Biological Explanations for Sexual Orientation

How much of our sexual orientation is biologically hardwired, and how can we know? These are big, ethically complex questions. A principal force driving these questions in society are political and religious organizations that require scientific evidence of the biological origin and nature of sexual orientation as a prerequisite for ceding rights or acceptance for lesbian, gay, and bisexual people. Therefore researchers have engaged in scientific studies exploring the nature and origin of human sexual orientation. Based on this research, a majority of scientists have come to the conclusion that it develops through a complex series of causes - predominantly biological - and thus independent of conscious choice - and is therefore not likely to be alterable through any form of therapy.

We’ll explore some of the evidence for that assertion in a minute, but first let’s look at the numbers. According to the National Survey of Sexual Health and Behavior, an average 10% of the population reports experiencing some kind of same-sex attraction over their lifetime, with 5 to 7% of the population acting on those attractions at some point in their life and between 3 and 4% of adult males and 1 to 2% adult females not only feeling it and acting on it, but also self-identifying as homosexual, with an average of 2% of all adults identifying as bisexual. If we apply these average percentages to the global population, we get a worldwide estimate of approximately 630,000,000 homosexual and bisexual people – which is twice the population of the United States. So while we are talking about a minority, it is still a significant portion of the population. It should also be noted that these percentages have largely held consistent for the last few decades, with some increases reported in a few countries.

And now the science: One of the more enduring methods employed by scientists in this field has been the method of comparing what are called sexually dimorphic traits or traits that normally differ between the two biological sexes - male and female. These traits are caused by hormone secretions within a fetus as it grows - in embryo. This makes them noteworthy in this line of research because they are not affected by any factors occurring after birth; therefore, it is understood that any other traits correlated with these traits would also be biologically formed and not a product of factors occurring after birth.

So it is significant that scientists have found that the sexually dimorphic traits of gay men often resemble those of heterosexual females, while those of lesbian women often resemble heterosexual males. Therefore, many scientists have deduced that the fact that these traits - which, again, are determined before birth - correlate with sexual orientation, suggests that sexual orientation is also determined before birth. Let’s take a look at a few of the studies that illustrate these notable corollaries.

First stop the brain, the hypothalamus to be specific. The sexually dimorphic trait we’ll look at here - again meaning they are different between men and women - is the volume or density of a part of the hypothalamus called INAH-3. This means that the volume of the INAH-3 differs between heterosexual males and heterosexual females. Yet, beginning with studies in 1991 and later in 2001, neuroscientists found that the volume of INAH-3 in homosexual males actually appears closer to that of heterosexual females than it does in heterosexual males. So, in other words, statistically speaking, the less densely packed the INAH-3 in a male’s hypothalamus, the more likely he is to experience homosexuality. 

Another sexually dimorphic trait is finger-length ratio, and here again, the ratios of homosexual males and females tend to look more like the opposite sex than their own. Typically a heterosexual male’s ring finger is longer than their index finger and a heterosexual female’s ring finger is the same length as their index finger. But homosexual male ring and index fingers tend to be the same length (like hetersexual females) and homosexual female ring fingers tend to be longer than their index finger (like heterosexual males). And what’s more, since this dimorphic trait is determined by the level of testosterone, which is measurable, its development can be observed while the fetus is still in embryo.

There’s another sexually dimorphic trait that can provide a fun experiment for you and your friends. It’s called pre-pulse inhibition or PPI and is a non-learned mechanism that controls how we react when we hear a loud noise. Here homosexual females react similarly to all males. A fun way to test this is to set a heterosexual male and female in chairs next to each other. Secretly pop a balloon behind them and watch how both the male and female startle. A few moments later, pop another balloon and notice how the female is startled again, same as before, but the male is less startled. This is because his startle-response has been inhibited. But when this exercise is performed with homosexual females, their startle-response has also been inhibited so they respond similarly to males i.e. startle less.

Other studies of neurocognitive profiles or how well people perform on certain thinking and memory tests, have also been analyzed for their sexually dimorphic differences and have once again shown that homosexuals perform more like their opposite sex counterparts. For example, the object location memory test. First, a heterosexual male and female look at a table covered in various objects. Then the objects are removed and the participants are asked to replace the objects in their original position. Typically, heterosexual females perform well on this task and heterosexual males not so well, but as you might expect, homosexual males tend to perform well - just like heterosexual females - and homosexual females tends to underperform - again, just like heterosexual males. Similar results also tend to occur with tests of synonym fluency, category fluency, and spatial rotation.

And finally, pheromones; aromatic chemicals produced via hormones by both males and females that induce non-learned, inborn brain activity in members of the opposite sex. For example, the testosterone found in a male’s sweat typically evokes activity in a region of a heterosexual female’s hypothalamus that is highly involved in sexual attraction and arousal. But as you might suspect, homosexuals exhibit this same brain activity and sexual arousal in response to pheromones produced by members of their same sex.

