Neuroscience and sexual orientation

Sexual orientation is an enduring pattern of romantic or sexual attraction (or a combination of these) to persons of the opposite sex or gender, the same sex or gender, or to both sexes or more than one gender, or none of the aforementioned at all.^[1][2] The ultimate causes and mechanisms of sexual orientation development in humans remain unclear and many theories are speculative and controversial. However, advances in neuroscience explain and illustrate 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 illustrating prenatal hormone exposure, maternal immunity, and developmental instability. Other proposed factors include genetic control of sexual orientation. No conclusive evidence has been shown that environmental or learned effects are responsible for the development of non-heterosexual orientation.^[3]

As of 2005, sexual dimorphisms in the brain and behavior among vertebrates were accounted for by the influence of gonadal steroidal androgens as demonstrated in animal models over the prior few decades. The prenatal androgen model of homosexuality describes the neuro-developmental effects of fetal exposure to these hormones.^[3] In 1985, Geschwind and Galaburda proposed that homosexual men are exposed to high androgen levels early in development and proposed that temporal and local variations in androgen exposure to a fetus's developing brain is a factor in the pathways determining homosexuality.^[3] This led scientists to look for somatic markers for prenatal hormonal exposure that could be easily, and non-invasively, explored in otherwise endocrinologically normal populations. Various somatic markers (including 2D:4D finger ratios, auditory evoked potentials, fingerprint patterns and eye-blink patterns) have since been found to show variation based on sexual orientation in healthy adult individuals.^[3]

Other evidence supporting the role of testosterone and prenatal hormones in sexual orientation development include observations of male subjects with cloacal exstrophy who were sex-assigned as female during birth only later to declare themselves male. This supports the theory that the prenatal testosterone surge is crucial for gender identity development. Additionally, females whose mothers were exposed to diethylstilbestrol (DES) during pregnancy show higher rates of bi- and homosexuality.^[4]

Variations in the hypothalamus may have some influence on sexual orientation. Studies show that factors such as cell number and size of various nuclei in the hypothalamus may impact one's sexual orientation.^[5]

Brain structure

Multiple areas of the human brain have been found to display structural differences associated with sexual orientation. Several of these can be found in the hypothalamus, particularly the sexually dimorphic nucleus of the preoptic area (SDN-POA) in mammals. Researchers have shown that the SDN-POA aides in sex-dimorphic mating behavior in some mammals, which is representative of human sexual orientation.^[6] In humans, its equivalent is the interstitial nucleus of the anterior hypothalamus (INAH-3), which has demonstrated size differences between heterosexual and homosexual men in early post-mortem studies.^[7][6][8] There are also other POA-like brain structures in the human brain which differ between sexual orientations, such as the suprachiasmatic nucleus and the anterior hypothalamus have similarly shown structural variations linked to sexual orientation.^[9][6] Using meta-analysis of neuroimaging, researchers have concluded that these areas are linked to sexual preferences in humans, which would explain why they may differ based on sexual orientation.^[8]

Neuroimaging studies have also identified differences in other brain regions. Meta-analyses suggest that areas involved in emotional processing and sexual arousal, such as the thalamus and amygdala, exhibit distinct patterns between heterosexual and homosexual individuals.^[10] Another area of the brain which demonstrates sexual orientation differentiation is the thalamus, which is a structure involved in sexual arousal and reward. Earlier research found that the thalamus tends to be larger in heterosexual individuals compared to homosexual individuals.^[9][11] The placement of connections in the amygdala have been demonstrated to differ between heterosexual and homosexual individuals.^[12] The posterior cingulate cortex, a part of the occipital lobe, the region of the brain that processes visual information, has also been demonstrated to have differences based on sexual orientation.^[12]

Additionally, hemispheric connectivity differs by sexual orientation: the anterior commissure was reported to be wider and the corpus callosum larger in homosexual men than in heterosexual men.^[9][12]

Some areas of the brain which researchers looked at but did not find differences in structure between sexualities are the temporal cortex, hippocampus and putamen.^[12]

Multiple neuroimaging studies have reported structural brain differences associated with sexual orientation. Early research by LeVay (1991) found that the third interstitial nucleus of the anterior hypothalamus (INAH-3) was significantly smaller in homosexual men compared to heterosexual men, though methodological limitations and small sample sizes limited conclusions.^[13]

Subsequent studies using MRI techniques observed variations in hemispheric asymmetry and amygdala connectivity patterns, suggesting that homosexual men exhibit brain structural and functional patterns more similar to heterosexual women, and vice versa for homosexual women.^[14]

