Behavior is intricately woven of genetics, environment, and hormones. The interplay of the nervous and endocrine systems and the influence of sex hormones on behavior are examined here in depth. Hormones are intimately tied to sex characteristics and to behavior. The behavioral implications of the presence or lack of certain hormones at critical periods of development is explored. Hormonal influence on female cycles, reproduction, nursing, sexual behavior and sex differences are also examined. The intimate relationship between psychology and physiology becomes apparent.

Sex Hormones, Gender Differences, and Behavior

Organisms monitor the external state of the world and integrate the information they receive with their internal state. It is through the nervous system that humans contact our environment, the external world, and are adapted to function in it (Mani, Blaustein & O’Malley, 1997). Awareness of the tangible, through the network of nerves, spinal cord and brain, becomes awareness of information ceaselessly conveyed. Messages are carried along the millions of nerve pathways to the brain, and are then transformed in some mysterious way into information.

Progesterone is synthesized in the ovary, testes, and adrenal glands from cholesterol and pregnenolone (Barfield, Glaser, Rubin & Etgen, 1984). Women need increasing amounts of progesterone with pregnancy as the uterus prepares for implantation of a fertilized egg (Barfield et al., 1984). Progesterone is involved in the production of many hormones including: cortisol, aldosterone, estradiol, DHEA, testosterone, estriol and estrone as well as androstenedione (Mani et al, 1997). Normal males secrete 1-5 mg of progesterone daily, and the level is only slightly higher in females during the egg releasing stage of the cycle (Mani et al, 1997). During the luteal phase, progesterone amounts of 20-30 mg/day are released into the circulation to signal fertility and preparation for implantation in the stage known as the premenstrual period. This phase of the menstrual cycle occurs from about 10 days to one week before menstruation. This monthly cycle occurs only if the egg does not implant into the uterus (Mani et al, 1997).

Menstruating women start to produce progesterone in their ovaries after the lutenizing hormone (LH) surge around the time of ovulation (Barfield et al., 1984). The progesterone level rises and falls just before the bleeding begins. This premenstrual time is called the progesterone withdrawal phase. For some women this period is felt as an extremely anxious, nervous, "temperamental mood." Some women find it to be an extremely depressing time with tears and feelings of low self-esteem (Schechter, Strasser, Endicott, et al., 1996).

Current research also highlights oxytocin's role in breastfeeding. For example, one study indicates that the secretion of oxytocin is a conditioned response, meaning that a mother's body may produce oxytocin in response to familiar sights, sounds, or activities, not just from the direct stimulation of breastfeeding (Insel & Shapiro, 1992). In a small sample of nursing mothers, all showed an increase in oxytocin before the baby was put to their breast. However, levels of prolactin, the other hormone produced by nursing, did not increase until the baby actually sucked at the breast (Insel & Shapiro, 1992).

The area in the brain that regulates female and male reproductive behavior is the hypothalamus (Mani et al., 1997). This tiny structure at the base of the brain connects to the pituitary, the master endocrine gland. It has been shown that a region of the hypothalamus is visibly larger in male rats than in females and that this size difference is under hormonal control (McEwen et al., 1987). Scientists have also found parallel sex differences in a clump of nerve cells in the human brain, part of the interstitial nucleus of the anterior hypothalamus, that is larger in men than in women (McEwen et al., 1987). Even sexual orientation and gender identity have been related to anatomical variation in the hypothalamus. Scientists have observed another part of the hypothalamus to be smaller in male-to-female transsexuals than in a male control group (Pfaff, 1999). These findings are consistent with suggestions that sexual orientation and gender identity have a significant biological component. Studies have found that hormonal manipulation during the critical period could alter these behaviors (Pfaff, 1999). Depriving newborn males of sex hormones by castrating them or administering hormones to newborn females resulted in a complete reversal of sex-typed behaviors in the adult animals. Treated males behaved like females and treated females, like males (Pfaff, 1999).

Some topics in this essay:
Goy McEwen, Bermont Davidson, Hypothalamus Glands, Hormones Hormones, Jones Pfaff, Rubin Etgen, Influences Behavior, System Interplay, Knobil Neill, Insel Shapiro, et al, sexual behavior, sex hormones, goy mcewen, goy mcewen 1980, mcewen 1980, ledoux 1996, endocrine system, et al 1997, mani et al, mani et, al 1997, pituitary gland, insel shapiro 1992, bermont davidson 1974,