Reposted from the American Sexuality magazine online.
The Biology of Sexual Orientation
Insight from animal research about what turns us on
What determines sexual orientation? What makes a person gay, bisexual, or straight? These sort of questions hold an undeniable interest to the general public and the answers are still hotly debated both by the experts in the field and by society at large. There are some powerful reasons for this universal appeal: First, the vast majority of us have experienced in the course of our lives some sort of sexual attraction toward other human beings, and this attraction in turn exerts a powerful influence in our mood, our behavior, our social interactions, and on the image that we have of ourselves. Since this plays such a key role in our lives, it is only natural that we would be interested in knowing at some point about its origins, and why we are oriented only toward people with certain characteristics and not others.
The examination of sexual orientation also holds relevance to specific social policies. Whether sexual orientation is the result of a conscious choice by the individual, as opposed to just another trait that comes "built-in" in our system, helps determine if it should be categorized as a "moral problem" or not. This seems to matter a lot in shaping out attitudes toward sexual minorities. Specifically (and for better or worse), the public appears to be more sympathetic to variations from the norm, in this case strict heterosexuality, if they are convinced that the individual has no "say" on this departure since it is the product of biological determination. On the other hand, sexual minorities have traditionally regarded this argument with suspicion. They fear that scientific research may open the door to treating these variations as little more than a disease and that efforts will be made to reduce or eliminate incidences of homosexuality in human populations.
Still, as it often happens with contentious issues that have such an impact for everyday life, society has turned to scientific research in order to get some answers. Despite the many occasions in which it has been proved otherwise, science still holds in the public conciousness the image of an unbiased actor whose answers are based solely in the pure application of a rational methodology and are therefore beyond the usual "contaminations" introduced by those who have specific interests in directing the public opinion toward their side of the field.
The question of sexual orientation has received special attention from biologists from early on. In part, this is due to the relevance that this trait is supposed to have for the survival of animal species with two (or more) separate sexes: If reproduction depends on successful mating with a member of another sex, then being attracted and actively attempting to interact with them would seem to be important to ensure that the genes of one generation be well represented in the next.
Plenty of experimental work has been done using lab animals to try to figure out how this is established, and how they develop an attraction for potential mates belonging to a sex other than their own. Most of it has been carried out in rodents (rats, mice, hamsters) for the simple reason that they breed well and adapt easily to a laboratory environment. Their proverbial capacity to deliver plenty of litters in a short time also comes handy at the time of doing an experiment.
The evidence derived from lab animals points directly to hormones derived from the gonads (testes in males, ovaries in females), specifically testosterone, in the determination of sexual orientation. When male pups are castrated at birth, they no longer seek the company of females after they have gone through puberty. Conversely, when females are injected with testosterone early in life, they later show an attraction toward other females, just like a male.
This is often referred to as the "organizational" effect of hormones, meaning that hormones trigger changes in the brain circuit, so that the brain develops in a particular way making animals predisposed to seek the company of one sex over another after reaching sexual maturity. In addition, the levels of hormones that they have in adulthood are very important to maintain this preference: If the gonads are removed, the preference quickly disappears, no matter how strong it may have been before the surgery. So the evidence is strong for a "built-in" mechanism in the determination of sexual orientation in rodents. Whether they will be attracted to females or to males when they grow up seems to be largely determined by the presence of functional testes or ovaries early in life (or even before they are actually delivered by the mother). The million-dollar question, the one that continues to generate heated debates both within and outside the scientific community, is whether this can be extrapolated at all to humans. And the answer is far from being trivial, since there are indeed powerful reasons that call for caution when doing so.
Sexual behavior in rodents is strictly associated with reproductive function, to the point that females will normally accept to mate with a male only during a particular stage of the estrous cycle: immediately after ovulation. Attempts by the male to initiate copulation during any other stage of the cycle are generally met with rejection, and this can turn into downright aggression. It is easy to see why this should be so. By limiting sexual activity to the period in which the female is actually fertile, the waste of energy that mating not resulting in pregnancy represents is actually avoided. Not surprisingly, gonadal hormones control the coordination between these two events (ovulation and sexual receptivity). Cycling ovaries release estradiol and progesterone to the bloodstream, which triggers ovulation and sexual receptivity.
