On the Science of Changing Sex

Etiological Conjectures, Part 1

Posted in Transsexual Theory by Kay Brown on December 22, 2012

androgynous faceWe all love to speculate on what made us the way we are.  We all love to generate models for how the transgendered world came to be.  I’m certainly not immune to those speculations, and neither are many of the scientists who conduct research on the trans-phenomena.  About two decades ago, I formulated a model of how MTF transkids (HSTS) and conventional gay men were similar and dissimilar.  At the time, it was purely based on personal observation and conjecture.  Imagine my surprise and delight when I read a recent paper based on genetic manipulation (gene “knock-out”) on sexual dimorphic behaviors in mice that would support part of my conjecture.

Many animals who are bisexual (i.e. that come in two and only two sexes) also often have sexually dimorphic behaviors.  Typically, they involve reproductive behavior, sexual uniting of gametes, mating, and rearing or protection of their young.  The range of such behaviors found in the animal world are so diverse, that I would fill up an entire multi-volume set of books just to list them all.  However, for mammals, many of these behaviors are similar enough that we can use some animals as stand-ins for researching what is likely to be also true for humans, especially in the evolutionarily close relatives in primates and rodents.  The most useful, due to their short lifespans, small size, and ease of maintaining, are mice and rats.

My personal model has been that many sexually dimorphic behaviors are independently evolved and genetically encoded.  They are developmentally controlled by similar mechanisms such as sex hormone receptors on neurons.  This implied that masculinity and femininity (to be defined below) are not a “one shot deal” nor a simple one dimensional, nor even a two dimensional behavioral space.  That is to say, that many of the sexually dimorphic behaviors may be “switched on or off” independently.  In fact, when describing this model to others, I often asked my listener to imagine a long row of switches, which may be up or down.  Some of these switches control masculine behaviors and some feminine.  In theory, they could be in any combination, but during development, processes come into play such that the vast majority of people have all of one type, masculine or feminine in the on, while the opposite type are in the off positions.  However, in a small number of individuals some of the switches are flipped to the “wrong” state.  In an even smaller number of individuals, quite a few of the switches are flipped to the wrong state.  Some of those switches have only small effects.  But some of the switches have rather dramatic effects.

To say that a given behavior is masculine or feminine is to say that that behavior is more likely to be produced by one sex than the other.  For example, in common rabbits, a female is far more likely to pull hair from its belly to line an underground nest in preparation for caring for kits (newborn rabbits).  Thus, in rabbits, nest lining would be a “feminine” behavior.  In rodents, females are far more likely than males to exhibit lordosis, arching of the spine to tilt and raise the pelvis, than males, usually in the presence of adult male.  So we can describe lordosis as also being “feminine” behavior.  Conversely, mounting behavior is usually only seen in males, and thus may be described as a “masculine” trait.

Now, at least in mice, we have confirmation that it is possible to switch “off” individual genes that are associated with such behaviors, both masculine and feminine.  This study did not demonstrate turning “on” a cross-sex behavior, but that has been demonstrated repeatedly, if crudely, by administering cross-sex hormones to neonatal rats.   Further, in sheep, we have seen that a mix of masculine and feminine traits can coexist in that one finds male sheep who preferentially mount (masculine) other male sheep for sex (feminine).  (I needn’t provide references, given that these are well known in the literature.)

In humans, there are a range of behaviors that show varying levels of sexual dimorphism.  Simple observation would suggest that the single most sexually dimorphic trait in humans is the propensity for sexual attraction to men.  In women, approximately 98% exhibit sexual attraction to men, while in men perhaps only 5-10% are attracted to other men, and only 3% are exclusively so.  Thus, sexual attraction to men would, by our definition, be a “feminine” trait.  Interestingly, there appears to be analogs to “mounting behavior” and “lordosis” in humans.   Men who are sexually attracted to other men, also exhibit a preference for mounting (active or “top”) or lordosis (passive or “bottom”).  It is my thesis here that in gay men, independent sexually dimorphic behaviors have been feminized while others have not, and that this independent switching has occurred in varying combinations in individual men.  That is to say, that a gay man could be quite feminized in at least one behavior (androphilia) but show a range of other behaviors that may or may not also be feminine.

Which brings us to MTF transkids.

Transkids are universally attracted to men.  They are also universally obligate “bottoms”.  In fact, they are also universally “avoidant” as well.  That is to say, that they refuse to allow a partner to take notice of, or touch, their pre-op genitalia.  Transkids are also, by definition, persistors, while most gay men were at least somewhat gender atypical as young children and may or may not have also been gender dysphoric, yet they desisted being so by the time they were eleven or twelve.  Transkids remain behaviorally feminine in voice production, motor movements, etc.  I’ve often noted that many transkids are especially interested in small children and babies.  As a speculative conjecture, might the difference between conventional gay men and transkids be the number, or a key subset, of the sexually dimorphic behaviors that are possible?  That it is not so much that they are “more feminine” than most gay men, but that they are “feminine in more ways”?  So much so, that they find it far more comfortable and advantageous to transition?

Etiological Conjectures, Part 2

Etiological Conjectures, Part 3


X. Xu et al. “Modular Genetic Control of Sexually Dimorphic Behaviors”


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