A 2020 study by Wang et al. examined cortical thickness, surface area, and gray matter volume in 29 gay men, 29 straight men, 17 gay women, and 17 straight women. The results showed complex interactive effects of biological sex and sexual orientation. Notably, gay men exhibited increased cortical thickness in regions such as the right anterior cingulate cortex (ACC), superior frontal gyrus, and precuneus when compared to both heterosexual men and women. In contrast, straight women displayed "female-typical" cortical thickness in parietal regions. Also, gay men demonstrated a mixture of sex-atypical (cross-sex) characteristics in midline brain areas (ACC, frontal cortex) alongside some sex-typical patterns. The study also emphasized differences in the left inferior frontal gyrus and temporal lobes, suggesting these regions may play a critical role in the sexual differentiation of the human brain.^[15]

Functional neuroimaging studies have examined whether sexual orientation is associated with differences in brain activation patterns during cognitive tasks known to show sex differences. Folkierska-Żukowska et al. (2020) investigated mental rotation ability, confirming established sex differences in parietal and frontal regions, with men showing greater parietal activation and women greater frontal activation. Among gay men, the results indicated variability linked to childhood gender nonconformity. Subgroups of gay men with gender-nonconforming histories displayed partial cross-sex shifts in brain activation, while others did not. The study suggested the presence of subgroup variation within sexual orientation categories, with functional differences emerging primarily in specific sub-populations.^[16]

While some studies have reported subtle neuroanatomical and functional differences related to sexual orientation, others have urged caution in interpreting these findings. A systematic review by Frigerio et al. (2021) emphasized that most neuroimaging features of homosexual individuals resembled those of same-sex heterosexuals rather than opposite-sex individuals. The review noted that reported effects were often inconsistent, small in size, and frequently failed replication.^[17] For example, Kranz et al. (2014) found that sex differences in white-matter microstructure persisted after adjusting for sexual orientation.^[18] These findings have led some researchers to argue that evidence for orientation-linked brain structure is limited and that any real effects may be confined to subtle network-level patterns detectable only in larger, more powered studies.^[19][20]

A 2021 voxel-based morphometry MRI study involving approximately 500 adults identified focal gray matter volume differences associated with sexual orientation. Votinov et al. reported that heterosexual men exhibited larger thalamic volumes than homosexual men, while homosexual women had larger putamen volumes than heterosexual women. Additionally, volume differences were noted in the precentral gyrus. The study concluded that sexual orientation is "reflected in brain structure characteristics" in a sex-specific manner.^[21]

A large-scale imaging and genetics study conducted by Abe et al. (2021) analyzed brain MRI and genetic data from 18,645 individuals through the UK Biobank project. Using a data-driven partial least squares (PLS) approach, the study found that brain features typically distinguishing male and female participants were attenuated in non-heterosexual individuals. Specifically, non-heterosexual women exhibited brain-activation and structural patterns shifted toward male-typical profiles. Additionally, higher polygenic scores for same-sex sexual behavior were associated with increased volume in occipital and temporo-occipital cortices. The authors concluded that the study provided multivariate neuroanatomical correlates of same-sex sexual behavior, suggesting a potential biological basis for sexual orientation.^[22]

More recently, a 2022 study by Clemens et al. utilized resting-state functional magnetic resonance imaging (fMRI) combined with machine learning techniques to examine patterns of intrinsic functional connectivity (FC) in relation to sexual orientation. The study involved 86 adult participants and demonstrated that a classifier trained on FC data could predict participants’ sexual orientation with approximately 92% accuracy, a rate significantly above chance. Key brain regions contributing to this classification included the precentral gyrus, precuneus, and prefrontal cortex, among others. The study highlighted that patterns of connectivity within distributed brain networks, rather than isolated brain structures, carry information associated with sexual orientation. In contrast, analyses based solely on gray matter volumes achieved a lower predictive accuracy of approximately 62%.^[23]

