-experience theories require the application of very different techniques and thus tend to engage researchers with different training and backgrounds. Biological theories of sexual orientation have a long history. Magnus Hirschfeld, the German sexologist and gay-rights pioneer, promoted such theories early in the 20th century. Still, Freudian, behaviorist, and social-constructionist thinking dominated thinking on the topic for most of the century. Only in the 1980s and 1990s did biological ideas re-emerge in a significant way. This re-emergence paralleled a remarkable increase in tolerance and acceptance of gay people in many Western societies. It seems likely that these parallel trends reflected a two-way interaction: increasing acceptance of (and familiarity with) gays fostered a belief in biological theories, and vice versa. Sexual orientation is a gendered trait: most men are sexually attracted to women more than they are to men, and most women are sexually attracted to men more than they are to women. Homosexual people are sex-atypical, at least with respect to their sexual orientation. Biological theories of sexual orientation commonly, though not always, include the idea that

sexual orientation is embedded within a larger constellation of gendered traits, and that biological factors influence multiple gendered traits simultaneously. Whatever ultimate biological factors influence a person to become homosexual, these factors may promote the development of other characteristics— anatomical, physiological, molecular-genetic, or psychological— that are sex-atypical. Given that the ultimate factors may not be directly detectable (if they operated during fetal life, for example), the presence of other sex-atypical traits in gay people may be taken as an indicator that those undetectable factors were in fact at work. Still, the presence of sex-atypical traits in gay people doesn’t always compel a biological interpretation—it might be that certain life experiences promote both homosexuality and other sex-atypical characteristics. To give a concrete example: it’s been well documented that gay people, on average, display some sex-atypical psychological characteristics during childhood (Bailey and Zucker 1995). Gay men, for example, tend to report that they had less interest in rough-and-tumble sports than other boys. A prospective study showed that boys who are very strongly gender-nonconformist have a high likelihood of developing into gay or bisexual adults (Green 1987). But this connection between childhood gendernonconformity and adult homosexuality could arise for genetic reasons (genes promoting a spectrum of gender-nonconformist traits including homosexuality) or for environmental reasons (e.g., parental encouragement these same traits). It’s also possible that genes cause childhood gender-nonconformity and that environmental factors (e.g. the hostile reactions of peers) cause gender-nonconformist children to become gay. Thus the fact that there is a correlation between homosexuality and some other trait doesn’t in itself distinguish between different possible causes. Genes Animal studies. In the fruit fly, Drosophila, sexual orientation appears to be under the control of a single gene named fruitless (“fru”) (Demir and Dickson 2005). fru is alternatively spliced (read off into messenger RNAs and proteins in a different fashion) in males and females. If a female fruit fly is engineered to splice fru in the male-specific fashion, she will approach and court other females and attempt to copulate with them. If a male fruit fly is engineered to splice fru in the female-specific fashion, he will fail to approach or court females. A chain of neurons in the male fly’s

nervous system expresses fru and splices it in the male-specific fashion (Stockinger, Kvitsiani et al. 2005). These neurons include olfactory receptors that are probably involved in the detection of female sex pheromones, as well as other neurons that are synaptically connected with these olfactory neurons and with each other. There are no obvious anatomical differences between these neurons in male and female flies: thus, they probably differ in some physiological or chemical attribute that causes them to generate (in males) the male-specific sexual behavior. Comment: In insects, sex differentiation is cell-autonomous, so neurons are caused to splice fru in male-specific or femalespecific fashion by the sex-determining genes in those same cells. Thus homosexuality (a dissociation between anatomical sex and sex-typical sexual orientation) is unlikely to occur, and in fact has not been observed outside of molecular-genetics labs. The situation in humans is different in that sexual differentiation is not cell-autonomous but depends in large part on circulating sex hormones. This may allow for greater variability in sexual orientation, whether we are talking about genetic or non-genetic causes. Sibling studies Most of the evidence for a genetic influence on human sexual orientation comes from family and twin studies. Homosexuality clusters in particular families, especially among siblings. Thus, the brothers of gay men are reported to have about a 22 percent chance of themselves being gay, whereas the brothers of heterosexual men have only about a 4 percent chance of being gay (Pillard and Weinrich 1986). Similarly, the sisters of lesbians have an increased chance of being lesbian (Bailey and Benishay 1993). This clustering in largely sex-specific: the existence of a lesbian in a family has little effect on the chances that her brothers will be gay, or vice versa. Comment: Family clustering is consistent with a genetic influence, but it does not by itself distinguish between genetic and environmental causes. For example, a mother who treats one son in such a way as to make him gay might well do the same with another son. To the extent that the clustering does have a genetic cause, the sex-specificity of the clustering would imply that different genes contribute to male and female homosexuality. This is hardly surprising since they are really different phenomena: male homosexuality is sexual attraction to males and female homosexuality is sexual attraction to females.

