Chapter 12: Sex: Evolutionary, Hormonal & Neural Bases

Sex: Evolutionary, Hormonal & Neural Bases begins by outlining the four major stages of reproductive behavior—sexual attraction, appetitive behavior, copulation, and postcopulatory behavior—and examines how sexual selection shapes traits to maximize reproductive fitness. The text explores the physiological regulation of mating, contrasting the activational effects of hormones in adulthood with the organizational effects that permanently sculpt the nervous system during critical developmental periods. Significant attention is given to the neural circuitry regulating sexual behavior in rodent models, detailing how the ventromedial hypothalamus (VMH) controls the female lordosis response and how the medial preoptic area (mPOA) and pheromonal input from the vomeronasal organ govern male mounting behaviors. The discussion moves to sexual differentiation, explaining how the SRY gene on the Y chromosome initiates a cascade of events involving the Wolffian and Mullerian duct systems to determine gonadal and genital sex. The chapter further analyzes the aromatization hypothesis, describing how testosterone is converted into estradiol to masculinize the developing brain in many rodents, protected by alpha-fetoprotein. Human sexual development is explored through clinical conditions that decouple genetic sex from phenotypic appearance, such as Androgen Insensitivity Syndrome (AIS), Congenital Adrenal Hyperplasia (CAH), and 5-alpha-reductase deficiency. Finally, the chapter addresses the biological origins of sexual orientation and gender identity, reviewing evidence from neuroanatomical structures like the sexually dimorphic nucleus of the preoptic area (SDN-POA) and the interstitial nuclei of the anterior hypothalamus (INAH-3), as well as biological markers including digit ratios and the fraternal birth order effect.