On the Science of Changing Sex

False (Scent) Trail

Posted in Brain Sex by Kay Brown on September 22, 2010

shrinking brainMuch has been made of a study that seemed to prove that MTF transsexuals have female brains because they respond to human sexual pheromones the same as heterosexual women do, and unlike heterosexual men. The experiment used fMRI to image neural activity in the brain while subjects were exposed to several different odorants. When exposed to androstedienone, the transsexuals brains showed a pattern similar to those of the women, and unlike the non-transsexual, heterosexual men. As controls, the researchers used several odorants that were assumed not to be human pheromones. The experimental results seem all the more powerful proof that MTF transsexuals must all have female brains, because the MTF transsexuals were all pre-HRT and all exclusively gynephilic. As I will point out later, this last point is actually the most intriguing factor of the study, but can only be understood in the full context of the history of pheromone research in mammals.

There are some very troubling aspects of the research, right from the start.

First, not everyone can smell androstedienone to the same degree. Only a small percentage of people can smell it in reasonable concentrations. Most people can’t smell it until it is very concentrated. A similar distribution is found for the ability to smell androstenone, another sometime putative human sexual pheromone. If something is important to a species, something as basic as reproductive behavior, that trait is strongly conserved. A species only loses a trait or has a range of traits, for something that is not important. Thus, the ability to sense this putative pheromone would not be found in only a small subset of a population, if it were a pheromone.

In studies that show that androstedienone has a measurable effect on the levels of cortisol in heterosexual women, it only does so when a man is present. Oops… a live, actual man has to be present? So, which is it, the androstedienone or the visual, aural, and likely tactile stimulation that a man, sitting close, taking saliva samples, is what straight women are responding to?

Most researchers that are trying to track down and prove pheromones are involved in mammalian sexual response usually point to the vomeronasal accessory olfactory system, a second “sniff” system if you will, believed by some to be specialized to detect pheromones. But in humans, the vomeronasal system is vestigial, completely non-functional !

Finally, after many years of study, the very concept of a mammalian “pheromone” is being seriously challenged. Although there can be no doubt that olfaction is an important part of mammalian conspecific (and occasionally of exspecific, e.g. skunks) communication; It is being shown that it is no more important than, and must be learned in the same manner as, visual and aural communications.  Further, the theory of how odorants are detected and identified is in flux, and that in mammals, it may not be possible to uniquely identify a specific molecule from another of similar properties.

But first, we must explore the nature of mammalian olfaction, and our current as well as historic understanding of how it operates and then examine how pheromones might operate to effect human behavior.

When odorant molecules are inhaled, sniffed, into the nasal passages, chemosensory neurons lining the roof respond to their presence. These neurons connect to special processing centers, glomeruli just on the other side of the skull bones, in the olfactory bulbs, before they are sent to the brain. In the popular imagination, because the connection from these centers to the brain is to various so called “primitive” areas, including the amygdala, known to process emotional states, it is thought that olfaction provides a direct path, around our higher cognitive processing, able to effect emotions and sexual arousal directly, perhaps without any conscious knowledge of the odorant.

Historically, there are two primary theories about how olfaction transduction takes place. In one theory, based on the observation that the odor of a chemical highly correlates with the infrared absorption spectra of that chemical, is that there is a mechanism for detecting the molecular vibrations, which cause the absorption of infrared light, of the odorant. This seemed improbable to many biologists in the mid-20th Century, as they couldn’t imagine biologically based micro-infrared-spectroscopes hiding away in the nose. The other theory was that the shape of the odorant molecule fit “lock and key” style into another molecule on the surface of the neurons. Given that this mechanism is known to be correct for the operation of inter-neuron communications using neurotransmitters in the synapses, as well as how hormones are detected by cells of all types, the shape theory of olfaction seemed more likely and was accepted as being “true”, even though it had not been shown to be.

However, recently the vibrational theory has been gaining ground because it not only is better able to predict the odor of a given chemical, but we now have a plausible biological mechanism whereby quantum mechanical effects at the molecular level can give rise to the ability to detect molecular vibrations.

Wikipedia has excellent overviews on these two theories and the current state of flux in this scientific mystery.



