How Testosterone Affects the Brain

We last met the steroid hormone testosterone in fish and birds where it was rendering them more vulnerable to parasites by exaggerating their sexual ornaments. In recent years more and more evidence has been found that testosterone affects not just ornaments and bodies but also brains. Testosterone is an ancient chemical. found in much the same form throughout the vertebrates. Its concentration determines aggressiveness so exactly that in birds with reversed sex roles, such as phalaropes and ...