Is There a Homosexual Brain?

Vittorio Gallo & Phyllis R Robinson. The Gay & Lesbian Review Worldwide. Volume 7, Issue 1. January, 2000.

Questions about the origins of homosexuality would be of little interest if it were not a stigmatized behavior. We do not ask comparable questions about “normal” sexual preferences, such as preferences for certain physical types or for specific sexual acts that are common among heterosexuals. Still, many gay people welcome biological explanations and, in recent years, much of the search for biological components in homosexuality has been carried out by gay researchers. ~ Ruth Hubbard and Elijah Wald, Exploding the Gene Myth

In 1997 Simon LeVay, better known for his much-publicized research on post-mortem gay men’s brains, published the novel Albrick’s Gold. In it, researchers in the laboratory of Dr. Guy Albrick at the imaginary Levicitan University work on modifying homosexual behavior of young students by using a combined biological and psychological approach. This comes from Dr. Albrick’s belief that homosexuality is “a change in molecular development, in the centers that regulate sexuality, there is no doubt about that, in my mind.”

The goal of the “therapy” is therefore to change the cellular structure and nerve circuitry of specific areas of the brain, namely those that control sexual orientation. This is achieved by injecting embryonic monkey brain cells into a region of the brain, the hypothalamus, of young gay men. This therapy is based on published research demonstrating that one of the human hypothalamic nuclei (a nucleus is an aggregate of a variable number of nerve cells in the brain) was smaller in the brains of gay men who died of AIDS than in the brains of reportedly heterosexual men who died of a variety of causes. The premise of Albrick’s Gold is also based on current research in developmental neuroscience demonstrating that embryonic mammalian nerve cells can adapt to a new environment because they are still immature, and can be transplanted into a host brain where they integrate and function as mature cells. The scenario depicted in LeVay’s novel is therefore not that farfetched: a technique to modify the size of specific brain nuclei by direct transplantation of foreign brain cells.

For many years, neuroscientists have been engaged in an intense effort to raise the level of public awareness about the importance of understanding how the brain works. Research was directed toward elucidating the nature of the healthy brain and neurological disorders. In the last decade, the most striking discoveries in the neurosciences made headlines in the popular press.

Genetic research in the same period has contributed to our current understanding of the complexity of the brain, and it has been proved to be a powerful tool for elucidating the nature of some hereditary diseases, such as Huntington’s chorea and cystic fibrosis. Unfortunately, poorly reported and poorly understood results have mistakenly led the general public to believe that complex behaviors can easily be reduced to a single gene, and one of the behaviors that has raised the greatest interest and passion is homosexuality. Although research on the biology of homosexuality is over a hundred years old, both the scientific community and the general public paid more attention to this field in the 1990’s. A few studies on this topic were published in authoritative scientific journals and were sensationalized by the popular press. Consequently, we are now approaching the new millennium with the premature notion that scientists have established a direct causal link between sexual orientation, specific areas of the brain, and ultimately a particular set of genes.

Neurobiology represents the interface between genes and behavior, the science that stands to establish links between specific behaviors and brain circuitry. This is not a trivial undertaking, because several hierarchical levels exist between the structure of the brain and behavior. Therefore, as concerned neuroscientists, we feel that it is imperative to understand the complexities and the implications of research attempting to establish a biological basis for homosexuality. We wish to review the present state of knowledge of the neurobiology of homosexuality, and to share some critical views on the subject. Finally, we will discuss a possible biological model based on changes in the brain induced by ongoing life experience, which includes an interaction between the brain and the environment.

Gender Differences: Size Isn’t Everything

Recall the fictionalized account in Albrick’s Gold, where scientists base their work on presumed differences in the size of specific areas of the brain between homo- and heterosexual men. The underlying premise here is that the brain is a sexually dimorphic structure, i.e., that certain areas of the brain display phenotypic differences between males and females, and that this dimorphism is associated with functional and behavioral differences between the sexes. Based on the questionable notion that homosexuals display “intersexual” behavior, it is further assumed that homosexuals will have brain structures resembling those of the opposite sex. This naturalization of gender makes homosexuality a form of gender dysfunction. A corollary to this logic is that it is possible to predict sexual orientation based on certain “brain markers.”

