Biology and same-sex love

February 25th, 2015

This article is excerpted from chapter three of the author's recent book Science, Scripture, and Same-Sex Love (Abingdon Press, 2014). 

Science is a human enterprise seeking to understand the universe in which we live. To put it more directly, science tries to figure out how things work, with the assumption that if we can figure out how something works, we can predict how it will work in the future. There is no doubt that this enterprise has been exceedingly successful for good and for bad. So when science comes at the question of human sexual orientation and behavior and, specifically, same-sex orientation and behavior, it seeks to figure out why it occurs as a human phenomenon and the mechanics behind it. In its purest form science does not make value judgments about the objects of its inquiry. That is to say, it does not declare that a condition or behavior is right or wrong. It simply seeks to describe, categorize, and understand how the condition emerged or how the behaviors are practiced.

If our universe is an open system, then at a natural process level, there is vulnerability in the freedom of nature to unfold in diverse ways. It means that sometimes the universe can be horribly destructive from the viewpoint of humans living. Tsunamis, earthquakes, volcanoes, tornados, asteroids, all the result of natural processes—while holding a certain beauty in them—can devastate human communities. At one level, this is the vulnerability of contingency.

It also means that nature may express diversity in how life emerges. I have red hair, a somewhat unusual human trait and one that caused me much suffering as child. Being different from the norm carries with it some danger. Unfortunately, sometimes nature’s diverse unfolding will produce children with physical dysfunctions. And sometimes nature’s diverse unfolding will produce people who are not heterosexual. Within the form and freedom of nature, there is always a range of expression with a diversity that can go in many directions. When thinking about homosexuality, this is intended not as a value judgment but as an observation. It is not intended to suggest that homosexuality is a mistake of nature but merely is part of the range of variation that emerges in nature. When process is open and therefore free, process will express itself in diverse ways. Indeed—and this is a value judgment—it is this freedom that is expressed in such diversity that creates so much beauty and wonder in our universe; it also gives us the possibility of individuality and a unique expression of humanity, which our culture values so highly.

Recent research into the origin of homosexuality within the scientific and mental health research community has turned away from nonbiological origins as the primary force in determining a person’s sexual orientation. That means it turns to biology—to nature. More specifically the disciplines involved in the research include neuroscience, genetics, endocrinology, and evolutionary biology. The field of psychology is involved on both sides of the discussion (nonbiological and biological), though mainstream psychology recognizes the biological element as a primary factor. Wilson and Rahman’s book Born Gay: The Psychobiology of Sex Orientation explores how all of these fit together. In their introduction, they state the following about biology: “Biology in this case means at least either hormones or genes or both. Turns out, it is both, and together they influence critical developmental processes of a fetus in utero. One element of that influence is that set of processes that shapes the sex and gender of a person” (10).

Hormone Exposure and Sexual Orientation and Anatomical Sex

There are multiple biological theories offered to explain how humans develop their sexual anatomy and orientation. Perhaps the strongest, or at least most influential, is that sexual orientation develops primarily during fetal neural development. Within this framework are models that include prenatal hormone exposure—which can include a genetic element, maternal immunological reactions, and an unstable prenatal development environment.

The prenatal hormone exposure theory proposes that sexual anatomy and orientation are determined by hormone exposure at two critical moments in fetal development. The first is the differentiation of the sexual organs, and the second occurs at a later date in the prenatal process wherein hormones affect brain structure. It is this latter process that is believed to be the origin of many differences, including sexual orientation (Garcia-Falgueras and Swaab, 24; LeVay). Sexual differentiation of the genitals takes place in the first couple of months of pregnancy. Sexual differentiation of the brain occurs in the second half of a pregnancy during a period of major and rapid brain development. This is significant because the gap means that the two processes can be influenced separately and influence development differently. In most of the human (and animal) population, the two processes are in synch. Male genitals align with typical male sexual behavior that prefers females due to the masculinization of the sexual behavior control areas of the brain. Conversely, the same is true for females. But it also means that sometimes they get out of synch. The result is “discrepancies between physical sex and aspects of sexual behavior that are sexually differentiated (orientation, gender identity)” (Balthazart, 155). A fetus that develops male genitalia may not develop a masculinized brain. In some cases the result is a transgender person; in others, a gay, lesbian, or bisexual person.

