Bisexuality
Bisexuality is a sexual orientation defined by the capacity for emotional, romantic, and/or physical attraction to more than one sex.[1][2] Empirical studies, including physiological arousal research, provide robust evidence that self-identified bisexual individuals, particularly men, exhibit distinct patterns of response to both male and female stimuli, countering earlier skepticism that bisexuality merely reflects transitional or unstable preferences between exclusive heterosexuality and homosexuality.[3][4] Population surveys indicate prevalence rates of bisexual identification around 4-5% among U.S. adults, with higher figures—up to 15%—among younger generations like Gen Z, alongside 2-10% reporting same-sex behavior regardless of identity; these patterns suggest both stable orientations and some fluidity, with about 1 in 11 adults shifting identities over time.[5][6][7] Biologically, genetic analyses reveal variants associated with bisexual behavior in men correlate positively with reproductive success (e.g., higher offspring numbers) and risk-taking propensity, unlike exclusive same-sex behavior, which shows negative correlations, implying evolutionary advantages under certain conditions.[8][9] Bisexual individuals often face unique challenges, including elevated risks for mental health disorders, substance use, and poorer physical outcomes compared to heterosexuals or exclusive homosexuals, potentially linked to minority stress and identity invalidation from both mainstream and LGBTQ+ communities.[10][11]Definitions and Terminology
Etymology and Historical Usage
The term bisexuality derives from the prefix bi-, meaning "two" from Latin bis and Greek di-, combined with sexuality, rooted in Latin sexus denoting sex or division into sexes.[12] The adjective bisexual first appeared in English around 1793 to describe organisms possessing characteristics of both sexes, such as hermaphroditic plants or animals.[13] In its earliest documented usage, bisexuality referred exclusively to biological phenomena involving dual sexual characteristics, as introduced in 1859 by anatomist Robert Bentley Todd to describe the possession of both male and female physical traits in human development.[14] This biological connotation persisted through the mid-19th century, aligning with emerging anatomical studies of intersex conditions and embryonic development, where "bisexuality" denoted a primordial state of undifferentiated sexual organs before specialization.[12] By the late 19th century, amid the rise of sexology, the term shifted toward denoting sexual attraction to both males and females. The first application in this psychological sense occurred in 1892, when Charles Gilbert Chaddock used bisexual in his English translation of Richard von Krafft-Ebing's Psychopathia Sexualis to characterize individuals exhibiting both heterosexual and homosexual inclinations, framing it as a form of psychosexual hermaphroditism.[15] Krafft-Ebing himself had employed bisexuell in the original 1886 German edition to describe mixed sexual perversions, influencing early classifications of human eroticism beyond strict monosexuality.[16] This usage marked a departure from purely anatomical meanings, embedding bisexuality within debates on innate drives and pathology in works by figures like Sigmund Freud, who later invoked it to argue for universal bisexual potential in psychic development.[14]Contemporary Definitions and Distinctions
Bisexuality is defined in contemporary psychological literature as a sexual orientation involving the capacity for romantic, emotional, romantic, and/or sexual attraction to persons of more than one sex.[17] The American Psychological Association describes bisexual individuals as those with the potential to form attractions and/or relationships to more than one gender, encompassing a spectrum of experiences rather than exclusive preferences.[18] Peer-reviewed definitions emphasize attractions to both male and female categories, distinguishing bisexuality from monosexual orientations by its dual-directed nature.[2][1] In distinction from heterosexuality, which entails enduring attraction primarily to the opposite sex, and homosexuality, which involves attraction primarily to the same sex, bisexuality is empirically characterized by attractions spanning both sexes, often measured via self-reported patterns or physiological responses in research settings.[2] These distinctions are not always binary in practice, as bisexual attractions may vary in intensity across individuals, with some exhibiting stronger preferences toward one sex without exclusivity.[19] Empirical studies highlight that bisexuals do not consistently occupy an intermediate position between heterosexuals and homosexuals in arousal metrics, underscoring bisexuality as a distinct category rather than a mere midpoint.[19] Contemporary debates differentiate bisexuality from pansexuality, with the latter defined as attraction irrespective of gender or sex, potentially extending to non-binary or transgender individuals without regard for biological sex.[20] Traditional bisexual definitions focus on attraction to two or more genders within a binary framework (male and female), whereas pansexuality explicitly rejects gender as a factor in attraction.[21] However, overlap exists, as some self-identified bisexuals report attractions encompassing all genders, leading researchers to view pansexuality as a subset or semantic variant of bisexuality rather than a fundamentally separate orientation.[22] These distinctions rely heavily on self-identification, with empirical validation challenging due to subjective reporting and cultural influences on terminology.[20]Biological and Neurological Foundations
Genetic Correlates
