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All you need is love: brain sex and the mating game.

The individual most responsible for legitimating the modern distinction between sex and gender was the anthropologist Margaret Mead. Based on studies of native people in Samoa, Mead argued that the enormous variability of male and female behavior suggests that innate, or biologically predetermined, behaviors are almost nonexistent in our species. She then concluded that the fundamental determinant of gender identity is not nature, or what we are as a result of biological inheritance, but nurture, or what we are as a result of the socialization process. "We may safely say," wrote Mead, "that many if not all of the personality traits which we call masculine and feminine are as lightly linked to sex as are the clothing, the manners and the form of headdress that a society at a given period assigns to sex."(1)

The Mead doctrine occasioned a revolution in our thinking about gender identity for the same reasons that the theories of Copernicus, Darwin, and Einstein occasioned revolutions in thought. It was derived in accordance with research methodologies and rules of evidence designed to produce objective and value-free knowledge. Although a large body of research on sex-specific behavior that could not be explained by learning per se began to accumulate not long after the doctrine was formulated, this evidence appeared "soft" in the absence of biological explanations. We now know that the biological factors that contribute to these behaviors are differing levels of sex hormones and sex-specific differences in the human brain.


Until recently, much of the knowledge about the sex-specific human brain was derived from postmortem dissections, and virtually nothing was known about the relationship between sex-specific anatomical differences and actual brain function. During the last two decades, however, studies in neuroscience have shown that these differences condition a wide range of human behavior. Brain science has also provided bold new insights into an aspect of our lives where the attempt to ignore or transcend gender differences occasions the most confusion and conflict--romantic love relationships.

There are chemicals in our brains, called neurotransmitters, that make the experience of eros far more universal than we previously imagined. Normally produced at various stages in love relationships, these chemicals occasion similar emotional and physical states in all human beings. Mind- or mood-altering drugs have molecular structures that resemble those of neurotransmitters. For example, cocaine resembles dopamine, acts on the dopamine receptors, and tricks the brain into operating as if enormously high levels of this neurotransmitter were present. Similarly, valium reduces anxiety by augmenting the effects of GABA, and Prozac alleviates depression by enhancing the action of serotonin.

Given the destructive influence of artificial substances that induce altered states, why did mutations that produce these states survive in the gene pool? The answer is that the powerful neurotransmitters associated with being in love enhanced the prospect of mating and of successfully rearing children. And as anyone who has been in love knows firsthand, these love potents propel us well out of the range of normative emotional responses.

Studies of the initial stages of "being in love" indicate that the love object typically becomes the center of the individual's universe, and that even the most mundane and trivial characteristics of the magical other are a source of utter fascination. A large number of respondents in one study said that their thoughts and feelings were fixated on the love object from 85 to almost 100 percent of the time. In the presence of the love object, both men and women said they trembled, felt flushed, stammered, and feared losing control over basic faculties and skills. There was also common agreement about the primary reward for this confusion--95 percent of the males and 91 percent of the females indicated that the best thing about being in love was sex.(2)

The principal neurotransmitter contributing to these behaviors is an excitant amine called phenylethylamine, or PEA. This endogenous amphetamine, or speed, saturates the brain when we fall in love, and generates feelings of elation and euphoria. When lovers are giddy, absent-minded, optimistic, gregarious, wonderfully alive, and full of extraordinary energy, they are riding a natural high that results from the action of PEA and possibly two other natural amphetamines--dopamine and norepinephrine. A brain flooded with PEA can override the impulse to sleep and allow lovers to dance the night away in both figurative and literal terms.

