Estrogen is a hormone that plays a major role in the development and daily function of both female and male bodies—that’s correct, male bodies as well as female bodies. (Estrogen may have more to do with creating male aggression than testosterone, a finding that helps squash the cultural myth that testosterone drives male behavior and estrogen drives female behavior.) In women, estrogen is responsible for female sexual characteristics, including breast development, fat distribution in the hips, legs, and breasts, and the development of reproductive organs. Estrogen plays a major role in how female reproductive organs function, in how the breasts function, in the changes that occur in the endometrium of the uterus during the menstrual cycle, in ovulation, in supporting the epithelial mucosa cells of the vagina and vulva, in the development and maintenance of healthy bones, in the production of collagen, and in helping lower the risk of coronary artery disease. As for estrogen in men’s bodies, men have estrogen receptors in their brains, penis, and balls. Men wouldn’t be able to get erections without estrogen, and it is estrogen rather than testosterone that causes the masculinization of the male infant’s brain during pregnancy. Estrogen and testosterone are so similar that testosterone can easily be converted into estrogen, and we are just beginning to learn how estrogen and testosterone might influence human behavior.