Many perspectives on the nature of sexual orientation exist, each one asserting its point of view as vehemently as the next. Contemporary explanatory models of sexuality have been based more on cultural and religious norms and social stereotypes than on actual objective, scientific facts. Over the last 20 years or so, however, a growing body of research on the biology of sexual orientation has been accumulating. Researchers have explored the relationships between sexual orientation and factors such as hormones, prenatal stress, cerebral asymmetry, neuroanatomy, otoacoustic emissions, anthropometrics, genetics, fraternal birth order, and developmental instability (Mustanski, Chivers, & Bailey, 2002). Although results have been mixed, a number of interesting findings have emerged.
Research has provided evidence that prenatal factors may be involved in sexual orientation. The neurohormonal theory attempts to explain sexual orientation in terms of prenatal testosterone levels which are involved in differentiating physiological, neural, and anatomical structures of both sexes. Swaab and Hofman (1990) found clear differences in the superchiasmatic nucleus (SCN) of the hypothalamus between homosexual and heterosexual men (as cited in Alexander, 2000). Adam et al. (2007) compared neural activation to preferred sexual stimuli and nonpreferred sexual stimuli in heterosexual and homosexual men and discovered that within the amygdala the latter had greater activity for preferred sexual stimuli than the former, suggesting the possibility that male homosexual brains may be characterized by atypical patterns of neural activity. Whether this result was a cause or consequence of the participant's homosexuality, however, is debatable. Other research has postulated that developmental instability in utero may be related to homosexuality. For example, Miller, Hoffmann, and Mustanksi (2008) found that overall index of bodily fluctuating asymmetry (FA) was positively correlated with homosexuality in men. Research by Martin, Puts, & Breedlove (2008), however, failed to confirm the results of the latter. In fact, the researchers reported that FA was lower in homosexual men than heterosexual men.
The most favourable finding to have emerged from the neurohormonal research is the fraternal birth order effect, which was first documented by Blanchard and Bogaert (1996) (as cited in Bogaert, 2006). The researchers found that there was a significant correlation between homosexuality in males and the number of biological older brothers in a Canadian sample. Since then several other studies have confirmed these results in samples from across the world. For example, Blanchard and Lippa (2007) reported that results from an online BBC survey involving 159,779 respondents revealed that older brothers increased the odds of homosexuality in men. Another study (Rahman, Clarke, & Morera, 2009) found evidence of the fraternal birth order effect in a sample of 100 heterosexual and 100 homosexual men. It appears that the fraternal birth order effect has no bearing on female sexuality (Bogaert, 1997). Bogaert (2006) demonstrated that only the number of biological older brothers, and not any other siblings, such as non-biological brothers, significantly predicted homosexuality in men. Moreover, rearing time with older siblings, whether biological or nonbiological, had no effect on sexual orientation, suggesting a prenatal origin to the fraternal birth order effect. Recent studies have indicated that the fraternal birth order effect is evident in right-handed males but not in non-right-handed males (Blanchard & Lippa, 2007; Blanchard, 2008).
The maternal immune system hypothesis was developed to explain these results and posits that the fraternal birth order effect reflects progressive immunization of some mothers to male-antigens with each successive male foetus (Bogaert, 2002). With respect to the interaction of handedness and the fraternal birth order effect, two explanations have been proposed (Blanchard, 2008): It could be that non-right handed foetuses are insensitive to the presence of maternal male-antigens or that mothers of non-right handed foetuses do not produce anti-male antibodies. Although many researchers agree that this explanation, on the surface, seems plausible, it is lacking concrete, empirical support (James, 2004).
Research has also attempted to indentify genetic factors that influence sexual orientation. Concordance rates of homosexuality tend to be higher in monozygotic twins (who share 100% of the same genes) than dizygotic twins (who share 50% of the same genes) (Kendler at al., 2000; Kirk et al., 2000). Ellis et al. (2008) looked at blood type and Rh factor in both heterosexual and homosexual participants and discovered that male and female homosexuals were significantly more likely to be Rh negative than heterosexuals. The researchers also found that female homosexuals exhibited higher incidences of blood A type than male homosexuals, compared to heterosexual male and females who exhibited identical frequencies of the blood type A. Thus, chromosomes 9 and 1, which are responsible for blood type and Rh factor, may be involved in sexual orientation.