People with low levels of PEA are often romance junkies literally "addicted to love." But this is an abnormality.(3) The function of the PEA high in evolutionary terms is to promote mating and the transmission of genes to subsequent generations. After this is accomplished, it is not evolutionarily advantageous to remain in an altered state that could threaten survival. This explains why the brains of most people can sustain high levels of PEA for only about two to three years.(4)

However, it would not be evolutionarily advantageous for parents who must care for children well into the teenage years to terminate their relationship when the PEA high subsides. Therefore, as this high diminishes, the brain compensates by increasing the levels of morphine-like substances, endorphins, that create feelings of calmness, security, and well-being. This is the biological component in the transition from passionate love to companionate love, or from eros as illogical need and obsession to eros as mutual affirmation and acceptance. The discomfort and anxiety felt by those in long-term love relationships when separated from a partner could be due in part to the rapid decrease in endorphin levels.(5)

In a study of divorce statistics in various cultures, anthropologist Helen Fisher found a correlation between the two- to three-year period during which the human brain can sustain the PEA high and the years in a marriage when most couples divorce.(6) In societies as diverse as Finland, Russia, Egypt, South Africa, and Venezuela, divorces generally occur early in marriage, reach their peak during the fourth year of marriage, and gradually decline in later years. Although there are variations from the four-year peak in some of these cultures, Fisher believes this is due to the influence of cultural variables.

For example, Fisher believes that cultural variables account for variations in the four-year peak in the United States. During the period from 1960 to 1980, when the divorce rate doubled, the incidence of divorce peaked in and around the second year of marriage. Did this have anything to do with couples living together or being in some sense married before becoming legally married? Apparently not. Fisher found that while the number of American couples living together tripled in the 1970s, the peak year for divorce among married couples remained the same.

The cultural variables that explain this pattern could be the attitudes of Americans toward marriage. While people in traditional cultures typically marry for economic, social, or political reasons, Americans marry, says Fisher, "to accentuate, balance out, or mask parts of our private lives."(7) If Americans do not feel as pressured to remain married, it should follow that they are more likely to dissolve marital relationships at the point at which the PEA high subsides.


Neuroscience has also provided some insights into a phenomenon that has long puzzled social scientists--love at first sight. Even though there are a myriad of potential mates, powerful attraction between prospective lovers is about as rare as it is spontaneous. From across the crowded room, or at the end of the checkout aisle, there suddenly emerges that special smile, face, body type that is like no other.

This magical moment for males is accompanied by stiffening of the muscles, increase in heart rate, a flushed face, and dilated pupils. Signs of love at first sight for females are tingling palms, hardening nipples, quick and shallow breathing, and dilated pupils. Although we may sense in this situation that some cosmic matchmaker is at work, there is another, more prosaic explanation.

The organization of neuronal patterns in our brains from the time of infancy to adolescence is determined in no small part by environmental stimuli. The totality of our experience is encoded in those patterns, and their dynamic interplay constitutes our subjective realities. Within this maze are neuronal patterns associated with members of the opposite sex that constitute a kind of gestalt image that includes physical features, subtle behavioral clues, and powerful emotional inputs. When we encounter a member of the opposite sex toward whom we feel instant sexual attraction, our brains are constructing an image of femaleness or maleness that activates neuronal assemblages corresponding with this gestalt image, or with what are normally called search images.

The search images that most fundamentally condition sexual attraction develop in childhood and derive from interactions with those closest to us in physical and emotional terms. The boy or girl next door can be a source of these images. But the primary source is normally opposite-sex family members. When we consider that these people share half our genes, the well-known fact that we tend to marry people like ourselves begins to make scientific sense.

Efforts to assess resemblances in the physical appearance and behavior of married couples of Len involve an index called the correlation coefficient. Although this is a statistical measure, it can be described in simple numerical terms. Imagine putting a hundred couples in a room and lining up males and females according to one characteristic, such as age. If a married couple ends up at the same place in the line, say at No. 33, the correspondence is perfect and the correlation coefficient is plus 1. Minus I designates a perfect opposite match, as in youngest woman is married to oldest man. If the correlation is random, as in youngest women is just as likely to be married to a younger as older man, the coefficient is zero.