In humans, on the other hand, the situation is radically different. Despite several attemps that have been made over the years to measure variations in sexual desire in women during the cycle, there is virtually no evidence to support such a claim. In humans, and other selected mammalian species, the willingness to engage in sexual activities seems to be dissociated from the hormonal status, and therefore not limited to be a mere prerequisite for succesful reproduction. On the contrary, we are all familiar with the multiple roles that sex plays in human societies, ranging from the expression of affection to the validation of social status. Even in non-human primates we can see some clear evidence of this emancipation of sex from its primitive role: Pygmy chimpanzees (or bonobos) are famous for using sexual intercourse to regulate many aspects of their social interactions, including greeting each other, resolving conflicts between members of the same clan, and exchanging food and other commodities.
Does this diminished role of gonadal steroids in the regulation of sexual activities also translate to the determination of sexual orientation in humans? With perhaps a single exception, researchers have in general failed to find a link between this characteristic and exposure to gonadal hormones at any point during the life of the individual. The exception is a study that showed women affected by congenital adrenal hyperplasia (CAH), a syndrome that caused their adrenal glands to excrete an excesive amount of sex steroids during their development, exhibited a higher proportion of individuals reporting same-sex sexual orientation compared to the general population. Notice, however, that this is not the same as saying that all the women exposed to high hormonal levels as a consequence of their syndrome became homosexual, which suggests that if hormones are playing a role here, they have to be doing it in combination with other factors. On the other hand, there is not a single study to this date that has conclusively proved that gay men are exposed to subnormal levels of testosterone or other sex hormones during development.
So, the evidence for gonadal hormones as the determining factor for sexual orientation in humans seems to be much less abundant than does the evidence in animal models. Perhaps, in part, this is because in the former case studying the phenomenon under strict experimental conditions is impossible, and thus researchers have to rely almost exclusively in the so-called "experiments of nature," clinical syndromes such as CAH in which it is particularly difficult to control for other variables—such as psychosexual history, genetic and social background—that may affect the outcome.
Another reasonable consideration is that with the evolution of higher cognitive functions, our sexuality became a much more complex phenomenon, with multiple purposes beyond mere reproduction and also with multiple variables affecting its different aspects, and this includes of course sexual orientation. And yet, it is difficult to shake off the feeling that some deeply ingrained biological root exists that determines who we feel sexually attracted to. When asked, most people declare having no recollection of making a conscious choice about this issue at any point in their lives. Instead, there is a strong feeling of having been "made" in certain way, which implies an underlying biological cause that overrules any attempts to modify it by conscious decision. (This has been and often still is the cause of a heavy psychological burden for gay/lesbian individuals raised in an environment that does not tolerate their sexual orientation and blames them for it.) But regardless of whether improved experimental methodology and more advanced technology would allow us one day to shed some light on the elusive biological factors that determine sexual orientation in humans, it is important to ask ourselves if we are interested in finding an answer to it, and thus if it should continue to be the object of scientific enquiry. As mentioned at the beginning, sexual minorities have repeatedly expressed concerns about this, since they fear that it may actually increase discrimination practices against them.
It is easy to sympathize with this point of view, especially considering the many instances in which supposedly neutral scientific knowledge was used in the past to justify racist policies or to deny women their civil rights. But at the same time it is perfectly reasonable to wonder if such an attitude is not putting the blame in the wrong place. Instead of making researchers scapegoats, we should ask why society at large would use the tools they create to enforce discriminative policies.
There is a wide amount of variation in human traits, ranging from some that have an obvious external manifestation (eye, hair, or skin color) to others that are virtually impossible to recognize without resorting to specific test tools (blood type), and they are known in many cases to have an evident genetic component. Even though human societies seem to have the unfortunate tendency to use these variations to discriminate, we have made remarkable progress in exposing this tendency as irrational and as the cause of much suffering, so that what was once universally accepted and justified is today relegated to the fringes. There is no reason to believe that this would not also be achieved in the case of our attitudes toward sexual orientation, even if an agreement on its biological causes is eventually reached as a consequence of further scientific research on the subject. By embracing too quickly the other option, seeking to prevent scientists from looking for the causes because of fear of what we may do with such knowledge, we may indeed find ourselves sharing our views with very strange bedfellows.
Cristian C A Bodo was born in Buenos Aires, Argentina. He received his Ph.D. in neuroscience from the University of Virginia in August 2007. The topic of his dissertation research was the role of gonadal steroids in the sexual differentiation of the mouse brain.