Biological Basis of Sexual Orientation: Finding Needle in Haystack

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Sexual orientation is one of the animal kingdom’s most pronounced gender differences. The Overwhelming majority of people are heterosexual with few exceptions: most males want Females as sexual partners and vice versa. Knowing whether someone has a male or female sexual preference is one of the most reliable behavioural predictors of the biological sex of that individual, perhaps second only to gender identity, i.e. the sense of being male or female.

Although heterosexuality is the norm, a small but significant proportion of people (2–6 %) report mainly homosexual attractions (Diamond, 1993).

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There are some interesting differences in the distribution of men and women between the two extremes of sexual orientation (completely heterosexual versus completely homosexual). The search for sexual orientation’s biological basis is not a recent undertaking.

For instance, in the mid-19th century, it was generally accepted that innate intellectual inferiority described why certain classes of people were socially disadvantaged. The brains and bodies of these groups, which included the poor, women, persons of colour, and homosexuals, were assumed to reflect their lower status. Richard von Krafft-Ebing, a notable Viennese sexologist, believed that the homosexual behaviour was a product of defective development (Krafft-Ebing, 1965). By the late 19th and early 20th centuries, the discourse had altered somewhat. The bodies of homosexuals were still seen as different, but they were now characterized as a third sex. Homosexuals were seen as inverts in this context, i.e., gay men were thought to have some

Innately feminine tendencies, while lesbian women were more bent to express masculine Characteristics. Though homosexuals are no longer considered a separate sex, the model of inversion continues to impact how homosexuality research is presented, especially in words of neurological correlates.

Behavior Genetics

Some of the most persuasive initial studies of the biological basis of sexual orientation were the result of the emerging field of behavior genetics. This area of research focusing on homosexuality continued in following decades, with comparable findings from study to study. For example, in another paper, Pillard et al. (1981) found that both lesbians and gay men were more probable than heterosexual men and women to have gay siblings, with monozygotic twins surrendering the highest agreeing rates in orientation, as would be expected assuming a genetic susceptibility to the trait.

Shortly thereafter, Pillar et al. (1982), studying a sample of 50 heterosexual and 50 homosexual men, found out that roughly 25% of the gay men’s brothers were also accounted to be gay. Pillard and Weinrich (1986) found gay men reported having roughly four times as many gay brothers as heterosexual men did, suggesting a familial aggregation of genes underlying the trait’s variation in men. Bailey and Benishay (1993) found lesbians had a higher proportion of lesbian sisters and (though non-significantly) a higher proportion of gay brothers than heterosexual women.

These data suggest that male and female homosexuality may be cofamilial to some extent. The question of whether this is true to the same degree for males was chased several years later, with the finding of Bailey et al. (1999) that between 7% and 10% of brothers and 3% and 4% of sisters of gay men were gay themselves. Both of these spans are higher than anticipated, suggesting a role for familial aggregated genes. On the other hand, Bailey et al. (1995) found a negative result when exploring whether young men are more likely to be gay if their fathers are: less than 10% of the sons were gay.

Sex Hormones

Studies examining the relations between sexual orientation and sex hormone exposure have helped to put light on its biological components, like research on the behavior genetics of sexual orientation. Early research proposing links between adult sexual orientation and both prenatal sex hormone signalling (e.g., Ward, 1972; Ellis et al., 1988) and gender-related behaviour during childhood spawned research into possible associations among all three phenomena.

Because gender unconventionality during childhood may be associated with adult homosexuality, if prenatal hormone exhibition affects childhood gender atypicality, this might suggest a link between prenatal sex hormones and adult homosexuality. Childhood gender nonconformity (CGN) is among the best foretellers of adult homosexuality for men, both in retrospective and in prospective studies. Research into this possibility engages both “natural experiments” and biomarkers in an effort to establish links between early sex hormone exposure and sex-atypical behaviour & psychology.

Simon LeVay’s Neurobiology

Sex steroids exercise their influence by regulating gene expression in their target tissues (Nelson, 2005). The target tissues would presumably be the nervous system for amendments in gene expression to affect sexual orientation, and such changes would influence neural development. In other words, we would anticipate to differ in male and female neuroanatomy and/or neurophysiology, along with gay and heterosexual individuals.