The highest correlations, typically around plus 0.9, are for age, race, ethnic background, religion, socioeconomic status, and political views. Measures of personality, such as extroversion or introversion, and IQ levels normally fall out at around plus 0.4. This much seems obvious. But what about physical characteristics? Statistically significant correlations have been found between a large number of physical traits that most of us would never imagine had anything to do with the sources of our sexual attraction.

Correlations of about plus 0.2 have been discovered between length of earlobes, lung volumes, circumferences of wrists and ankles, and distances between eyes in married couples from cultures as diverse as Chad and Poland. In some instances, such as the length of middle fingers, the correlation is plus 0.61.(8) The best explanation for these results is that the gestalt image that informs our attraction to members of the opposite sex is based upon the images of those who share half our genes--opposite-sex members of our family.


The nonverbal language of love also attests to the legacy of mate selection among our hunter-gatherer ancestors. Women in cultures as diverse as those in Amazonia, Japan, Africa, France, Samoa, and Papua flirt using virtually the same sequence of expressions. These women first display sexual interest by smiling at the potential love object with eyebrows lifted and eyes opened wide. They then drop the eyebrows, tilt their heads down and to the side, and look in another direction.(9)

Given the importance of eye contact for the mating game of hunter-gatherers, the fact that gazing is the most obvious and universal flirtation signal should come as no great surprise. Men and women in all cultures stare intently into the eyes of potential sexual partners for several seconds, and extreme attraction is signaled by dilated pupils. This is followed by an impulse to close the eyelids, drop the gaze, and look away. Looking back in the direction of the source of this attraction tends to be furtive and is typically accompanied by meaningless gestures that signal anxiety, like fondling objects, fidgeting, and touching hair.(10)

If the physiological responses associated with love at first sight do not prove too disabling and conversation ensues, another indicator that a sexual liaison may be in the offing comes into play. The gestures made by men and women tend to mirror one another, or to become more synchronous. When he lifts his drink and turns his head right, she lifts her drink and turns her head left. When she touches her hair, he touches his hair, and so on.

Increased physical proximity, like leaning forward and positioning arms and legs closer together, is another sign of increased sexual intimacy. The prospect of further intimacy is typically assessed by "casually" touching a wrist, a shoulder, or a forearm. If the party that is touched does not touch in return, or reacts to being touched by moving out of intimate space, this signals reluctance to become more sexually intimate. But if the potential lover mirrors this laying on of hands behavior, a major obstacle on the road to sexual intercourse may have been eliminated. While there are cultures where sexual mores forbid displays of mirroring behaviors, they exist in every society where men and women are free to choose one another as mates.(11)

If we can believe the results of studies of nonverbal sexual interaction, most American cultural narratives that celebrate male sexual prowess are in need of revision. Researchers have found that American women initiate nonverbal flirtation cues, including the critical first touch, over two-thirds of the time. And follow-up interviews with these women revealed that they were very aware that this was the case.(12) Studies of cross-cultural sexual practices confirm that women normally take the initiative in making sexual advances in virtually any society where they are allowed to do so.(13)

There are also transcultural patterns in wooing or courtship rituals. In all human societies males offer females food and gifts in the hope of winning sexual favors. The food offering might be a fish, beer, or sweets instead of dinner at an overpriced restaurant, and the gifts might be cloth, tobacco, and hand-carved figures instead of cards and flowers. But the nonverbal messages conveyed by these enticements are not terribly dissimilar.