The extant research on sexual orientation has not gone without its fair share of criticism. Because the prevalence of homosexuals is relatively low within a given population, random sampling is not a viable option in experiments. Instead, researchers often have to place ads in homosexual magazines, recruit participants from recognized gay areas, or rely on word of mouth, all of which increase the chances of sampling biases. Another problem is researchers tendency to define sexuality in "either or" terms; that is, participants are classified as either gay or straight. Sexual orientation, like gender, is not a fixated entity for which criteria can easily be defined. For example, it is not uncommon for male prisoners to have sex with other males yet still consider themselves "straight". Also, although some individuals do not engage in sexual relations with the same sex, they do frequently fantasize about it. Thus, it is more fruitful for researchers to place sexual orientation along a continuum, with homosexuality and heterosexuality at the extreme ends and bisexuality in the middle. Furthermore, Alexander (2000) comments that research on human sexuality should employ the "double confirmation method", whereby sexual dimorphism is established before sexual orientation differences. In other words, a firm understanding of the biological differences between the sexes is necessary before researchers can even consider the possible prenatal factors involved in sexual orientation. It's also been noted that studies exploring the etiology of female homosexuality are lacking (Mustanski, Chivers, & Bailey, 2002). The most profound limitation of the literature on sexual orientation is its predominant usage of correlational studies, which make casual attributions next to impossible.
Despite the considerable amount of research that has been generated over the last two decades on the biology of sexual orientation, psychology's understanding of the topic is far from complete. This is not surprising, though, when one takes into consideration the complexity of the issue at hand and the fact that in the not so distant past homosexuality was classified in the DSM-III as a mental disorder. Researchers studying human sexuality are faced with two daunting tasks. Not only do they have to find consistent, biological differences between people of different sexual orientations, but they have to clearly demonstrate that these biological differences (i.e., number of biological older brothers, blood type), and not other confounding variables (i.e., social upbringing, sexual experiences), account for sexual orientation. For example, neurological differences between homosexuals and heterosexuals could be caused by prenatal factors, but it's also just as likely that postnatal sexual experiences may have caused the differences in brain structures (Alexander, 2000).
Although it's not likely that future research will be able to identify the exact etiological pathway from which sexual preferences in human beings develops, the articles summarized above should make it clear that biology influences a person's sexual orientation to some extent. Future research should take a scientific interactionist approach to studying sexual orientation, which acknowledges the importance of both biology and the environment. Such an approach is likely to produce more cogent findings than the current "nature or nurture" approaches.
Research has provided evidence that prenatal factors may be involved in sexual orientation. The neurohormonal theory attempts to explain sexual orientation in terms of prenatal testosterone levels which are involved in differentiating physiological, neural, and anatomical structures of both sexes. Swaab and Hofman (1990) found clear differences in the superchiasmatic nucleus (SCN) of the hypothalamus between homosexual and heterosexual men (as cited in Alexander, 2000). Adam et al. (2007) compared neural activation to preferred sexual stimuli and nonpreferred sexual stimuli in heterosexual and homosexual men and discovered that within the amygdala the latter had greater activity for preferred sexual stimuli than the former, suggesting the possibility that male homosexual brains may be characterized by atypical patterns of neural activity. Whether this result was a cause or consequence of the participant's homosexuality, however, is debatable. Other research has postulated that developmental instability in utero may be related to homosexuality. For example, Miller, Hoffmann, and Mustanksi (2008) found that overall index of bodily fluctuating asymmetry (FA) was positively correlated with homosexuality in men. Research by Martin, Puts, & Breedlove (2008), however, failed to confirm the results of the latter. In fact, the researchers reported that FA was lower in homosexual men than heterosexual men.
The most favourable finding to have emerged from the neurohormonal research is the fraternal birth order effect, which was first documented by Blanchard and Bogaert (1996) (as cited in Bogaert, 2006). The researchers found that there was a significant correlation between homosexuality in males and the number of biological older brothers in a Canadian sample. Since then several other studies have confirmed these results in samples from across the world. For example, Blanchard and Lippa (2007) reported that results from an online BBC survey involving 159,779 respondents revealed that older brothers increased the odds of homosexuality in men. Another study (Rahman, Clarke, & Morera, 2009) found evidence of the fraternal birth order effect in a sample of 100 heterosexual and 100 homosexual men. It appears that the fraternal birth order effect has no bearing on female sexuality (Bogaert, 1997). Bogaert (2006) demonstrated that only the number of biological older brothers, and not any other siblings, such as non-biological brothers, significantly predicted homosexuality in men. Moreover, rearing time with older siblings, whether biological or nonbiological, had no effect on sexual orientation, suggesting a prenatal origin to the fraternal birth order effect. Recent studies have indicated that the fraternal birth order effect is evident in right-handed males but not in non-right-handed males (Blanchard & Lippa, 2007; Blanchard, 2008).
