Table of Contents:

i. Introduction/Mini-Summary

PART I: GENDER IDENTITY, SEX AND SEXUAL ORIENTATION

Section 1: Gender Identity

1. Pseudohermaphroditism: The Curious Case of Machihembras in Las Salinas, Dominican Republic

2. Bruce, Brenda and David Reimer: The John/Joan Case

Section 2: Sex and Sexual Orientation

3. Sexual Desire and Human Estrus

4. Sexual Orientation: The Mystery of Homosexuality and Transgendered Individuals

5. The Mystery of Fetishes

PART II: LOVE AND PARENTING

6. The Maternal Instinct

7. Women in Love

8. Men in Love

PART III: LOVE AS AN ADDICTION, AND INFIDELITY

9. Love as an Addiction

10. Infidelity: Cheating and Adultery

11. Conclusion

 i. Introduction/Mini-Summary

Love and sex play a central role in the human drama. But when we talk about the emotions and decisions that we make in connection with them, we tend to remain strictly at the macro level, referring to people, and relationships, and our freely made choices. However, in their new book The Chemistry Between Us: Love, Sex and the Science of Attraction social neuroscientist Larry Young and journalist Brian Alexander contend that our biology and chemistry play a much bigger role in love and sex than most of us ever acknowledge (since Larry Young is the scientist behind the book [and responsible for the ideas therein], I will refer to him as the main author throughout). Young explores everything from gender identity (and sexual orientation), to romantic relationships (and parenting), to monogamy (and infidelity), taking us inside our bodies to investigate the genes and hormones that influence our approach to love, sex and relationships. While the focus here is on us humans, the evidence comes not only from our own species but from a host of other animals that exhibit similar biology and behavior.

Young begins by way of smashing the notions that gender identity is constructed by culture, and that sexual orientation is a matter of choice. The foundations of these phenomena, the author argues, are laid down in utero by the specific hormones that wash over the fetus as it develops. Interestingly, we learn that the genes and hormones that are responsible for genital development are active at a different time than those that are responsible for gender-specific behavior, thus explaining how the two can become separated from each other.

While the foundations of gender and sexual orientation may be laid down in utero, it is also the case that they are capable of being influenced to a degree by learning and culture, thus explaining cross-cultural differences in the manifestation of gender, as well as such phenomenon as fetishes.

When it comes to a woman’s gender identity, Young explores the hormones that explain maternal behavior, and why women differ in regard to just how maternal they are–as well as what effect this has on their children. Interestingly, we also learn that a woman’s love for a man appears to have been built on the same brain mechanisms responsible for her maternal behavior. This fact helps explain a number of baffling phenomena (including, incredibly, the size of women’s breasts, and men’s penises!).

While men are capable of experiencing romantic love just as strongly as women (if not more so), we learn that a man’s love is built on an entirely different biological mechanism. Specifically, a man’s love is built on the ancient mechanism responsible for territoriality. This helps explain such phenomenon as male possessiveness and jealousy; but it also helps explain why men are more paternal than the males of most other species.

While love may have a different biological basis in men and women, it takes on a strikingly similar form in both. In short, it is an addiction–not at all unlike a drug addiction. Indeed, like a drug addiction, a romantic relationship starts out as a high, then morphs into an experience whereby the lover cannot stand to be away from their love, and experiences deep stress when this occurs. Even the brain chemistry of using drugs, and the way the brain changes as a drug user becomes addicted, is the same as occurs in the progression of a romantic relationship.

While men and women in love may be addicted to one another, this does not mean they are incapable of cheating on one another. And, indeed, the prevalence of adultery in all times and places (despite the near ubiquity of social mores opposed to the practice) indicate that it is a deep part of our biology. Young explores this biology, and also why some people are more disposed to the practice than others.

*To check out this book at Amazon.com, or purchase it, please click here: The Chemistry Between Us: Love, Sex, and the Science of Attraction

What follows is a full executive summary of Larry Young’s The Chemistry Between Us: Loves, Sex, and the Science of Attraction.

