Your Brain: The Other Sex Organ

“The brain is a full-fledged sexual organ,” wrote Nicholas Wade in the New York Times Science section (1). Although the not first bodily structure to spring to mind—I can think of a few more prevalent organs—this idea follows logically with what we know about the brain, as well as how the brain relates to other systems within the body. Like the gonads, the brain has an active role in the endocrine system. There are physical differences between the brains of the two sexes, just like the genitalia, which lead to differences in sexual behavior. Our sex behaviors, whether involving the I-function or not, all stem from the brain.

Reduced to its most basic structures, the brain is a gland. The brain secretes hormones that control sexual processes. In both males and females, the pituitary gland releases luteinizing hormone, and follicle stimulating hormone to initiate puberty and start the transition to sexual maturity (2). In females, these hormones regulate the menstrual cycle. The brain has receptors for the sex steroids—progesterone, estrogen, and testosterone—secreted by the gonads, and it will respond to these signals, so the gonads modify the brain and vice versa (3). Even in utero, testosterone, or lack thereof, controls the formation of the brain. The activation of the SRY gene on the Y-chromosome causes the development of the testes, which then release testosterone in the fetus. This surge of testosterone masculinizes the brain, causing the observable differences in the adult brain (4).

The physical differences between male and female brain exist from birth and contribute to all sorts of behaviors throughout life. For example, women have a thicker cortex compared to men, and they also have a proportionally larger hippocampus, which helps form initial memories, compared to men (5). The anterior commissure which connects the left and right hemispheres of the brain is also bigger in women than in men, implying that women have more of a connection between the analytic and language parts of the brain to the emotional and intuitive portions (6). In rats, two different centers that regulate the sexual behavior have been found, the medial preoptic area (MPA) in males and the ventromedial nucleus of the hypothalamus (VMN) in females. Therefore, when the MPA is ablated in male rats they cease to show mounting behaviors, and when the VMN is ablated in females they cease to show lordosis (7). Sexual dimorphisms of the brain influence its function in behaviors, just like the more ‘typical’ sex organs do.

Physical differences in the brain aren’t only between men and women; there are also some dimorphisms between homosexual and heterosexual men. Like women, homosexual men have a larger anterior commissure than heterosexual men, but it is not quite as large as it is in the female brain (6). Homosexual men have a less masculinized brain, but it’s not completely feminine. Perhaps the strongest implication of this observation is that the human brain is wired to be either homosexual or heterosexual. The center for sexual orientation, if it is as simple as one particular area, resides in the brain. The removal of the sexual organs, for example the penis of a heterosexual man, does not automatically make him become homosexual even if this removal happened in infancy (8). A prime example of this is Joan/John, who had his penis amputated in an accident as an infant and was raised as a girl, but even through his childhood he had more of a male sexual identity. The brain has a sexual identity and orientation independent of the presence or absence of certain sexual structures.

Men and women show obvious differences in sexual behavior, and, as is the case with all behaviors, this can be traced back to the brain. Traditionally and historically, men are the ones who seek out sexual partners, while women either accept or reject advances made by a potential partner (1). This could be a possible explanation as to why men show much more strongly defined sexual preferences for either men or women, and women are more amorphous in who they find attractive. Preferences and the power of choice reside in the brain, so when some one points out who they find attractive, their choice processed by the brain. Variations in sex drive also exist between men and women. Men are typically ready to go all the time, with obvious evolutionary benefits, while women’s sex drive bounces all over the place and doesn’t necessarily peak when she is most fertile (1). Why wouldn’t women always have their highest sex drive when they are most fertile, when really they should be more sexually driven when it’s more likely that they will get pregnant? Homosexuality isn’t evolutionarily beneficial, but it still exists, so maybe a woman whose peak sex drive isn’t when she’s most fertile is the same way—no obvious benefit, but exists all the same, most likely mediated by the brain. The brain serves as the first step in the chain of attraction and arousal.

All neurons act through activation and inhibition. Some neurotransmitters activate the neuron, while others inhibit; if there is enough activation the neuron will fire. Sexual arousal operates in much the same way. The Kinsey Institute at the University of Indiana is primary location for this research on the excitation/inhibition of sexual urges. I participated in their on-line study called the sExIn Study (9). The survey consists of a series of statements and you respond to them on a scale of ‘strongly disagree’ to ‘strongly agree.’ Two examples are as follows: “It is easier for me to become aroused with someone who has ‘relationship potential,’” and “When I have a distracting thought, I easily lose my arousal.” These statements and the others in the survey, all ask about situations in which you might become sexually excited or inhibited, to determine how strong your excitatory and inhibitory mechanisms are (8). The variation from person to person is huge; having a strong excitatory impulse does not mean your inhibitory mechanism is also strong. The simplest neuron and something as complicated as sexual arousal function in almost the same way, only one uses electrochemical signaling while the other uses more complicated internal and external inputs, which could also be reduced to chemical signaling. Sexual behavior is rooted in the electrochemical communication used by neurons throughout the body, but specifically the brain because eventually that’s where all the inputs get processed.

What comes first arousal, the sexual physical response, or desire, the conscious wish for some sort of sexual relation? If this question were asked about 60 years ago, the answer would have been desire then arousal, but now the tables have turned. When women are shown any sort of sexual image—homosexual, heterosexual, even between bonobos—they will show physiological arousal responses to all of these images, which showed no correlation to which images they found to be arousing (8). Women are capable of physiological arousal without feeling any sort psychological desire; they can be aroused without the I-function. This is like any sort behavior that we would term as a reflex reaction. A ball is thrown at you, so you try to catch it even though you’re not consciously aware of putting your hand in from of your face. Arousal apparently works in the same manner: mediated by the brain but not to the point where it reaches the I-function.

The brain is a sexual organ. Physically it behaves like other sexual organs—secreting and receiving hormones, showing differences in behavior between men and women—but its involvement goes beyond the physical. Someone’s sexual orientation resides in the brain along with the ability to become aroused or to inhibit arousal, with or without the I-function. If the brain has this much involvement in sexual behaviors to the point where it could be considered a sexual organ, why not think of it as another digestive organ or respiratory organ? Through the nervous system communicating with the rest of the body, the brain mediates all our behaviors, sexual or otherwise. Saying the brain is another sexual organ implies that the brain is an extension off of all our organs, acting as the control center for behavior.

WWW Sources
1. Wade, Nicholas. New York Times. “Pas de Deux of Sexuality Is Written in the Genes.”
2. Grenier, Mary; James Kerrigan. Pediatric Annals. “PUBERTY: Timing Is Everything.”
3. Keefe, David. Archives of Sexual Behavior. “Sex hormones and neural mechanisms.”
4. Gooren. Molecular Cell Endocrinology. “Androgens and male behavior.”
5. Shute, Nancy. U.S. News & World Report. “His Brain, Her Brain.”
6. Thomas, Earl. Lecture, Psychology 218 Behavioral Neuroscience. Bryn Mawr College, 2006. www.brynmawr.edu/psychology
7. Carlson, Neil R. Physiology of Behavior 9th Ed. Boston: Pearson, 2007.
8. Angier, Natalie. New York Times. “Birds Do It. Bees Do It. People Seek the Keys to It.”
9. The Kinsey Institute. Participate in a Study.