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Review of the study: Brain response to putative pheromones in homosexual men

Warren Throckmorton, PhD
May 10, 2005

Research Review
By Warren Throckmorton, PhD

Title: Brain response to putative pheromones in homosexual men
Authors: Ivanka Savic*†‡, Hans Berglund§, and Per Lindstro¨m
Reference: Proceedings of the National Academies of Science, pages: 7356–7361, May 17, 2005, vol. 102, no. 20.

Abstract (by authors of the article):
The testosterone derivative 4,16-androstadien-3-one (AND) and the estrogen-like steroid estra-1,3,5(10),16-tetraen-3-ol (EST) are candidate compounds for human pheromones. AND is detected primarily in male sweat, whereas EST has been found in female urine. In a previous positron emission tomography study, we found that smelling AND and EST activated regions covering sexually dimorphic nuclei of the anterior hypothalamus, and that this activation was differentiated with respect to sex and compound. In the present study, the pattern of activation induced by AND and EST was compared among homosexual men, heterosexual men, and heterosexual women. In contrast to heterosexual men, and in congruence with heterosexual women, homosexual men displayed hypothalamic activation in response to AND. Maximal activation was observed in the medial preoptic area_anterior hypothalamus, which, according to animal studies, is highly involved in sexual behavior. As opposed to putative pheromones, common odors were processed similarly in all three groups of subjects and engaged only the olfactory brain (amygdala, piriform, orbitofrontal, and insular cortex). These findings show that our brain reacts differently to the two putative pheromones compared with common odors, and suggest a link between sexual orientation and hypothalamic neuronal processes.

What the study investigated:

The researchers exposed heterosexual men, homosexual men, and heterosexual women to compounds including a testosterone derivative and an estrogen-like steroid. Various neutral smells and compounds were used as control conditions. The testosterone derivative is proposed as a pheromone activating sexual arousal in the hypothalamic regions for women and the estrogen compound propose to create like responses for men. Further, the researchers wondered if the testosterone compound would demonstrate sexual reactions in gay men on par with straight women as viewed by PET scanning. The researchers also examined subjective preferences and blood level hormone levels. There were matched groups of 12 each in the straight male, gay male and straight female groups.

What the study found:

• The gay males and the straight females demonstrated arousal in the hypothalamic regions in reaction to the testosterone compound. Lesbians were not included in the study.
• Straight males did not demonstrate the same arousal to the testosterone compound but were aroused by the estrogen compound.
• Gay and straight males demonstrated similar arousal to the estrogen compound near the amygdala and other structures but not near the hypothalamus
• The study asked participants for subjective reactions to the smells and there were no significant differences between any of the groups.
• The study assessed hormonal differences in reaction to the smells and there were no differences in blood levels among any of the groups.

What are the implications?

This paragraph from the research report states the author’s appropriate caution concerning their findings:

The difference between HoM (homosexual males) and HeM (heterosexual males) could reflect a variant differentiation of the anterior hypothalamus in HoM, leading to an altered response pattern. Alternatively, it could reflect an acquired sensitization to AND (testosterone – male pheromone) stimuli in the hypothalamus or its centrifugal networks, due to repeated sexual exposure to men (35). A third possibility is that HeW (heterosexual women) and HoM associated AND with sex, whereas HeM made a similar association with EST (estrogen – female pheromone). These tentative mechanisms are not mutually exclusive, nor can they be discriminated on the basis of the present PET data. (Ivanka, et al, 2005, p. 7361).
In lay terms, this means the study does not mean these differences are of necessity due to hard wiring of the brain. Researcher Dr. Ivanka Savic was quoted in the May 9, 2005, Chicago Tribune saying: "I want to be extremely cautious - this study does not tell us anything about whether sexual orientation is hardwired in the brain. It doesn't say anything about that.”
Savic and colleagues cite a 2002 study from Nature demonstrating that smell is highly subject to learning and habituation. It seems quite possible that gay men and heterosexual men and women learn to respond to chemical signals as a result of heightened sensitivity engendered by sexual activity. The gay male participants were all rated a Kinsey 6 for exclusive homosexual activity and fantasy with the straight participants are reporting exclusive heterosexual preferences. Although the authors do not give a sexual history of the participants, one assumes from their caution that all subjects had sexual experience.

The study appears to be well done with many appropriate controls. However, the number of participants was small and the absence of lesbians makes other interesting comparisons not possible. Further, I would be interested to know more about the individual responses of the subjects. Concerning individual participant responses, the authors write: “According to the method applied, the material was sufficient to generate inference at group level, implying that each subject was representative of his or her designated group…” (p. 7360). While this is fine for statistical work, I am very curious about any variation within these groups.

This study could help us better understand why sexual attractions seems so natural and at time illogical. If stimulating this area of the hypothalamic region influences a subjective experience of sexual arousal, then imperceptible “chemicosignals” could be, metaphorically speaking, the little fiery darts that trigger sexual temptations. Engaging in sexual activity could set in motion brain changes that are difficult to reverse, although not impossible, once established.

So in summary:

• The study does show involuntary hypothalamic response associated with self-assessed sexual orientation
• The study shows that gay males do react to the estrogen condition but in a different manner than they react to the testosterone condition
• The study cannot shed light on the complicated question of whether sexual orientation of the participants is hard wired.
• The brains of these participants may have acquired a sexual response to these chemicals as the result of past sexual experience. In other word, the response described in this study could well have been learned.
• If these results hold up, this could explain why varying sexual attractions seem so “natural.” Also, such conditioning could give insight into why changing sexual attractions is often experienced by those changing sexual preferences as a process of unlearning responses to environmental triggers.



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