The most recently evolved part of our brain, and the last layer of MacLean’s triune brain, is the neomammalian brain. Composed of the neocortex, it is the thin rind covering the brain’s outermost wrinkled layer. Two features that distinguish Homo sapiens from other animals are the extent of the neocortex and the brain’s outsized frontal lobes. Together, they are primarily responsible for language, foresight, reason, judgment, and delayed gratification.
Again, myelination begins from the bottom up, coating first the neurons of the reptilian brain, essentially completing this phase within the first months of life. The spinal cord and brain stem are the first parts of the brain to receive their coats, beginning at twenty-two weeks in utero. The paleomammalian brain is next, as myelination proceeds from back to front, a process mostly completed by the end of the first five years. The neurons in the visual cortex, residing in the occipital lobes at the rear of the brain, receive their insulation first; then the process moves forward to the temporal and parietal lobes. The frontal lobes, along with a few other, highly specialized neocortical patches, are the last to be completely myelinated.14
A subtle differential exists between the myelination of the right and left sides of the brain. In utero, the visio-spatial right hemisphere develops earlier than the logical-language left one.15 A more recent evolutionary addition, the highly specialized left hemisphere undergoes its major growth spurt beginning in the ninth month of pregnancy and continues this accelerated phase through the first four years.16
The most highly specialized and dichotomous brain belongs to a right-handed heterosexual male. Over 97 percent of the language centers of this subset are located in these men’s left hemispheres. This extreme skewing of brain function is not present in the brains of either right- or left-handed females, gays and lesbians, or left-handers of either sex. All three of these categories have their language centers (and other cortical functions) more evenly distributed between the two hemispheres.
In right-handed people in general and right-handed heterosexual males in particular, the left frontal lobe is the seat of the Executive Function. Accepted as a useful concept by many neuroscientists, the Executor is the final assembly location that assimilates information from all other areas of the brain, and then evaluates risks, judges outcomes, overrides urges, delays gratification, and makes decisions.* After contemplating multiple risk-benefit scenarios, it chooses among several courses of action. Then the Executor exerts its “will,” instructing the motor centers to carry out its command. Having decided to drink a beer that is located in the refrigerator, for example, the Executor will order certain muscles to twitch in an orderly prescribed sequence, propelling the body toward the kitchen.†
Natural Selection found it advantageous to the species to place an incompletely myelinated left frontal lobe’s Executor in charge of the rapidly growing, gangly body of a teenage boy. Add to this “accident waiting to happen” the fact that the Executor, like the rest of the adolescent boy’s brain, is pickling in a potent brew of testosterone brine.
This sets up a unique situation. Postpubescent boys will assume extraordinarily dangerous risks for the sheer thrill of it. After their left frontal lobes have been completely myelinated in their mid-twenties, they will look back and shake their heads in disbelief, reminiscing about the impulsively foolish, even life-endangering stunts they performed on instructions from their faulty Executors.
From an evolutionary standpoint, however, it was a pubescent boy’s willingness to engage in risky behavior, combined with a young woman’s unique requirement for a constant supply of iron, that played a paramount role in the development of our species. The proximity of a young woman can make a young man behave stupidly. He will try to impress her with his prowess or reckless behavior—attributes that fortuitously put him in harm’s way in the pursuit of iron on-the-hoof.
A girl’s left frontal lobe is also the last part of her brain to myelinate, but she does not depend on it as much as boys do. Nor is she being driven as mad by hormones. A female’s greatest risk revolves around getting pregnant. If her Executor fails, there remains the safety cushion of her anovulatory cycles. The greatest threat to a young ancestral woman’s life was a premature pregnancy; the greatest threat to an ancestral adolescent boy was premature injury or death when hunting. The two great themes of the earliest art preserved on cave walls and chiseled in stone are hunting and fertility. Iron and sex bind the two tightly together.
Once puberty begins, male testosterone levels soar. Thoughts of sex are never far from a young male’s mind.
Chapter 12
Premenstrual Tension/
Masturbatory Tension
Males act as kind of a genetic sieve: Only the best males get to breed and the reproductive extinction of bad males constantly purges bad genes from the population. From time to time it has been suggested that that this is the purpose of males, but that commits the fallacy of assuming evolution designs what is best for the species.
—Matt Ridley1
We come to blows, I think, because sex is the only behavior that to be fully satisfying requires another person. You can find food and eat alone, you can sleep, think, and play alone. But to have a decent sexual interaction you need the enthusiastic participation of another person. And this is the problem.
—Meredith Small2
The two traits paired in the chapter title appear to be uniquely human. No other animal manifests either of them to the degree to which they crop up in our species. From their observable behavior, the other thirty menstruating primate species do not appear to experience the symptoms women have come to associate with the onset of their periods. Nor does any other male animal masturbate as often or as intensely as a human. To appreciate why these two rooted and then flourished in our genome, an understanding of three physiological concepts—internal milieu, homeostasis, and stress—is necessary.
