The American Academy of Pediatrics (2004) has stated that "sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences." The amount that each influence plays is highly debated. One study on Swedish twins suggested that there was a moderate, primarily genetic, familial effect, and moderate to large effects of the non-shared environment (social and biological) on same-sex sexual behaviour.

 

The American Psychological Association (2007) has stated that: "there are probably many reasons for a person's sexual orientation and the reasons may be different for different people." It also stated that for most people, sexual orientation is determined at an early age.

 

The American Psychiatric Association (2000) has stated that, "...to date there are no replicated scientific studies supporting any specific biological aetiology 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.

 

Biological Explanations

In 1993, Dean Hamer found the genetic marker Xq28 on the X chromosome. Hamer's study found a link between the Xq28 marker and male homosexuality; but the original study's results have been disputed. Several mutations have been identified in flies, such as changes in the fruitless gene, cause male flies to court and attempt to mate with other males; however, when a modified male fruit fly is isolated with only female fruit flies, then he will attempt to mate with them.

 

Twin studies give indications that genes may predispose some men to seek partners of the same sex. Hamer commented: "From twin studies, we already know that half or more of the variability in sexual orientation is not inherited. Our studies try to pinpoint the genetic factors, not to negate the psychosocial factors." One common type of twin study compares the monozygotic (or identical) twins of people possessing a particular trait to the dizygotic (non-identical, or fraternal) twins of people possessing the trait. Michael Bailey & Richard Pillard (1991), in a study of gay twins, found that 52% of monozygotic brothers and 22% of the dizygotic twins were concordant for homosexuality. Recently Niklas Långström, Qazi Rahman, Eva Carlström & Paul Lichtenstein (2008) reported on a study of 3,826 twin pairs, comprising all twins between the ages of 20 and 47 in Sweden. Their results showed that genetic factors explained about 34-39% of the variation whereas specific environmental effects explained the remaining 61-66% in men. In women, the genetic part of the variation was 18-19%, with 16-17% for shared environmental and 64-66% for unique environmental factors.

 

Some studies have shown that homosexual men react to certain substances believed to be human pheromones differently from heterosexual men.

 

Prenatal Hormonal Theory                                                                                                                             

The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in foetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult.

 

Prenatal maternal stress                                                                                                                                      

It has been hypothesised that elevated maternal stress during pregnancy can increase the probability that the child will be homosexual. R H Anderson, D E Fleming, R W Rhees & E Kinghorn (1986) found that stressing female rats during  pregnancy lowered the level of sex hormones produced which in turn affected the sexual development of the male pups.

 

However, Alfred Kinsey, Wardell Pomeroy & Clyde Martin in the Kinsey Report (1948) reported that boosting the levels of androgens increased sexual desire but did affect the sexual orientation of the recipient of the treatment.

 

Physiological differences in gay men and lesbians                                                                                       

Recent studies have found notable differences between the physiology of gay people and non-gay people. There is evidence that:-

 

• The average size of the INAH-3 (part of the anterior hypothalamus) in the brains of gay men is approximately the same size as INAH 3 in women, which is significantly smaller, and the cells more densely packed than in heterosexual men's brains.
  Simon LeVay (1991) discovered this from studying the brain structures of deceased homosexuals. Neil Carlson (1993) has proposed that this hints at possible prenatal influences when the brain is still undergoing development.
   
• The suprachiasmatic nucleus was found by D F Swaab & M A Hoffman (1995) to be larger in gay men than in non-gay men; the suprachiasmatic nucleus is also known to be larger in men than in women.
   
• The anterior commissure is larger in women than men and was reported to be larger in gay men than in non-gay men, but a subsequent study found no such difference.
   
• Gay men report, on an average, slightly longer and thicker penises than non-gay men.
   
• Gay men's brains respond differently to fluoxetine, a specific serotonin reuptake inhibitor.
   
• The functioning of the inner ear and the central auditory system in lesbians and bisexual women are more like the functional properties found in men than in non-gay women. (The researchers argued this finding was consistent with the Prenatal Hormonal Theory of sexual orientation).
   
• The startle response (eye blink following a loud sound) is similarly masculinised in lesbians and bisexual women.
   
• 3 regions of the brain (medial prefrontal cortex, left hippocampus, and right amygdala) are more active in gay men than non-gay men when exposed to sexually arousing material.
   
• Gay and non-gay people emit different armpit odours.
   
• Gay men are more likely to have a counter-clockwise hair whorl pattern.
   
• Gay and non-gay people's brains respond differently to two human sex pheromones (AND, found in male armpit secretions, and EST, found in female urine).
   
• Finger length ratios between the index and ring fingers may be different between non-gay and lesbian women.