The maternal immune system hypothesis was developed to explain these results and posits that the fraternal birth order effect reflects progressive immunization of some mothers to male-antigens with each successive male foetus (Bogaert, 2002). With respect to the interaction of handedness and the fraternal birth order effect, two explanations have been proposed (Blanchard, 2008): It could be that non-right handed foetuses are insensitive to the presence of maternal male-antigens or that mothers of non-right handed foetuses do not produce anti-male antibodies. Although many researchers agree that this explanation, on the surface, seems plausible, it is lacking concrete, empirical support (James, 2004).
Research has also attempted to indentify genetic factors that influence sexual orientation. Concordance rates of homosexuality tend to be higher in monozygotic twins (who share 100% of the same genes) than dizygotic twins (who share 50% of the same genes) (Kendler at al., 2000; Kirk et al., 2000). Ellis et al. (2008) looked at blood type and Rh factor in both heterosexual and homosexual participants and discovered that male and female homosexuals were significantly more likely to be Rh negative than heterosexuals. The researchers also found that female homosexuals exhibited higher incidences of blood A type than male homosexuals, compared to heterosexual male and females who exhibited identical frequencies of the blood type A. Thus, chromosomes 9 and 1, which are responsible for blood type and Rh factor, may be involved in sexual orientation.
The extant research on sexual orientation has not gone without its fair share of criticism. Because the prevalence of homosexuals is relatively low within a given population, random sampling is not a viable option in experiments. Instead, researchers often have to place ads in homosexual magazines, recruit participants from recognized gay areas, or rely on word of mouth, all of which increase the chances of sampling biases. Another problem is researchers tendency to define sexuality in "either or" terms; that is, participants are classified as either gay or straight. Sexual orientation, like gender, is not a fixated entity for which criteria can easily be defined. For example, it is not uncommon for male prisoners to have sex with other males yet still consider themselves "straight". Also, although some individuals do not engage in sexual relations with the same sex, they do frequently fantasize about it. Thus, it is more fruitful for researchers to place sexual orientation along a continuum, with homosexuality and heterosexuality at the extreme ends and bisexuality in the middle. Furthermore, Alexander (2000) comments that research on human sexuality should employ the "double confirmation method", whereby sexual dimorphism is established before sexual orientation differences. In other words, a firm understanding of the biological differences between the sexes is necessary before researchers can even consider the possible prenatal factors involved in sexual orientation. It's also been noted that studies exploring the etiology of female homosexuality are lacking (Mustanski, Chivers, & Bailey, 2002). The most profound limitation of the literature on sexual orientation is its predominant usage of correlational studies, which make casual attributions next to impossible.
Despite the considerable amount of research that has been generated over the last two decades on the biology of sexual orientation, psychology's understanding of the topic is far from complete. This is not surprising, though, when one takes into consideration the complexity of the issue at hand and the fact that in the not so distant past homosexuality was classified in the DSM-III as a mental disorder. Researchers studying human sexuality are faced with two daunting tasks. Not only do they have to find consistent, biological differences between people of different sexual orientations, but they have to clearly demonstrate that these biological differences (i.e., number of biological older brothers, blood type), and not other confounding variables (i.e., social upbringing, sexual experiences), account for sexual orientation. For example, neurological differences between homosexuals and heterosexuals could be caused by prenatal factors, but it's also just as likely that postnatal sexual experiences may have caused the differences in brain structures (Alexander, 2000).
Although it's not likely that future research will be able to identify the exact etiological pathway from which sexual preferences in human beings develops, the articles summarized above should make it clear that biology influences a person's sexual orientation to some extent. Future research should take a scientific interactionist approach to studying sexual orientation, which acknowledges the importance of both biology and the environment. Such an approach is likely to produce more cogent findings than the current "nature or nurture" approaches.