Differences in hypothalamic structure between heterosexual and homosexual men

Differences in hypothalamic structure between heterosexual and homosexual men
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In this article, “Differences in hypothalamic structure between heterosexual and homosexual men”, the author aims to investigate the biological basis of sexual orientation by studying the anterior hypothalamus, a structure involved in regulating sexual behavior. In particular, four neuron groups (the interstitial nuclei of the anterior hypothalamus (INAH)) were measured in volume. In a previous study (Allen and Gorski, 1992), both INAH 2 and 3 were found to be considerably larger in men than in women.

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The author thus hypothesized that either INAH 2 or INAH 3 is large in individuals with a sexual preference for women, and small in those sexually oriented towards men. Test subjects constituted of homosexual men, heterosexual men and heterosexual women. Only INAH 3 was found to be twice as large in heterosexual men than in both homosexual men and heterosexual women. Therefore INAH 3 is dimorphic with sexual orientation in men, and provides evidence for the idea that sexual orientation is of a biological nature.

LeVay wanted to test whether either one of both of the nuclei showed a size dimorphism with sexual orientation. His main thesis states that INAH 2 or INAH 3 is large in those with a sexual orientation towards women and small in individuals with a sexual preference for men (LeVay, 1991). In nonhuman primate studies, lesions in the comparable region of the anterior hypothalamus have successfully eliminated heterosexual behavior while keeping sex drive intact.

The study by Allen et al. investigating this region of the human hypothalamus found INAH 2 and 3 both to exhibit size dimorphisms, being significantly larger in men than in women. This suggests that INAH 2 and INAH 3 could be responsible for typical male sexual behavior, a thesis which the author again puts to the test (Allen and Gorski, 1992).

The study by LeVay has failed to replicate the finding that INAH 2 is twice as large in men than in women, but it did confirm that INAH 3 is dimorphic with sexual orientation. In heterosexual men, INAH 3 was a twofold larger than in homosexual men and heterosexual women. Data from homosexual women could not be obtained, therefore this line of reasoning only applies for men. The data used in the study has come from subjects who have died of AIDS, complications of AIDS or other, unspecified causes.

The author mentions the possibility that the small size in homosexual men could be due to side effects of AIDS or its complications. However, he dismisses this problem because the size difference was significant even when comparing the homosexual and heterosexual AIDS patients. AIDS neither had any effects on the other groups of nuclei studied, and finally no correlation was found in the entire subject sample between the volume of INAH 3 and the length of their survival time from the moment of diagnosis. Furthermore, Byne et al. mention that in HIV positive men and women, only INAH 1 was found to be influenced, having increased about 8% in size (Byne et al., 2001).

Another problem the author expresses is the fact that AIDS patients could be an unrepresentative part of the population, with a tendency to engage in reckless sexual behavior with many partners, or a strong preference for being the receptor in anal intercourse, both of which are great risk factors for acquiring AIDS. The author also mentions that this type of behavior was at least until recently common among most gay men, but nevertheless he poses that more detailed information is needed about the patient’s sexuality before correlations can be made between brain structure and sexual diversity.

Finally, he states that exceptions have been found, that is, heterosexual men with a small INAH 3. He possibly attributes this to technical shortcomings or subject mismanagement, but also mentions as a possible reason that nucleus size is not the only determinant in sexual orientation. However, the finding that for the majority of the sample, the INAH 3 size difference between heterosexual and homosexual men was significant, supports the fact that it is indeed feasible to study sexual orientation at the biological level, and creates possibilities for further research.

At first sight the information seems convincingly supportive. However, further research points out that the arguments made by the author contain some flaws. To begin with, Byne et al. tried to replicate this study’s findings, but they found that even though INAH 3 indeed tended to have a smaller volume in homosexual men than in heterosexual men, no difference was to be observed as far as the number of neurons within the nucleus was concerned. Presumed heterosexual male individuals have an average of approximately 60% more neurons in their INAH 3 than females. In homosexual males, the number of neurons in INAH 3 did not differ from heterosexual males. LeVay’s classification was based only on volume and not on neuronal number. Volume can be susceptible to several external pre- and post-mortem factors, such as differences in agonal state and fixation time (Byne et al., 2001). However, shrinkage during tissue fixation seems an unlikely possibility since no other INAH were affected in the same way and neuron size was identical between homosexual and heterosexual males, eliminating the possibility of neuron shrinkage.

