Biological Foundations of Adult Sexual Development
According to Wickens (2005), "Although sexual behavior satisfies no vital tissue need, nor is it necessary for individual survival, from an evolutionary perspective it is crucial, as without it, we would not produce offspring or continue the survival of the species" (p. 202). Hormones play a key role in the biological foundation of adult sexual development and significantly influence gender-specific behavior (Wickens, 2005). Sexual activity is a powerful and complex force in human evolution, and from an evolutionary standpoint, the mechanics of which are deeply embedded into the human psyche (Wickens, 2005). Deciphering the complex biological foundations of human sexuality is an important goal of biological psychology in an effort to promote the understanding of our instinctual and sexual drives and enable the treatment of its dysfunction (Wickens, 2005).
Biological Psychology Processes on Sexual Differentiation
Sex hormones exert two main types of effects: organizational and activational (Wickens, 2005). During early development, hormones determine one's gender and help organize the structure of the brain (Wickens, 2005). In the womb, the secretion of specific hormones produces the reproductive organs and also plays a role in organizing the nervous system for male or female behavior and later adult sexual development (Wickens, 2005). Both prenatal and neonatal organizational effects of gender hormones are a factor in determining differences in the cognitive and perspective abilities for adult performance (Halari et al., 2005). The neural circuits that govern sexual responses are organized early in development, although sexual behavior is not produced until later in development (Wickens, 2005).
Regarding the structural organization of the gendered brain, adult females have a larger anterior commissure (that connects the right and left temporal lobe) and corpus callosum than do males, which may indicate that the two cerebral hemispheres are better connected in women (Wickens, 2005). This may also explain the gendered differences in cognitive skills between males and females (Halari et al., 2005, Wickens, 2005). In males, there is a larger bed nucleus of the stria terminalis, which connects the amygdala with the hypothalamus (Wickens, 2005). Although implication is only theorized in sexual differentiation, it is assumed that the hypothalamus and amygdala may play significant roles in sexual behavior (Wickens, 2005). In the preoptic area of the hypothalamus, two of the nuclei are much larger in males than females and in homosexual men, which may indicate an influence on sexual orientation (Wickens, 2005). Another brain part near the preoptic part of the hypothalamus that may be implicated in homosexual identification is the suprachiasmatic nucleus, which is involved in the maintenance of circadian rhythms and is larger in homosexual men than heterosexual males and females (Wickens, 2005).
Key Biological Changes in Sexual Development
Apart from the obvious differences in gendered anatomy, males and females are similar at birth and until puberty, when secondary sexual characteristics begin to develop (Wickens, 2005). At this time the testes and ovaries are controlled by the hypothalamus and pituitary gland (Wickens, 2005). Sex hormones are released by the hypothalamus, which causes menstruation in females, and in males, causes the testes to produce testosterone (Wickens, 2005). During puberty, hormones play a more activational role producing specific gender-related behaviors (Halari et al., 2005) with testosterone producing male characteristics and estrogen and progesterone producing female characteristics (Wickens, 2005). At this stage of sexual development, the adolescent develops into a fully reproductive adult (Wickens, 2005). Both sexes experience growth in height and males exhibit maturation of the testes, testosterone production, an increase in skeletal and muscle mass, the growth of body and pubic hair, deepening voice, and the capability to ejaculate (Wickens, 2005). In females, sex hormones produce the fuller female body shape, breasts, wider hips, menstruation, and the ability to ovulate (Wickens, 2005).
Hormones and Their Relationship to Behavior
Regarding the effect of hormones on behavior, Wickens (2005) states that it is simplistic to equate testosterone with aggression, but easier to link it with "success or failure in competition, and in the pursuit of dominance [and] aggressiveness" (p. 219). Wickens (2005) also describes a study in which testosterone was injected into less aggressive hens. The injection of the hormone increased their aggressive behavior, they crowed (not typical behavior for hens) and some even began courting other hens (Wickens, 2005). Other hormones affect sexual activity, thoughts, and fantasies in men given testosterone, and the association of higher testosterone levels has been linked to dominance and success in competitions (Wickens, 2005).
In relation to female hormones and their effect on behavior, according to Sherwin (2008), 90 percent of estrogen is produced in the ovaries, and the ovarian stroma produce one third of the testosterone. Sherwin's (2008) finding were the first from controlled studies to demonstrate the importance of testosterone in affecting women's sexual desire, as much as it affects the same in men. Conventional wisdom has long understood that sex hormones affect women's moods (Sherwin, 2008). Physiological fluctuations in hormone levels during pregnancy, menstruation, and menopause often cause changes in behavior ranging from mild to extreme symptoms, such as irritability, anger, and even psychosis (Sherwin, 2008). Although medical science has been fairly entrenched in the notion that estrogen levels determined such behavior, evidence suggests that a combination of hormones affects the production of serotonin, which elevates mood (Sherwin, 2008). In later studies, Sherwin (2008) determined that estrogen, although not testosterone nor progesterone was "causally related to improvement in memory performance" (p. 112), and played a significant role in maintaining verbal and working memory in both young and older, postmenopausal women.
The endocrine system and its hormonal effect in the human body plays a significant role in the quality of life, and the sex hormones play an even more significant role in the determination of gender and its subsequent set of behaviors and characteristics (Wickens, 2005). Both organizational and activational characteristics of sex hormones develop and enhance gender specific behavior and contribute to lifelong tendencies (Wickens, 2005). Similar to neurotransmitters, miniscule amounts of hormones radically alter our ability to function according to our sexual determination, and dictate our quality of life, health, well-being, and even cognitive function (Sherwin, 2008). Hormones play a primary role in the development and activation of specific gender-related behaviors, although Wickens (2005), states the brain and its highly complex neural circuitry is as crucial for all aspects of sexual behavior. This stunning and complex combination of the hormonal and nervous systems create the usually unambiguous gendered nature of humans.
Halari, R., Hines, M., Kumari, V., Mehrotra, R., Wheeler, M., Ng, V., & Sharma, T. (2005). Sex Differences and Individual Differences in Cognitive Performance and Their Relationship to Endogenous Gonadal Hormones and Gonadotropins. Behavioral Neuroscience, 119(1), 104-117. doi: 10.1037/0735-7044.119.1.104
Sherwin, B. B. (2008). Hormones, the brain, and me. Canadian Psychology/Psychologie canadienne, 49(1), 42-48. doi: 10.1037/0708-5522.214.171.124
Wickens , A.P. (2005). Foundations of Biopsychology (2nd ed.). New York: Pearson/Prentice Hall