Causes

The exact causes for the development of a particular sexual orientation have yet to be established. To date, a lot of research has been conducted to determine the influence of genetics, hormonal action, development dynamics, social and cultural influences—which has led many to think that biology and environment factors play a complex role in forming it. It was once thought that homosexuality was the result of faulty psychological development, resulting from childhood experiences and troubled relationships, including childhood sexual abuse. It has been found that this was based on prejudice and misinformation.

Biologyedit

Research has identified several biological factors which may be related to the development of sexual orientation, including genes, prenatal hormones, and brain structure. No single controlling cause has been identified, and research is continuing in this area.

Although researchers generally believe that sexual orientation is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences, with biological factors involving a complex interplay of genetic factors and the early uterine environment, they favor biological models for the cause. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males. Scientists do not believe that sexual orientation is a choice, and some of them believe that it is established at conception. Current scientific investigation usually seeks to find biological explanations for the adoption of a particular sexual orientation. Scientific studies have found a number of statistical biological differences between gay people and heterosexuals, which may result from the same underlying cause as sexual orientation itself.

Genetic factorsedit

Genes may be related to the development of sexual orientation. A twin study from 2001 appears to exclude genes as a major factor, while a twin study from 2010 found that homosexuality was explained by both genes and environmental factors. However, experimental design of the available twin studies has made their interpretation difficult.

In 2012, a large, comprehensive genome-wide linkage study of male sexual orientation was conducted by several independent groups of researchers. Significant linkage to homosexuality was found with genes on chromosome Xq28 and chromosome 8 in the pericentromeric region. The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in each of these regions contributes to development of the important psychological trait of male sexual orientation." It was the largest study of the genetic basis of homosexuality to date and was published online in November 2014.

Hormonesedit

The hormonal theory of sexuality holds that just as exposure to certain hormones plays a role in fetal sex differentiation, hormonal exposure also influences the sexual orientation that emerges later in the adult. Fetal hormones may be seen as either the primary influence upon adult sexual orientation or as a co-factor interacting with genes or environmental and social conditions.

For humans, the norm is that females possess two X sex chromosomes, while males have one X and one Y. The default developmental pathway for a human fetus being female, the Y chromosome is what induces the changes necessary to shift to the male developmental pathway. This differentiation process is driven by androgen hormones, mainly testosterone and dihydrotestosterone (DHT). The newly formed testicles in the fetus are responsible for the secretion of androgens, that will cooperate in driving the sexual differentiation of the developing fetus, including its brain. This results in sexual differences between males and females. This fact has led some scientists to test in various ways the result of modifying androgen exposure levels in mammals during fetus and early life.

Birth orderedit

A significant volume of research has demonstrated that the probability of a male growing up to be gay increases with each older brother he has from the same mother. Known as the fraternal birth order (FBO) effect, scientists attribute this to a prenatal biological mechanism – specifically a maternal immune response to male fetuses – since the effect is only present in men with older biological brothers, and not present among men with older step-brothers and adoptive brothers. This process, known as the maternal immunization hypothesis (MIH), would begin when cells from a male fetus enter the mother's circulation during pregnancy. These cells carry Y-proteins, which are thought to play a role in brain masculinisation (sex-differentiation) during fetal development. The mothers immune system builds antibodies to these Y-proteins. These antibodies are later released on future male fetuses and interfere with the masculinization role of Y-proteins. leaving regions of the brain responsible for sexual orientation in the 'default' female–typical arrangement, causing the exposed son to be more attracted to men over women. Biochemical evidence for this hypothesis was identified in 2017, finding that mothers with a gay son, especially those with older brothers, had significantly higher levels of anti-bodies to the NLGN4Y Y-protein than mothers with heterosexual sons.