An alternative possibility is that the homosexual group has suffered a reduction in neuropil within INAH 3, but this remains open to further research. Apart from the biological factors, histological procedures and section thickness can also be of influence. Therefore the importance of using a variable free of such influences such as neuronal density is again emphasized. Based on sexual orientation, no variations in INAH 3 were found (Byne et al., 2001).

Sex differences between males and females have indeed been found, but as a previous study by Byne et al. pointed out, “sex difference in volume was attributable to a sex difference in neuronal number and not in neuronal size or density” (Byne et al., 2000).

A factor that may contribute to the neuronal sex difference in INAH 3 is neuronal survival that differs in males and females. On top of that, as the surviving neurons integrate in functional circuits, the sex related differences may emerge later in development as a result of the latter.

Swaab and Hofman go even further in describing the factors influencing human sexual orientation. In a clarifying overview, they mention genetic factors, as results from studies on twins, which found genetic factors “to be of influence in both male and female sexual orientation” (Bailey et al., 2000). Unfortunately, exactly what part of the brain was responsible for the influence remains unknown.

The study showed that there were significantly more homosexual sisters than brothers, suggesting that male and female homosexuality are causally independent at least to a certain extend. The curiosity here is that female homosexuality appears to be a lot less common than male homosexuality, which suggests that the findings from male homosexuality studies cannot merely be extended to females. Rather, female homosexuality should be studied separately.

A linkage between DNA markers on the X chromosome and male sexual orientation has also been found (Hamer et al., 1993). They relate the influence of sex hormones on sexual orientation, due to the high proportion of bisexual and homosexual girls suffering from congenital adrenal hyperplasia. For another, the increased occurrence of diethylstilboestrol (DES), a compound related to estrogens, in bisexual and homosexual girls supports the influential role of sex hormones.

The first biological difference in the human brain Swaab and Hofman found relating to sexual orientation was in the vasopressin-containing subnucleus of the suprachiasmatic nucleus (SCN), the brain structure responsible for organizing the endogenous circadian rhythms. It was found to be twice as large in homosexual men than in heterosexual individuals. More precisely, it was 1.7 times as large and contained 2.1 times as many cells as the SCN of the heterosexual group. This also implies that the difference in volume could not be due to tissue shrinkage during the histological procedure. Sex differences in the shape in the SCN have also been found, it being elongated in women and more spherical in men (Swaab and Hofman, 1995). However, the exact role of the SCN in sexual orientation remains to be investigated.

Maternal stress and social factors are also thought to be involved in the increase of homosexuality in both boys and girls. However, no convincing evidence has yet been found. Children raised by lesbian or transsexual couples are generally found to have a heterosexual orientation (Golombok and Tasker, 1996), which decreases the importance of the social environment as an influencing factor for sexual orientation.

Golombok does mention that those who had grown up in lesbian families, and especially those families in which the parents have openly shown affection towards each other, were more likely to consider the possibility of having a lesbian or a gay relationship. However, the number of young adults to actually do so was not significant, and therefore the assumption that children raised by lesbian mothers will themselves become involved in a lesbian or a gay relationship is not supported by her findings.

When reviewing the plentiful literature on the topic, the main academic debate at this point seems to be between the biological specialists, who try to link sexual orientation to brain anatomy, genetic factors and sex hormones, and the social theorists, who claim sexual orientation to be dependent on environmental and cultural influences. However, the experience-based theories have not done well in the empirical context (Bem, 1996) and even in the biological literature, many contradictory claims are made and research often cannot be exactly replicated.

In his paper “Exotic becomes erotic: A developmental theory of sexual orientation” Daryl J. Bem proposes a theory of romantic attraction providing “the same basic account for opposite-sex and same-sex desire in both men and women”. The essence of his paper comes down to the following: feeling different from same-sex individuals causes attraction to those in later life.

He suggests that biological factors such as genes, hormones and brain neuroanatomy codes childhood temperaments rather than sexual orientation. During childhood, a child will feel a preference for sex-typical or sex-atypical friends and activities, which guide children in their later feelings towards opposite- or same-sex peers. When perceiving the dissimilar peers as strange and exotic a heightened sense of arousal will be felt which will ultimately be interpreted as erotic. The theory claims to integrate both the biological and the social aspects.

I will provide a short overview of the theory. To begin with, biological variables code the child’s temperaments such as aggression or activity level.

Then, the child’s temperaments cause him or her to prefer certain activities over others. Some children will enjoy male-typical activities such as competitive sports and rough play, others will enjoy female-typical play such as playing with dolls. These preferences are assumed to be independent of actual sex. Children enjoying sex-typical activities are gender conforming, children preferring sex-atypical activities are gender nonconforming.