Twin studies Most twin studies have focused on the concordance rate for homosexuality. This is the likelihood that, if one twin is gay, his or her co-twin will be gay too. If genes influence sexual orientation, the concordance rate should be higher for twin pairs who are monozygotic (“identical”) than for twin pairs who are dizygotic (“fraternal”). That’s because monozygotic twins share all the same genes, whereas dizygotic twins share only about half their genes. If genes absolutely determined sexual orientation the concordance rate for monozygotic twins should be 100%. One early study did report a near-100% concordance rate for male monozygotic twins (Kallmann 1952). More recent studies have come up with much lower figures, but have generally reported higher concordance rates for monozygotic than for dizygotic twins, consistent with a genetic influence on sexual orientation. In one study the concordance rate was 52% for male monozygotic twins compared with 22% for male dizygotic twins (Bailey and Pillard 1995). A comparable study of female twins came up with concordances of 48% and 16% respectively (Bailey, Pillard et al. 1993). Although these studies suggest that there is a substantial influence of genes on sexual orientation in both men and women, there are problems of interpretation. For one thing, it is difficult to get from the concordance rates to a measure of heritability (meaning, simply put, the fraction of the total causation of homosexuality that is genetic). If it is the case that monozygotic twins experience a more similar environment than do dizygotic twins (being treated more similarly by their parents, for example), and these environmental factors influence sexual orientation, then the concordance rate would be higher for monozygotic twins for that reason alone. There is in fact no reason to think that this scenario is the case, but it is a theoretical possibility. Another problem has to do with ascertainment bias. Typically, researchers do these twin studies by advertising for individuals who are gay and have a twin, then they check on the other twin’s sexual orientation. But if the likelihood that a person responds to the ad is affected by whether his/her twin is also gay or not, this could throw off the statistics. To get away from this problem, Bailey and colleagues did one study using a pre-existing twin registry (Bailey, Dunne et al. 2000). This study came up with lower concordance rates than previous studies, especially in

women. Interestingly, the researchers found that childhood gender nonconformity—a common precursor of adult homosexuality—was significantly heritable in both sexes. There is one small study of monozygotic twins reared apart (Eckert, Bouchard et al. 1986). Of four female pairs in which one twin was lesbian, none of the co-twins were lesbian. Of two male pairs in which one twin was gay, one of the co-twins was also gay, while the other was bisexual. Comment: There remains considerable uncertainty about the heritability of homosexuality: it is probably significantly heritable in men but may be only slightly heritable or not heritable at all in women. Candidate-gene study One approach to the question of genes influencing sexual orientation is to pick a gene that might conceivably play a role and to compare its DNA sequence in gay and straight people. One group of researchers picked the androgen receptor gene, a gene that plays the key role in mediating testosterone’s influence on the body and brain (Macke, Hu et al. 1993). They could not find any differences between gay and straight men, however. Linkage studies A contrasting approach is to scan part or all of the genome, looking for sites where pairs of gay siblings inherit the same DNA more frequently than would be expected on a chance basis (“linkage analysis”). Dean Hamer’s group reported finding (in pairs of gay brothers) such a site on the X chromosome—in a region called Xq28 (Hamer, Hu et al. 1993). They concluded that a gene influencing male sexual orientation was probably located in this region. (The choice of the X chromosome for study was motivated by family data suggesting that gay men inherit a predisposition to homosexuality from their mothers — the X chromosome is the only chromosome that males inherit exclusively from their mothers.) Hamer’s group replicated the finding in a second sample but there has not been an independent confirmation, and in fact one group has reported failing to replicate the finding (Rice, Anderson et al. 1999). Thus the claim of a “gay gene” on the X chromosome remains unverified.

In 2005 a group led by Brian Mustanski (and including Hamer) reported on a genome-wide linkage scan (Mustanski, Dupree et al. 2005). They did not confirm the Xq28 linkage but they did find evidence for linkage at three other sites — on chromosomes 7, 8, and 10. The researchers were not able to perform a statistical analysis to evaluate whether these results were due to chance or to the actual existence of genes influencing sexual orientation at those three locations. Comment: Even in the case of clearly “biological” traits such type 2 diabetes, which is known to be under genetic influence, the search for the responsible genes has proven frustratingly difficult. Thus is should be no surprise that researchers have had a hard time pinning a complex trait like sexual orientation down to specific genes. It may be that a number of genes have effects that are individually weak and therefore difficult to detect, or that certain genes do have strong effects but only in certain families or under certain environmental conditions. Given the evidence that sexual orientation is indeed partly inherited, at least in men, the continued search for the responsible genes and their mechanism of action is certainly warranted. Genomic imprinting This is the phenomenon whereby some genes acquire different molecular labels depending on whether they are inherited from the mother or the father; this labeling affects gene expression and development in the offspring. An article from Dean Hamer’s lab speculates that imprinting could play a role in the development of sexual orientation (Bocklandt and Hamer 2003). X-inactivation Bocklandt and Hamer reported that women with gay sons— especially those with two gay sons—are more likely than women without gay sons to show extreme skewing of X-inactivation (that is, more than 90 percent of their cells show inactivation of the same X chromosome)(Bocklandt, Horvath et al. 2006). Comment: These data could be taken to strengthen Hamer’s 1993 conclusion that a gene or genes on the X chromosome influence male sexual orientation. However, the current study depends primarily on the same subject set that was used for that earlier study. Given that subsequent studies, including Mustanski’s, haven’t confirmed the linkage reported in the 1993