 

Cognitive differences in gay men and lesbians                                                                                             

Recent studies suggest the presence of subtle differences in the way gay people and non-gay people process certain kinds of information. Researchers have found that:-

 

• Gay men and lesbians are significantly more likely to be left-handed or ambidextrous than are non-gay men and women. LeVay (1996) argues that because "[h]and preference is observable before birth... [t]he observation of increased non-right-handness in gay people is therefore consistent with the idea that sexual orientation is influenced by prenatal processes," perhaps heredity.
   
• Gay men and lesbians are more verbally fluent than heterosexuals of the same sex - but 2 other studies did not find this result.
   
• Gay men may receive higher scores than non-gay men on tests of object location memory (no difference was found between lesbians and non-gay women).

 

Fraternal birth order                                                                                                                                                

There is evidence from numerous studies that homosexual men tend to have more older brothers than do heterosexual men, known as the ’fraternal birth order effect’. One study reported that each older brother increases the odds of being gay by 33%. The fraternal birth order effect is the strongest known predictor of sexual orientation. Interestingly, this relation seems to hold only for right-handed males. There has been no observed equivalent for women. Peter Bearman (2002) repeated the experiments but found no correlation between older brothers and male homosexuality and questions the sampling methods used.

 

The effect has been found even in males not raised with their biological brothers, suggesting an in-utero environmental causation. To explain this finding, a maternal immune response has been hypothesized. Male foetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates.

 

Environmental Factors

Researchers have found childhood gender nonconformity to be the largest predictor of homosexuality in adulthood. Daryl Bem's (1996) ‘Exotic Becomes Erotic’ theory  suggests that some children will prefer activities that are typical of the other sex and that this will make a gender-conforming child feel different from opposite-sex children, while gender-nonconforming children will feel different from children of their own sex, which may evoke physiological arousal when the child is near members of the sex which it considers as being ‘different’; this will later be transformed into sexual arousal. Researchers have suggested that this nonconformity may be a result of genetics, prenatal hormones, personality, parental care or other environmental factors. Bearman showed that males with a female twin are twice as likely to report same-sex attractions, unless there was an older brother. He says that his findings support the hypothesis that less gendered socialisation in early childhood and preadolescence shapes subsequent same-sex romantic preferences. He suggests that parents of opposite-sex twins are more likely to give them unisex treatment, but that an older brother establishes gender-socialising mechanisms for the younger brother to follow.

 

From their research on 275 men in the Taiwanese military, Bin Ching Shu & For Wey Lung (2006) concluded that "...paternal protection and maternal care were determined to be the main vulnerability factors in the development of homosexual males." Key factors in the development of homosexuals were "paternal attachment, introversion, and neurotic characteristics." Other researchers have also provided evidence that gay men report having had less loving and more rejecting fathers, and closer relationships with their mothers, than non-gay men. Whether this phenomenon is a cause of homosexuality, or whether parents behave this way in response to gender-variant traits in a child, is unclear.

 

Some authorities have suggested that homosexuality may be an expression of non-sexual problems, such as fear of adult responsibility, or may be triggered by various experiences, such as having sexual relationships with members of one's own sex at an early age that prove to be very satisfying. Arguments regarding the roots of lesbianism include disappointing heterosexual love experience, a father who displays distaste for men who express interest in his daughter, and memories of abusive relationships with men.

 

Henry Gleitman, Alan J Fridlund & Daniel Reisberg (1999) have called for longitudinal data to be gathered via studies following both parents and children over several decades - though some might argue this would be overly intrusive for the participants.

 

Innate bisexuality

Innate bisexuality (or predisposition to bisexuality) is a term introduced by Sigmund Freud (1920, based on work by his associate Wilhelm Fliess) that expounds that all that expounds that all humans are born bisexual but through psychological development (which includes both external and internal factors) become monosexual, while the bisexuality remains in a latent state.

 

Pepper Blumenstein & Phillip Schwartz (1977) proposed 3 factors in the development of bisexuality:-

1. Experimentation within a friendship setting (particularly among female bisexuals)
2. A liberal, hedonistic environment in which sexual activities and practices can be freely tried without the fear of persecution  or judgement
3. A general philosophy or outlook on life that is open and embraces eroticism – with attitudes to sex being just one element

 

Freudian and Freudian-derived theories of homosexuality have included the following:-

• homosexuality is a regression to the earliest (Oral) stage of development
• most families of homosexuals are characterised by an overprotective mother and an absent father
• homosexuals fear engulfment by a dominant mother in the pre-Oedipal phase.

 

Irving Bieber (1962) used Freudian concepts to explain the emergence of sexual orientation. He suggested that homosexuals were likely to have overly-protective and doting mothers but their fathers were likely to be aggressive and unfeeling fathers. Bieber’s views came from a study he led of maladjusted adults undergoing psychiatric treatment who additionally recalled events from their childhood. These factors raised concerns for some commentators about the reliability of their accounts.

 

Nature or Nurture?