Fraternal birth order effect

Main article: Fraternal birth order and male sexual orientation

Neuroscience has been implicated in the study of birth order and male sexual orientation. A significant volume of research has found that the more older brothers a man has from the same mother, the greater the probability he will have a homosexual orientation. Estimates indicate that there is a 33–48% increase in chances of homosexuality in a male child with each older brother, and the effect is not observed in those with older adoptive or step-brothers, indicative of a prenatal biological mechanism.^[3] Ray Blanchard and Anthony Bogaert discovered the association in the 1990s, and named it the fraternal birth order (FBO) effect. The mechanism by which the effect is believed to operate states that a mother develops an immune response against a substance important in male fetal development during pregnancy, and that this immune effect becomes increasingly likely with each male fetus gestated by the mother. This immune effect is thought to cause an alteration in (some) later born males' prenatal brain development. The target of the immune response are molecules (specifically Y-linked proteins, which are thought to play a role in fetal brain sex-differentiation) on the surface of male fetal brain cells, including in sites of the anterior hypothalamus (which has been linked to sexual orientation in other research). Antibodies produced during the immune response are thought to cross the placental barrier and enter the fetal compartment where they bind to the Y-linked molecules and thus alter their role in sexual differentiation, leading some males to be attracted to men as opposed to women. Biochemical evidence to support this hypothesis was identified in 2017, finding mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than other samples of women, including mothers of heterosexual sons.^[24][25]

The effect does not mean that all or most sons will be gay after several male pregnancies, but rather, the odds of having a gay son increase from approximately 2% for the firstborn son, to 4% for the second, 6% for the third and so on.^[24][26] Scientists have estimated that 15–29% of gay men owe their sexual orientation to this effect, but the number may be higher, as prior miscarriages and terminations of male pregnancies may have exposed their mothers to Y-linked antigens. In addition, the effect is nullified in left-handed men. As it is contingent on handedness and handedness is a prenatally determined trait, it further attributes the effect to be biological, rather than psychosocial.^[27] The fraternal birth order effect does not apply to the development of female homosexuality.^[27] Blanchard does not believe the same antibody response would cause homosexuality in firstborn gay sons – instead, they may owe their orientation to genes, prenatal hormones and other maternal immune responses which also influence fetal brain development.^[25]

The few studies which have not observed a correlation between gay men and birth order have generally been criticized for methodological errors and sampling methods.^[28] J. Michael Bailey has said that no plausible hypothesis other than a maternal immune response has been identified.^[28]

Research directions

As of 2005, research directions included:^[29]

• finding markers for sex steroid levels in the brains of fetuses that highlight features of early neuro-development leading to certain sexual orientations
• determine the precise neural circuitry underlying direction of sexual preference
• use animal models to explore genetic and developmental factors that influence sexual orientation
• further population studies, genetic studies, and serological markers to clarify and definitively determine the effect of maternal immunity
• performing neuroimaging studies to quantify sexual-orientation-related differences in structure and function in vivo
• carrying out neurochemical studies to investigate the roles of sex steroids upon neural circuitry involved in sexual attraction

Since the early 2000s, research in this area has expanded considerably. Key recent developments include a shift towards larger, data-driven studies using multivariate analyses and machine learning techniques. Notably, the UK Biobank imaging-genetics project has combined brain MRI, polygenic risk scores, and multivariate statistical approaches in large population samples to investigate neuroanatomical correlates of same-sex sexual behavior.^[30]

Another trend has been a reevaluation of binary models of brain "sex" and "gender". Several contemporary studies have emphasized the importance of considering gender identity, social context, and environmental stressors rather than focusing solely on anatomical dimorphism. One commentary published in 2025 noted that decades of male-versus-female brain research had not fully delivered on early expectations, and called for greater attention to how factors such as stigma and stress influence brain development.^{[citation needed]}

Additionally, whole-brain network analyses and pattern classifiers have been increasingly applied to neuroimaging data. Recent studies suggest that orientation-related patterns may be more accurately detected through distributed network connectivity rather than isolated regional volume or thickness differences alone.^[31]

Recent research directions also emphasize greater inclusivity and intersectionality. Compared to early work, contemporary studies aim to include a wider range of identities. Analyses of the literature indicate that, although LGBTQ-related neuroscience studies increased during the 2010s, they remained unevenly distributed: approximately 32% focused on gay men and 24% on transgender people, while lesbians and bisexual individuals were notably underrepresented.^[32] New initiatives call for broadening samples and considering variables such as ethnicity, gender expression, and minority stress. This reflects a broader shift toward more diverse, representative studies and an intersectional lens in neuroimaging research.

The field has also adopted a more cautious approach regarding interpretation and reproducibility. Scholars in the 2020s have openly questioned whether robust, replicable brain correlates of sexual orientation will emerge, given historical replication difficulties. Some researchers also express concern about the ethical implications of attempting to "explain" sexual orientation through neuroscience.^[33][34] As a result, recent studies increasingly emphasize effect sizes, confidence intervals, and transparency in reporting, while systematic reviews and meta-analyses frequently highlight limitations and call for careful, nuanced interpretation of findings.^[35]

See also

• Biology and sexual orientation
• Neuroanatomy of intimacy
• Neuroscience of sex differences
• Heterosexuality
• Homosexuality and psychology