"Cerebral responses to putative pheromones and objects of sexual attraction were recently found to differ between homo- and heterosexual subjects. Although this observation may merely mirror perceptional differences, it raises the intriguing question as to whether certain sexually dimorphic features in the brain may differ between individuals of the same sex but different sexual orientation...
HeM [heterosexual men] and HoW [homosexual women] showed a rightward cerebral asymmetry, whereas volumes of the cerebral hemispheres were symmetrical in HoM [homosexual men] and HeW [heterosexual women]. No cerebellar asymmetries were found. Homosexual subjects also showed sex-atypical amygdala connections. In HoM, as in HeW, the connections were more widespread from the left amygdala; in HoW and HeM, on the other hand, from the right amygdala...
The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects. The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities."
National Geographic, on June 16, 2008, published the article "Gay Men, Straight Women Have Similar Brains," by James Owen, which stated:
"Differences both in the brain activity and anatomy were observed in a study involving 90 men and women, including homosexuals and heterosexuals of both genders. The researchers monitored neural activity in the brain by charting blood flow. The scans were carried out when the volunteers were resting and exposed to no external stimuli. Researchers focused in particular on the amygdala, an almond-shaped structure inside each brain hemisphere associated with processing and storing emotions. In homosexuals, brain activity most closely matched that of heterosexuals of the other sex. For example, the study found that straight men and gay women are both wired for a greater 'fight or flight' response than gay men or straight women, the team reports this week in the journal Proceedings of the National Academy of Sciences. Also, homosexual men and straight women showed significantly more neural connections across the two brain hemispheres than heterosexual men did."
"Only biological older brothers (reared with or not) and no other sibling characteristic, including nonbiological older brothers and the time reared with older biological or older nonbiological brothers, predicted men's sexual orientation. ...
These results support a prenatal origin to sexual orientation development in men...
A mother's body may have a memory for male (but not female) fetuses because she herself is female, and thus, her immune system may interpret and remember male ... fetuses as foreign. ...
[T]he link between the mother's immune reaction and the child's future sexual orientation would probably be some effect of maternal anti-male antibodies on the sexual differentiation of the brain."
"Brain Response To Putative Pheromones in Lesbian Women", Proceedings of the National Academy of Sciences
May 8, 2006
"Our study can't answer questions of cause and effect. We can't say whether the differences are because of pre-existing differences in their brains, or if past sexual experiences have conditioned their brains to respond differently."
Dr. Savic additionally noted in a May 8, 2006 New Scientist article:
"Lesbian and heterosexual women respond differently to specific human odours, a brain-scanning study has found. The homosexual women showed similar brain activity to heterosexual men when they inhaled certain chemicals, which may be pheromones, the researchers say.
When the heterosexual women smelled AND [a chemical produced by males] their brains showed activity in the anterior hypothalamus, a region of the brain thought to process sexual cues. But EST [a chemical produced by females] only produced activity in the olfactory region of their brains, the area that processes smells. The lesbians, however, only showed activity in the olfactory region whichever odour they smelled."
"We show that male [gene] splicing is essential for male courtship behavior and sexual orientation. More importantly, male [gene] splicing is also sufficient to generate male behavior in otherwise normal females. These females direct their courtship toward other females (or males engineered to produce female pheromones).
The splicing of a single neuronal gene thus specifies essentially all aspects of a complex innate behavior."
"These findings show that our brain reacts differently to the two putative pheromones compared with common odors, and suggest a link between sexual orientation and hypothalamic neuronal processes." May 2005 Ivanka Savic-Berglund
The New York Times noted in a May 10, 2005 article "For Gay Men, an Attraction to a Different Kind of Scent," by Nicholas Wade:
"Using a brain imaging technique, Swedish researchers have shown that homosexual and heterosexual men respond differently to two odors that may be involved in sexual arousal, and that the gay men respond in the same way as women. The new research may open the way to studying human pheromones, as well as the biological basis of sexual preference. Pheromones, chemicals emitted by one individual to evoke some behavior in another of the same species, are known to govern sexual activity in animals, but experts differ as to what role, if any, they play in making humans sexually attractive to one another."
