The neuropsychology of transsexualism

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     I have stated at the beginning of this book that it is the prenatal hormone perfusion which is the more critical in establishing sexual identity and sexual orientation,  and probably most of the subtle psychological differences between the sexes as well.   One way to help figure out the extent to which sex differences can result from a secondary hormone bath, at adolescence or even later,  is the study of psychological, behavioral and neuropsychological differences between people before and after a sex change -which always includes hormone treatment as an adjunct.   It turns out,  not so surprisingly,  that one of the traits which depends on the hormones of adulthood is sexual desire.   Men who become phenotypic women report a decrease in libido and aggressiveness and women who become phenotypic men report an increase in libido and aggressiveness.  But more intriguingly,  there are more subtle traits which switch around as well,  traits which these people could not be aware of -even if they wanted to.    Some studies have found that the sex-change operation significantly redistributes the verbal and visuospatial aptitudes in the direction of the targeted sex, -even though the respondents are not aware that such sex differences exist (Van-Goozen et al, 1995).  One investigation comprised an evaluation of 22 individuals (mean age 25 years) seeking sex-change surgery and diagnosed as transsexual by a Gender Identity Research Team.  These persons underwent extensive neuropsychological and cognitive testing prior to surgery. On the Wechsler Adult Intelligence Scale, the subjects performed in congruence with their biologic sex rather than gender identity. In contrast, on a measure of conceptual styles, the subjects performed in congruence with their gender identity.   Recall that homosexual men tend to be last borns. And recall that all maternal steroïd hormones are in lesser supply  with each pregnancy.  Maternal supply of testosterone during gestation must perhaps be high enough (among other things) for normal sexual orientation to be implanted in her male fetus’s brain.    What then is the situation with transsexuals ? Homosexual male gender dysphorics (male transsexuals who prefer men as sexual partners)  have a later birth order: each older brother increases odds by 45% (similar to a prior study for gay men). Male heterosexual transsexuals have an opposite effect: each older brother decreases the odds! Regarding birth order, then, homosexual male gender dysphorics are more like homosexual men without gender dysphoria than they are to heterosexual male gender dysphorics. The situation is much less complicated for biological females with gender dysphoria, who are almost always attracted to biological females,  though the very small minority who are attracted to biological males have been much less well studied.  All of this supports the notion,  explained above,  according to which the maternal hormonal status (particularly testosterone) during pregnancy influences development of brain circuits responsible for sexual orientation in male offspring. 

To me, such findings are very important in constraining our neurobiological theories of sex differences.  My reading of the literature on the neurobiology of sex differences is that it is too dry,  not wet enough.   Let me explain.   Wet physiology is the physiology of fluids and chemicals in the body  -also known as pharmacology.   There are complex interactions between steroid hormones and neurotransmitters in the brain.   Some of these interactions are fixed before birth.  Some of these interactions have a structuring effect on brain development.  However,  some of these interactions continue to take place and to influence the mind and behavior throughout life.    I will try to make a case for this point of view in the upcoming section on the neuropsychology of the menstrual cycle.     Incidentally, you may be wondering what  “dry” physiology might be ?   It is electrophysiology.

Most studies of transsexuals do not compare the person to himself (or herself) before and after the sex-change operation.  It is important to understand that when the research design is not a pre-post operation design,  what is being studied is more likely to result from particularities of transsexuals' brains which were in fact prenatally determined and fixed.  One large scale study of transsexuals (Orlebeke et al, 1992) investigated handedness  -but not before and after any sex-change operation. A statistically significant incidence of left handedness was found.  In fact,  the transsexuals were twice more at risk for sinistrality (left handedness) than were the heterosexual controls.  Interestingly,  the largest handedness study in homosexuals that I am aware of,  carried out on 1,612 cases,  found an incidence of left handedness which was just barely statistically significant (Becker et al, 1992).  The authors of the study happened (to their credit) to mention that a proportion of their sample was also transsexual in addition to being homosexual,  and of course,  as is usually the case with these studies, all the subjects were men.  The higher incidence of sinistrality in transsexuals is a finding which has been recently replicated by several research teams.   It can therefore be considered quite robust.  Incidentally,  the latest of this string of studies has found that female-to-male transsexuals (not just the more commonly studied male-to-female transsexuals) are also significantly more often sinistral than either sex with the standard sexual identity.  Another interesting recent finding is that male-to-female transsexuals have finger ridge asymetry (of the right and left hand) which significantly differs form men with a normal sexual identity.   However,  female-to-male transsexuals do not differ from women with a normal sexual identity.  One study looked at dichotic listening in male-to-female operated transsexuals (Cohen & Forget, 1995).  It found that the profile of ear asymmetries was female-typical, that is, less asymmetric than observed in normal men.   Such findings could be explained just as easily in terms of interhemispheric communication as in terms of hemispheric specialization.  However,  the one brain imaging study that I am aware of which looked at the corpus callosum (the major commissure linking the two hemispheres of the brain) in transsexuals and controls found no difference at all (Emory et al, 1991).   On the other hand,  the sampling was a bit crude in that all subjects were right handed,  and sex differences in corpus callosum morphology are typically observed only when handedness variation is also taken into account.