Turner’s syndrome, Klinefelter’s syndrome and cognition.

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  Turner’s syndrome and Klinefelter’s syndrome are both gonosomal aberrations involving the X chromosome.   In Turner’s syndrome,  there is only one X gonosome.  In Klinefelter’s syndrome,  there is a Y gonosome as well as at least two X chromosomes. A straightforward (and simplistic) gonosomal theory of sex differences in cognitive abilities would predict that Turner women should manifest a relatively more male-typical cognitive abilities profile than normal women (because they have a more male-typical gonosomal arrangement),  and that Klinefelter men should manifest a relatively more female-typical profile than normal men (because they have a more female-typical gonosomal arrangement).    For example,  Turner women would be predicted to have certain high visuospatial skills and certain low verbal skills,  and Klinefelter men would be expected to show the opposite pattern.    Well,  any such straightforward simplistic gonosomal model of sex differences in cognitive profile holds no water.    In fact,  the opposite of the prediction occurs:  Turner women have low visuospatial abilities,  whereas Klinefelter men have low verbal abilities.    

Obviously,  a higher level of complexity of neurobiological theorizing is called for here.   Such a model,  more sophisticated than the one outlined just above,  has been proposed by two developmental specialists named Charles Netley and Joanne Rovet.    These authors proposed that there is an effect of the X gonosome on the differential rate of prenatal maturation of the brain hemispheres.    A deletion of the X chromosome results in relative slowness of development of the right hemisphere,  whereas a superfetatory (extra) X gonosome results in a relative slowness of development of the left hemisphere.    There are a number of problems with this model.  It does not explain normal sex differences and even goes in the opposite direction. Furthermore it does not explain why XXX females usually have low-normal intelligence with no apparent difference in verbal and non-verbal abilities.  Finally,  all attempts to uncover a relative physiological or anatomical weakness of the right hemisphere in Turner women, or of the left hemisphere in Klinefelter men,  have failed.   

My own position on gonosomal contribution to sex-specific cognitive ability profiles is that such an effect seems to exist,  but has not yet been adequately explained.   As much as I would love to explain it myself,  I am unable to.  It is known that untreated Turner women have abnormally low sex hormones,  both androgens and the “female” sex hormones.  Nyborg found that the visuospatial deficit of Turner women is much reduced after estradiol treatment.   In these patients as in normals, part of the estradiol is aromatized (metabolically converted) into androgens.    And Nyborg believes that it is the new availability of androgens to the brain that results in the marked improvement of visuospatial skill following treatment.    As for Klinefelter patients,  the most frequently occurring hormonal imbalance consists of testosterone insufficiency.   Many Klinefelter patients have now received testosterone replacement therapy from an early age on.   There have been several reports of the long term effects of such replacement therapy.  Overall,  the results are favorable:   academic performance and personality seem to both benefit.  The effect is obtained even in cases where the replacement therapy is initiated after puberty.  In short,  it appears that  the specific cognitive deficits in these two gonosonal syndromes,  Turner’s and Klinefelter’s syndromes,   are primarily due to hormonal deficiencies occurring post-natally rather than to prenatal decelerations of development of one or the other brain hemisphere.