Alan Bell, Martin Weinberg & Sue Hammersmith (1986) studied approximately 1,000 gay men and lesbians from the large gay community in and around San Francisco recruited via newspaper adverts and from venues popular with gays. As well as questions about their sexual behaviour, the respondents were also asked about their past and present relationships with members of their family, together with childhood experiences.

 

Although no one clear explanation of homosexuality emerged, several contributing factors appeared to be significant. These included identification with parents, early sexual encounters, and gender roles in childhood. It appeared that participants’ sexual preferences were not influenced by the extent to which they identified with the opposite-sex parent as a child, nor did the participants report being more likely than heterosexuals to have had first sexual experience with someone of the same sex. However, gender nonconformity in childhood was significantly related to sexual orientation in both the males and females. A major factor also seemed to be that respondents experienced ‘sexual feelings’ some years prior to homosexual behaviour. Both males and females reported difficult relationships with their fathers.

 

Bell et al concluded that sexual preference may be brought about to some extent by biological influences, as the participants appeared to ‘resist’ cultural and social factors. These influences may determine the way that gender and sexual development happens.

 

Other recent studies, while not directly supporting biological explanations for homosexuality, suggest that it may be a predisposition that can be detected at an early age among children who do not appear to have traditional gender identification. Whether it can be easily detected or not, most theorists agree that homosexual orientation tends to arise at an early age. Substantially fewer studies of homosexuality have been performed among lesbians, perhaps because of the greater stigma which is often attached to male homosexuality in many Western cultures.

 

Malleability of sexual orientation                                                                                                                

The American Psychiatric Association has stated: "...some people believe that sexual orientation is innate and fixed; however, sexual orientation develops across a person’s lifetime." In combination with other major American medical organisations, they have put out a statement which said: "Sexual orientation develops across a person's lifetime - different people realise at different points in their lives that they are heterosexual, gay, lesbian, or bisexual." A report from the Centre for Addiction & Mental Health states: "For some people, sexual orientation is continuous and fixed throughout their lives. For others, sexual orientation may be fluid and change over time.” One study has suggested "considerable fluidity in bisexual, unlabeled, and lesbian women's attractions, behaviours, and identities."

 

However, they have said: "...most people experience little or no sense of choice about their sexual orientation." The Centre has further stated therapy cannot change sexual orientation, and have expressed concerns over potential harms. The director of the APA's LGBT Concerns Office explained: "I don't think that anyone disagrees with the idea that people can change because we know that straight people become gays and lesbians.... the issue is whether therapy changes sexual orientation, which is what many of these people claim." The American Psychiatric Association has stated: "To date, there are no scientifically rigorous outcome studies to determine either the actual efficacy or harm of ‘reparative" treatments”, and supports research to further determines risks versus its benefits. Similarly, United States Surgeon General David Satcher issued a report in 2001 stating that "there is no valid scientific evidence that sexual orientation can be changed".

 

Pathological model of homosexuality

Homosexuality is no longer regarded as a mental illness by most of the scientific community. In 1973 the American Psychiatric Association removed homosexuality as a disorder from the Sexual Deviancy section of the Diagnostic and Statistical Manual of Mental Disorders, the DSM-II. The World Health Organization's International Classification of Diseases, ICD-9 (1977), listed homosexuality as a mental illness - though in 1990 a resolution was adopted to remove it in the ICD-10 (1993). The ICD-10 added ego-dystonic sexual orientation to the list, which refers to people who want to change their gender identities or sexual orientation because of a psychological or behavioural disorder (F66.1). Groups that advocate reparative therapy, which includes both secular organizations such as NARTH and religious organizations such as Exodus International, do not accept the mainstream medical position.

 

Homosexual Behaviour in Animals

Homosexual sexual behaviour occurs in the animal kingdom, especially in social species and particularly in marine birds and mammals, monkeys and the great apes. Homosexual behaviour has been observed among 1,500 species, and in 500 of those it is well documented. This discovery constitutes a major argument against those calling into question the biological legitimacy or naturalness of homosexuality, or those regarding it as a meditated social decision. For example, male penguin couples have been documented to mate for life, build nests together, and to use a stone as a surrogate egg in nesting and brooding. In a well-publicised story from 2004, the Central Park Zoo in the United States replaced one male couple's stone with a fertile egg, which the couple then raised as their own offspring.

 

The genetic basis of animal homosexuality has been studied in the fly drosophila melanogaster. Here multiple genes have been identified that can cause homosexual courtship and mating. These genes are thought to control behaviour through pheromones as well as altering the structure of the animal's brains. These studies have also investigated the influence of environment on the likelihood of flies displaying homosexual behaviour.

 

Georgetown University professor Janet Mann (2000) has specifically theorised that homosexual behaviour, at least in dolphins, is an evolutionary advantage that minimizes intraspecies aggression, especially among males. Studies indicating prenatal homosexuality in certain animal species have had social and political implications surrounding the gay rights debate.