Endometriosis is driven by relatively low levels of prenatal and postnatal testosterone. Testosterone affects the developing hypothalamic–pituitary–ovarian (HPO) axis (defined below), and at low levels and it can result in an altered trajectory of reproductive and physiological phenotypes that can mediate the symptoms of endometriosis. A phenotype is defined as a person’s observable characteristics resulting from the interaction of their genetic makeup and the environment. Polycystic ovary syndrome, by contrast, is known to be caused primarily by high prenatal and postnatal testosterone, and it demonstrates a set of phenotypes that are diametrically opposite to those found in endometriosis.