For many years, neuroanatomists have used “morphometric analysis,” an experimental approach that measures the size of specific regions, groups of cells, or even single cells in the brain. Morphometric analysis can provide important information about differences in the pattern of organization of specific brain areas or structures. However, it does not by itself provide any functional information, even at the most basic physiological level. The difference between structure and function is an important distinction. For example, drastic differences in anatomical measurement obtained by morphometric analysis could have no impact on the physiology and “network properties” of a specific cerebral circuit. Conversely, finding no anatomical differences in the size of a specific cerebral area or group of cells would not rule out important functional deviations. It is entirely possible that different anatomical structures could perform the same cerebral function, or that like structures could yield different results.

Nevertheless, studies on two cerebral structures, the planum temporale and the corpus callosum, show how controversial scientific evidence can be accepted as fact when structure and function are conflated. In a 1968 study, Geshwind and Levitsky reported that the planum temporale, a part of the temporal lobe, is larger on the left side of the brain in the majority of people. A few years later, Wada et al. confirmed this finding and also described a difference in the proportion of men and women with a reversed asymmetry. These studies are widely cited in research on anatomical differences of homosexual brains to validate morphometric analysis as a valid scientific approach. They have also attracted much attention, because the planum temporale is involved in language functions, and in most individuals these functions are lateralized and reside on the left side of the brain. None of these studies, however, has addressed the functional implications potentially associated with the anatomical findings.

Another part of the human brain extensively analyzed and measured by neuroanatomists is the corpus callosum, a thick bundle of nerves connecting the left and the right cerebral hemispheres. The corpus callosum, long a hot area in brain research, has been hypothesized to be the part of the human brain involved in integrating tasks performed by the two hemispheres. Different laboratories have measured its cross-section area in dead brain tissue of men and women, and reported strikingly inconsistent results.

Is There a “Homosexual Brain”?

For all the weakness of anatomical approaches when it comes to sexual dimorphism, research on the neurobiology of homosexuality has marched on over the past decade, searching for size differences of specific areas of the brain between homo- and heterosexual subjects. The majority of neuroscience research on sexual orientation relied on the validity of the “central nervous system hermaphroditism” paradigm and searched for differences in size of specific areas of the brain between homo- and heterosexual individuals. Most of the attention has focused on the hypothalamus, a part of the brain involved in controlling a variety of functions (many still largely unknown), including sexual and reproductive behavior. In the late 1970’s, it was reported that an aggregate of cells in the hypothalamus, later designated as the sexually dimorphic nucleus of the preoptic area (SDN-POA), was larger in male than in female rats. Based on rather inconclusive studies of rodents, the preoptic area was hypothesized to be directly involved in sexual behavior, and its size and cellular organization to be controlled by male hormones (androgens).

In humans, the so-called interstitial nuclei of the anterior hypothalamus (INAH1-4) are the areas proposed to correspond to the SDN-POA in rodents. These nuclei were extensively studied in search of possible differences between men and women, and between male heterosexuals and homosexuals. Conflicting results were obtained from different laboratories, and no unambiguous and reproducible difference in these structures has been firmly established to date.

Besides the interstitial hypothalamic nuclei, the size of other areas of the brain was examined for possible correlation with sexual orientation. Allen and Gorsky reported that a structure known as the anterior commissure (a group of fibers interconnecting the temporal lobes and located near the hypothalamus) was larger in women and in homosexual men than in heterosexual men. When interviewed by the national gay newsmagazine The Advocate (October, 1991), Allen pushed her results even further and generalized that her study indicates that “the brain is organized differently [in homosexual men], not just the region that affects sexual behavior.” According to Allen, gay men share the attraction to men with heterosexual women because their entire brains are differently structured to be more feminine. This claim was never substantiated and, if anything, neuroanatomical analysis has so far demonstrated that gay men’s brains are organized identically to those of heterosexual men. Furthermore, in the Allen and Gorsky study, it is not clear how differences in the size of the anterior commissure correlate with cognitive function and sexual orientation.