An evident place to look for such differences is the brain. Some researchers have theorized that the sexual preferences and behavior of homosexuals may be ruled by the structure of the brain—particularly if the brains of homosexual men, for example, can be displayed to resemble those of heterosexual women more than they resemble those of heterosexual men. For alterations in gene expression to affect sexual orientation, the target tissues would presumably be the nervous system and such alterations would affect neural development.

In other words, we would expect to vary in male and female neuroanatomy and/or neurophysiology, as well as in gay and heterosexual individuals. This is what LeVay found out: “INAH 3 was more than twice as large in the heterosexual men as in the women. Also, more than twice as large in the heterosexual men as in the homosexual men. This finding proves that INAH is dimorphic with sexual orientation (i.e., shows a difference in structure between homosexuals and heterosexuals), at least in men, and recommends that sexual orientation has a biological substrate.”

Fraternal Birth Order

Research on biological components of sexual orientation is related with a phenomenon that may appear to have a very little connection with sexual orientation at first look: birth order. Specifically, a repeated finding has been that the number of a man’s (but not a woman’s) older brothers rises his likelihood of being gay, each older brother increasing the odds by around 33% above the base rate of 2–3%.

This is named as the fraternal birth order effect. With a host of studies since the mid-1990s replicating this finding, the fraternal birth order effect is one of the most well-established close correlates of sexual orientation. What is it about the number of older brothers that connects to a man’s chances of being gay? One hypothesis is socialization for example, that being reared in a home with older brothers in some way “demasculinizes” a boy.

This seems not to be the case, as only the number of biological brothers from the same mother predicts a man’s likelihood of homosexuality, despite of the duration of rearing together, or even whether the brothers were reared together at all. Gay men with older brothers have also been shown to have lower birth weights than have heterosexual men with older brothers (Blanchard & Ellis, 2001), and since birth weight is unforeseeable on gestational events, this suggests that whatever phenomenon is responsible for the trend applies its influence early, during gestation.

Blanchard and Klassen (1997) put forward the maternal immune hypothesis to showcase the fraternal birth order effect. According to the hypothesis, when a mother gives birth to a son, maternal fetal blood mix, and the mother’s immune system is uncovered to Y-linked (male-specific) antigens. The mother produces antibodies to these male-defined antigens, which can cross the placental barrier in later pregnancies. These antibodies then in some as yet undefined way affect the neural development of every following male fetus.

Because the production of anti-male antibodies is supported by each subsequent delivery of a son, the mother’s immune system can “recollect” the number of sons she has previously delivered, increasing the probability of later homosexuality by about one-third of the population base rate with each successive son. This theory is admittedly conjectural at this point, though it is the only widely accepted explanation of the fraternal birth order effect. Two recent estimates of the percentage of male homosexuality attributable to the fraternal birth order effect are 15.1% (Cantor, Blanchard, Paterson, & Bogaert, 2002) and 28.6% (Blanchard & Bogaert, 2004). Nonetheless, the fraternal birth order effect remains the best held up of all proximate explanations for the etiology of homosexuality.


This paper has discussed chief avenues of study into biological aspects of sexual orientation it is reasonably well-established that (1) male and female homosexuality are at least partially heritable traits that are (2) sometimes linked with childhood gender nonconformity and (3) are found at homogeneous frequencies across many culturally divergent populations. Some evidences suggest that early developmental processes influencing female sexual orientation could include androgen signaling.

It has been noted that the sexual orientation of men is associated with fraternal birth order, which could reflect a maternal immune response to male-specific antigens. Less is known at the supreme level, although it appears that the most promising explanatory hypothesis at present is a pleiotropic genetic model. However, most theories at either stage need not be mutually exclusive.

Furthermore, the philosophy of homosexuality is often studied separately by social and biological scientists, rather than from an interdisciplinary perspective, which may yield additional insights into the proximate and ultimate roots of sexual orientation. Much of the needed research may come from genomics, neuroscience, and related fields. Meticulous exploration of the fraternal birth order effect, and specifically testing of the maternal immune hypothesis, might clarify the aetiology of sexual orientation for some individuals.

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Biological Basis of Sexual Orientation: Finding Needle in Haystack. (2019, Oct 21). Retrieved from