study, one may wonder if that subject set was atypical in some way. Thus replication of the current findings with an entirely new sample would be desirable. Gay genes and evolution The existence of genes promoting homosexuality is counterintuitive, since such genes should reduce their owner’s reproductive success and thus, over many generations, they should be eliminated from the gene pool. A number of people have considered the various ways in which gay genes might persist (Wilson 1978; Weinrich 1987; Ruse 1988; Hamer and Copeland 1994; Ridley 1994; Bailey 2003). Here are some of the ideas that have been put forward: 1. Gay genes might persist if gay people, though having few children themselves, promote the reproductive success of their siblings (“kin selection”). 2. A gene might cause homosexuality and thus reduce reproductive success when present on two homologous chromosomes (homozygous state) but have some other, positive effect when present on one chromosome (heterozygous state). The analogy is to the sickle-cell gene which causes anemia when homozygous but confers resistance to malaria when heterozygous. If the heterozygous advantage is sufficiently great the gene will persist in the population. 3. A gene for sexual attraction to men would cause homosexuality in men but might cause a “hyper-heterosexuality” in women, thus increasing their reproductive success—and vice versa. The positive effect on the reproductive success of one sex might balance the negative effect in the other sex. Consistent this hypothesis, an Italian study reported that the female maternal relatives of gay men have more offspring than those of heterosexual men, as if a gene predisposing simultaneously to male homosexuality and female “hyper-heterosexuality” were being transmitted on the X chromosome (Camperio-Ciani, Corna et al. 2004). 4. It’s possible that, through much of human evolution, people have been socially compelled to marry and have children regardless of their sexual orientation. In this case, the negative effect of a gay gene on reproductive success might be small, and might be outweighed by some other, unknown benefit conferred by the gene.

5. The elimination of gay genes from the population (by nonreproduction of gay people) might be balanced by the occurrence of new mutations. For this to be the case, the mutation rate for gay genes would have to be exceptionally high. Comment: None of these theories are particularly persuasive. The evolutionary value of gay genes may become clearer if and when such genes are identified and their mechanism of action determined. Hormones Adult hormone levels Most studies have failed to find significant differences in the levels of circulating sex hormones between homosexual and heterosexual adults of the same sex (Meyer-Bahlburg 1984). Prenatal hormones: background In experimental animals it’s been well established that the sexual differentiation of the body and brain results primarily from the influence of sex hormones secreted by the testes or ovaries (Arnold 2002). Males have high levels of testosterone in fetal life (after functional development of the testes) and around the time of birth, as well as at and after puberty. Females have low levels of all sex hormones in fetal life, and high levels of estrogens and progestagens starting at puberty. High prenatal testosterone levels organize the brain in a male-specific fashion; low levels testosterone permits it to organize in a female-specific fashion. Hormones at puberty activate the circuits laid down in prenatal life but do not fundamentally change them. Thus, the range of sexual behaviors that adult animals can show is determined in large part by their prenatal/perinatal hormone exposure— manipulating these hormone levels can lead to atypical sex behavior or preference for same-sex sex partners as well as a range of other gender-atypical characteristics. Nevertheless, prenatal/perinatal hormones may not be the entire story. Changes in adult hormone levels can change brain anatomy in some cases (Cooke, Tabibnia et al. 1999). Furthermore, some aspects of the prenatal sexual differentiation of the brain seem to be independent of sex hormones and probably reflect the direct effects of the brain’s chromosomal sex on its own development (Arnold 2003). Whether these direct