The studies suggesting anatomical differences in gay men’s brains unavoidably led to the question, How are these differences formed during human development? The favorite hypothesis of neurobiologists has been that hormones or other factors can change the cellular structure or the “wiring” of the brain during critical developmental stages, in particular before birth. Observations made in laboratory animals (predominantly rats) demonstrated that exposure to different sex hormones at early developmental stages induces male or female mating behavior, and affects the gender selection of the mate in adults, and can profoundly modify the structural organization of some areas of the brain. This observation was extrapolated to humans, and it was hypothesized, for example, that fetal exposure to high prenatal androgen levels would result in male heterosexuality and female homosexuality. Conversely, low levels of the same hormones would lead to male homosexuality and female heterosexuality, accompanied by appropriate similarities in brain structure. This model also implies that, once the brain circuitry involved in sexual orientation has developed, its structural organization is fixed and cannot change in adult life.

Will We Ever Find a “Homosexual Brain”?

The brain is made of cells that communicate with each other and integrate information. The function of brain cells depends on the expression of thousands of different genes that encode proteins. Therefore, brain function, psychological phenomena, and behavioral patterns are ultimately linked to biological processes and genes. However, at this time we cannot firmly conclude whether someone’s sexual orientation is anatomically represented in his or her brain structure.

Putting aside the theoretical difficulties of demonstrating a link between neurobiological factors and sexual orientation, the research to date has been called into question on grounds of experimental design and reproducibility of results. The latter problem, for example, has dogged the much-publicized study by Dean Hamer on the genetics of homosexuality. Hamer and colleagues claimed to have found a region of the X28 chromosome that’s involved in male homosexual identity, the so-called “gay gene.” And while this finding got all the media attention, a Canadian research team headed by George Ebers that set out to replicate Hamer’s work was unable to do so.

Studies on the brain and homosexuality, to date, suffer from serious limitations in design. The brain is a malleable organ, which changes during development and retains its potential to change well into adulthood. Several processes can shape brain structure and function. For example, the number of connections between brain cells (neurons) and the efficiency of transmission of information can be drastically altered with use and experience. Recent studies have indicated that the mature nervous system retains the ability to be modified by circulating hormones, indicating that “morphological sex differences in the human brain are likely to reflect an interaction between developmental influences, experience and hormone actions on the mature brain.” In the reports discussed previously, morphometric analysis of post-mortem brains was obviously performed only at one time point in an individual’s life, and did not reflect the dynamic nature of the brain.

Finally, a potential problem in the research design of most neurobiological studies is that they’re based on post-mortem brains, so that their morphology may reflect the disease process that led to death, or the effects of prolonged use of brain-altering medications before death. For example, in LeVay’s study the majority of the gay men died of AIDS, and HIV infection of the brain is known to affect its anatomy and function.

The question remains whether a neurobiological analysis of a complex behavior such as homosexuality is even theoretically possible. The first problem is simply how to define homo- and heterosexuality so that a given population can be sampled adequately. Psychiatrist Richard Isay set forth this problem in his influential book, Being Homosexual:

Because sexual behavior may be inhibited by societal pressure or by internal conflict, a man need not to engage in sexual activity to be homosexual. Those who have homosexual contacts but, because of censorious social pressures, intrapsychic conflict, or both, are unable to accept that they are gay are also homosexual. There are others who may not even have conscious access to their homoerotic fantasies because they repress, suppress, or deny them. Their fantasies become more available to them during a properly conducted analysis or therapy, and I also consider them to be homosexual.

Thus, for example, in the studies under discussion, is it possible that the heterosexual “control” group included men who had sexual fantasies about other men that were never acted out? Or perhaps they were acted out, but the men still never identified themselves as “gay” or “bisexual.” Furthermore, cross-cultural research shows, among myriad other variations in sexual categorization, that there are cultures in which men have heterosexual nuclear families and are happy with this construction, yet they also have sexual relationships with other men, without identifying as either homosexual or bisexual. As Roger Sperry, one of the great neuroscientists of this century, has pointed out: “From the standpoint of brain function, it is clear that a person’s or a society’s values directly and constantly shape its actions and decisions. Any given brain will respond differently to the same input, and will tend to process the same information in quite diverse ways depending on its particular system of value priorities. In short, what an individual or a society values determines very largely what it does.”