In the next step, gender-conforming children will perceive opposite-sex peers as exotic. On the other hand, gender nonconforming children will feel dissimilar from same-sex peers, and ultimately consider them as exotic.

Heightened autonomic arousal in the presence of alienated peers is produced by these feelings of unfamiliarity, even though at this stage they are likely not yet to be feelings of affection, or even to be consciously felt.

However, as the child develops, these feelings are transformed into romantic or erotic feelings. These last two steps envelop the psychological mechanisms transforming exotic into erotic.

Evidence for this theory comes from a study by Bell et al., which shows that childhood gender conformity or nonconformity is not only the strongest, but also the only reliable childhood predictor of later sexual orientation for both men and women (Bell et al., 1981).

In order to describe the integration of biological and social factors in the theory, there are three essentialist assumptions underlying the theory which must be examined (Bem, 1996). First, the assumption that childhood temperaments are partially coded in the genes, second, those temperaments can influence a child’s attraction towards sex-typical or sex-atypical activities, and third, the assumption that psychological processes transforming “exotic into erotic” are universal human properties. These are the biological aspects. They influence sexual orientation only indirectly, through childhood temperament. All other influences can be accounted for by cultural factors, even including the concept of sexual orientation itself.

I very much liked to read about the subject. I have never thought about the origins of sexual orientation, or considered the possibility of it being directly coded in one’s brain anatomy. While I do not consider this to be the only decisive factor, due to the many inconsistent research outcomes, I do feel that Bem’s theory of indirect biological influence holds a core of truth. During puberty, many mixed and even contradictory feelings are experienced which can only come from deep within. However, how these feelings are then interpreted or whether the individual actually pays heed to them, that is in my opinion a psychological, conscious process which can certainly be influenced by external pressure and cultural morals about what is right and what is wrong.

The articles I examined did not have a shock value for me; I just found it truly interesting to read about the many approaches to sexual orientation, and the many factors that can be of influence. I have learned that there is no straightforward approach, and in my further professional career I will most certainly be open to many different views. I feel a broad perspective will enhance my thinking and, consequently, my decisions.

(Allen and Gorski, 1992) Allen, L. and Gorski, R. (1992). Sexual Orientation and the Size of the Anterior Commissure in the Human Brain. Proceedings of the National Academy of Sciences, 89(15):7199-7202.

(Bailey et al., 2000) Bailey, J., Dunne, M., and Martin, N. (2000). Genetic and Environmental Influences on Sexual Orientation and Its Correlates in an Australian Twin Sample. Journal of Personality and social Psychology, 78(3):524-536.

(Bell et al., 1981) Bell, A., Weinberg, M., and Hammersmith, S. (1981). Sexual

preference, its development in men and women. Indiana University Press.

(Bem, 1996) Bem, D. (1996). Exotic Becomes Erotic: A Developmental Theory of Sexual Orientation. Psychological review New York, 103:320-335.

(Byne et al., 2000) Byne, W., Lasco, M., Kemether, E., Shinwari, A., Edgar, M.,  Morgello, S., Jones, L., and Tobet, S. (2000). The interstitial nuclei of the human anterior hypothalamus: an investigation of sexual variation in volume and cell size, number and density. Brain Research, 856(1-2):254-258.

(Byne et al., 2001) Byne, W., Tobet, S., Mattiace, L., Lasco, M., Kemether, E., Edgar, M., Morgello, S., Buchsbaum, M., and Jones, L. (2001). The Interstitial Nuclei of the Human Anterior Hypothalamus: An Investigation of Variation with Sex, Sexual Orientation, and HIV Status. Hormones and Behavior, 40(2):86-92.

(Golombok and Tasker, 1996) Golombok, S. and Tasker, F. (1996). Do Parents Influence the Sexual Orientation of Their Children? Findings From a Longitudinal Study of Lesbian Families. Developmental psychology, 32:3-11.

(Hamer et al., 1993) Hamer, D., Hu, S., Magnuson, V., Hu, N., and Pattatucci, A. (1993). A linkage between DNA markers on the X chromosome and male sexual orientation. Science, 261(5119):321-327.

(LeVay, 1991) LeVay, S. (1991). A difference in hypothalamic structure between heterosexual and homosexual men. Science, 253(5023):1034-1037.

(Swaab and Hofman, 1995) Swaab, D. and Hofman, M. (1995). Sexual differentiation of the human hypothalamus in relation to gender and sexual orientation. Trends in Neurosciences, 18(6):264-270.