Epistemology/Psychology

Developmental stability
Developmental stability is achieved when an organism is able to withstand genetic and environmental stress, to display the bilaterally symmetrical traits determined by its developmentally programmed phenotype. To measure an individual's developmental stability, the FA measurements of 10 traits are added together, including ear width, elbows, ankles, wrists, feet, length of ears and fingers. This is achieved by: (L - R)trait 1 + (L - R)trait 2 + ......(L - R)trait 10. This provides a good overall measure of body FA, as every individual has some features that are not perfectly symmetrical.

Common environmental pressures leading to lower developmental stability include exposure to toxins, poison and infectious diseases, low food quality and malnutrition. Genetic pressures include spontaneous new mutations, and "bad genes" (genes that once had adaptive functions, but are being removed through evolutionary selection). A large fluctuating asymmetry (FA) and a low developmental stability suggests that an organism is unable to develop according to the ideal state of bilateral symmetry. The energy required for bilateral symmetry development is extremely high, making fully perfect bilateral symmetry functionally nonexistent in natural organic creatures. Energy is invested into survival in spite of the genetic and environmental pressures, before making bilaterally symmetrical traits. Research has also revealed links between FA and depression, genetic or environmental stress and measures of mate quality for sexual selection.

A correlation has also been reported between FA and human social dominance. However, research is proving less consistent with other personality factors, with some finding some weak correlations between low FA and conscientiousness and openness to experience, and others finding no significant differences between those with high or low FA.

Health-risk behaviours
It has been suggested that individuals with lower levels of FA may engage in more biologically costly behaviours such as recreational drug use and risky body modifications such as piercings and tattoos. These ideas have been proposed in the context of Zahavi's handicap principle, which argues that highly costly behaviours or traits serve as signals of an organism's genetic quality. The relationship between FA and behaviours with high health risks has received mixed support. Individuals with body piercings and tattoos (which increase risk of blood-borne infections) have been shown to have lower levels of FA, but individuals with lower FA do not engage in any more recreational drug use than those with higher FA levels.

Mental health in humans
Higher levels of FA have been linked to higher levels of some mental health difficulties. For instance, it has been shown that, among university students, higher FA is associated with higher levels of schizotypy. Depression scores have been found to be higher in men, but not women, with higher levels of FA. One study by Shackelford and Larsen found that men and women with higher facial asymmetry reported more physiological complaints than those with lower facial asymmetry, and that both men and women with higher asymmetry experienced higher levels of psychological distress overall. For example, men with higher facial asymmetry experienced higher levels of depression compared to men with lower facial asymmetry. Fluctuating asymmetry has also been studied in relation to psychopathy. One study looking at offenders and non-offenders found that, although offenders had higher levels of FA overall, psychopathic offenders had lower levels of FA compared to offenders who did not meet the criteria for psychopathy. Additionally, offenders with the highest levels of psychopathy were found to have similar levels of FA to non-offenders.

Antisocial behaviours
Some studies suggest a link between FA and aggression, but the evidence is mixed. In humans, criminal offenders show greater FA than nonoffenders. However, other studies report that human males with higher FA show less physical aggression and less anger. Females show no association between FA and physical aggression, but some research has suggested that older female adolescents with higher facial FA are less hostile. The type of aggression being studied may account for the mixed evidence that is seen here. For example, one study found that females with higher FA demonstrated higher levels of reactive aggression in response to high levels of provocation, whereas high FA males showed more reactive aggression under low levels of provocation.

Research is also mixed in other animals. In Japanese scorpionflies (Panorpa nipponensis and Panorpa ochraceopennis), FA differences between members of the same sex competing for food determines the outcome of interspecific contests and aggression better than body size or ownership of food. Furthermore, cannibalistic laying hens (Gallus gallus domesticus) demonstrate more asymmetry than normal hens. However, this link between FA and aggression in hens is questionable, as victimised hens also showed greater asymmetry. Furthermore, when prenatally injecting hen eggs with excess serotonin (5-HT), the hens later exhibited more FA at 18 weeks of age, but displayed less aggressive behaviours. It is suggested that the stress introduced during early embryonic stages via certain factors (such as excess serotonin) may create developmental instability, causing phenotypic and behavioural variations (such as increased or decreased aggression).

Minor Physical Anomalies

 * Moffitt writes, "Minor physical anomalies, which are thought to be observable markers for hidden anomalies in neural development, have been found at elevated rates among violent offenders and subjects with antisocial personality traits." Neural development in the fetus may also be affected by maternal drug abuse, poor prenatal nutrition, or prenatal/postnatal exposure to toxic agents. Minor physical anomalies (MPAs) are features such as low-seated ears, furrowed tongue, and adherent ear lobes.  Evidence supporting the link between minor physical anomalies and antisocial behavior shows that the link only exists when adverse environmental factors are present.