effects are significant for the development of any gendered traits in humans is unknown. Based on this animal research a number of scientists, especially the German neuroendocrinologist Günter Dörner, have promoted a prenatal hormonal theory of homosexuality (Dörner 1969). This theory postulated that in human fetuses destined to become homosexual adults, the sexual differentiation of the brain proceeds in a sex-atypical direction. The cause could be atypical levels of sex hormones (e.g., unusually low levels of testosterone in the case of a male fetus, or unusually high levels in a female fetus) or some difference in the way the brain responds to hormones, such as a genetic peculiarity of the androgen receptor (see above). Dörner initially presented his theory as part of a pathological conception of homosexuality and even as a tool for preventing it through medical means. This did not endear him to the gay community. There is no intrinsic reason why his theory should be seen as less gay-friendly than other theories, however (LeVay 1996). Although the prenatal hormonal theory has not been proved or disproved in the decades since Dörner proposed it, a body of supportive evidence has accumulated, and it is probably the dominant idea among those who think about sexual orientation from a biological perspective. Attributing sexual orientation to prenatal hormone levels is not an ultimate explanation, because the question remains as to how those levels (or the brain’s response to them) come to be different in pre-gay and pre-straight fetuses. At one extreme, the reason for these differences might be genetic, as with the androgen receptor hypothesis mentioned above or the case of congenital adrenal hyperplasia, discussed below. At the other extreme the reason might be environmental, as with Dörner’s maternal stress theory, discussed below. It’s also possible that essentially random developmental processes could be responsible. In species such as rats where the mother carries multiple fetuses simultaneously, a female fetus that happens to be located next to a male fetus can absorb testosterone from its neighbor, resulting in some masculinization of her sexual behavior in adulthood (Clemens, Gladue et al. 1978; Meisel and Ward 1981). Since the sex of a fetus’s neighbor is random, the ultimate cause of the masculinized behavior is also random. There are probably countless such random processes occurring prenatally, even in fetuses who are singletons. (Human females who had a male twin are not thought to be especially likely to be

lesbian. Some studies have reported other gender-atypical traits in these women (McFadden 1993), but negative findings have also been reported (Henderson and Berenbaum 1997)). Congenital adrenal hyperplasia (CAH) This condition is caused by a genetic defect in one of the enzymes that are involved in the synthesis of corticosteroid hormones. It is marked by excessive levels of androgens (testosterone-like hormones) that are secreted by the adrenal glands during fetal life. (The condition is generally recognized and successfully treated after birth.) Affected girls are often born with some degree of masculinization of the external genitalia, in which case the condition is considered a form of intersexuality. Numerous studies have reported that CAH-affected girls tend to display a variety of gender-atypical traits (Berenbaum, Duck et al. 2000), though the effects may be small (Henderson and Berenbaum 1997). When adult they are much more likely to have experienced or to wish for homosexual relationships that comparison groups of women such as their unaffected sisters (Dittmann, Kappes et al. 1992). Comment: Although it’s been suggested that the tendency toward gender-atypicality and homosexuality in CAH-affected females is an indirect effect caused by their (or their family’s) reaction to the partially masculinized genitalia, it seems more likely to be a direct effect of the prenatal androgens on brain development. Supporting this conclusion is the observation that there seems to be no relationship between the degree of genital masculinization and the degree of psychological genderatypicality (Berenbaum and Bailey 2003). Of course, CAH is a rare condition and plays no role in the psychosexual development of most lesbian or bisexual women, but it supports the hypothesis that atypical prenatal hormone levels can influence adult sexual orientation. Diethylstilbestrol (DES) exposure DES is a synthetic, non-steroidal drug that activates estrogen receptors. It was widely prescribed to pregnant women before 1971. Women who were exposed to the drug during fetal life are significantly more likely to experience same-sex attraction than comparison groups such as their unexposed sisters, according to one small study (Meyer-Bahlburg, Ehrhardt et al. 1995), but a recent larger study found that exposed women were actually slightly less likely to have experienced a same-sex relationship

(Titus-Ernstoff, Perez et al. 2003). Comment: The equivocal or possibly non-existent effect of prenatal DES on female sexual orientation, which contrasts with the strong effects seen in CAH, may reflect the fact that DES does not activate androgen receptors. Although androgens are normally converted to estrogens in the brain and thus activate estrogen receptors as well as androgen receptors, the direct activation of androgen receptors may be more important for this particular gendered trait, and perhaps for others too. Prenatal stress theory of male homosexuality Stressing pregnant rats (for example, by close confinement and exposure to bright lights) causes the male offspring of those pregnancies to display atypical sex behavior in adulthood: they are relatively unwilling to mount females and they may show a female-type response (“lordosis”) to being mounted by males (Ward 1972; Ward, Ward et al. 1994). The reason is that the stress activates the fetuses’ stress hormones which in turn lead to a diminution in the levels of testosterone during a critical period of brain development. On the basis of Ward’s findings and his own animal studies, Dörner proposed that the mothers of homosexual men were exposed to severe stress during pregnancy, and he carried out retrospective studies that seemed to offer strong support for the hypothesis (Dörner, Geier et al. 1980; Dörner, Schenk et al. 1983). But more recent studies have either completely failed to confirm Dörner’s hypothesis (Schmidt and Clement 1990; Bailey, Willerman et al. 1991) or have provided very equivocal support for it (Ellis, Ames et al. 1988). Prenatal stress also has no effect on the development of gender role behavior in boys (Hines, Johnston et al. 2002). Comment: The prenatal stress theory of male homosexuality seems to be incorrect, in spite of the animal results. Rats and humans probably differ in their stress-response mechanisms. Effects on anatomy, brain structure and function, and cognition Prenatal sex hormones have numerous effects on the developing body and brain. Thus, if there are differences in the levels of these hormones between pre-gay and pre-straight fetuses, one might expect to see other differences between gay and straight adults than simply their sexual orientation. Presumably, this would particularly likely for traits that differ between the sexes.

