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The Neuroscience of Sexual Orientation



Table of Contents

Overview
Sexual orientation refers to an individual’s personal and social identity involving behaviors and/or emotions involving sexual attraction to men, women, or both sexes. Homosexuality (attraction to members of the same sex), heterosexuality (attraction to the opposite sex), and bisexuality (attracted to members of both sexes) are the three classes of sexual orientation. The ultimate causes and mechanisms of sexual orientation development in humans remain unclear and many theories are speculative and controversial. However, advances in the sciences of the brain (neuroscience) are providing evidence, support, and explanation of these ideas and are illustrating characteristics linked to sexual orientation. Studies have explored structural neural-correlates, functional and/or cognitive relationships, and developmental theories relating to sexual orientation in humans.

Developmental Neurobiology
Many theories concerning the development of sexual orientation involve fetal neural development, with proposed models involving prenatal hormone exposure, maternal immunity, and developmental instability. Other proposed factors include genetic control of sexual orientation while no conclusive evidence has been shown that environmental or learned effects are responsible for the development of non-typical sexual orientation.

Prenatal Androgen Model
Sexual dimorphisms in the brain and behavior among vertebrates is accounted for the by the influence of gonadal steroidal androgens as demonstrated in animal models over the past few decades. The prenatal androgen model of homosexuality describes the neurodevelopmental effects of fetal exposure to these hormones. In 1985, Geschwind and Galaburda proposed that homosexual men are exposed to particularly high androgen levels very early in development, explaining both their tendency to be less right-handed and by extension the hyper-masculinized traits observed in this population. It is currently argued that temporal and local variations in androgen exposure to a fetus’s developing brain is a factor in the pathways determining homosexuality. Recent research has been performed to find somatic markers for prenatal hormonal exposure which have been found to show variation based on sexual orientation in healthy adult individuals.

2D:4D Digit Ratio
The best, non-invasive, marker of prenatal hormone exposure is the ratio of the second and fourth finger lengths (2D:4D ratio), a known sexually dimorphic measure (males showing lower ratios than females). Patients with androgen over-exposure (such as in congenital adrenal hyperplasia) show lower 2D:4D ratios, providing evidence linking prenatal androgen exposure as key to this feature. Additionally, the 2D:4D ratio has been shown to be affected by variation in the androgen receptor gene and the ratio of testosterone to estrogen in amniotic fluid is found to be negatively correlated with the 2D:4D ratio.

Independent studies indicate that homosexual women have masculinized (lower) ratios and homosexual men show either hyper-masculinized or feminized ratios. These findings reinforce the prenatal androgen model - abnormal prenatal hormone exposure is related to the development of human homosexuality.

Auditory Evoked Potentials
Studies of the central nervous system processing of auditory sensation, aspects of which has been linked to prenatal androgen exposure, to click-stimuli have shown that homosexual women have masculinized responses while homosexual men have hyper-masculinized responses.

Fraternal Birth Order Effect
Studies show that homosexual men have higher numbers of older male siblings. This finding led to the development of the Fraternal Birth Order effect theory of homosexuality, stating that a mother becomes progressively immunized to successive male children, leading to increased chances of homosexuality in later male children. The mechanism involves the mother producing increasing anti-male antibodies to male-specific antigens expressed in male fetuses. These antibodies are thought to block the full masculinization of the fetal brain by “binding to and inactivating male-specific molecules located on the surface of fetal brain cells” thus preventing the morphogenesis of masculinized sexual preferences2 Estimations indicate that there is a 33% increase in chances of homosexuality in a male child with each older male sibling. A BBC study reported that the fraternal birth order effect does not apply to left-handed homosexual males. Support for this theory is given by data indicating that the effect holds true only for biological brothers and the chances of male homosexuality is not increased by the number of older stepbrothers or adoptive siblings. This theory does not apply to the development of female homosexuality.

Developmental Instability and Handedness
The chances of being left-handed are increased in homosexual populations. Homosexual men have an approximate 34% odds ratio of being non-right-handed while lesbian women have 91% odds of being so, both compared to heterosexual populations. It has been proposed that non-right-handedness (including ambidexterity) is related to homosexuality through developmental instability. Developmental instability refers to the level of vulnerability to environmental and genetic stresses and perturbations during development.

Structural Differences
Postmortem and imaging studies over the past two decades have revealed structural differences in both global structures and sexually-related brain structures between heterosexual and homosexual subjects.

Cerebral Asymmetry
The size of the brain’s hemispheres is a sexually dimorphic trait in which men tend to show asymmetry in the volumes of their hemispheres while women show volumetric symmetry. A recent volumetric MRI study indicated that homosexuals showed sex-atypical symmetry: homosexual men showed hemispheric volumes to be symmetric similar to heterosexual women and homosexual women showed asymmetry in hemispheric volumes as heterosexual men do. These findings demonstrate a global neurological difference in brain structures showing sex-atypical characteristics associated with sexual orientation.