Sociosexuality
Individuals who are sociosexually unrestricted tend to score higher on openness to experience, and be more extraverted, less agreeable, lower on honesty-humility, more erotophilic, more impulsive, more likely to take risks, more likely to have an avoidant attachment style, less likely to have a secure attachment style, and score higher on the Dark Triad traits (i.e. narcissism, Machiavellianism, psychopathy). Higher masculinity and eveningness in women is related to unrestricted sociosexuality. High self-monitoring is also associated with unrestricted sociosexuality, regardless of gender or sexual orientation.

Individuals with an intrinsic religious orientation (i.e., religion as an end) tend to be sociosexually restricted, while those with an extrinsic religious orientation (i.e., religion as a means to achieve non-religious goals) tend to be unrestricted. Unrestricted women report engaging in more social interactions with men on a daily basis than restricted women. However, unrestricted individuals rate their interactions with their best friends (non-romantic) as lower in quality (i.e., as less pleasant and satisfying) than restricted individuals.

Gender differences and sexual orientation
Men tend to have higher SOI scores and be more unrestricted than women across a variety of cultures. However, there is more variability in scores within each gender than between men and women, indicating that although the average man is less restricted than the average woman, individuals may vary in sociosexuality regardless of gender.

Bisexual women are significantly less restricted in their sociosexual attitudes than both lesbian and heterosexual women. Bisexual women are also the most unrestricted in sociosexual behavior, followed by lesbians and then, heterosexual women. Gay and bisexual men are similar to heterosexual men in sociosexual attitudes, in that they express relatively unrestricted attitudes relative to women. However, gay men are the most unrestricted in sociosexual behavior, followed by bisexual men and then, heterosexual men. This may be because gay men have more potential partners who prefer short-term, casual sexual encounters.

Unrestricted sociosexuality is associated with early life experiences with sex, more frequent sexual activity and a greater number of lifetime sex partners. Unrestricted men tend to have greater rape myth acceptance, past sexual aggression and more conservative attitudes about women than restricted men. Unrestricted women tend to have more sexual fantasies involving having power or control over another person and lower levels of sexual conservatism than restricted women.

Hormones
Individuals who are partnered typically have lower testosterone levels than individuals who are single. However, this was found to apply solely to individuals who have a restricted sociosexuality. Partnered, unrestricted men and women's testosterone levels are more similar to the levels of single men and women.

Implications
Possessing an unrestricted sociosexuality seems to increase the likelihood of having a son by 12-19% in American samples. This may be explained by the generalized Trivers-Willard hypothesis, which states that parents who possess any heritable trait that increases males' reproductive success above females' will have more sons, and will have more daughters if they possess traits that increase females' reproductive success above males'. Since unrestricted sociosexuality increases the reproductive fitness of sons more than daughters (as males have the potential to have more offspring through casual sex), unrestricted parents have a higher-than-expected offspring sex ratio (more sons).

Bisexuality
There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual or homosexual orientation. Although scientists favor biological models for the cause of sexual orientation, they do not believe that the development of sexual orientation is the result of any one factor. They generally believe that it is determined by a complex interplay of biological and environmental factors, and is shaped at an early age. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males. There is no substantive evidence which suggests parenting or early childhood experiences play a role with regard to sexual orientation. Scientists do not believe that sexual orientation is a choice.

The American Psychiatric Association stated: "To date there are no replicated scientific studies supporting any specific biological etiology for homosexuality. Similarly, no specific psychosocial or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse." Research into how sexual orientation may be determined by genetic or other prenatal factors plays a role in political and social debates about homosexuality, and also raises fears about genetic profiling and prenatal testing.

Magnus Hirschfeld argued that adult sexual orientation can be explained in terms of the bisexual nature of the developing fetus: he believed that in every embryo there is one rudimentary neutral center for attraction to males and another for attraction to females. In most fetuses, the center for attraction to the opposite sex developed while the center for attraction to the same sex regressed, but in fetuses that became homosexual, the reverse occurred. Simon LeVay has criticized Hirschfeld's theory of an early bisexual stage of development, calling it confusing; LeVay maintains that Hirschfeld failed to distinguish between saying that the brain is sexually undifferentiated at an early stage of development and saying that an individual actually experiences sexual attraction to both men and women. According to LeVay, Hirschfeld believed that in most bisexual people the strength of attraction to the same sex was relatively low, and that it was therefore possible to restrain its development in young people, something Hirschfeld supported.