Once men and women enter the mind-altered state of the PEA high, behavioral tendencies are translated into actual behaviors in accordance with the rites and rituals of love within particular cultural contexts. And the stories, myths, legends, and songs that script these behaviors are clearly not universal. In some cultures, like the Mangaians of Polynesia and the Bem-Bem of the New Guinea highlands, the construct of "being in love" does not even exist. And yet behaviors associated with this altered state, like suicide among males who are not allowed to marry girlfriends and elopement among star-crossed couples, are not uncommon in these cultures. Also, multiple aspects of romantic love as it is conceived in the West exist, according to one recent anthropological survey, in 87 percent of 168 very diverse cultures.(14)


American popular culture creates the impression that most of us are constantly preparing for, engaging in, or recovering from promiscuous sex. But the results of what may be the first truly scientific survey of American sexual behavior, The Social Organization of Sexuality, present a very different picture. Based on face-to-face interviews with a random sample of almost thirty-five hundred Americans, ages 18 to 59, researchers found that the average American male has six sexual partners over a lifetime and the average American female has two sexual partners.(15)

Equally significant, adultery appears to be much more the exception than the rule. Nearly 75 percent of married men and 85 percent of married women surveyed said they had never been unfaithful. As for frequency of sexual intercourse, almost 40 percent of married people indicated they had sex twice a week, and only 25 percent of single people had sex that often.(16)

Obviously, a host of cultural and personal variables contribute to these behaviors. The legacy of our evolutionary past does, however, condition these behaviors. This legacy lives on in selectively advantageous traits that encourage powerful emotional bonding between potential parents--face-to-face coitus, concealed ovulation, private sex, and female orgasm. Hence the mating game in our species is framed around biological regularities that favor interdependence, cooperation, and long-term involvement.

This does not mean, of course, that evolution is a moral philosopher that dictates the terms of successful love relationships. On the other hand, behavioral tendencies associated with the sex-specific human brain do have something to say about ways in which we might seek to sustain and improve these relationships. Since the human brain cannot sustain the PEA high for more than a few years, the idea that this altered state is a precondition for a healthy love relationship is not in accord with biological reality. And yet we are incessantly bombarded with messages in print and electronic media that the opposite is true.

That the vast majority of those who fall in love and enter long-term relationships elect to have children also makes sense from a biological perspective. The PEA high evolved in our species not merely because it facilitated frequent intercourse and impregnation. This biological mechanism also evolved because it encouraged the emotional bonding required to raise big-brained infants to the point at which they, too, could bear offspring.

The fact that the brain generates an increased level of morphine-like endorphins as the PEA high subsides is another lesson of evolution that we should take seriously. The transition from passionate love to companionate love, as previous generations seem to have known far better than our own, is not only natural and necessary but can also signal the beginning of a very satisfying phase in love relationships.

This does not mean that sex between marital partners ceases to play a central and vitally important role in sustaining relationships, or that the excitement of being in love is forever lost. But it does suggest that the feelings of peace, security, and well-being occasioned by higher levels of endorphins are probably more conducive to maintaining relationships between responsible adults who are raising children.

Evolution also has something to say about the fact that teenagers tend to be more victimized by sexual love. Since the biological clock of ancestral females ran more slowly due to dietary differences, these females reached puberty several years later on average than contemporary females. But since ancestral hunter-gatherers began to mate and reproduce shortly after reaching puberty, all of the mechanisms that facilitate this process are powerfully at work in the lives of teenagers.

Concern about sexually transmitted diseases, particularly AIDS, has resulted in numerous campaigns to promote the use of condoms. And many of these campaigns suggest that virtually all the dangers associated with adolescent sexual behavior can be eliminated by consistent use of condoms. From the perspective of evolution, however, sex is not a form of recreation or a game that can be played with no liabilities on the part of the players.

The biological mechanisms of human sex evolved under special conditions in accordance with the most fundamental compulsion of life--passing on genes to subsequent generations. And we have done untold violence to teenagers by failing to make them sufficiently aware of the terrible force of this compulsion--and the enormous difference between sex as a biological reality and sex in popular culture.

Knowing the codes of evolution in flirtation behavior and the dating game also has its advantages. Obviously, sexual attraction is powerful, and men and women will not keep scorecards to check out their progress. Given that the biological predispositions in the sex-specific human brain are quite malleable in the learning process, the scorecard approach could do more harm than good. On the other hand, familiarity with the biological codes does provide a larger awareness of the difference between a response that is merely warm or friendly and one that signals sexual attraction.