Twin studies estimate the broad-sense heritability of same-sex sexual behavior, which encompasses both exclusive same-sex behavior and bisexual behavior, at approximately 30%.[8] This heritability figure derives from analyses of monozygotic and dizygotic twins, indicating a moderate genetic contribution alongside environmental influences, though specific heritability for bisexuality as distinct from exclusive homosexuality remains less precisely quantified due to overlapping phenotypes in datasets.[23] Genome-wide association studies (GWAS) have identified polygenic signals associated with same-sex sexual behavior, accounting for 8-25% of variance in males and females combined, but these signals do not localize to a single "bisexuality gene" and show limited predictive power for individuals.[24] A 2024 analysis disentangled bisexual behavior (defined as non-exclusive same-sex activity) from exclusive same-sex behavior, revealing that genetic variants linked to male bisexual behavior positively correlate with reproductive success, including higher offspring counts, unlike variants tied to exclusive same-sex behavior which show negative reproductive correlations.[8] This suggests that alleles predisposing to bisexual patterns may persist evolutionarily due to fitness advantages, potentially through increased mating opportunities across sexes.[25] In males, bisexual behavior-associated genetic variants also correlate with risk-taking propensity, a trait that may enhance reproductive outcomes by promoting boldness in social and sexual contexts, though no such correlation appears in females.[8] Female bisexual behavior, while heritable, exhibits weaker links to reproductive metrics in available data, possibly reflecting sex-specific evolutionary pressures or measurement challenges in self-reported behaviors.[26] Overall, these findings underscore a polygenic architecture where bisexual tendencies differ genetically from exclusive homosexuality, with the former potentially adaptive in population-level dynamics, though environmental and cultural factors modulate expression and confound causal inference.[8][24]Arousal Patterns and Physiology
Studies of genital arousal patterns indicate category-specific responses in men, where self-identified bisexual men typically exhibit physiological arousal concordant with either heterosexual or homosexual patterns rather than intermediate bisexual responses. In a 2005 study by Rosenthal, Gorzalka, and Bailey involving 30 heterosexual, 33 bisexual, and 38 homosexual men, penile plethysmography measured genital arousal to male and female erotic stimuli; bisexual men showed either predominantly heterosexual or homosexual arousal profiles, with no distinct bisexual pattern differentiating them from monosexual groups.[27] This finding aligns with broader evidence that male sexual arousal is highly gender-specific, potentially reflecting evolutionary adaptations for mate selection.[28] In contrast, women's genital arousal, measured via vaginal photoplethysmography (VPA) for changes in vaginal pulse amplitude, demonstrates greater non-specificity across orientations. Chivers, Rieger, Latty, and Bailey's 2004 research exposed heterosexual, bisexual, and lesbian women to erotic films; both heterosexual and lesbian women displayed significant VPA responses to both male and female stimuli, though subjective arousal remained orientation-concordant, suggesting a decoupling between physiological and self-reported arousal in females.[29] Bisexual women in subsequent studies, such as Bouchard et al. (2015), endorsed facets of bisexuality (e.g., orientation, fantasy) and showed less differentiated genital responses to gender cues compared to monosexual women, with female stimuli eliciting stronger responses overall, but patterns varying by bisexuality endorsement level.[30] Pupil dilation serves as a non-invasive physiological indicator of arousal, correlating with autonomic nervous system activation. A 2012 study by Rieger et al. found that men's pupil responses to sexual stimuli were category-specific, mirroring genital patterns, while women's were more bisexual regardless of identity; bisexual men displayed dilation primarily to one sex, often linked to higher self-reported curiosity toward the non-preferred sex rather than equivalent arousal.[31] A 2021 meta-analysis by Imhoff et al. confirmed bisexual men's greater pupil dilation to male stimuli in some contexts but emphasized that overall patterns in men lack robust bisexuality, unlike women who exhibit broader responsiveness.[32] These physiological measures highlight sex differences: male arousal aligns closely with stated orientation in a binary fashion, whereas female patterns allow for more fluidity, potentially influenced by contextual or hormonal factors, though direct causal links remain under investigation.[33] Longitudinal data from Jabbour et al. (2022) on arousal stability over time further showed bisexual individuals reporting greater change in patterns than monosexuals, but genital measures in men persisted as non-bisexual.[34]Neurological and Hormonal Factors
Studies utilizing functional magnetic resonance imaging (fMRI) have identified distinct neural activation patterns in bisexual individuals during responses to sexual stimuli. In a 2017 study of women, heterosexual participants exhibited stronger activation in sex-specific brain regions—such as the thalamus and ventral striatum for heterosexual women viewing male stimuli—while bisexual women displayed intermediate or bilateral activations not strictly aligned with either exclusive pattern, suggesting a unique neural processing of mixed-gender attractions.[35] Similarly, structural MRI research has revealed variations in cortical thickness and subcortical volumes among bisexuals, with some evidence of patterns intermediate between heterosexual and homosexual groups, though sample sizes for bisexual participants remain small and replication is limited.[36] Prenatal hormone exposure, particularly androgens like testosterone, is implicated in shaping sexual orientation, with atypical levels potentially contributing to bisexuality. Animal models and human proxy measures, such as the 2D:4D digit ratio (reflecting prenatal testosterone), correlate with non-exclusive orientations, where bisexual individuals often show ratios intermediate between heterosexual and homosexual averages.[37] A 2017 study linked prenatal progesterone exposure—used in treatments like those for threatened miscarriage—to increased rates of bisexual or homosexual identification in adulthood, with exposed women 29% more likely to report same-sex attractions compared to unexposed controls (n=3,034).[38] Circulating adult sex hormones show inconsistent differences; for instance, one analysis found lesbian and bisexual women with elevated testosterone and progesterone relative to heterosexual women, but overall evidence indicates minimal divergence in baseline levels across orientations.[39][40] These findings underscore that neurological and hormonal factors likely interact with genetic and environmental influences, but bisexuality-specific research lags due to smaller cohorts and definitional challenges in distinguishing it from exclusive orientations or behavioral fluidity.[41] Peer-reviewed studies emphasize prenatal over postnatal hormonal effects, with adult hormone therapies showing negligible impact on core orientation.[42] Limitations include reliance on self-reported orientation, which may conflate identity with physiology, and potential publication biases favoring novel over null results in understudied subgroups.[43]Psychological and Developmental Aspects