Hirschfeld created a ten-point scale to measure the strength of sexual desire, with the direction of desire being represented by the letters A (for heterosexuality), B (for homosexuality), and A + B (for bisexuality). On this scale, someone who was A3, B9 would be weakly attracted to the opposite sex and very strongly attracted to the same sex, an A0, B0 would be asexual, and an A10, B10 would be very attracted to both sexes. LeVay compares Hirschfeld's scale to that developed by Kinsey decades later.

Sigmund Freud, the founder of psychoanalysis, believed that every human being is bisexual in the sense of incorporating general attributes of both sexes. In his view, this was true anatomically and therefore also psychologically, with sexual attraction to both sexes being an aspect of this psychological bisexuality. Freud believed that in the course of sexual development the masculine side of this bisexual disposition would normally become dominant in men and the feminine side in women, but that all adults still have desires derived from both the masculine and the feminine sides of their natures. Freud did not claim that everyone is bisexual in the sense of feeling the same level of sexual attraction to both genders. Freud's belief in innate bisexuality was rejected by Sándor Radó in 1940 and, following Radó, by many later psychoanalysts. Radó argued that there is no biological bisexuality in humans.

Alan P. Bell, Martin S. Weinberg, and Sue Kiefer Hammersmith reported in Sexual Preference (1981) that sexual preference was much less strongly connected with pre-adult sexual feelings among bisexuals than it was among heterosexuals and homosexuals. Based on this and other findings, they suggested that bisexuality is more influenced by social and sexual learning than is exclusive homosexuality. Letitia Anne Peplau et al. wrote that while Bell et al.'s view that biological factors may be more influential on homosexuality than on bisexuality might seem plausible, it has not been directly tested and appears to conflict with available evidence, such as that concerning prenatal hormone exposure.

Human bisexuality has mainly been studied alongside homosexuality. Van Wyk and Geist argue that this is a problem for sexuality research because the few studies that have observed bisexuals separately have found that bisexuals are often different from both heterosexuals and homosexuals. Furthermore, bisexuality does not always represent a halfway point between the dichotomy. Research indicates that bisexuality is influenced by biological, cognitive and cultural variables in interaction, and this leads to different types of bisexuality.

In the current debate around influences on sexual orientation, biological explanations have been questioned by social scientists, particularly by feminists who encourage women to make conscious decisions about their life and sexuality. A difference in attitude between homosexual men and women has also been reported, with men more likely to regard their sexuality as biological, "reflecting the universal male experience in this culture, not the complexities of the lesbian world." There is also evidence that women's sexuality may be more strongly affected by cultural and contextual factors.

The critic Camille Paglia has promoted bisexuality as an ideal. Harvard Shakespeare professor Marjorie Garber made an academic case for bisexuality with her 1995 book Vice Versa: Bisexuality and the Eroticism of Everyday Life, in which she argued that most people would be bisexual if not for repression and other factors such as lack of sexual opportunity.

Brain structure and chromosomes
LeVay's (1991) examination at autopsy of 18 homosexual men, 1 bisexual man, 16 presumably heterosexual men and 6 presumably heterosexual women found that the INAH 3 nucleus of the anterior hypothalamus of homosexual men was smaller than that of heterosexual men and closer in size of heterosexual women. Although grouped with homosexuals, the INAH 3 size of the one bisexual subject was similar to that of the heterosexual men.

Some evidence supports the concept of biological precursors of bisexual orientation in genetic males. According to John Money (1988), genetic males with an extra Y chromosome are more likely to be bisexual, paraphilic and impulsive.

Evolutionary theory
Some evolutionary psychologists have argued that same-sex attraction does not have adaptive value because it has no association with potential reproductive success. Instead, bisexuality can be due to normal variation in brain plasticity. More recently, it has been suggested that same-sex alliances may have helped males climb the social hierarchy giving access to females and reproductive opportunities. Same-sex allies could have helped females to move to the safer and resource richer center of the group, which increased their chances of raising their offspring successfully.

Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved do not drive them to complete rejection of heterosexuality.

Also, in a 2008 study, its authors stated that "There is considerable evidence that human sexual orientation is genetically influenced, so it is not known how homosexuality, which tends to lower reproductive success, is maintained in the population at a relatively high frequency." They hypothesized that "while genes predisposing to homosexuality reduce homosexuals' reproductive success, they may confer some advantage in heterosexuals who carry them" and their results suggested that "genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population."

In Scientific American Mind, the scientist Emily V. Driscoll stated that homosexual and bisexual behavior is quite common in several species and that it fosters bonding: "The more homosexuality, the more peaceful the species". The article also stated: "Unlike most humans, however, individual animals generally cannot be classified as gay or straight: an animal that engages in a same-sex flirtation or partnership does not necessarily shun heterosexual encounters. Rather, many species seem to have ingrained homosexual tendencies that are a regular part of their society. That is, there are probably no strictly gay critters, just bisexual ones. Animals don't do sexual identity. They just do sex."