Numerous studies have compared body anatomy, brain anatomy, brain function, and cognitive and personality traits between gay and straight men and between lesbian and heterosexual women. The results of some of these studies are reviewed in the following sections. Anatomy Penis size According to a re-analysis of old Kinsey Institute data derived from about 5,000 men, the penises of gay men are slightly but significantly longer (6.46 vs. 6.14 inches measured along the top surface) and fatter than those of straight men (Bogaert and Hershberger 1999). Comment: These findings are open to criticism because the measurements were made by the subjects themselves at home and not by an independent observer. (Gay men might be more tempted to exaggerate than straight men, or they might be more aroused by the sight of their erect penises, thus causing stronger erections.) If correct, the result is inconsistent with the simplest form of the prenatal hormone hypothesis, which would predict gay men’s penises to be smaller. There are various ways one could make the findings fit the hypothesis, but it may not be worth dwelling on this until a replication study has been done— which could be a while.

Finger length ratios In men the index finger (D2) is usually significantly shorter than the ring finger (D4), whereas in women D2 is nearly as long as D4. In other words, the D2:D4 ratio is usually lower in men than in women. Presumably this results from hormonal differences between males and females during development of the fingers. This idea is supported by the observation that CAH-affected individuals, who were exposed to high prenatal androgen levels, have low D2:D4 ratios (Brown, Hines et al. 2002). Several groups have reported that the D2:D4 ratio is lower in lesbians than in heterosexual women (Williams, Pepitone et al. 2000; McFadden and Shubel 2002; Rahman and Wilson 2003), consistent with the prenatal hormone theory. It’s also been reported that only one subgroup of lesbians, namely those who

self-identify as “butch” (masculine), has low D2:D4 ratios (Brown, Finn et al. 2002). This is one of the very few biological studies that look at the important differences that exist within the categories of “gay” and “lesbian.” One study failed to confirm the relationship between D2:D4 ratios and sexual orientation in women, citing ethnicity as a confounding variable (Lippa 2003). The finding was confirmed, however, in another recent study that focused on female monozygotic twins who were discordant for sexual orientation: the lesbians twins had a lower 2D:4D ratio than their heterosexual co-twins (Hall and Love 2003). This suggests that the 2D:4D effect is independent of genes. Data for men have been inconsistent: gay men have been reported to have a D2:D4 ratio that is lower (McFadden and Shubel 2002; Rahman and Wilson 2003), higher (Lippa 2003) or the same (Williams, Pepitone et al. 2000) as in straight men. Fingerprints A 1994 study reported a difference in the fingerprint patterns of gay and straight men: more specifically, the ratio of the numbers of ridges on the fingers of the left and right hands, which usually favors the right hand, was reported to be left-shifted in gay men (Hall and Kimura 1994). Two subsequent studies have failed to replicate this finding, however (Forastieri, Andrade et al. 2002; Mustanski, Bailey et al. 2002). Comment: None of these anatomical studies inspire tremendous confidence, though the D2:D4 findings in women seem the best documented, and are consistent with the prenatal hormone theory.

Brain studies—anatomy Suprachiasmatic nucleus (SCN) This is a small group of cells in the hypothalamus that plays a key role in the generation of circadian rhythms. A Dutch group has reported that the SCN is larger in gay men than in straight men (Swaab and Hofman 1990).