Classification in Sexual Orientation Models
Bisexuality is positioned intermediately in early dimensional models of sexual orientation, such as the Kinsey scale developed in 1948, which ranges from 0 (exclusively heterosexual) to 6 (exclusively homosexual), with ratings of 1-5 indicating varying degrees of bisexuality based on reported attractions, behaviors, and fantasies.[44] This continuum framework implies bisexuality as non-exclusive attraction rather than a discrete category, influencing subsequent self-report measures.[45] However, the Kinsey scale conflates multiple constructs, including physiological arousal, emotional preferences, and overt behavior, limiting its precision for classifying bisexuality empirically.[45] The Klein Sexual Orientation Grid, introduced in 1978 and expanded in 1985, refines this approach by evaluating seven dimensions—attractions, sexual fantasies, emotional preferences, social preferences, sex partners, self-identification, and lifestyle—across past, present, and ideal/future time periods on a 1-7 scale akin to Kinsey's.[46] Cluster analyses of the grid reveal bisexual respondents forming subgroups distinct from heterosexual and homosexual clusters, with intermediate but not averaged patterns, supporting bisexuality's classification as a multifaceted, potentially fluid orientation rather than a simple midpoint.[46] This multidimensionality accommodates evidence that bisexual individuals often report non-binary patterns, such as stronger emotional attractions to one gender alongside bisexual behavioral histories.[47] Contemporary debates contrast categorical models, treating heterosexuality, homosexuality, and bisexuality as taxonic classes, against dimensional spectra where bisexuality emerges as intermediate variation.[48] Physiological data, including pupillometry and genital plethysmography, demonstrate that bisexual-identified men display category-specific arousal to both male and female stimuli, distinct from monosexual patterns and refuting prior claims of rarity or inauthenticity in male bisexuality.[3] Similarly, genetic analyses indicate bisexual behavior correlates with unique polygenic signals separate from exclusive same-sex or opposite-sex orientations, bolstering evidence for bisexuality as a distinct phenotypic class rather than mere spectrum overlap.[8] Taxometric studies further suggest underlying latent structure favors categorical boundaries for primary orientations, with bisexuality occupying a valid, non-transient position, though fluidity in self-labeling complicates rigid assignment.[49]Stability and Fluidity Empirical Data
Longitudinal studies indicate that bisexuality demonstrates relative stability in the direction of attractions—characterized by non-exclusive patterns toward both sexes—but greater fluidity in self-reported identity labels compared to exclusive heterosexual or homosexual orientations.[50] In a 10-year prospective study of 79 non-heterosexual women tracked from adolescence to adulthood, 67% changed their sexual identity labels at least once, with one-third shifting two or more times; bisexual and unlabeled identities were adopted more frequently than lesbian or heterosexual ones, while the overall distribution of same-sex versus other-sex attractions remained stable, and declines in same-sex behavior ratios occurred across the sample.[50] This pattern supports models of bisexuality as a distinct orientation with fluid labeling rather than a mere transitional phase to heterosexuality or homosexuality.[50] Gender differences emerge consistently, with women exhibiting less stability in daily attractions and self-reported orientation than men.[51] [34] In a study of 294 adults aged 18–40 using 30-day attraction diaries, women's attractions to preferred and less-preferred genders showed weaker day-to-day correlations (e.g., G more-preferred = .05, p < .05; G less-preferred = .10, p < .01) than men's, while bisexual individuals displayed lower stability in less-preferred gender attractions (G differences up to -.33, p < .001) and reported larger post-adolescent shifts compared to exclusive orientations (p < .001 for less-preferred).[51] Bisexuals across genders also evidenced lower relative stability and more mean changes in orientation identity over one-year intervals than heterosexuals or homosexuals.[34] In youth cohorts, fluidity manifests as higher mobility in identity endorsement, particularly among females.[52] Analysis of self-reports from ages 12–23 in a large sample found females with greater overall mobility (M=0.125 for ages 12–17 versus M=0.081 for males), though sexual minorities—including bisexuals—showed elevated mobility (0.5–0.8) regardless of gender; bisexual identification rose with age (e.g., females from 0.6% at 13 to 2.1% at 23), and initial "unsure" youth often resolved to heterosexual rather than minority identities.[52] Physiological measures highlight a disconnect from subjective reports, underscoring stability in core responses.[34] Genital arousal patterns to sexual stimuli in 52 men and 67 women over one year correlated highly across sessions with minimal change, irrespective of shifts in self-reported orientation, which were more pronounced in women and bisexuals.[34] These findings suggest that while bisexuality involves dynamic elements—especially in identity and attraction intensity—underlying arousal specificity and non-exclusive attraction profiles persist, distinguishing it from greater rigidity in monosexual orientations.[34] [50]Personality and Behavioral Associations
Bisexual individuals exhibit distinct patterns in Big Five personality traits compared to heterosexual and homosexual counterparts. Meta-analytic evidence indicates that bisexuals report lower conscientiousness than both heterosexuals and homosexuals, a trait associated with self-discipline and impulse control.[53] Bisexual women specifically show elevated neuroticism and openness to experience relative to heterosexual women, alongside reduced conscientiousness, while bisexual men display similar trends in openness but less pronounced differences in other traits.[54] These patterns hold across multiple studies, with heterosexuals demonstrating lower neuroticism and openness but higher agreeableness than bisexuals and homosexuals.[55] Bisexuals also correlate with higher Dark Triad traits—psychopathy, Machiavellianism, and narcissism—than heterosexuals, reflecting tendencies toward manipulativeness, self-centeredness, and disregard for social norms. Bisexual women, but not homosexual women, exhibit elevated scores on these traits relative to heterosexual women, with bisexual women showing higher Machiavellianism (p=.020, d=0.22), psychopathy (p<.001, d=0.41), and narcissism (p<.001, d=0.29); they also display slightly higher psychopathy than homosexual women (p=.055, d=0.40). Bisexuals score particularly high on psychopathy and narcissism subscales.