In the late nineteenth century, French physician Claude Bernard proposed the existence of an invisible field within the body that he called the “internal milieu.” Influenced by physicist Michael Faraday’s earlier discovery of the insensate electromagnetic field, Bernard broke with previous thinkers who had compared the human body to a machine. He proposed that within all the tissues of each organism there exists an optimal state the organism strives to maintain. Throughout the human body, all systems tend toward stability.
For example, the concentration of sodium ions (determining the blood’s saltiness) circulating in the big toe is essentially the same as that making the loop around the skull. The body seems to know what is going on in its far-flung corners and deep internal recesses, and it manages to titrate the levels of its vital components, keeping them all within a narrow optimal range. Homeostasis is the process by which the body regulates and maintains its delicate internal equilibrium through a continual series of feedback loops. An organism experiences stress whenever some factor disturbs its internal milieu’s homeostasis. Stress comes in two varieties: acute and chronic. Acute stress is sudden and precipitous. Chronic stress intrudes over an indefinite period of time.
The human response to acute stress is mediated by the reptilian part of the brain. It is involuntary, immediate, and instinctual, and in all but the most extreme cases, homeostasis is quickly restored. Chronic stress results when the factor in question disturbs homeostasis for a longer period and cannot be ameliorated by reflexive instinctual measures. The higher brain centers must respond by developing a strategy to thwart its source.
Seemingly minor changes in the internal milieu can ruffle homeostasis. Sensors scattered throughout the body identify which particular component of the internal milieu is disturbing the peace, and they, in turn, send out a cascade of nerve impulses and hormones, forcing the organism to take action. The example of water regulation is both apt and familiar, since water is the most common constituent of the human body.
Within the sealed waterproof bag we call our skin, feedback sensors positioned strategically around the body monitor
water levels in the blood as it filters past. Whenever water loss exceeds intake—for example, through excessive perspiring—water levels in the blood fall, the blood becomes thicker, and the symptoms of dehydration appear.
Warned by the sensors of the problem, homeostatic regulators immediately institute safety measures: The kidneys begin to conserve water, causing concentrated urine to appear darker; sweating diminishes; and the brain secretes hormones that parch the throat, creating the sensation of thirst. Overseeing the movements of the body from its perch in the frontal lobes, the Executor advances thirst to the top priority slot on its to-do list.
A thirsty person will then interrupt whatever he or she is doing and take the necessary steps to locate a water source. Continued concentration on the task at hand becomes increasingly difficult until it approaches impossibility. If something prevents the person from drinking, a condition of acute stress is activated. Internally, red lights flash, sirens sound, and every bodily system is marshaled toward achieving a single goal.
Once water has been imbibed, the stomach absorbs it and transfers it into the bloodstream. The water level in the internal milieu begins to rise, resembling a filling reservoir. When fluid levels return to normal, the water sensors deactivate.
Much like the shipping lanes of a vital port, the internal milieu is a busy place. An organism must consume a constant stream of raw materials to build, repair, and maintain the integrity of its tissues. Every twenty-four hours, the pancreatic cells that secrete the highly corrosive digestive enzymes die and must be replaced. Essentially, the body must manufacture a new secretory pancreas every day. The cells lining the stomach last but three days, and most white blood cells live for only five days.* Homeostasis is an incredibly dynamic process.
Defending the integrity of the internal milieu is an organism’s highest priority. The process called life is a ceaseless struggle against the dour second law of thermodynamics. Also known as the “principle of entropy,” this adamantine rule states that all things in the universe tend to run down and decay. In our world, the arrow of time points in only one direction, and a system that begins with a high degree of order, such as a brand-new car, will, with the passage of time, pass to a condition of less order. New cars inevitably become clunkers, demonstrating the harsh truth of the second law.
Just as there are mechanisms that drive an animal to quench its thirst and satisfy hunger, there are those that drive a creature to correct more subtle imbalances. Iron loss will result in a desire for foodstuffs high in iron. When a battered Spartan warrior returned home after battle, friends and family greeted him with a goblet of red wine containing a rusting iron nail. Apparently they understood, as did many ancient peoples, the relationship between blood, iron, and vitality.
The most extreme example of the homeostatic mechanism controlling iron is the strange medical syndrome of “pica,” the overwhelming craving to eat dirt. In many malnourished cultures, physicians have observed people, mostly children and women, eating handfuls of soil. The most plausible explanation for this anomaly is a diet deficient in the elements commonly found in soil, notably iron. It shows how important the body considers iron homeostasis that, in a desperate attempt to correct a deficiency, humans will resort to such a drastic measure.*
Though the components of the internal milieu are basically the same for men and women, several are more specific to one sex than the other. Women need significantly more iron than men do; men have incredibly high testosterone levels, which create intolerable sexual tensions demanding release. Although at first glance seemingly unrelated, the disequilibrium of these two features of human sexuality conveniently complement each other.