Comment: This finding hasn’t been replicated (or refuted) by other labs. If it is correct, its significance is unclear since the SCN is not known to play a role in the generation of sexual feelings or behaviors. Third interstitial nucleus of the anterior hypothalamus (INAH1) This small group of cells lies in a region of the hypothalamus called the medial preoptic area, which is known from animal studies to be involved in the generation of male-typical sex behavior. It is generally larger in men than women (Allen, Hines et al. 1989; LeVay 1991; Byne, Tobet et al. 2001). In a 1991 autopsy study, I reported that INAH3 was smaller, on average, in gay men than in straight men (LeVay 1991). A more recent study replicated this finding, although the magnitude of the difference was less (Byne, Tobet et al. 2001). This latter study also reported that there was a difference in cell density—a higher density (more cells per cubic millimeter) in the gay men. The researchers commented that the total number of cells in INAH3 may be the same in gay and straight men, but are packed more closely in the gay men, perhaps because they did not form so many synapses during development. Comment: There has been concern that the small size of INAH3 in the gay men might be a consequence of the disease (AIDS) from which most of them died, rather than their sexual orientation. However, neither I nor Byne’s group found any evidence that AIDS by itself has any effect on the size of INAH3. The findings on INAH3 support the prenatal hormone theory, because it’s known that manipulating testosterone levels in rats, if performed during a critical prenatal/perinatal period of development, affects the ultimate size of the analogous cell group in the rat’s hypothalamus, (Rhees, Shryne et al. 1990), as well as causing atypical sex behavior in adulthood (Grady, Phoenix et al. 1965). Still, the findings don’t absolutely compel us to accept that prenatal events influence sexual orientation, since (as mentioned above) there is evidence that some sexually dimorphic brain structures can be modified by hormonal or other changes in adulthood. A group at the Oregon Health Sciences University recently reported analogous findings for sheep (Roselli, Larkin et al. 2004). A hypothalamic cell group that may be the sheep equivalent of INAH3 was reported to be larger in rams than ewes, but smaller in rams that mate exclusively with other rams (“homosexual rams”) than in heterosexual rams. The cell group

also expressed lower levels of aromatase—the enzyme that converts testosterone to estrogen—in the homosexual rams than in the heterosexual rams. Comment: Of course I like this study because it offers such a close parallel to my own human study. Why some rams are homosexual is not known—exclusive homosexuality seems to be rare in the animal kingdom—but there are other studies suggestive of a neuroendocrinological mechanism (Pinckard, Stellflug et al. 2000). There is a report that the anterior commissure, a fiber bundle connecting the left and right sides of the cerebral cortex, is larger in women than men, and larger in gay men than in straight men (Allen and Gorski 1992). Comment: This report has not been replicated or refuted. The significance of the finding, if correct, is unclear, though it might be related to the cognitive differences that have been reported between gay and straight men (see below). The Karolinska group (Ivana Savic and Per Lindstrom) reported on the relative sizes of the left and right cerebral hemispheres in gay and straight men and women, as determined from MRI scans (Savic and Lindstrom 2008). In straight men, the right hemisphere is about 2 percent larger than the left hemisphere, on average, whereas in straight women the two hemispheres are the same size. In gay people, it’s the reverse: gay men have symmetrical hemispheres, and lesbians have a larger right hemisphere. In the same paper the researchers examined the connections of the left and right amygdalas—brain regions that are involved in the processing of emotions. The connections were determined by PET scanning. (If two brain regions are interconnected, cerebral blood flow—which PET measures—tends to be correlated in time between the two regions.) The connectivity patterns of the two amygdalas were asymmetrical and different between heterosexual men and women. For example, the right amygdala has more connections than the left in heterosexual men, whereas in women the left amygdala has more connections that the right amygdala. Again, these asymmetries were sex-atypical in gay men and women. Comment: This seems to offer two more examples of sex-atypical brain organization in gay people. The findings are consistent with the idea that some common driver, such as prenatal hormone levels, guides the development of a package of gender-related brain systems in a sex-atypical direction in fetuses that later

become gay men and women. Brain studies—function Auditory system There are differences between men and women in the functional properties of the inner ear and the central auditory system, as assessed by measurement of otoacoustic emissions (sounds produced by the inner ear) and auditory evoked potentials (recordings of brain activity following a brief sound). Dennis McFadden and his colleagues (McFadden 2002) have reported that lesbian and bisexual women have partially masculinized otoacoustic emissions and auditory evoked potentials. They also report that women who had male twins (and who may therefore have been exposed to testosterone from their twin during prenatal life) are likewise masculinized in otoacoustic emissions, as mentioned above. They therefore interpret their findings as consistent with the prenatal hormonal theory of sexual orientation, i.e., that pre-lesbian or pre-bisexual fetuses are exposed to atypically high levels of androgens. Interestingly, the researchers found no difference in the otoacoustic emissions of gay and straight men, but they observed that some aspects of the evoked potentials of gay men were shifted in a “hypermasculine” direction—which if true is the opposite of what would be expected on the basis of the prenatal hormone theory, at least in its simplest form. A British group (Rahman, Kumari et al. 2003) recently reported differences in the startle response (eyeblink following a loud sound) of lesbians compared with heterosexual women. It was previously known that men and women differ in the extent to which the startle response is inhibited when the loud sound is preceded by a weaker sound: this “prepulse inhibition” (PPI) is typically less evident in women than men. The researchers reported that the PPI was greater (i.e., masculinized) in the lesbian subjects, a finding that they interpreted in terms of the prenatal hormone theory. They did not find any difference between the startle responses of gay and straight men. Comment: These specific findings await independent replication, but the Rahman and McFadden studies are consistent with each other. Sexual arousal