If women tend to initiate the first touch in flirtation behavior, men should be well aware of this fact. And if women tend to be the decision makers in the initial stages of dating behavior, then men should know this as well. The absence of mirroring behavior may or may not signal sexual responsiveness, and the presence of this behavior is not a green light for sexual intimacy. But by knowing that this behavior is usual, both men and women could check sexually inappropriate behavior. More important, much of the mythology in this culture about male sexual process is not in sync with biological reality.

For the past thirty years, we have lived with the assumption that the sexual personae of men and women is learned in particular cultural contexts in gender-neutral minds. In marriage counseling, in advice columns in newspapers, and on prime-time talk shows, the idea that the price for political correctness in male-female love relationships is gender sameness is either explicit or implied. And even when the differences between the sexual personae of men and women are recognized, the usual refrain is that they are entirely a consequence of learning.

Since what we are as men and women is primarily a product of learning, the assumption of gender sameness has served us well in the attempt to control and eliminate offensive sexist behaviors. It has been particularly useful in disclosing that the myths of male dominance that sanction sexual abuse and use of women are utterly indefensible and morally bankrupt. Obviously, we are a long way from achieving the goal of full sexual equality, and the struggle to eliminate learned sexist attitudes and behaviors must continue. But since there is a linkage between biological reality and gender identity, the idea that men and women in love relationships must behave as if this linkage does not exist is clearly in need of revision.

(1.) Margaret Mead, Sex and Temperament in Three Primitive Societies (New York: William Morrow, 1935), 280.

(2.) D. Tennov, Love and Limerance: The Experience of Being in Love (New York: Stein and Day, 1979).

(3.) M.R. Liebowitz, The Chemistry of Love (Boston: LittleBrown, 1983), 200.

(4.) J. Money, Love and Love Sickness: The Science of Sex, Gender Difference, and Pair-Bonding (Baltimore: Johns Hopkins University Press, 1980), 65.

(5.) Liebowitz, The Chemistry of Love.

(6.) Helen Fisher, "The Four-Year Itch" Natural History (October 1989): 22-23.

(7.) Helen Fisher, Anatomy of Love: The Natural History of Monogamy, Adultery, and Divorce (New York: W.W. Norton, 1992), 109-111.

(8.) Jared Diamond, The Third Chimpanzee: The Evolution and Future of the Human Animal (New York: HarperCollins, 1992), 101-102.

(9.) Irenaus Eibl-Eibesfeldt, Ethology: The Biology of Behavior (New York: Holt, Rinehart and Winston, 1970).

(10.) E.H. Hess, The Tell-Tale Eye (New York: Van Nostrand Reinhold, 1975).

(11.) D.B. Givens, Love Signals: How to Attract a Mate (New York: Crown, 1983), and Fisher, Anatomy of Love.

(12.) T. Perper, Sex Signals: The Biology of Love (Philadelphia: ISI Press, 1985).

(13.) C.S. Ford and F.A. Beach, Patterns of Sexual Behavior (New York: Harper & Brothers, 1951).

(14.) W.R. Jankowiak and E.F. Fisher, "A Cross-Cultural Perspective on Romantic Love, Ethnology 31:2 (1992): 149-55.

(15.) John Gagnon, Robert Michael, and Stuart Michaels, The Social Organization of Sexuality (Chicago: University of Chicago Press, 1994).

(16.) Gagnon, Michael, and Michaels, The Social Organization of Sexuality.

Robert L. Nadeau is a professor at George Mason University. His article "Brain Sex and the Language of Love' was published in the November 1997 issue of The World & I. This article, like the last, is based on his book S/he Brain (Praeger, 1996).
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Author:Nadeau, Robert L.
Publication:World and I
Date:Mar 1, 1998
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