Meanwhile, research on female homosexuality suggests that women’s sexual identity is more fluid than men’s and does not fall neatly into hetero-/homosexual dichotomies. For example, Carla Golden found in a study on women’s homosexuality that “there is no simple relationship between sexual desire, experience, orientation, and identity, and that for some women, choice is seen as influencing their sexual decisions.” Furthermore, “the claim that sexual orientation is clearly established by adolescence and stable across the life-course is not consistent with the research conducted with women.” Comparable extensive studies have not explored this issue in men. If this aspect of male sexuality were carefully investigated, it might be found that male sexual identity is equally complex. In short, simple comparative studies positing neat boundaries on sexual orientation are an oversimplification, and the findings from those studies are inevitably inconclusive.

Future Research on the “Homosexual Brain”

In light of the above, we would like to advance an interactive hypothesis as a neurobiological model of sexual orientation, one that allows for a continuous interplay between brain and environment. It is known that environmental factors and experience influence the development and function of neural circuits in the brain. We can therefore refer to other well-studied systems, such as the visual cortex, in which the formation of specific neural circuits is strictly dependent on interactions with the environment at critical times in development.

The visual cortex is an area of the brain that processes information received from both eyes. It has been established in animal models and in humans that the appropriate processing of visual information depends on the correct formation of a specific and unique pattern of connections between cells of the visual cortex during early development. The anatomical changes in cellular connections triggered by the first sets of stimuli received by the visual cortex become the substrate for subsequent visual processing. If the interaction between the visual cortex and the environment does not occur in this self-reinforcing fashion, then this part of the brain does not become properly organized to decode images.

We suggest a complex neurobiological model of sexual orientation, one that’s centered on specific neural circuits. Since sexual attraction depends on a multitude of external stimuli as well as individual biological substrates, it is likely that more than a single area of the brain is involved. We suggest that both biological factors and experience could modify these circuits, and that the confluence of both is what reinforces specific sexual behaviors. This could occur through the continuous interactive relationship between behavior and hormones, and/or through the development of cognitive skills and specific brain structures.

We feel that any future neurobiological analysis of homosexuality should be based on brain function and physiology, and should also take into account the complex interactions occurring between the brain and the environment. We recognize that this is a very difficult and complex task, at least in part because no adequate animal models exist for homosexual behavior in humans. Consequently, the types of experiments that have been done on the visual cortex cannot be transferred to the problem of sexual orientation.

The recent development of high-resolution techniques—MRI (magnetic resonance imaging), CT/CAT (computerized axial tomography), and PET (positron emission tomography) scanning—allow researchers to visualize specific areas of the brain and their association with particular behaviors. These will be instrumental in defining the brain areas involved in sexual orientation and erotic behavior. At present, these techniques cannot analyze individual groups of neurons but only much larger parts of the brain, but the resolution may soon get down to highly specific structures. The challenge will then be to determine how different behavior patterns are linked to distinct chemical and biological processes in specific neural circuits. Even then, it should still be kept in mind that biological analysis might not provide causality. That will require a complex analysis of the function of possibly several areas of the brain simultaneously, and long-term longitudinal and cross-cultural studies.

As an obvious consequence, we are concerned about the political use of this research. Part of the gay and lesbian community has enthusiastically and uncritically embraced the research done in the field of the genetics and neurobiology of homosexuality. Based on the argument that homosexuality is predetermined and is therefore not a choice, a biological explanation has been used as support for liberationist aspirations of many gays and lesbians in their private homes and in public.

And yet, it is not at all obvious to us how establishing a biology of homosexuality would translate into increased social acceptance for gays and lesbians. As Ruth Hubbard and Elijah Wald have pointed out: “Grounding difference in biology does not stem bigotry. Quite the contrary, African Americans, Jews, people with disabilities, and also homosexuals have been persecuted for biological ‘flaws,’ and even exterminated to keep them from spreading biological `contamination.’” Scientific evidence has often been selectively used to justify prejudice, or ignored in political discourse if its findings proved inconvenient. LeVay’s novel develops into a nightmare scenario involving the extermination of homosexuals once the genes responsible for it are discovered. If such a configuration of genes were to be found—still a big “if”—it’s still an open question whether that would inaugurate an era of human understanding or something closer to Albrick’s Gold grim outcome.