A brief report from Bailey and Mesulam’s groups described the patterns of brain activity in gay and straight men while they were viewing sexually arousing and non-arousing images (Barch, Reber et al. 2003). The regions selectively active during arousing stimuli were generally the same ones in the two groups, but three regions (medial prefrontal cortex, left hippocampus, and right amygdala) were more selectively active in the gay men. Comment: Probably the main message of this study is the similarity of activity patterns in the gay and straight men during sexual arousal, in spite of the differences in the kind of images they find arousing. Neurotransmitter function A group at the University of Chicago (Kinnunen, Moltz et al. 2004) compared the brain’s metabolic response to oral administration of a selective serotonin reuptake inhibitor, fluoxetine, in gay and straight men. Using fluorodoxyglucose positron emission tomography, they found differences in response in several brain regions. In the hypothalamus, the heterosexual men showed a significantly greater reduction of glucose metabolism in response to the drug, though only on the right side. Comment: It could be that the serotonergic input to INAH3 is less extensive in gay men, corresponding to the smaller size of this cell group. However, PET imaging does not permit resolution of individual hypothalamic nuclei. Also, the lateralization of the effect is puzzling. The researchers did not test for a basic sex difference in fluoxetine responses. Odor responses A group at the Karolinska Institute has previously reported differences in the patterns of brain activity between heterosexual men and women when smelling two putative human sex pheromones, 4,16-androstadien-3-one (AND)(a component of male armpit secretions) and estra-1,3,5(10),16-tetraen-3-ol (EST)(found in female urine). In PET scans, men showed activity in the anterior hypothalamus during exposure to EST but not to AND (Savic, Berglund et al. 2001), while women showed hypothalamic activity during exposure to AND but not EST (The location of activity within the hypothalamus was slightly different.) In a recent follow-up the research group included homosexual men in a similar experiment (Savic, Berglund et al.

2005). They found that gay men had a pattern of hypothalamic activity resembling that of heterosexual women: activity was elicited by AND and not EST. In contrast, activity patterns in the olfactory cortex were similar in all three groups. The researchers suggest that the hypothalamus is organized to respond to sex pheromones in a sex-atypical way in homosexual men. Recently the Swedish group reported on a similar study comparing heterosexual and lesbian women (Berglund, Lindstrom et al. 2006). As might be expected, they found that AND elicited activity in the anterior hypothalamus of heterosexual women but not lesbians. The results with EST were more equivocal but were more similar to those observed in heterosexual men than in heterosexual women. In a related study, researchers at the Monell Chemical Senses Center tested the preferences of heterosexual men, homosexual men, heterosexual women, and homosexual women for armpit secretions pooled from six-member panels representing these same four subject groups (Martins, Preti et al. 2005). They reported differences in preferences related to both the sex and the sexual orientation of both the donor panels and the judging panels. For example, gay men preferred odors from gay men to those from other groups, whereas straight men like the odor from gay men less than from other groups. The authors suggest that biological factors lead to recognizably different odors in persons of the same sex but different sexual orientation, and that homosexual men and women assess the attractiveness of these odors differently. Comment: The question of human sex pheromones has been controversial, in part because the biological system that detects sex pheromones in mammals (the vomeronasal system and its molecular receptors and neural connections) appear to be missing, vestigial, or non-functional in humans. It should be noted that the “pheromones” in the Swedish study were presented in very high and uncontrolled concentrations, so that they could be consciously perceived as odors. Thus the results, though consistent with the idea that a sex-pheromone system is wired up differently in homosexual and heterosexual men, could also be interpreted as reflecting sexual arousal or non-arousal induced by odors that the subjects have come to recognize as belonging to their preferred or non-preferred partners—similar to the activity patterns that might be elicited by, for example, erotic videos featuring men or women. Nevertheless, it has been shown that both AND and EST activate the human hypothalamus when

presented at concentrations below the threshold for conscious perception. If the Swedish results related to sexual orientation could be replicated with sub-threshold concentrations of the odorants, this would bolster the idea that a pheromonal mechanism is at work. The Monell study suggests that gay and straight people emit different armpit odors, possibly as a result of biological differences in the major histocompatibility complex (MHC) genes or something similar, but other explanations are possible. Given that the donor panels were so small these results need to be replicated. The preference shown by gay men for the odors derived from gay men, if it holds up, seems more likely to be a learned response that an innate biological mechanism, because there is no evidence that gay men have an innate preference for gay male partners over straight male partners. Cognitive studies General Men and women differ in a number of cognitive traits. Men tend to outperform women in certain kinds of visuospatial tasks, such as mental rotation and targeting, as well as in mathematical reasoning, whereas women tend to outperform men in tests of verbal fluency (speed at coming up with words that correspond to some category), speed of calculation, recognition of facial expressions, and memory of object location (Kimura 1999). In navigation tasks, women typically use landmark cues more than men do. Men are also typically more competitive and aggressive than women. There is evidence that these sex differences result at least in part from differences in prenatal sex hormone levels (Collaer and Hines 1995). Thus, in terms of the prenatal hormone hypothesis it makes sense to ask whether gay people differ from straight people of the same sex in any of these traits.

