he recent controversy over the South African runner Caster Semenya's gender illustrates the complexity of how sex is assigned in humans. Experts must decide whether DNA, genitalia, or hormones should serve as the determining characteristic. Although there are cases of genetically XX females with male genitalia and vice versa, the three sex identifiers are aligned in most people. This is because, in humans and most mammals, genetic sex (i.e., whether you are XX or XY) controls development of a testis or ovary during fetal life, and all secondary sex characteristics (genitalia, musculature, sex ducts) are controlled by hormones and other secretions from the testis or ovary.1
In many animals, sexual characteristics are quite plastic—even in adult life. In some species of fish, all it takes is a glance, or lack thereof, to cause an adult female to change her sex and become male. When the dominant male goes out of ...