It should be acknowledged that comparing sexually dimorphic traits has not revealed a specific biological cause of sexuality. Still, the high number of corollaries between sexually dimorphic traits and one’s sexual orientation (explained above) have led a majority of scientists to believe that there must be some kind of biological genesis of sexual orientation formation.

And these are not the only studies that suggest a biological cause for sexual orientation. For example, several studies have shown that individuals, both men and women, who are left handed or ambidextrous, a trait that develops while in embryo, are more likely to be homosexual. And studies of birth order have shown that men who have several older brothers born to the same mother, are more likely to be homosexual. The theory of why this tends to be the case is that with each male fetus, the mother grows and accumulates antibodies that attack and somehow “feminize” (biologically speaking) the male fetus. Therefore, with each additional male fetus, the likelihood that the child will experience same sex attraction increases.

Sibling studies have offered some insight into potential genetic influences on sexual orientation formation. Since the early 1990s, various identical and fraternal twin studies have investigated how often twins experience the same sexual orientation. Several studies have shown as high as 75% of identical twins and 24% of fraternal twins experience the same sexual orientation. This is compared to as low as 9% of non-twin siblings and 6% of adoptive siblings. While these numbers do not conclusively prove that there is a biological cause for sexual orientation, they do indicate that there could be some genetic component to it and that homosexuality in both males and females, may also run in families.

These types of statistics have led to studies looking for the so-called gay gene. Beginning in 1993 and again in 2014, scientists looked at the X chromosome of pairs of homosexual brothers.  These studies revealed that brothers who are both homosexual share the same genetic markers along two regions of their X chromosome. Scientists have speculated that one or a combination of these markers could influence the development of a same-sex orientation in men. These studies continue to lead some to believe that one day scientists might isolate a “gay gene.”

But rather than looking solely for a gay gene, many scientists are instead coming to believe that one’s sexual orientation is formed as a result of a complex combination of interactions between genes, levels of sex hormones, and the cells of a developing fetal brain and body. This is where the relatively new field of epigenetics comes into the picture. Scientists who study epigenetics have found that, depending on the presence or absence of steroidal sex hormones that activate or repress certain genes, as well as when they turn genes on or off in the developmental process of a fetus, the child may develop any number of different biological traits. So according to this field of research, there may be a chemical switch or even a series of switches attached to genes that when turned on or off by hormones, in the right sequence, may increase the likelihood of a child being born with same-sex attraction. In fact, male and female fetuses begin their development with the same genetic code for brain networks that potentially make them attracted to both men and women. Depending on which network is activated in embryo—and at what time it is activated, a baby may be born with same sex or opposite sex attraction.

This process can explain, for example, how some identical twins may differ in their sexual orientation. Though they may possess the same genes, they each may have been exposed to or deprived of hormones at different stages of fetal development that would have turned on or off genetic markers. For example, two twins, Sam and Fred share the same genes in every single cell of their bodies. In Sam, a particular gene we’ll call A, sitting in the nucleus of one of his brain cells, is turned on by the presence of a hormone on day 40 of his fetal development. But in Fred’s case, that same gene (A) is activated in a different brain cell on day 41 of his fetal development. This seemingly subtle difference in the twins’ developmental process, replicated billions of times as they grow - even in the same embryo, can produce very different effects when they are born.

And while scientists continue to investigate the potential biological and genetic origins of sexual orientation formation, other researchers have been looking at how homosexuals and heterosexuals physiologically experience romantic love.

These studies have revealed that the physiological and neurological manifestations of love in the brains of both heterosexuals and homosexuals are the same. Specifically, brain scans of people in love with an opposite sex partner, and individuals in love with a same sex partner look the same; the dopamine reward systems become intensely activated in both heterosexual and homosexual pairs. This finding confirms that all humans, almost universally, experience romantic love as a basic neurological drive in the brain; energizing and directing our behavior intensely toward acquiring what our brain sees as a basic biological need.

This drives both heterosexuals and homosexuals towards one of life’s greatest prizes: a person to whom they are attracted and with whom they desire to exclusively pair bond, or in more popular terms, fall in love and form a long-term relationship. And unlike mere emotions or “preferences and tendencies", both homosexuals and heterosexuals experience this neurological drive towards romantic love tenaciously, and can experience acute emotional distress and even physiological pain and suffering when it cannot be obtained.

So while we don’t yet know definitively how homosexual orientation is formed, it certainly appears to function as a fundamental drive in the same way heterosexual orientation does; meaning it is experienced as a compelling and persistent force with biological causes and profound psychological and physiological consequences. This is not the only inborn trait that is experienced as a persistent force with biological causes that is resistant to change. For example, research is finding that traits like pain-threshold and happiness-threshold are also inborn and not likely to change.

But it should also be noted that for some people— amongst both homosexuals and heterosexuals, there is a possibility of what is referred to by researchers as sexual fluidity – meaning spontaneous shifts in attractions to include a person not consistent with their sexual orientation. So far, the research on sexual fluidity indicates that the potential level of fluidity an individual may experience also appears to be inborn and not likely alterable by any known form of intervention. Furthermore, amongst those who do encounter some degree of sexual fluidity, such shifts tend to widen their overall range of sexual attraction rather than change or eradicate their existing sexual orientation. In other words, the research does not show that people change from one sexual orientation to another, they just expand their potential attractions.