The effect becomes stronger with each successive male pregnancy, meaning the odds of the next son being gay increase by 38–48%. This does not mean that all or most sons will be gay after several male pregnancies, but rather, the odds of having a gay son increase from approximately 2% for the first born son, to 4% for the second, 6% for the third and so on. Scientists have estimated between 15% and 29% of gay men may owe their sexual orientation to this effect, but the number may be higher, as prior miscarriages and terminations of male pregnancies may have exposed their mothers to Y-linked antigens. The fraternal birth order effect would not likely apply to first born gay sons; instead, scientists say they may owe their orientation to genes, prenatal hormones and other maternal immune responses which also influence brain development. This effect is nullified if the man is left-handed. Ray Blanchard and Anthony Bogaert are credited with discovering the effect in the 1990s, and Blanchard describes it as "one of the most reliable epidemiological variables ever identified in the study of sexual orientation". J. Michael Bailey and Jacques Balthazart say the FBO effect demonstrates that sexual orientation is heavily influenced by prenatal biological mechanisms rather than unidentified factors in socialization.

Environmental factorsedit

In the field of genetics, any factor which is non-genetic is considered an environmental influence. However, environmental influence does not automatically imply that the social environment influences or contributes to the development of sexual orientation. There is a vast non-social environment that is non-genetic yet still biological, such as prenatal development, that likely helps shape sexual orientation.:76 There is no substantive evidence to support the suggestion that early childhood experiences, parenting, sexual abuse, or other adverse life events influence sexual orientation. Hypotheses for the impact of the post-natal social environment on sexual orientation are weak, especially for males. Parental attitudes may affect whether or not children openly identify with their sexual orientation.

Influences: professional organizations' statementsedit

The American Academy of Pediatrics in 2004 stated:

The mechanisms for the development of a particular sexual orientation remain unclear, but the current literature and most scholars in the field state that one's sexual orientation is not a choice; that is, individuals do not choose to be homosexual or heterosexual. A variety of theories about the influences on sexual orientation have been proposed. Sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences. In recent decades, biologically based theories have been favored by experts. Although there continues to be controversy and uncertainty as to the genesis of the variety of human sexual orientations, there is no scientific evidence that abnormal parenting, sexual abuse, or other adverse life events influence sexual orientation. Current knowledge suggests that sexual orientation is usually established during early childhood.

The American Psychological Association, the American Psychiatric Association, and the National Association of Social Workers in 2006 stated:

Currently, there is no scientific consensus about the specific factors that cause an individual to become heterosexual, homosexual, or bisexual – including possible biological, psychological, or social effects of the parents' sexual orientation. However, the available evidence indicates that the vast majority of lesbian and gay adults were raised by heterosexual parents and the vast majority of children raised by lesbian and gay parents eventually grow up to be heterosexual.

The Royal College of Psychiatrists in 2007 stated:

Despite almost a century of psychoanalytic and psychological speculation, there is no substantive evidence to support the suggestion that the nature of parenting or early childhood experiences play any role in the formation of a person's fundamental heterosexual or homosexual orientation. It would appear that sexual orientation is biological in nature, determined by a complex interplay of genetic factors and the early uterine environment. Sexual orientation is therefore not a choice, though sexual behaviour clearly is.

The American Psychiatric Association stated:

No one knows what causes heterosexuality, homosexuality, or bisexuality. Homosexuality was once thought to be the result of troubled family dynamics or faulty psychological development. Those assumptions are now understood to have been based on misinformation and prejudice.

A legal brief dated September 26, 2007, and presented on behalf of the American Psychological Association, California Psychological Association, American Psychiatric Association, National Association of Social Workers, and National Association of Social Workers, California Chapter, stated:

Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation – heterosexuality, homosexuality, or bisexuality – is determined by any particular factor or factors. The evaluation of amici is that, although some of this research may be promising in facilitating greater understanding of the development of sexual orientation, it does not permit a conclusion based in sound science at the present time as to the cause or causes of sexual orientation, whether homosexual, bisexual, or heterosexual.