Anterior Commissure
The anterior commissure, a bundle of white matter fibers connecting the two cerebral hemispheres, was found by Allen and Gorski to be larger in homosexual men and heterosexual women than in heterosexual men. This finding provides a possible anatomical basis for higher inter-hemispheric functional connections in homosexuals explaining why homosexual men and heterosexual women show language circuit functional symmetry in out performing heterosexual men in verbal tests.

Functional Differences
Recent studies have begun exploring the functional and cognitive substrates of sexual orientation, ultimately a behavioral manifestation. Neural processing in response to specific stimuli and sexually-biased cognitive tasks have been found to be correlated with an individual’s sexual orientation.

Response to Pheromones
Two proposed human pheromones – the progesterone derivative 4,16-androstadien-3-one (AND) and an estrogen-like steroid estra-1,3-5(10),16-tetraen-3-ol (EST) – have been shown to have sexual orientation specific responses in activating the neural circuits of the anterior hypothalamus in homosexual and heterosexual subjects. The anterior hypothalamus is involved in processing reproductive functions and recent evidence suggests it helps integrate hormonal and sensory cues involved in sexual behavior and sexual preference.

Recent functional neuro-imaging experiments have shown that the presentation of AND, found in male sweat, as a olfactory stimuli produced normal olfactory responses in heterosexual men and homosexual women, while activating the anterior hypothalamus in homosexual men and heterosexual women. The proposed pheromone EST, found in the urine of pregnant women, produces normal olfactory activation in homosexual men and heterosexual women while homosexual women and heterosexual men demonstrated sexually-related hypothalamic responses.

Homosexual men showed the same sexually-related functional responses to these stimuli as heterosexual women and homosexual women responded like heterosexual men. This research conducted by Berglund and Savic indicates overall that AND and EST induce “sex-specific effects on the autonomic nervous system” and that the stimuli elicited a response pathway that was dependent on the subject’s sexual orientation rather than phenotypic sex.

Functional Cerebral Asymmetry
Differences in neural processing and cognitive tasks have been found in relation to sexual orientation. In a 1987 review on cognition, cerebral lateralization, and sexual orientation, Sandes and Ross-Field suggested that prenatal hormonal events would lead to functional cerebral asymmetries related to sexual orientation.

Several cognitive tasks are known to be sexually dimorphic. The better verbal ability of women is associated with reduced lateralization of language tasks while the male advantage in spatial reasoning corresponds to marked cerebral lateralization. Sexual orientation effects in some of these tasks have been found in recent studies.

In the Vincent Mechanical Diagrams test, a dot detection divided field measure of functional cerebral asymmetry, homosexual men performed the same as heterosexual women with both scoring lower than heterosexual men displaying less asymmetry. Additionally, homosexual men display higher verbal performance IQ scores on subtests of the Wecshler Adult Intelligence Scale, in concordance with female testing patterns. On several other tests, including a male-biased targeted throwing task and a female-biased Purdue pegboard test, the performance of homosexual men and heterosexual women showed no statistical difference from each other, while both significantly differed from heterosexual men.

Additionally, reduced asymmetry was found in a magnetoencephelographic study in which MEG-based source location estimates of an auditory evoked signal is found to be hemispherically symmetric in heterosexual women and homosexual men, while asymmetric in heterosexual men.

Response to Visual Sexual Stimuli
A recent functional magnetic resonance imaging study has demonstrated that upon viewing of both heterosexual and homosexual erotic visual stimuli, only those images corresponding to the subject’s sexual orientation produced hypothalamic activation patterns associated with sexual arousal. The response of heterosexuals viewing heterosexual adult videos showed the same pattern of sexual arousing neural processing as homosexuals viewing same-sex adult videos, while the viewing of the opposite orientation’s images did not elicit the same response.

Related Studies
Various animal and insect models have been used to explore sexual orientation and brain characteristics. One experiment involved genetically altering male Drosophila causing them to have feminized brain structures involved in processing sexually dimorphic contact pheromones. Transformed males showed increase homosexual courtship behaviors to wild-type male flies, and there a correlation was found between the courtship behavior and the expression of the altered gene in the sexually related brain regions.

A 2004 study exploring the difference in brain metabolism between heterosexual and homosexual males found that homosexuals had a significant reduction in hypothalamic glucose metabolism compared to that of heterosexuals with other brain regions including the prefrontal association cortex and portions of the cingulate cortex showing differences in the measured activation.