It is not hormone levels, according to Carlson (2007) that affect sexual orientation. Hormone levels in heterosexual and homosexual individuals have been found to be similar. However, instead, it is exposure to hormones prenatally may be the cause.
Prenatal exposure to androgens is believed to have an effect on the structure of the brain, which in turn affects sexual orientation. Kruijver and Swaab (2002) and Motluck (2003) both found significant differences in three subregions of the brain when examining deceased heterosexual men, heterosexual women, and homosexual men. Both pieces of research show that the suprachiasmatic nucleus is larger in homosexual men than it is in heterosexual men or women. In contrast, heterosexual women and homosexual men have a smaller sexually dimorphic nucleus of the hypothalmus, when compared to heterosexual men. The third subregion both sets of research noted differences in was the anterior commissure. Interestingly, in homosexual men and heterosexual women, the anterior commissure was found to be larger than that of heterosexual males.
The Interaction between Hormones, the Body, and Behavior, Including Sex
Hormones are critical to not only the sexual development, sexual maturation, and body development, as discussed earlier, but they also have an important role to play when it comes to behavior, including sex. Once again, it is hormones that once the gender is determined by chromosomes that instructs the body on how to proceed to produce sexual dimorphism in a species. Internal and external sexual organs are all determined by hormones. As also discussed, it is hormones that turn on and guide the sexual maturation process. Although both genders produce a small amount of the other's hormone, it is the gender-specific hormone that creates the masculinity of males and the femininity of females. It's not surprising that it is hormones that also control behavior, including sex.
Carlson (2007) discusses the three categories of male mammal sexual behavior -- intromission, pelvic thrusting, and ejaculation. Each of these behaviors is affected by hormones. Oxytocin facilitates orgasm. Prolactin is the hormone that affects the refractory period in males. The hormones of progesterone and estradiol, according to Carlson, as well as Agmo, Choleris, Kavaliers, Pfaff, and Ogawa (2008), affect a females receptivity and proceptivity to sex, as well as their attractiveness to males.
Impact of Environment on Sexual Orientation
The debate of nurture vs. nature when it comes to sexual orientation has waged for decades. Quoting previous research by Bell, Weinberg, and Hammersmith, Carlson (2007) notes that their research did not find any correlation between an individual's environment and their sexual orientation. Coghlan (2008) confirms these findings with his study that found that homosexual men had symmetric brains, like heterosexual women, and homosexual women had asymmetrical brains, like heterosexual men. As research into this topic continues, more and more studies support the fact that environment has very little, if anything at all, with sexual orientation. Instead, it is the physiological differences in the brain that are present from birth.
Sexual development and orientation are fascinating areas of study, as they effect every single living being on the face of the planet. From the moment the two strands of DNA combine to form a new identity and determine the gender of a new life, sexual development begins. Hormones are the instigator, organizer and conductor of the process. From the development of physical organs to the catalyst for sexual behavior, it is hormones that are behind it all. Even sexual orientation may be affected by hormones. Rather than environmental factors, as some may believe, instead hormonal effects on brain structure development may actually be the key to understanding sexual orientation.
Agmo, A., Choleris, E., Kavaliers, M., Pfaff, D, & Ogawa S. (Feb 2008). Social and sexual incentive properties of estrogen receptor ?, estrogen receptor ?, or oxytocin knockout mice. Genes, Brain & Behavior, 7(1). Retrieved December 9, 2009, from Academice Search Complete database.
Bianchi, N. (2002). Akodon sex reversed females: The never ending story. Cytogenetic and Genome Research, 96(1-4). Retrieved December 9, 2009, from MEDLINE database.
Carlson, N.R. (2007). Physiology of behavior, 9th edition. Upper Saddle River, NJ: Pearson.
Coghlan, A. (20 Dec 2008). Born gay? New Scientist Archive, 200(2687/2688). Retrieved December 9, 2009, from New Scientist Archive database.
Kruijver, F. & Swaab, D. (2002). Sex hormone receptors are…