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How humans develop sexual orientations is a question that people have pondered for at least a century. In the later part of the twentieth century, scientists began to articulate sophisticated biological theories of how sexual orientations develop. This research paper critically surveys contemporary biological theories of the development of sexual orientations. In particular, it examines three recent studies of how sexual orientations develop and their theoretical underpinnings. The focus is on these studies, because not only are they positively cited by almost every scientist trying to develop a biological theory of sexual orientation, but also because they are typical in their assumptions and methodology.
1. What Is A Sexual Orientation?
A person’s sexual orientation concerns his or her sexual desires and fantasies towards others in virtue of their sex (gender). However, a person’s sexual orientation is only one part of a person’s sexual interest generally. People have a wide range of sexual tastes. Some are attracted to people of certain ages, people of certain body types, of certain races, of certain hair colors, of certain personality types, of certain professions, as well as to people of a certain sex and a certain sexual orientation. Further, people are not only sexually interested in certain sorts of people, some also have quite speciﬁc interests in certain sorts of sexual acts, certain venues for sex, or a certain frequency of having sex. We recognize that people can be sorted into all sorts of groups in virtue of their sexual interests, but most contemporary scientiﬁc studies focus only on the sex of the people a person is sexually attracted to as an essential feature about him or her. Doing so may be culturally salient but it is not scientiﬁcally justiﬁed.
2. What Makes A Theory A Biological One?
To say that sexual orientation is biologically based is an ambiguous claim; there are various senses in which it is trivially true that sexual orientation is biological. Everything psychological is biologically based. Humans can have sexual orientations while inanimate objects and one-celled organisms cannot because of our biological/psychological make-up. The same sort of claim is true with respect to having a favorite type of music: humans but not single-celled organisms can have a favorite type of music. Even though a preference for classical music seems a paradigmatic example of a learned trait, such a preference is also biological in that a certain cognitive complexity is required in order to have such a preference. Sexual orientation is at least biologically based in the sense that musical preferences are. The central claim of biological research on sexual orientation makes a much bolder claim. It says a person’s sexual orientation is inborn or determined at a very early age and, as a result, a person’s sexual orientation is ‘wired into’ his or her brain. To understand the signiﬁcance of such a claim, it is useful to contrast three models of the role genes and other biological factors might play in sexual orientation (Byne 1996, Stein 1999, pp. 123–7).
According to the permissive model, genes or other biological factors inﬂuence neuroanatomical structures on which experience inscribes sexual orientation, but biological factors do not directly or indirectly determine sexual orientation. Something like the permissive model correctly describes the development of musical preferences. Various genetic and biological factors make it possible for our experiences with various kinds of music to shape our musical preferences.
Contrast the permissive model with the direct model, according to which genes, hormones, or other biological factors directly inﬂuence the brain structures that underlie sexual orientation. According to the direct model, the neurological structures responsible for the direction of a person’s sexual attraction toward men or women develop at a very early age as a result of a person’s genes or other biological factors. One version of the direct model sees genes in the q28 region of the X chromosome as coding for a set of proteins that causes the INAH-3 region of the hypothalamus to develop so as to determine a person’s sexual orientation (LeVay and Hamer 1994).
The direct and the permissive models can be contrasted with the indirect model, according to which genes code for (and or other biological factors inﬂuence) temperamental or personality factors that shape how a person interacts with his or her environment and experiences of it, which, in turn, aﬀects the development of his or her sexual orientation. On this view, the same gene (or set of genes) might predispose to homosexuality in some environments, to heterosexuality in others, and have no eﬀect on sexual orientation in others. An example of such a theory is Daryl Bem’s theory of sexual orientation according to which biological factors code for childhood personality types and temperaments (for example, aggressiveness, willingness to engage in physical contact, and so on) (Bem 1996, Peplau et al. 1998). In societies like ours where there are signiﬁcantly diﬀerent gender roles typically associated with men and women, these diﬀerent personality and temperament types get molded into gender roles that, in turn, play a crucial role in the development of sexual orientation. The three biological theories of the development of sexual orientation discussed below accept the direct model. However, what evidence there is for these theories is equally consistent with the indirect model.