PART I: GENDER IDENTITY, SEX AND SEXUAL ORIENTATION

Section 1: Gender Identity

Beginning with the rise of behaviorism in the 1920’s, and lasting for much of the next 50 years, the social sciences were dominated by the view that human nature is essentially a tabula rasa. That is, a baby is born as a blank slate, without any preformed instincts or inclinations, and is just waiting to be imprinted by the stamp of learning and culture. This included, of course, the baby’s gender identity. As Young explains, it was believed that “gender behavior was imposed by parents, society, and culture. It was a position of nature over nurture”. By the 1970’s, though, cracks were beginning to form in this theory.

1. Pseudohermaphroditism: The Curious Case of Machihembras in Las Salinas, Dominican Republic

One of the earliest cracks in the theory that gender identity is constructed by culture took shape in the 1960’s and 1970’s when scientists began studying the curious case of people in Las Salinas, Dominican Republic, whom the locals referred to as machihembras (literally ‘first woman, then man’ [loc. 184]). As the name suggests, machihembras were born looking, to all eyes, like girls, with “female-appearing genitals, complete with labia and clitorises” (loc. 177). Once they reached puberty, however, machihembras became masculinised. Specifically, “their labia turned into scrotums filled with testicles. Their voices deepened and they grew muscles—a picture of one machihembra, around nineteen years old, shows the carved physique of a strutting middleweight boxer” (loc. 184).

As it turns out, machihembras are pseudohermaphrodites. What this means is that they are actually boys all along (biologically, anyway, in that they are XY chromosomally), even though they may appear to be girls for the first part of their life (loc. 240). Normally what happens is that XY fetuses produce the hormones testosterone, and dihydrotestosterone (DHT), which activate cells to form “the prostate, penis and scrotum” (loc. 249). In machihembras, though, there is a glitch in the gene that is responsible for triggering the release of these hormones during development. As a result, the organs in question fail to fully form (loc. 249). (The male organs are actually there all along, but remain underdeveloped to the point where the individual appears to be female [237].)

Now, when XY’s reach puberty another rush of male hormones is triggered (loc. 252). In the machihembras, this extra kick of hormones was enough to trigger the development of the sexual organs that had failed to take place in utero: “Voila!—girls appeared to turn into boys” (loc. 252).

Given that the machihembras appeared to be females all the way up to puberty, they were naturally raised as girls (loc. 177). As Young explains, “they wore bows in their hair and dresses—if they had any. They did the domestic chores girls in Las Salinas were expected to do while the boys roughhoused together and had fun exploring away from home” (loc. 180).

If gender identity really is a matter of nurture, then, we would expect that by the time the machihembras had reached puberty that they would have formed a strong sense of identity as a female. Indeed, a sense of female identity strong enough that it would not be easily wiped away by a shift in physiology (even if this shift in biology was a major one). This being the case, we would expect that the machihembras would continue to hang on to at least some aspects of their female identity well beyond the time when they had become masculine in physiology; or, at the very least, that they would experience a very real conflict in their gender identity.

Now, it cannot be said that the metamorphosis that machihembras experienced was in any sense an easy transition. This is particularly the case given the social stigma that they endured: “imagine the taunts an adolescent schoolboy might suffer if his fellows knew he’d once been a girl” (loc. 188). As difficult as this transition was, though, issues of conflict in gender identity were not a part it. Instead, the shift in physiology in machihembras signaled an immediate and seamless shift in their behavior and gender identity. Indeed, as Young explains, once the transition occurred, the machihembras “walked with a macho bearing, joined the village boys in male play, and eventually started chasing girls. Most married. Some fathered children… More importantly, they accepted themselves as male” (loc. 191). This makes it look like gender identity in the machihembras was being governed by their hormones (and specifically the hormone rush that they experienced when they hit puberty), rather than their upbringing.