Masculinization
Masculinization of women and hypermasculinization of men has been a central theme in sexual orientation research. There are several studies suggesting that bisexuals have a high degree of masculinization. LaTorre and Wendenberg (1983) found differing personality characteristics for bisexual, heterosexual and homosexual women. Bisexuals were found to have fewer personal insecurities than heterosexuals and homosexuals. This finding defined bisexuals as self-assured and less likely to suffer from mental instabilities. The confidence of a secure identity consistently translated to more masculinity than other subjects. This study did not explore societal norms, prejudices, or the feminization of homosexual males.

In a research comparison, published in the Journal of the Association for Research in Otolaryngology, women usually have a better hearing sensitivity than males, assumed by researchers as a genetic disposition connected to child bearing. Homosexual and bisexual women have been found to have a hypersensitivity to sound in comparison to heterosexual women, suggesting a genetic disposition to not tolerate high pitched tones. While heterosexual, homosexual and bisexual men have been found to exhibit similar patterns of hearing, there was a notable differential in a sub-group of males identified as hyperfeminized homosexual males who exhibited test results similar to heterosexual women.

Prenatal hormones
The prenatal hormonal theory of sexual orientation suggests that people who are exposed to excess levels of sex hormones have masculinized brains and show increased homosexuality or bisexuality. Studies providing evidence for the masculinization of the brain have, however, not been conducted to date. Research on special conditions such as congenital adrenal hyperplasia (CAH) and exposure to diethylstilbestrol (DES) indicate that prenatal exposure to, respectively, excess testosterone and estrogens are associated with female–female sex fantasies in adults. Both effects are associated with bisexuality rather than homosexuality.

There is research evidence that the digit ratio of the length of the 2nd and 4th digits (index finger and ring finger) is somewhat negatively related to prenatal testosterone and positively to estrogen. Studies measuring the fingers found a statistically significant skew in the 2D:4D ratio (long ring finger) towards homosexuality with an even lower ratio in bisexuals. It is suggested that exposure to high prenatal testosterone and low prenatal estrogen concentrations is one cause of homosexuality whereas exposure to very high testosterone levels may be associated with bisexuality. Because testosterone in general is important for sexual differentiation, this view offers an alternative to the suggestion that male homosexuality is genetic.

The prenatal hormonal theory suggests that a homosexual orientation results from exposure to excessive testosterone causing an over-masculinized brain. This is contradictory to another hypothesis that homosexual preferences may be due to a feminized brain in males. However, it has also been suggested that homosexuality may be due to high prenatal levels of unbound testosterone that results from a lack of receptors at particular brain sites. Therefore, the brain could be feminized while other features, such as the 2D:4D ratio could be over-masculinized.

Sex drive
Van Wyk and Geist summarized several studies comparing bisexuals with hetero- or homosexuals that have indicated that bisexuals have higher rates of sexual activity, fantasy, or erotic interest. These studies found that male and female bisexuals had more heterosexual fantasy than heterosexuals or homosexuals; that bisexual men had more sexual activities with women than did heterosexual men, and that they masturbated more but had fewer happy marriages than heterosexuals; that bisexual women had more orgasms per week and they described them as stronger than those of hetero- or homosexual women; and that bisexual women became heterosexually active earlier, masturbated and enjoyed masturbation more, and were more experienced in different types of heterosexual contact.

Research suggests that, for most women, high sex drive is associated with increased sexual attraction to both women and men. For men, however, high sex drive is associated with increased attraction to one sex or the other, but not to both, depending on sexual orientation. Similarly for most bisexual women, high sex drive is associated with increased sexual attraction to both women and men; while for bisexual men, high sex drive is associated with increased attraction to one sex, and weakened attraction to the other.

Sociosexuality
Richard A. Lippa proposed that there exist two dimensions of sexual orientation: a gender typicality dimension, and a monosexuality dimension. With the gender typicality dimension being associated with the heterosexual-homosexual distinction, while the sociosexuality dimension has many behavioral effects. He proposes someone who would be at any point in the heterosexual-homosexual spectrum will become bisexual if they are high on the sociosexuality dimension. This dimension being associated with higher sociosexuality, higher neuroticism, lower aggreeableness, lower honesty-humility, and higher openness to experience, and a minor degree of gender nonconformity. He proposes this as explaining phenomena such as increased juvenile delinquency among bisexuals, increased mental health issues and substance use disorder among bisexuals, increased dark triad traits among bisexual women. Critics of this theory have described elements observed as coming from experiences of biphobia, but Lippa couners that these phenomena are present even among heterosexual identifying people with some same sex attraction, who would likely be heterosexual passing.