3. Is Sexual Orientation Wired Into The Brain?
In 1991, Simon LeVay published a study of the size of INAH-3, a particular cell group in the hypothalamus (LeVay 1991). Starting from the assumption that there are neurological diﬀerences between men and women, LeVay decided to look for sexual-orientation differences in some of the areas of the hypothalamus that seem to exhibit sex diﬀerentiation. He reasoned as follows: given that most people who are primarily attracted to women are men and most people who are primarily attracted to men are women, in order to discover where sexual orientation is reﬂected in the brain, we should look in parts of the brain that are structured diﬀerently for men and women. This picture is based on seeing gay men as having female-typical characteristics and seeing lesbians as having maletypical characteristics. Seeing gay men and lesbians as gender inverts has a certain cultural salience but its scientiﬁc merit has been subject to serious criticism (Stein 1999, pp. 202–5, Byne 1994).
To examine the hypothalamus, LeVay had to study portions of human brain tissue that are accessible only after the person has died. Further, LeVay needed to know the sexual orientations of the people associated with the brain tissue he was studying. LeVay’s study was made possible as a result of the AIDS epidemic, which had the result of making available brains from people whose self-reported sexual histories are to some extent part of their medical records. LeVay examined 41 brains: 19 of them from men LeVay presumed to be gay because they had died of complications due to AIDS and their medical records suggested that they had been exposed to HIV (the virus that causes AIDS) through sexual activity with other men; six of them from men of undetermined sexual orientation who also died of AIDS and who LeVay presumed were heterosexual; 10 of them from men of undetermined sexual orientation who died of causes other than AIDS and who were also presumed to be heterosexual; and six of them from women all of whom were presumed to be heterosexual, one whom died of AIDS and ﬁve who died from other causes. LeVay found that, on average, the INAH-3 of the presumed gay men were signiﬁcantly smaller than those of the presumed heterosexual men and about the same size as those of the women. From this, he inferred that gay men’s INAH-3 are in a sense ‘feminized.’ Although LeVay rather cautiously concluded that his ‘results do not allow one to decide if the size of INAH-3 in an individual is the cause or consequence of that individual’s sexual orientation or if the size of INAH-3 and sexual orientation co-vary under the inﬂuence of some third unidentiﬁed variable,’ he also said that his study illustrates that ‘sexual orientation in humans is amenable to study at the biological level’ (LeVay 1991, p. 1036). In media interviews after the publication of his study he made even stronger claims; he said, for example, that the study ‘opens the door to ﬁnd the answer’ to the question of ‘what makes people gay or straight’ (Gelman 1992).
4. Is Sexual Orientation Inherited?
Various studies suggest that sexual orientation runs in families (e.g., Pillard and Weinrich 1986). Such studies show that a same-sex sibling of a homosexual is more likely to be a homosexual than a same-sex sibling of a heterosexual is to be a homosexual; more simply, for example, the brother of a gay man is more likely to be gay than the brother of a straight man. These studies do not establish that sexual orientation is genetic because most siblings, in addition to sharing a significant percentage of their genes, share many environmental variables, that is, they are raised in the same house, are fed the same meals, attend the same schools and have many of the same adult role models. For these reasons, disentangling inherited and environmental inﬂuences requires more sophisticated studies.
Heritability studies done by Michael Bailey and Richard Pillard assessed sexual orientation in identical twins, fraternal twins, nontwin biological siblings, and similarly-aged unrelated adopted siblings (Bailey and Pillard 1991, Bailey et al. 1993). If sexual orientation is genetic, then, ﬁrst, all identical twins should have the same sexual orientation and, second, the rate of homosexuality among the adopted siblings should be equal to the rate of homosexuality in the general population. If, on the other hand, identical twins are as likely to have the same sexual orientation as adopted siblings, this suggests that genetic factors make very little contribution to sexual orientation.
In both twin studies, subjects were recruited through ads placed in gay publications that asked for homosexual or bisexual volunteers with twin or adoptive siblings. Volunteers were encouraged to reply to the ad ‘regardless of the sexual orientation’ of their siblings. In both of these studies, the percentage of identical twins who are both homosexual is substantially higher than the percentage with respect to fraternal twins. For example, 48 percent of the identical twins of lesbians were also lesbians, 16 percent of the fraternal twin sisters were lesbians, 14 percent of the nontwin biological sisters were lesbians, as were 6 percent of adoptive sisters. These results show that sexual orientation is at least partly not the result of genetic factors. However, the higher concordance rate is consistent with a genetic eﬀect because identical twins share all of their genes while fraternal twins, on average, share only half of their genes. Also consistent with a genetic eﬀect is the result that the concordance rates for both types of twins are higher than the concordance rates for adopted siblings.