[56] In domain-specific traits, bisexual men and women report elevated sexual sensation seeking, sexual curiosity, and sexual excitability, which may facilitate non-exclusive attractions independent of fixed orientations toward one sex.[57][58] Behaviorally, bisexual women engage in a broader range of sexual activities and express more permissive attitudes toward sex than heterosexual women, including higher frequencies of behaviors like oral sex and casual encounters.[59] Some evidence suggests bisexuals experience heightened sex drive and seek more partners across sexes, potentially linked to lower conscientiousness and elevated sensation seeking.[60] These traits contribute to patterns of relationship instability, with bisexuals facing stereotypes in clinical settings of greater proneness to identity confusion and relational difficulties, though empirical validation remains mixed.[61] Bisexual women also experience elevated rates of intimate partner violence victimization, with lifetime prevalence of rape, physical violence, or stalking by an intimate partner at 61%, compared to 44% for lesbian women and 35% for heterosexual women.[62] Mental health outcomes reveal heightened vulnerabilities among bisexuals, with meta-analyses confirming elevated risks of depression, anxiety, borderline personality disorder (RR 3.82 for bisexuality compared to heterosexuals; Shu et al., 2024), and other disorders compared to heterosexuals, and often exceeding those in gay/lesbian populations.[63][64][65] Bisexuals report more general life stressors and poorer overall mental health than monosexual minorities, attributable in part to minority stress from biphobia and identity marginalization, though bisexual-specific stressors like concealment from both heterosexual and homosexual communities exacerbate disparities.[66][67] Recent data from 2025 underscores these trends, with bisexuals showing higher odds of mood disorders independent of gender identity factors.[68] Such associations persist after controlling for demographics, pointing to intrinsic links between bisexual orientation, personality, and behavioral risks rather than solely external discrimination.Epidemiology and Prevalence
Self-Reported Identification Rates
In the United States, a 2025 Gallup poll of more than 14,000 adults reported that 5.2% self-identified as bisexual, comprising the largest subgroup within the 9.3% overall LGBTQ+ identification rate, with straight/heterosexual identification at 85.7%. [69] This marked an increase from earlier surveys, such as the 2013 National Health Interview Survey (NHIS) by the Centers for Disease Control and Prevention (CDC), which found 0.7% of adults identifying as bisexual out of 1.6% gay or lesbian and 96.6% straight. [70] Bisexual identification has risen notably among younger cohorts, with over 20% of Generation Z adults (born 1997-2012) identifying as LGBTQ+ in Gallup data, predominantly as bisexual. [69] Gender disparities appear consistent across datasets, with women reporting higher bisexual identification rates than men; for instance, U.S. representative samples indicate 3.7% of women versus 1.6% of men. [71] In the United Kingdom, the Office for National Statistics (ONS) Annual Population Survey data showed bisexual identification doubling from 0.9% (approximately 457,000 people) in 2018 to around 1.8% by 2023 among those aged 16 and over, contributing to a total lesbian, gay, or bisexual (LGB) rate of 3.3% in 2022. [72] These trends reflect broader increases in non-heterosexual self-identification over time, potentially influenced by reduced stigma, though rates remain below 6% in most national adult populations. [69] [73]| Country/Survey | Year | Bisexual Identification Rate (%) | Total Sample/Notes |
|---|---|---|---|
| US (Gallup) | 2025 | 5.2 | >14,000 adults; highest among youth[69] |
| US (NHIS/CDC) | 2013 | 0.7 | National adults; earlier baseline[70] |
| UK (ONS) | 2023 | ~1.8 (doubled from 0.9% in 2018) | Aged 16+; part of 3.3% LGB total[72] |
Demographic Variations by Gender, Age, and Culture
Self-reported rates of bisexual identification exhibit notable variations by gender, with women consistently reporting higher prevalence than men in large-scale surveys. In the United States, a 2024 Pew Research Center analysis of adults found that 5% of women identified as bisexual, compared to 2% of men.[74] Similarly, Gallup polling from 2024 indicated that women are nearly twice as likely as men to identify as LGBTQ+ overall (8.5% versus 4.7%), with bisexuality driving much of the disparity, as it constitutes the predominant category within LGBTQ+ identifications for both genders but especially women.[75] This gender gap widens among younger cohorts; for instance, among Generation Z adults, Gallup data show bisexual identification rates exceeding 20% for women versus lower rates for men, where gay identification sometimes surpasses bisexual.[75] Age-related trends reveal a marked increase in bisexual identification among younger generations, reflecting generational shifts in self-reporting. Gallup's 2025 survey of U.S. adults reported that over half (59%) of LGBTQ+-identifying Generation Z individuals and 52% of Millennials identify specifically as bisexual, compared to declining proportions in older groups such as Generation X (44%).[69] Overall LGBTQ+ identification, heavily weighted toward bisexuality, reaches 22% among Generation Z but drops to under 2% among those over 50, per 2023 Pew data on lesbian, gay, or bisexual adults.[76] A 2023 analysis of U.S. representative data similarly found bisexual identification at over 6% for ages 18-29, falling below 2% for those over 40.[77] These patterns suggest influences from evolving social norms, though self-reports may capture fluidity or exploratory phases more readily in permissive environments. Cross-cultural data on bisexual identification remain limited and predominantly drawn from Western or urban samples, complicating direct comparisons due to varying stigma levels and survey methodologies. A 2019 study analyzing self-reported sexual orientation from a 2005 BBC internet survey of 191,088 participants across 28 nations (including Western, Asian, and other regions) reported average bisexual identification of 7.2% among women and 5.1% among men, with rates showing relative stability across countries (e.g., around 5-6% in the UK, Australia, and India) and no significant correlations with national gender equality, economic development, or individualism indices.[78] However, lower reporting in non-Western contexts, such as conservative Asian societies, likely reflects underreporting due to social penalties rather than true prevalence differences, as evidenced by smaller-scale studies like a 2020 Hong Kong survey where bisexuals comprised 30% of a cisgender LGB sample but faced heightened mental health risks amid lower acceptance.[63] In cultures with traditional gender roles, such as Samoa, same-sex behaviors occur but are often framed outside Western bisexual categories, integrating into third-gender roles rather than self-identified bisexuality.[79] Overall, empirical evidence indicates that while base attraction patterns may vary modestly, identification rates are heavily modulated by cultural tolerance for non-heteronormative expressions.Historical Context