Now, take a moment to view this video of Luca Turin, the modern proponent of the vibration theory:


{On a personal note:  I would dearly love to get know Mr. Turin, having read his book on olfaction and his blog regarding purfumes, which I adore.  Oh, and if you’re interested, my personal scent is Mariella Burani.}

Now we come to the historically coincidental development of the shape theory of olfaction and the recognition of chemical signaling in social insects and many other arthropods. Given that hormones are detected by their shape, and that odors were thought to be detected in the same manner, it would seem reasonable to consider external chemical signaling molecules to have developed as an evolutionary extension of the internal chemical signaling molecules. Given that it was thought that olfaction was a process of “lock&key” matching, it was thought that specific receptors for pheromones would have evolved that would selectively respond only to the pheromone, so that other odorants would not be confused for the pheromone.  If mammals were to evolve external chemical signaling molecules, it would seem most likely that they would be odoriferous metabolites of hormones.

Thus, researchers began to examine such odoriferous molecules or sometimes natural odors produced by test subjects, both experimental lab animals and in humans. And lo… many examples of odors being used as chemical signals were found. Clearly, many mammals have scent producing glands that are used to mark territory. But is this an example of a “pheromone”, an “external hormone” that effects members of that same species in a specific and unambiguously instinctive way? Or is this in the same category of signaling as bird song, wolf howls, and dog barks? And what are we to make of a skunk? Clearly the skunk produces an odorant that sends a very strong signal. But are we to suggest that the all of the mammals of the world evolved to understand a skunk pheromone? The very concept of what a pheromone is becomes critically important.

Finally, what about sexual pheromones that signal in an unambiguous way the sex of an individual in such a way as to deeply, instinctively, effect the behavior of another animal? In arthropods, we have many such odorants, the most famous of which is the one used by the Silk Moth female to lay down a scent trail for male Silk Moths to follow. Are there chemicals emitted by one or both sexes in mammals, and most especially, in humans, that has this effect?

Richard Doty in his recent book, The Great Pheromone Myth, explores all of the scientific papers published to date, and systemically demonstrates that each and every time that such a molecule has been held up to be such a pheromone in mammals, careful study to replicate the effect has failed. Each and every time, it has been discovered that either the experiment, or the analysis, was flawed. Many experiments seemed to show an effect, but when studied more closely, it turned out that it was a learned association between the odorant and the target sex. For example, in mice, the smell of male urine effects the behavior of female mice; But only if that female mouse has previously learned that male mice urine has that smell. Female mice raised in isolation, never having encountered a male mouse, are not effected.

This last is the reason that only gynephilic transsexuals were included in the study of the effect of androstedienone. It was in the vain hope that such transsexuals hadn’t associated the smell with sexual encounters with their preferred erotic target, as would be the case for androphilic transsexuals. (On a personal note, I will admit I like the smell of my husband’s skin.)  But wait, that can’t be correct, as humans aren’t lab animals, raised in isolation. Not only have these transsexuals been exposed to men in general, but they have been continuously exposed to that odorant on themselves!

One would be tempted to attribute the results of the transsexual fMRI study as being consequent on the autogynephilic nature of the subjects, perhaps an association with autoerotic AGP arousal, but it’s far more likely that the results are from a flawed experiment, the failure of the double blind, where both the experimenters and the subjects knew that this was about pheromones, and that these straight women and MTF transsexuals knew that something that smelled “musky” was the “male pheromone”, and that they should be reacting sexually to it, while the straight men knew they shouldn’t. Even if it was not deliberate, the expectation that it would happen would ensure that it did.

For more essays on trans-brains see Brain Sex.

Additional Reading on the Web:




H. Berglund, P. Lindström, C. Dhejne-Helmy, I. Savic, “Male-to-Female Transsexuals Show Sex-Atypical Hypothalamus Activation When Smelling Odorous Steroids”

Lundström, Johan N. (Uppsala University, Department of Psychology)
Ph.D. Thesis:Human Pheromones: Psychological and Neurological Modulation of a Putative Human Pheromone

Doty, Richard L., The Great Pheromone Myth, The Johns Hopkins University Press | 2010 | ISBN: 080189347X

Antti Knaapila, Hely Tuorila, Eero Vuoksimaa, Kaisu Keskitalo-Vuokko, ichard J. Rose, Jaakko Kaprio, Karri Silventoinen, “Pleasantness of the Odor of Androstenone as a Function of Sexual Intercourse Experience in Women and Men”


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