Recent evidence suggests that exposure to prenatal androgen (testosterone for males and estrogen for females) can influence later sexual orientation and behavior. It is proposed that hormone exposure works as a cofactor interacting with genes and environmental and social conditions (Wilson and Rahman). Research also demonstrates that within the prenatal time line, sexual differentiation and the development of the sexual centers of the brain (where orientation develops) occurring at different moments explains, in part, why some persons develop orientations that are different than their anatomical sex. A genetic male who develops male genitalia due to testosterone exposure may for some reason not experience the same testosterone exposure in the later brain development that would normally masculinize the brain. In other words, depending upon the moment and extent of hormone exposure at two different points in the sexual development process, a fetus may emerge as straight, gay, bisexual, or transgender.

The Brain

Sex is in your head, it is often said. More accurately, a person’s sexual orientation and behavior originate in the brain. Research into prenatal development proposes that the brains of males develop differently than those of females due to hormone exposure or lack thereof, as stated above. In the second half of prenatal development, the fetal brain develops in a male direction (masculinization) through a surge in testosterone on the forming nerve cells. A brain develops in the female direction (feminization) through the absence of the hormone surge. The default pattern of fetal development is female unless exposed to testosterone. This surge can be seen in the following graphic that compares the development of male versus female mice. Notice that the male has a spike before birth and for a little time after. Females do not have a hormonal spike until puberty.

Males typically experience two spikes in fetal hormone exposure, whereas females experience only one (Vandenbergh, 220).

The research into hormone exposure also found that some mice exhibit same-sex preferences. In these cases, at some point the brains of female mice were exposed to androgen hormones, which masculinized the brain. This can be seen in the following graph in which a female (XX chromosomes) is exposed to androgens just prior to birth and as a result exhibits same-sex behaviors. Though females anatomically, the mice were sexually attracted to other female mice (Vandenbergh 2003, 220).

A similar pattern occurs in humans. A 2010 study by Garcia-Falgueras and Swaab of the Netherlands Institute for Neuroscience, at the Royal Netherlands Academy of Arts and Sciences, concluded:

The fetal brain develops during the intra­uterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed or organized into our brain structures when we are still in the womb. There is no indication that social environment after birth has an effect on gender identity or sexual orientation. (22–35)

Garcia-Falgueras and Swaab believe the effects of hormones are permanent. The main mechanism determining sexual identity and orientation is the effect of exposure to testosterone on an intrauterine developing brain. Sexual organs develop first, and following that, the brain is differentiated “under the influence, mainly, of sex hormones such as testosterone, estrogen, and progesterone on the developing brain cells and under the presence of different genes as well” (23–24). During fetal development, gay, lesbian, and bisexual people experience a different hormone exposure affecting how certain areas of the brain that determine one’s sexual orientation and gender identity develop. A gay man may develop male genitalia, which form first, but when the brain is ready to develop, a change in hormone exposure—little or no testosterone—results in the sexual part of the brain forming in a way that is more typical of a straight female brain resulting in an orientation that is attracted to other males. This is referred to as the “feminization” of the brain. The converse is true for a lesbian wherein she experiences a surge in testosterone. Put simply, because of a change in hormone exposure, the sexual part of the brain that is the source of sexual attraction and behavior can develop such that a woman is attracted to a woman and a man is attracted to a man. The shape and development of the brain sets the gender identity and sexual orientation; that is their pattern of attraction. That is their natural state just as is the case for a straight person.

Other research shows that there are parts of the straight male brain that are differently shaped than the straight female brain. Those areas having to do with sex and gender are called sexually dimorphic areas, meaning “different forms.” (In other words, they have different shapes.) Some studies have demonstrated, however, that these same areas of the brain in gay men are more likely to be the same shape as heterosexual females. Conversely, these specific areas in a lesbian brain are similar in shape to a heterosexual male (Balthazart).