Ancient and Pre-Modern References
In ancient Greece, particularly from the Archaic period (c. 800–480 BCE) onward, elite adult males frequently participated in pederasty—erotic and mentorship relationships with adolescent boys—while simultaneously fulfilling marital and procreative obligations with women, reflecting widespread bisexual behavior among the upper classes.[80] This practice was idealized in literature and philosophy; for instance, Plato's Symposium (c. 385–370 BCE) explores male-male eros alongside heterosexual unions, portraying bisexuality as a normative aspect of male sexuality rather than deviant.[81] Historical figures such as Alexander the Great (356–323 BCE), who maintained a close bond with Hephaestion akin to Achilles and Patroclus while marrying Roxana and fathering children, exemplify this pattern, with ancient sources like Plutarch noting such dual attractions without condemnation.[80] Scholarly analysis indicates that while modern notions of fixed sexual orientation were absent, behavioral evidence suggests bisexuality was intrinsic to Greek elite culture, tolerated in art and myth, though penetrative roles among adult males carried stigma.[81] Roman society from the Republic (509–27 BCE) through the Empire mirrored Greek patterns, with male same-sex relations—often with youths or slaves—coexisting with heterosexual marriages and family duties, as documented in poetry by Catullus (c. 84–54 BCE) and Virgil (70–19 BCE), who depicted attractions to both sexes.[80] Emperors exemplified this: Hadrian (r. 117–138 CE) deified his male lover Antinous after his death while married to Sabina, and Elagabalus (r. 218–222 CE) reportedly sought marriage to men alongside women, per the Historia Augusta.[82] Roman law and mores emphasized dominance over gender, permitting bisexual acts if the freeborn male avoided the passive role, thus framing such versatility as a marker of virility rather than a distinct identity.[81] Evidence from artifacts like the Warren Cup (1st century CE), depicting male-male intercourse, underscores cultural acceptance of these practices within a broader heterosexual framework.[80] Beyond the Mediterranean, the Han Dynasty in China (202 BCE–220 CE) provides stark examples, where all ten emperors of the Western Han era maintained documented male favorites—termed "charioteers" or intimate companions—alongside empresses and concubines, with Emperor Ai (r. 7–1 BCE) notably elevating his lover Dong Xian to high office.[83] Court records and histories like the Book of Han portray these relationships as routine, integrated with imperial heterosexual reproduction, suggesting bisexuality as a societal norm among rulers without modern pathologization.[83] In ancient India, Vedic and epic texts (c. 1500–500 BCE) reference mythological figures with dual-gender attractions, such as Ardhanarishvara (Shiva's half-male, half-female form), implying cultural tolerance for bisexual expressions in ritual and narrative contexts, though human behavioral evidence remains sparser.[84] Pre-modern references wane under monotheistic influences, but sporadic evidence persists; for example, in medieval Islamic poetry (8th–13th centuries CE), figures like Abu Nuwas celebrated wine, boys, and women in verses, blending attractions without framing them as conflicting.[14] Overall, these historical instances document bisexual behaviors across civilizations, driven by social structures prioritizing reproduction and hierarchy over exclusive orientations, with primary sources like annals and literature providing behavioral rather than introspective attitudinal data.[80][83]Modern Conceptualization (19th-20th Century)
The term bisexuality first appeared in scientific discourse in the mid-19th century, initially denoting biological or anatomical duality rather than erotic attraction. In 1859, anatomist Robert Bentley Todd employed it to describe organisms exhibiting both male and female physical traits, reflecting a physiological understanding rooted in emerging evolutionary biology.[85] By the 1880s, sexologists began adapting the concept to psychosexual contexts, framing bisexuality as a potential stage in "sexual inversion," where individuals displayed mixed-gender traits alongside attractions to both sexes.[86] Pioneering psychiatrist Richard von Krafft-Ebing advanced this in the 1892 seventh edition of Psychopathia Sexualis, using "bisexual" to categorize persons with documented desires for both men and women, often as a variant of degeneracy or perversion within a taxonomy of pathological sexualities.[87] Krafft-Ebing's framework, influenced by earlier theorists like Karl Heinrich Ulrichs, viewed such attractions as congenital anomalies disrupting reproductive norms, though he documented cases empirically from clinical histories rather than moral judgment alone.[88] Havelock Ellis, in works like Studies in the Psychology of Sex (published serially from 1897), offered a less pejorative lens, portraying bisexuality as a common, non-pathological variation in human sexuality, supported by anthropological and historical examples of mixed attractions across cultures.[14] Sigmund Freud's psychoanalytic contributions in the early 20th century shifted emphasis toward universality. In Three Essays on the Theory of Sexuality (1905), Freud theorized an original bisexual constitution in infancy, where libidinal impulses toward both sexes precede differentiation, with outcomes shaped by developmental arrests or resolutions.[89] He reiterated in 1915 that "all human beings are bisexual" in the sense of distributed libido across sexes, though empirical validation remained speculative, relying on case studies rather than population data.[90] This model influenced subsequent psychology but faced critique for conflating psychic bisexuality (masculine/feminine dispositions) with object-choice, overlooking behavioral evidence.