Women's pleasures, women's pains: Oxytocin, orgasm, and endometriosis

The capacity for health and disease, and pleasure and pain, has an evolutionary legacy. Human females demonstrate a suite of unique as well as elaborated reproductive traits, such as invasive placentation, copious menstruation, and highly developed social cognition and cooperative breeding. These phenotypes involve coordinated psychological and physiological components that develop and express via environmentally sensitive neuroendocrine mechanisms, such as the oxytocin system and its interactions with gonadal hormones. An evolutionary medicine framework illuminates why these reproductive phenotypes are vulnerable to dysregulation, generating patterns of disease risk in women. I apply principles of evolutionary biology and evolutionary medicine to investigate female reproduction and sexual response as related to health and disease across four chapters. First, I review and synthesize several disparate bodies of literature to evaluate the hypothesis that elevated oxytocinergic activity jointly contributes to bipolar disorder and endometriosis, and their apparent comorbidity, in women. Second, I propose and test the hypothesis that endometriosis and polycystic ovary syndrome (PCOS) are diametric disorders of female hypothalamic-pituitary-gonadal axis development and functioning, with risk strongly mediated by opposite and extreme levels of testosterone. Diverse lines of evidence from endocrinology, developmental biology, epidemiology, reproductive physiology, and morphology confirm that endometriosis and PCOS involve opposite risk factors, symptoms, and correlates. Third, I collect and analyze data from women with endometriosis or PCOS to examine cognitive empathy ability in relationship to reproductive disorder status, pain levels, and medication usage. Finally, I shift my focus from disease to health, investigating the role of oxytocin-mediated interrelationships among coitus, birth, and lactation in the tradition of the late biologist, Niles Newton, to provide insights into women's capacity for extensive sexual pleasure. These findings offer a novel and unifying explanation of the causes of endometriosis, as well as a fresh and generative perspective concerning the origins and functions of female orgasm - and sexual pleasure - more broadly. Exploring how evolutionary dynamics shape interconnected psychological and physiological aspects of female reproduction increases our understanding of women's vulnerability to disease as well as women's capacity for health and wellbeing.