"The present study examined the eyeblink startle responses to acoustic stimuli of 59 healthy heterosexual and homosexual men and women. Homosexual women showed significantly masculinized PPI compared with heterosexual women, whereas no difference was observed in PPI between homosexual and heterosexual men. These data provide the first evidence for within-gender differences in basic sensorimotor gating mechanisms and implicate the known neural substrates of PPI in human sexual orientation....
Because the startle response is known to be involuntary rather than learned, this strongly indicates that sexual orientation is largely determined before birth."
"This study tested the hypothesis that some individuals whose sexual orientation is predominantly homosexual can, with some form of reparative therapy, become predominantly heterosexual...
The majority of participants gave reports of change from a predominantly or exclusively homosexual orientation before therapy to a predominantly or exclusively heterosexual orientation in the past year. Reports of complete change were uncommon. Female participants reported significantly more change than did male participants...
Thus, there is evidence that change in sexual orientation following some form of reparative therapy does occur in some gay men and lesbians."
Presented at the annual conference of the European Society of Human Reproduction and Embryology, Madrid, Spain
June 30, 2003
"Preliminary research suggests that 'Lesbians are more than twice as likely to suffer from a hormone-related condition [polycystic ovary syndrome], fueling theories that hormones play a role in developing their sexuality.'
...while there was no evidence that polycystic ovaries could be implicated as a cause of lesbianism, it was possible that this hormone imbalance could be linked to both the medical condition and sexuality."
"Scientists say they have found a way to 'switch' homosexual behavior on and off in male fruit flies.
The researchers were able to do this by temporarily disrupting synaptic transmissions in the flies.
Previous research indicated that the sexual orientation of fruit flies is genetically determined, but the brain pathways for controlling sexual preference weren't clear."
"We present the results of a survey of 882 dissatisfied homosexual people whom we queried about their beliefs regarding conversion therapy and the possibility of change in sexual orientation...
Of the 882 participants, 726 of them reported that they had received conversion therapy from a professional therapist or a pastoral counselor...
Before treatment or change, only 2.2% of the participants perceived themselves as exclusively or almost entirely heterosexual, whereas after treatment or change, 34.3% perceived themselves as exclusively or almost entirely heterosexual...
As a group, the participants reported large and statistically significant reductions in the frequency of their homosexual thoughts and fantasies that they attributed to conversion therapy or self-help. They also reported large improvements in their psychological, interpersonal, and spiritual well-being."
"The Relation Between Sexual Orienation and Penile Size", Archives of Sexual Behavior
June, 1999
"The relation between sexual orientation and penile dimensions in a large sample of men was studied...
Penile dimensions were assessed using five measures of penile length and circumference from Kinsey's original protocol. On all five measures, homosexual men reported larger penises than did heterosexual men.
Alterations of typical levels of prenatal hormones in homosexual men may account for these findings."
"Researchers say they have found the first strong evidence of a physical difference between lesbians and straight women -- a finding that the inner ears of gay women work more like those of men.
The discovery adds new support to the theory that sexual orientation may be predisposed at birth."
"DNA linkage analysis of a selected group of 40 families in which there were two gay brothers and no indication of nonmaternal transmission revealed a correlation between homosexual orientation and the inheritance of polymorphic markers on the X chromosome in approximately 64 percent of the sib-pairs tested.
The linkage to markers on Xq28, the subtelomeric region of the long arm of the sex chromosome, had a multipoint lod score of 4.0 (P = 10(-5), indicating a statistical confidence level of more than 99 percent that at least one subtype of male sexual orientation is genetically influenced."
14. J. Michael Bailey, PhD, and Richard C. Pillard, MD, et al.
Archives of General Psychiatry
Dec. 1991
"Of the relatives whose sexual orientation could be rated, 52% of monozygotic cotwins, 22% of dizygotic co-twins, and 11% of adoptive brothers were homosexual. Heritabilities were substantial under a wide range of assumptions about the population base rate of homosexuality and ascertainment bias."
"A Difference in Hypothalamic Structure Between Heterosexual and Homosexual Men" in Science
June, 1991
"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."