5. Is Sexual Orientation Genetic?
Building on heritability studies, Dean Hamer and his collaborators obtained DNA samples from gay brothers in families in which homosexuality seemed surprisingly common. These samples were analyzed using linkage analysis, a technique for narrowing the location of a gene for some trait, to see if there was any particular portion of the X chromosome that was identical in the pairs of brothers at an unexpectedly high frequency (Hamer et al. 1993). Hamer found that a higher than expected percentage of the pairs of gay brothers had the same genetic sequences in a particular portion of the q28 region of the X chromosome (82 percent rather than the expected 50 percent). In other words, he found that gay brothers are much more likely to share the same genetic sequence in this particular region than they are to share the same genetic sequence in any other region of the X chromosome. Hamer’s study suggests that the q28 region is the particular place where sexual orientation differences are inscribed. This study does not, contrary to popular belief, claim to identify any particular genetic sequence associated with homosexuality. At best, it has found that many of the pairs of homosexual brothers had the same genetic sequences in this portion of the X chromosome. When Hamer is at his most precise, he says that his study shows that ‘at least one subtype of male sexual orientation is genetically inﬂuenced’ (Hamer et al. 1993, p. 321). In other contexts, Hamer is less careful. For example, in his book, Science of Desire, he talks of ‘gay genes,’ going so far as to use this term in the book’s subtitle (Hamer and Copeland 1994).
6. Problems With These (And Other) Studies
6.1 Lack Of Conﬁrmation
Independent conﬁrmation is the earmark of the scientiﬁc method. LeVay’s results have not been independently conﬁrmed (Byne 1996) and Hamer’s study has been recently disconﬁrmed (Rice et al. 1999). Although various research teams have conﬁrmed the twin study results, a recent and more sophisticated study by Bailey undermines the methodology of early twin studies (Bailey et al. in press). Bailey systematically recruited subjects from a registry of identical and fraternal twins in Australia. In women, the percentage of identical twins of bisexuals and homosexuals who are also either bisexual or homosexual was between 24 and 30 percent (depending on how the boundaries of these groups are drawn), while the percentage of same-sex fraternal twins of bisexual and homosexual women was between 10 and 30 percent. Not only is the diﬀerence between identical and fraternal twins signiﬁcantly smaller than in previous studies but the percentages for identical twins with the same sexual orientations are dramatically lower. Although these results can be read as consistent with the direct model, the evidence is much weaker than earlier heritability studies suggested.
6.2 Problems With The Subject Pool
This Australian study shows that the results of earlier twin and family studies (Bailey and Pillard 1991, Bailey et al. 1993, Hamer et al. 1993), which recruited subjects through HIV clinics, lesbian and/or gay organizations, newspapers, and other nonsystematic methods, must have been inﬂated by sampling bias. In particular, it suggests that gay men and lesbians with identical twins of the same sexual orientation are more likely to participate in such studies. If an experiment makes use of a subject pool that is in some way biased, then this gives rise to doubts about the conclusions based on it. LeVay’s subject pool is also biased. In particular, the homosexual population in his study is made up exclusively of men who died from complications due to AIDS and who told hospital staﬀ that they had engaged in same-sex sexual activities.
6.3 Methods Of Classiﬁcation
A related problem with such studies concerns the determination of subjects’ sexual orientations. Conclusions based on studies that inaccurately assign sexual orientations to subjects are weak. LeVay assumed, on the basis of no particular evidence, that all the women in his study were heterosexual and that all the men in his studies whose hospital records did not indicate same-sex sexual activity were heterosexual. Other studies that rely on a family member to report a person’s sexual orientation may be similarly problematic. Further, by assuming that there are two sexual orientations—gay and straight—and that people can be easily and reliably classiﬁed as one or the other on the basis of their behavior or their self-report, scientiﬁc research accepts, without strong justiﬁcation, that our cultural presumptions about human sexual desires are scientiﬁcally valid (Stein 1999, pp. 201–13).
6.4 Undefended Assumptions
Generally, most biological research on sexual orientation accepts without argument a quite particular picture of sexual orientation. For example, many studies in the emerging research program unquestioningly accept the inversion assumption, according to which lesbians and gay men are seen as gender inverts and many studies assume the direct relevance of animal models of sexual behavior to human sexual orientation (Stein 1999, pp. 164–79). More crucially, such studies typically accept that a person’s sexual orientation is a deep scientiﬁc property about her and that sexual orientation is a ‘window into a person’s soul.’ This view of the centrality of sexual orientation to human nature is neither culturally universal nor scientiﬁcally established (Stein 1990, Stein 1999, pp. 93–116).
Although there is some evidence that is consistent with biological factors playing an indirect role in the development of sexual orientations, there is no convincing evidence that biological factors are a direct cause of sexual orientation. How human sexual desires develop is an interesting research question, but we are probably rather far from answering it.
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