Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.
The results of the present work indicated a different organization within the human brain with respect to relation of testosterone to skilled movement compared to hand preference. Namely, the serum testosterone was found to be associated mainly with right-hand skill in men and women. However, there were fundamental differences in the male and female patterns. Except for male right-handers with right-eye preference, the right hand skill was found to be directly related to serum testosterone in men, and inversely related to serum testosterone in women. In other words, the right-hand skill increased with serum testosterone in men, which contrasts with the Geschwind-Behan hypothesis. In women, the right-hand skill decreased with serum testosterone in accord with the Geschwind-Behan hypothesis. It is, however, interesting that mainly the left-cerebral hemisphere is involved in testosterone actions, except for men with right-hand and right-eye preference exhibiting involvement of both cerebral hemispheres. As expected form the above presented results, there was a direct relationship between the difference in skill between hands and serum testosterone in men. That is, as testosterone increased, the right-hand skill with testosterone causes an increase in L-R hand skill depending upon improvement in the left hemisphere functions for skilled movements with testosterone. In contrast, as testosterone increased, the difference in skill between hands decreased in women. This result was also expected, since right-hand skill deceased with testosterone in these subjects. Thus, testosterone seems to exert a negative effect on the left cerebral hemisphere for skilled movements. However, these effects were found to be also dependent on eye and foot preferences. Here again, men and women showed different patterns. In right-handed men with right-eye preference, the difference in skill between hands was found to be directly related to serum testosterone, but there was not a significant relationship between these parameters in women with right-eye preference. On the contrary, there was not a significant correlation between serum testosterone and L-R hand skill in men with right-foot preference, but there was a marginally significant inverse correlation between these two parameters in women with right-foot preference. In summary, the results of the present work showed that serum testosterone levels are associated with hand skill in right-handed young adults without FS. There were, however, fundamental sex-related differences in this organization, which were also associated with eye and foot preferences. Testosterone seems to be advantageous for the male brain, but detrimental for the female brain with regard to skilled movements. Moreover, testosterone seems to create a more asymmetrical brain in men, and a more symmetrical brain in women. (Tan, U. (1990) Testosterone and hand skill in right-handed men and women. International Journal of Neuroscience 53: 188-9)