A Homosexual Gene?

With the advent of gene science, there has been an effort to find a gene or genes that are the “cause” of homosexuality. From an evolutionary perspective, the frequency of homosexuality should decrease unless there is some evolutionary advantage. A decrease in the incidence of homosexuality among many species is clearly not the case. (Nor has it increased as a percentage, 3.4 percent, of the human population.) Biological scientists propose that there is some genetic predisposition that creates a positive adaptation and thus the maintenance of a certain percentage of populations that are LGBT. For example, a study conducted by Dean Hammer found that gay men had more gay uncles and cousins on their mother’s side than on their father’s side. More specifically, at least some portion of male homosexuality is an inherited trait through the mother’s side. Research has identified a location on the X chromosome (that comes from the mother) that is associated with sexual orientation, though no specific gene or genes have been identified. Studies attempting to discover if there is a genetic origin of male homosexuality found certain “markers” transmitted on the X chromosome (from the mother) in the Xq28 region (Balthazart, 156).

The sexual orientation of women is more complex. Nonetheless Balthazart notes that several studies “have identified an increased rate of non­heterosexuality (the term used to group homosexual and bisexual women) in girls, nieces, and cousins of the paternal lineage of lesbians” (145). Balthazart explains that this transmission could be linked to the X chromosome or it could come from either the father or the mother. More research is needed on female sexual orientation in general because the actual genetic mechanisms are not clearly understood at this time.

Just how genes affect sexual orientation—either heterosexual or homosexual (or anywhere along a spectrum in between)—is still being explored. One area of emerging research is the impact of epigenetics on sexual orientation. Epigenetics focuses on how genes are switched on or off, a process that can be influenced by environment—in this case the fetal environment (Rice, Friberg, and Gavrilets). This research is quite new and is yet to be tested, but if confirmed it would blur the nature/nurture distinction within a biological theory of sexual orientation. (For example, it may shed light on what otherwise looks in some cases to be random.)

This simplified explanation of the biological origins of sexual orientation shows that science leans heavily in the direction of a prenatal biological determination of a whole cluster of gender and sex-related traits. If this is true, then one’s sexual orientation is the result of genetic and hormonal processes that shape one’s orientation as well as a host of other gender and sex traits such as gender-conforming preferences and behaviors, some physical characteristics, and size and structure of sex-related areas of the brain. What our adult sexual orientation will be is largely determined in our mother’s womb.¹ We are born the way we are.


1. The research data available on female homosexuality is significantly less than on male homosexuality. There is enough information to know that female homo­sexuality (like, I suppose, female heterosexuality) is different than male homosexuality, though the basic prenatal mechanisms are assumed to be similar. It does leave one a tad frustrated when trying to understand how this is expressed and what the potential mechanisms are in half the human population.


Balthazart, J. 2011. The Biology of Homosexuality. Oxford Series in Behavioral Neuroendocrinology. New York: Oxford University Press.

Garcia-Falgueras, A., and D. F. Swaab. 2010. “Sexual Hormones and the Brain: An Essential Alliance for Sexual Identity and Sexual Orientation.” Endocrine Development. 17:22–35.

LeVay, S. 2010. Gay, Straight, and the Reason Why: The Science of Sexual Orientation. New York: Oxford University Press.

Rice, W. R., U. Friberg, and S. Gavrilets. 2012. “Homosexuality as a Consequence of Epigenetically Canalized Sexual Development.” The Quarterly Review of Biology 87 (4): 343–68.

Vandenbergh, J. G. 2003. “Prenatal Hormone Exposure and Sexual Variation.” American Scientist 91 (3): 218–25.

Wilson, G., and Q. Rahman. 2008. Born Gay: The Psychobiology of Sex Orientation. 2nd ed. London: Peter Owen Publishers.

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