[14] Mid-20th-century empiricism, particularly Alfred Kinsey's 1948 Sexual Behavior in the Human Male and 1953 Sexual Behavior in the Human Female, challenged categorical binaries through quantitative surveys of over 5,000 men and 6,000 women. Kinsey introduced a 0-6 scale, with 0 denoting exclusive heterosexuality and 6 exclusive homosexuality; positions 1-5 captured bisexual patterns, revealing that 37% of men and 13% of women reported orgasmic experiences with both sexes to some degree, underscoring sexuality's continuum.[44][91] This data-driven approach demedicalized bisexuality, emphasizing behavioral plasticity over innate fixity, though Kinsey's sampling (urban, volunteer-heavy) drew methodological scrutiny for potential skew toward non-normative respondents.[92] By the 1950s, these conceptualizations coalesced into viewing bisexuality as a distinct, non-pathological orientation on a spectrum, informing decriminalization debates and early identity politics, despite persistent academic marginalization.[93]Evolutionary Perspectives
Hypotheses for Origins
One prominent evolutionary hypothesis posits that genetic variants associated with bisexual behavior in males confer a reproductive advantage, facilitating the persistence of such traits despite same-sex attractions. A 2023 genome-wide association study analyzing data from over 450,000 individuals in the UK Biobank found that male bisexual behavior exhibits a positive genetic correlation with the number of offspring (r_g = 0.156, P = 0.019), unlike exclusive same-sex behavior, which shows a negative correlation (r_g = -0.404, P = 0.0022).[8] This advantage appears mediated by alleles linked to risk-taking propensity (r_g = 0.484 with male bisexual behavior, P = 1.4 × 10⁻⁸), which independently correlates with higher offspring counts (r_g = 0.366, P = 4.2 × 10⁻²³) and may enhance mating opportunities through bolder pursuit of partners.[8] No such reproductive correlation was observed for female bisexual behavior (r_g = -0.013, P = 0.855).[8] Hypotheses adapted from explanations for exclusive homosexuality also apply to bisexuality, with the key distinction that bisexuality incurs lower direct reproductive costs by retaining opposite-sex attractions. Under kin selection, bisexual individuals may allocate resources to enhance relatives' fitness, as observed in some non-human primates and paralleling patterns in homosexual males who provide alloparental care.[94] Sexually antagonistic selection suggests genes promoting male bisexuality could elevate fecundity in female carriers, maintaining alleles in the population despite partial expression in males.[94] Maternal immune responses, such as fraternal birth order effects, may similarly contribute by influencing androgen exposure prenatally, though evidence remains correlational and debated.[94] Social and signaling hypotheses emphasize bisexuality's role in group dynamics among large, interdependent populations. The prosociality hypothesis frames same-sex sexual attraction, including bisexual patterns, as an extension of traits selected for social integration and reduced aggression, akin to self-domestication in humans and bonobos, where it fosters alliances and conflict resolution with minimal reproductive penalty for non-exclusive individuals.[95] A related sociosexual model proposes that same-sex attractions enable non-reproductive bonding to secure cooperative benefits, with bisexuality's prevalence on the attraction spectrum reflecting adaptive flexibility.[95] In mutual sexual selection contexts, partial disinterest in the opposite sex may signal high mate value, triggering Fisherian runaway escalation that redirects preferences toward bisexuality while preserving some reproductive access, supported by comparable lifetime fecundity between bisexual and heterosexual individuals.[96] These mechanisms collectively suggest bisexuality's evolutionary viability through balanced costs and indirect benefits, though empirical testing remains limited by measurement challenges in self-reported data.[94]Reproductive and Adaptive Implications
Genetic analyses of large-scale genomic data from over 450,000 individuals have identified variants associated with bisexual behavior in males that positively correlate with the number of offspring produced, unlike variants linked to exclusive same-sex behavior, which show negative correlations.[8] These bisexual-associated alleles appear reproductively advantageous, potentially explaining their persistence in populations despite the reproductive costs of exclusive homosexuality.[8] The same variants are also linked to traits such as risk-taking and openness to experience, which may indirectly boost mating success by facilitating more social and sexual opportunities in ancestral environments.[8] From an adaptive standpoint, bisexual attraction enables greater mate choice flexibility, allowing individuals to form reproductive partnerships with the opposite sex while potentially gaining non-reproductive benefits from same-sex interactions, such as alliances or resource sharing observed in nonhuman primates.[97] This contrasts with exclusive homosexuality, where direct reproduction is rarer, though offset in some models by indirect fitness gains via kin selection. Empirical reproductive data support higher fertility in bisexual-identifying individuals compared to gay or lesbian counterparts; for instance, bisexual men report more children on average in population surveys, aligning with genetic predictions.[8] However, some behavioral studies note variability, with self-identified bisexuals occasionally showing intermediate fertility rates influenced by relationship patterns and societal factors.[94] Causal mechanisms likely involve pleiotropy, where genes influencing sexual orientation also affect traits enhancing overall fitness, such as sociability or extraversion, which promote heterosexual mating even amid same-sex attractions.[97] In females, evidence is sparser, but analogous patterns suggest bisexual behavior may similarly support adaptive strategies in fluid social contexts, though cultural stigma can suppress realized reproduction.[8] Overall, bisexuality's evolutionary viability stems from its compatibility with reproduction, avoiding the fitness penalty of exclusivity while leveraging behavioral versatility for survival and propagation.[8]Controversies and Scientific Debates
Validity and Measurement Challenges