Visuospatial tasks A number of studies have reported that gay men perform worse than straight men on a variety of visuospatial tasks, such as mental rotation, judgment of line orientation, and targeting (Gladue, Beatty et al. 1990; McCormick and Witelson 1991; Hall and Kimura 1995; Wegesin 1998; Neave, Menaged et al. 1999;

Rahman and Wilson 2003). In these studies the gay men performed at the female-typical level or at an intermediate level. Two studies failed to find differences between gay and straight men (Gladue, Beatty et al. 1990; Tuttle and Pillard 1991). In navigation tasks gay men, like heterosexual women, use landmark cues much more than heterosexual men do (Rahman, Andersson et al. 2005). Findings for women have been mixed: one recent large study found that lesbians are moderately better than heterosexual women at mental rotation, but the difference was only in speed of response, not accuracy (Rahman and Wilson 2003). Object location memory One recent large study of object location memory found that gay men do better than straight men, and about at the level of heterosexual women. No difference between the performance of lesbian and heterosexual women was found (Rahman, Wilson et al. 2003). Verbal fluency A 1991 study reported that gay men outperform heterosexual men in this trait (McCormick and Witelson 1991). Two subsequent studies came up with negative results (Gladue, Beatty et al. 1990; Neave, Menaged et al. 1999), but a recent large study reported that both gay men and lesbians have sex-atypical scores in verbal fluency tests (Rahman, Abrahams et al. 2003). Aggressiveness Gay men are reported to be less physically aggressive than straight men (Ellis, Hoffman et al. 1990; Gladue and Bailey 1995). No difference was found between lesbians and straight women (Gladue and Bailey 1995). Handedness There seems to be little or no difference in the handedness of heterosexual men and women (Lippa 2003), but most studies have found that gay men and/or lesbians are significantly more likely to be non-righthanded (i.e. left-handed or mixed-handed) than straight people of the same sex (Lalumiere, Blanchard et al. 2000; Mustanski, Bailey et al. 2002; Lippa 2003). Comment: Hand preference is observable before birth (Hepper,

Shahidullah et al. 1991), though it can change as a result of birth trauma and the like. The observation of increased nonrighthandness in gay people is therefore consistent with the idea that sexual orientation is influenced by prenatal processes. Birth order A considerable number of studies, mostly by a Canadian group of researchers, have reported that gay men tend to have more older brothers than do straight men (Blanchard and Bogaert 1996; Bogaert 2003). By way of explanation, the researchers hypothesize that some women develop antibodies to malespecific antibodies during early pregnancies with male fetuses, and that these antibodies affect the development of subsequent male fetuses in such a way as to increase the likelihood of homosexuality. This might happen through a general retardation of fetal growth with resulting small stature in postnatal life (Bogaert 2003). The Canadian group has reported that the birth order effect is seen only among right-handed men (Blanchard, Cantor et al. 2006). Being non-righthanded, even though by itself it increases the likelihood of being gay (see above), actually nullifies the gaypromoting influence of older brothers. The authors speculate that this might be because the two causal factors work to make a fetus gay through opposing mechanisms: by raising prenatal androgen levels in one case and lowering them in the other, for example, so that when the two causal factors are combined androgen levels end up within the range that promotes heterosexuality. Comment: The birth-order effect seems to be robust across numerous samples. It is not a particularly large effect, however: it would take an improbable number of older brothers (I think about 10) to give a boy even a 50:50 chance of being gay by the birth-order effect alone. In general, birth-order effects on psychological traits are explained by family dynamics (e.g., in this case, by parents “permitting” younger sons to be gay). However, the researchers have offered some good arguments why family dynamics are not likely to be the main explanation in the case of sexual orientation. For example, Bogaert reported that non-biological older brothers have no effect, while biological older brothers do have an effect even if they never lived with the proband (Bogaert 2006). A more direct test of their hypothesis, such as the detection of anti-male antibodies in the mothers of gay men, remains to be done.

General comments Although quite a few of the findings reported here are inconsistent between studies or await independent replication, my general conclusion is that biological processes, especially the prenatal, hormonally-controlled sexual differentiation of the brain, are likely to influence a person’s ultimate sexual orientation.