Many peoples across the globe have a distinct word in their vocabularies for “meat hunger.”3 (Other components of animal fats and proteins are beneficial to humans, but for the moment I wish to concentrate on the iron in meat.) A pregnant or lactating woman raising small children has evolved instincts that use male desire so that she can be assured of a reliable iron supply during her reproductive years. These instincts are activated at the onset of menarche, long before her health and the well-being of her offspring depend on that supply. Many other examples exist in nature of creatures, from ants to squirrels, that lay away supplies in good times to tide them through the lean ones.
This coincides with the period in a woman’s life prior to motherhood, when she is the most sexually desirable to most males. And so we return to another evolutionary benefit of the adolescent girl’s anovulatory cycle. The slow but steady blood loss caused by early menstrual cycles stimulates in her a desire to eat meat. She now has an incentive that did not exist before menarche to set about learning how to acquire iron-rich foods at the least risk to her and her future children’s safety. This knowledge will serve her well when pregnancy, delivery, and lactation create enormous demands on her internal mineral reserves.
By the time a woman manifests an iron-deficiency anemia, her iron stores are already exhausted. Maintaining sufficient reserves is critical to keeping her vital organs, especially her brain, fueled with an unvarying oxygen supply. Individual iron atoms come and go to the warehouses in the liver and bone marrow far more often in a woman than in a man.
Though Gyna sapiens does not experience estrus in the nonhuman-primate sense of the word, a faint imprint of her once exuberant sexual heritage persists. The symphony of hormones secreted during her monthly ovulatory cycles reaches a crescendo just prior to the moment of her ovulation, increasing her libido to its maximum. Other physiological changes serve to increase the likelihood that she will engage in sexual intercourse. Her body temperature rises by one degree, causing her to experience excessive warmth and remove more clothing, thus exposing more skin. In an attempt to reduce her body’s temperature further, her skin becomes ever so slightly flushed.4 These extremely subtle physiological changes tend to attract all males within visual range, even though they may not be consciously aware of what attracted them.
A woman becomes more restless during the time surrounding her ovulation. She travels farther and to more unfamiliar places. She is more willing to take risks, and to engage strange males in conversation.5 In one study, ovulating females’ taste in males tended more to the rugged, jut-jawed he-man; during the rest of their cycle, they gravitated toward softer, less masculine-looking men.6 This change in preferences, however, can also be seen as a paradox, because it has been abundantly demonstrated in numerous studies that high testosterone levels are antithetical to parenting behavior: Macho males are less likely to remain faithful or help with children.7 In another study, American women at midcycle entering a discothèque wore more jewelry and makeup and were more often touched by men than those at other points in their cycle.8 These behaviors make good evolutionary sense and are echoes of the archaic hormonal tug of primate estrus.
But many women experience a second libidinous peak just before and during the height of their period. Penelope Shuttle and Peter Redgrove in their book The Wise Wound refer to this phase of a woman’s cycle as the paramenstrum. This second lusty spike is more difficult to explain in terms of evolutionary theory, since sexual desire in other animals coincides with ovulation. In no other animal have females been observed to manifest a surge of sexual desire during a time of nonovulation. This feature of female human sexuality would most likely not have advanced unless it possessed an adaptive advantage. Why would a woman desire sex when she is least likely to conceive? What is the evolutionary payoff?
Therese Benedek, a Jungian analyst, teamed up with clinician Boris Rubinstein to study the differences between the dreams of women during these two antipodal spikes in monthly libido.9 On the nights on either side of ovulation, women reported sexual dreams in which they felt open and receptive. Romantic feelings welled up, and, in general, the subjects commonly reported a sense of goodwill toward males. A few women prone to masochistic fantasies reported their increased incidence at midcycle.
In contrast, during the paramenstrum, the subjects
described very different libidinal emotions. They felt more dominant and often initiated the sexual act. Many tended to regard men in a more negative light. Their dreams and fantasies, if not overtly sadistic, were more controlling, and were, in rare cases, tinged with cruelty.* Why would a woman be dangerous and antagonistic to a man, yet sexually aggressive at the same time? The answer, I believe, lies in her homeostatic response to her need for iron.
A major component of a woman’s libido is the relative concentrations of several quite different hormones, one of which is testosterone, a molecule that induces aggressive and libidinous behavior. A woman produces this androgenic substance in her ovaries, adrenals, and fat cells. Her testosterone peaks at ovulation at midcycle, and its level is about the same as those of both estrogen and progesterone. The precipitous fall in estrogen and progesterone, however, during a woman’s period allows the level of testosterone to exceed the levels of the other two hormones significantly for a few days. This changing balance serves a woman well, for she is about to lose a small but incrementally important amount of blood.
Approaching a man provocatively during her menses, a time when sexual intercourse is least appealing to a male, reverses for a few days each month the natural relationship between Homo and Gyna sapiens. Woman assumes the persona of the fierce hunter, and man the submissive gatherer.
The human male’s sexual drive is—for most men, particularly young men—stuck in permanent overdrive. Mr. Ever-Ready is not known to be particularly discriminating in his constant quest to alleviate his lust. For a young man, the prospect of engaging in sex that is ragged with risk appeals to his incompletely myelinated left frontal lobe. For him, sex with a menstruating woman is dangerous.
Sex, Time, and Power Page 19