Assessing the validity of bisexuality as a distinct sexual orientation faces challenges due to inconsistencies between self-reported attraction and physiological measures of arousal. Self-reports, the primary method for identifying bisexual individuals, are prone to biases such as social desirability, where respondents may overstate or understate attractions to align with perceived norms or personal narratives.[98] Physiological assessments, including genital arousal via penile plethysmography (PPG) for men and vaginal photoplethysmography for women, offer objective data but reveal discrepancies, particularly in males, where self-identified bisexuals often exhibit category-specific responses akin to monosexual orientations rather than balanced bisexual patterns.[27] Early research, such as a 2005 study by Rieger, Chivers, and Bailey involving 101 men, found that self-identified bisexual males displayed genital arousal to male stimuli comparable to homosexual men and weaker responses to female stimuli, suggesting bisexuality might reflect interpretive styles of arousal rather than a unique physiological profile.[28] This fueled debates on whether male bisexuality constitutes a genuine intermediate orientation or a transitional phase, with critics arguing that genital responses in men are typically bifurcated (heterosexual or homosexual) due to stronger evolutionary pressures for specificity.[99] Subsequent studies, including a 2011 analysis by Rosenthal et al., identified bisexual arousal in a subset of bisexual men but noted variability, attributing inconsistencies to sample selection or stimulus specificity.[100] A 2020 study by Jabbour et al., analyzing data from over 470 men across multiple experiments, reported "robust evidence" of bisexual genital and subjective arousal patterns in self-identified bisexuals, challenging prior skepticism by using refined stimuli and larger samples.[101] However, methodological critiques persist, including potential confounds from curiosity-driven participation—bisexual men scoring higher on sexual curiosity scales showed more varied arousal—and questions about whether aggregated patterns truly distinguish bisexuality from monosexuality without clearer dose-response gradients.[102] In women, arousal patterns are less category-specific, complicating validation as bisexuality may overlap with generalized responsiveness rather than targeted dual attraction.[103] Definitional ambiguities exacerbate measurement issues, with no unified criteria for bisexuality—ranging from equal attraction to both sexes, predominant same-sex with incidental opposite-sex interest, or behavioral history—leading to heterogeneous samples and non-replicable findings.[104] Longitudinal stability is another hurdle; self-reported bisexual identification often fluctuates, potentially reflecting fluidity, experimentation, or response to cultural shifts rather than innate orientation, as evidenced by higher rates of identity change in population surveys.[105] These challenges underscore the need for multimodal assessments integrating self-report, physiology, and behavior, though ethical constraints on invasive measures limit scalability. Peer-reviewed research on bisexuality remains sparse compared to monosexual orientations, with validation efforts hampered by small samples and reliance on convenience recruitment from LGBTQ+ communities, potentially inflating prevalence estimates.[106]Criticisms of Bisexuality as a Distinct Orientation
Some researchers have questioned whether bisexuality constitutes a stable, distinct sexual orientation, arguing instead that self-identified bisexual individuals often exhibit physiological and behavioral patterns more aligned with monosexual (heterosexual or homosexual) categories, potentially reflecting transitional phases, denial of homosexuality, or measurement artifacts.[27] Early physiological studies, such as Rieger et al. (2005), measured genital arousal in self-identified bisexual men exposed to male and female erotic stimuli, finding that their responses were category-specific—predominantly heterosexual or homosexual—rather than showing balanced, intermediate arousal to both sexes, challenging the notion of a unique bisexual response profile.[27][28] This pattern suggested that bisexuality might not represent a third, equidistant orientation on a continuum but could instead mask underlying monosexual preferences.[107] Longitudinal data further highlight instability in bisexual identification, with higher rates of change compared to heterosexual or homosexual labels, implying it may function more as a temporary or fluid state rather than a fixed trait. In a 10-year study of non-heterosexual women by Diamond (2008), while some bisexual attractions persisted, a significant portion shifted labels—67% of initial bisexuals changed to another identity by the end, with patterns of flux that exceeded monosexual stability, supporting critiques that bisexuality often serves as a developmental waypoint rather than an enduring category.[50] Similarly, analyses of large national panels, such as Katz-Wise et al. (2023), reported that bisexual individuals exhibited greater identity fluidity over seven years, with changes bidirectional but more frequent among bisexuals (contributing disproportionately to the 5.7% overall shift rate), raising questions about its categorical distinctiveness from situational or experimental attractions.[108] Savin-Williams and Ream (2007) tracked 156 gay, lesbian, and bisexual youths, observing multiple identity transitions, particularly among bisexuals, which correlated with psychological adjustment challenges and suggested external social influences over innate orientation.[109] From an evolutionary standpoint, critics argue that a truly distinct bisexual orientation lacks clear adaptive utility, as balanced attraction to both sexes would dilute reproductive specificity without the inclusive fitness benefits hypothesized for exclusive homosexuality (e.g., kin selection) or the direct fitness of heterosexuality. Genetic analyses, like those by Zietsch et al. (2024), link bisexual behavior to risk-taking traits rather than a dedicated genetic cluster for dual attractions, implying it emerges as a byproduct of variable mate-seeking strategies rather than a specialized orientation.[9] These empirical patterns—category-specific arousal, label instability, and non-distinct genetic correlates—fuel ongoing debate, though subsequent studies (e.g., Jabbour et al., 2020) have presented counter-evidence for bisexual arousal patterns, underscoring unresolved methodological tensions in arousal measurement and self-report reliability.[3] Despite such affirmations, the preponderance of fluidity data supports viewing bisexuality skeptically as a coherent, biologically discrete entity equivalent to monosexuality.[110]Biphobia, Erasure, and Societal Skepticism
