The grasp reflex from the right and left hand in human neonates indicates that the development of both cerebral hemispheres in males, but only the right hemisphere in females, is favoured by testosterone.

The grasp reflex was studied in relation to serum free testosterone levels in human neonates. In the total and -FS (no familial sinistrality) males, the grasp reflex (especially that from the right hand) significantly increased as testosterone increased. In the total and -FS females, there was a significant positive linear correlation between testosterone and the grasp-reflex strength from the left hand, but not from the right hand. In +FS males, there was a significant negative correlation between the grasp-reflex strength from the left hand and testosterone, but not from the right hand; there were no significant correlations in the +FS females. The right minus left (R - L) grasp-reflex strength linearly increased with testosterone in females with normal testosterone levels, but linearly decreased in females with high testosterone levels. The log R - L grasp reflex increased with testosterone in males. It was concluded that testosterone may be beneficial for the development of the left brain in -FS males and the left brain in -FS females, but may have detrimental effects on the right brain in +FS males. The growth-promoting asymmetric effects of testosterone on the brain may depend upon genetic organization of the brain.