In a series of influential papers Geschwind, Galaburda, and their colleagues proposed that excessively high levels of pre-natal foetal testosterone, a male sex hormone circulating during early maturation of the brain but present also in female foetuses in much smaller amounts, are associated with increased rates of left-handedness, and dyslexia. This was intended to explain not only an association between dyslexia and non-right handedness, but also the higher incidence of dyslexia and other learning difficulties in males than females. Furthermore, by acting on the thymus gland, involved in responses of the immune system, high levels of testosterone would account for a reported association between dyslexia, left-handedness and immune disease. According to the most recent version of this hypothesis, the left planum is usually larger than the right because of greater prenatal pruning of cortical connections on the right side. In the presence of "too much" testosterone, this process is arrested leading to symmetry of the plana on the two sides (Geschwind & Galaburda, 1985).
The hypothesis (known as the Geschwind-Behan-Galaburda or GBG hypothesis) generated a huge amount of research which was reviewed comprehensively by Bryden, McManus, and Bulman-Fleming (1994). It was not until 1995, however, that anyone was able to test the theory directly. Grimshaw, Bryden, and Finegan (1995) measured testosterone levels in the amniotic fluid of mothers undergoing amniocentesis at 16 gestational weeks. The levels of testosterone are assumed to reflect largely though not exclusively foetal rather than maternal levels of the hormone. The results were contrary to the predictions of the GBG hypothesis. When the children of the pregnancies which went to full term were tested at the age of 10 years, high levels of foetal testosterone were found to be associated with stronger right-handedness and stronger left lateralisation of language (measured by a non-invasive technique known as dichotic listening) than low levels of testosterone.
Although there is some research linking testosterone from the mother's umbilical artery to infant patterns of handedness (Tan & Tan, 1999, 2001) and testosterone levels in adult saliva to handedness (Moffat & Hampson, 1996), I know of no findings directly linking testosterone to dyslexia. Interestingly, however, testosterone has been linked (Moffat, Hampson, Wickett, Vernon, & Lee, 1997) to the size of another brain structure which some have seen as having a role to play in dyslexic symptomatology. This structure is the corpus callosum.
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