Biphobia refers to prejudice, stereotypes, and discrimination directed specifically at bisexual individuals, often manifesting as assumptions of promiscuity, confusion, or inevitable transition to exclusive homosexuality or heterosexuality.[111] Peer-reviewed studies indicate that biphobic attitudes are more prevalent than homophobia in some contexts, with bisexual people reporting higher rates of rejection from both heterosexual and homosexual communities.[112] For instance, a 2021 systematic review identified stereotypes such as bisexuals being "greedy" or "untrustworthy in relationships" as common drivers of exclusion.[113] Bisexual erasure involves the systemic denial or minimization of bisexuality as a legitimate orientation, often by attributing bisexual attractions to phases or experimentation rather than stable identity.[114] Evidence from surveys shows this erasure contributes to invisibility, with bisexual individuals frequently omitted from LGBTQ+ narratives or research despite comprising a majority of that population in some estimates—nearly 60% of U.S. LGBTQ+ adults identified as bisexual in a 2023 Gallup poll.[115] A 2025 review of 67 studies found that bi+ identities are delegitimized across family, media, and academic settings, with bi-erasure linked to minority stress and reduced service access.[113] [114] Societal skepticism toward bisexuality persists, fueled by doubts about its authenticity, particularly for men, where genital arousal studies have sometimes shown patterns closer to heterosexual or homosexual exclusivity despite self-identification.[116] A 2013 national survey revealed that 15% of respondents outright rejected bisexuality as a real orientation, with attitudes remaining negative overall.[117] More recent data from 2024 indicates bisexuals encounter discrimination at rates comparable to or exceeding other sexual minorities, with 36% of LGBTQ+ adults reporting orientation-based bias, though bisexual-specific invisibility amplifies underreporting.[118] [119] Only 20% of bisexual respondents in a 2016 Movement Advancement Project report perceived local social acceptance for LGB people, lower than rates for gay men (39%) or lesbians (31%).[120] Such skepticism, while rooted in empirical observations of sexual fluidity in some populations, correlates with health disparities, including elevated mental health risks from internalized binegativity.[121] [118]Social, Cultural, and Health Dimensions
Relationships, Stigma, and Mental Health Outcomes
Bisexual individuals predominantly form opposite-sex unions despite self-reported attractions to both sexes. Analysis of U.S. National Health Interview Survey data from 2013–2017 indicates that among married bisexual adults, 92.8% were in opposite-sex marriages, while 81.4% of cohabiting bisexuals were in opposite-sex partnerships.[122] This pattern holds across studies, with approximately 81% of partnered bisexuals in opposite-sex relationships.[123] Relationship satisfaction among bisexuals in mixed-sex couples is often lower than that of exclusively heterosexual or homosexual partners due to binegativity, disclosure tensions, and acceptance issues, though comparable in supportive contexts with high intimacy and outness.[124][125][126] Stigma against bisexuals, termed biphobia, encompasses prejudice, stereotypes, and discrimination from heterosexual, homosexual, and broader societal sources, including perceptions of bisexuality as a phase, indecision, or inherent promiscuity leading to infidelity.[111][127] Such binegative attitudes contribute to bisexual erasure, where bisexual experiences are invalidated or overlooked in both mainstream and LGBTQ+ contexts, fostering internalized binegativity and identity concealment.[118] Research links these stigmas to relational challenges, such as skepticism from potential partners about monogamy fidelity, though empirical data do not consistently support higher infidelity rates among bisexuals compared to monosexuals.[128] Bisexual individuals experience elevated mental health risks relative to heterosexuals and, in many cases, gay or lesbian individuals, with disparities attributed in part to minority stress from biphobia and non-disclosure. Recent genetic research indicates additional factors beyond stigma. Zietsch et al. (2024) identified genetic variants for bisexual behavior that correlate with risk-taking (r_g = 0.484, P = 1.4 × 10⁻⁸) and reproductive advantage in males, distinct from exclusive same-sex variants.[129] Genetic correlations exist between same-sex sexual behavior and depression (r_g = 0.33 in males, 0.44 in females; Ganna et al., 2019). These findings indicate that while stigma plays a role, it is not a sufficient or exclusive explanation; underlying genetic, temperamental, and behavioral factors appear to contribute independently to the elevated risk.[24] Mental health disparities persist in tolerant societies like the Netherlands at rates comparable to less tolerant nations.[130] Bailey (2020) proposes a reversed‑causation model, in which pre‑existing traits such as higher neuroticism increase both the likelihood of non‑heterosexual identification and the tendency to perceive and be affected by stigma, further supporting the view that mental health disparities are not solely products of external prejudice. A 2022 meta-analysis of population-based studies found bisexuals had 2.78 times higher odds of any mental disorder (95% CI: 2.34–3.21) compared to heterosexuals, exceeding the 2.16 odds for gay/lesbian individuals; specific risks included depression (OR 2.70), anxiety (OR 2.87), and suicidality (OR 4.81).[64] A 2024 analysis of the Nurses' Health Study II found bisexual women died an estimated 37% sooner and lesbian women 20% sooner than heterosexual women, attributed to a combination of factors, including structural and interpersonal marginalization (stigma, discrimination, reduced access to care), as well as intrinsic vulnerabilities correlated with sexual minority status (e.g., higher baseline rates of depression, risk-taking, and comorbid physical health burdens that persist even in highly tolerant societies). Females with autism spectrum disorder (ASD) also show elevated rates of bisexual identification, being three to four times more likely than neurotypical females, indicating potential neurodevelopmental links to both orientation diversity and associated mental health risks.[131][132]| Mental Health Outcome | Bisexual Women (%) | Heterosexual Women (%) | Lesbian Women (%) | Bisexual Men (%) | Heterosexual Men (%) | Gay Men (%) |
|---|---|---|---|---|---|---|
| Lifetime Mood Disorders | 58.7 | 30.5 | 44.4 | 36.9 | 19.8 | 42.3 |
| Lifetime Anxiety Disorders | 57.8 | 31.3 | 40.8 | 38.7 | 18.6 | 41.2 |
| Suicidality | 45.4 | 9.6 | 29.5 | 34.8 | 7.4 | 25.2 |