So we can safely say that a considerable scientific consensus exists around the theory that sexual orientation is established in the womb during critical periods of development, as a result of a complex combination of interactions between genes, levels of sex hormones, and the cells of the developing fetal brain and body and that it is felt as something both innate (i.e. inherent and inborn) and immutable (i.e. irreversible and unchangeable) and therefore is not a conscious choice, as implied by those who talk of homosexuality as a ‘lifestyle choice.’

In fact, as early as 1973, mental health organizations began asserting that same-sex romantic and sexual attractions are “natural, normal, and positive variations of human sexuality” and that there is no clinical pathology or disease associated with homosexuality.

But while all of these findings are important, they are not without their complications and critics. This research has also been scrutinized and criticized, with some characterizing it as inconclusive and narrow. For example, only a few studies have focused on lesbians which has rendered the existing data mostly focused on gay men and therefore not accurately reflecting the entire homosexual experience. Other critics warn that it is overly-simplistic to use one simple study or formula to prove that sexual orientation is biologically determined because there could actually be hundreds of possible biological ways of developing sexual orientation. Some sociologists are also leery of this research because it seems to ignore the role social environments may play in shaping our personalities and temperaments and how they might impact our attractions.

And some critics of the search for the biological origin of sexual orientation point to Darwinian theories contending that homosexuality cannot be biologically based because it is less conducive to reproduction and therefore evolutionarily disadvantageous. Yet others push back by citing studies that have shown that a gene on the X chromosome appears to promote homosexual orientation in males and fertility in females which is evolutionarily advantageous.

Still others raise cautions about how such research opens up challenging ethical questions. While some gay, lesbian and bisexual people may take comfort in finding proof that sexual orientation is biologically determined, others worry that because of persisting religious or cultural rejection of homosexual behavior, social engineers could use that research to invent a ‘cure’ to change people’s sexual orientation and drive some homosexuals to feel obligated or coerced into taking that cure. Furthermore, what would be the ethical ramifications if prenatal tests were able to show a predisposition toward homosexuality in a fetus?

And the implications of this line of inquiry may be far-reaching. Many contend that if it is proven that sexual orientation is an inborn and unchangeable biological trait, then religions and laws should not prohibit same sex relationships. Still others contend that a demonstrable biological origin of sexual orientation should not matter at all because all that really matters is the simple fact that people who experience same-sex attraction exist and therefore have the inalienable right to live fulfilling lives that include same sex emotional attachments and sexual intimacy. Yet others contend that it doesn’t matter what scientific studies might discover about how people form their sexual orientation. All that matters to them is that traditional religion has definitively declared that homosexual behavior is always morally wrong and no scientific explanation of the biological origins of sexual orientation will ever supercede what is believed to be a divine prohibition against homosexual behavior.

Before we finish, we need to clear up some myths and misconceptions about what might cause homoexuality. Some popular explanations for same sex attraction suggest that it is nothing more than a preference caused by defective parenting, sexual abuse, or early adolescent sexual experience and that it can be changed through increased spirituality and reparative or conversion therapy.  Still others maintain that there is no such thing as homosexuality (same sex attraction) and instead there is only homosexual behavior. They further assert that people simply choose homosexual or ‘deviant’ behavior with no underlying biological or psychological causation. These theories have largely been debunked by every major medical and mental health organization.

Others believe that people can become homosexual or be recruited into homosexuality through social interactions or in other words, that simply socializing with openly lesbian, gay, bisexual or transgender people can make someone homosexual. Yet studies conducted in the United States for the last 3 decades indicate that even though the percentage of Americans who know an openly LGBT person has gradually increased, the average number of LGBT people has actually remained the same. So while more and more people associate with LGBT people, their numbers are not multiplying. Furthermore, a recent study of adolescent sexual behavior found that while peer pressure heavily influenced some behaviors, it did not influence an adolescent's sexual orientation. Other studies have shown that homosexual parents are not any more likely to produce homosexual children than are their heterosexual counterparts. All these studies have underscored the fact that there is no such thing as “LGBT contagion” and that sexual orientation is not transmitted via close social interaction.

And sadly, others still equate homosexuality with pedophilia, fearing that children might be lured into sexually deviant behaviors which would create in them a homosexual orientation. This fear has been a key driving force behind social and religious condemnation of homosexuality.  Yet decades of research shows that almost all sexual abusers of children are heterosexual men who are often close male relatives of their victims. This research has also clarified that pedophelia is an age-based orientation rather than a gender based orientation; meaning a pedophile is usually focused on the age of the child and not their gender.

So in spite of any criticism and controversy, the overall preponderance of scientific research continues to lead scientists to an overwhelming consensus that there is a biological origin to human sexual orientation.

https://www.theatlantic.com/politics/archive/2012/05/americans-have-no-idea-how-few-gay-people-there-are/257753/