Given that this is the case, the case of the machihembras is tantalizing evidence in favor of the view that gender identity is more a matter of biology than upbringing and environment. Nevertheless, one could argue that the machihembras’ new physiology triggered others to treat them as males, which behavior triggered the machihembras themselves to both act and identify as males (loc. 284). However, other evidence indicates that this is almost certainly not what was going on here. Some of the most telling evidence comes from an episode known as the John/Joan case that unfolded simultaneously alongside the investigation into the machihembras (loc. 224).

2. Bruce, Brenda and David Reimer: The John/Joan Case

The case begins with the birth of twin boys in Canada in 1965, Bruce and Brian Reimer. Both boys were circumcised following birth; however, something went wrong during Bruce’s procedure, and Bruce’s doctor accidentally mutilated Bruce’s penis to the point where it could not be repaired (loc. 215).

This left Bruce’s parents in a very difficult position (to put it mildly). In their despair, the Reimer’s consulted the doctor and gender specialist John Money (loc. 215). Money ascribed to the view mentioned above that “biological sex [does] not dictate one’s ‘gender identity,’ a term he coined” (loc. 197). Given that this was the case, and given that Bruce’s penis was damaged beyond repair, Money recommended that the Reimer’s proceed with sex reassignment surgery (and hormone treatment) and raise Bruce as a girl. Putting their faith in the doctor’s expertise, “the Reimers followed Money’s advice. They had Bruce castrated and supplemented with estrogen. They raised him as a girl, changing his name to Brenda” (loc. 221).

At first, it appeared that Money’s plan was working, and by the early 1970’s Money was beginning to publicize the success of his experiment (loc. 194, 222). As Young explains, Money reported that “the male brother, Brian, behaved just the way we have come to think most eight-year-old little boys behave, he said, describing him as made of snips, snails and puppy-dog tails, and as enjoying rough play. Meanwhile, Brenda, all sugar and spice, reveled in her little dresses and dolls” (loc. 224).

As it turns out, though, Money’s assessment of Brenda’s behavior was more wishful thinking than anything, and it became increasingly clear that Brenda was rejecting her assigned sex (loc. 294). Indeed, in truth, “while living as Brenda, young Bruce Reimer hated dresses. When Brian refused to share his toy cars and trucks, Bruce/Brenda saved up enough allowance money to buy his own. He bought toy guns so he could play army with Brian” (loc. 294). By the age of 14, Brenda identified strongly as a male, and was deeply depressed by his confused reality.

Eventually, the Reimer’s were forced to tell Brenda the truth. As you can well imagine, the truth came as a painful shock to Brenda; but, at the same time, it was also a very real relief. Truth in hand, Brenda decided to go through with a second sex change (changing her name to David in the process) so he could live as a male henceforth (loc. 303). In the end, then, “Money’s grand experiment hadn’t merely not worked—it was a disaster” (loc. 249).

David went on to live a relatively normal life as a man. As Young notes, “he… took a job in a slaughterhouse, married a woman, and helped raise her children” (loc. 303). However, David continued to be haunted by his past. Ultimately, and tragically, “he could not fully excise his tormented history. David Reimer used a shotgun to commit suicide in 2004. It was his third attempt” (loc. 306).

Here is an excellent documentary about David Reimer’s experience (Part 1 is shown in the screen, while the links to parts 2-5 are available below):

Part 2

Part 3

Part 4

Part 5

David Reimer’s experience is strong evidence that gender identity is indeed more a matter of biology than upbringing and environment. What’s more, it makes it look very much like the machihembras transition to masculinity was itself a matter of biology, rather than being explicable in terms of the fact that they were suddenly treated as males by those around them. Together, the two cases make a powerful argument for the biological basis of gender identity.

Now, granted, it is true that many cultures encourage (or even, in certain cases, coerce) their members to adopt gender appropriate behavior; and it is true that this pressure can influence behavior. However, it is important to recognize than in applying this pressure these cultures are, in many cases, but reinforcing gender-specific roles that already have their foundations in human biology. As Young puts it, “there is a powerful interplay among culture, genes, upbringing, and our brains. But culture doesn’t create gender—it reflects it” (loc. 4142).

At the same time, it is also important to recognize that cultures can come to adopt slightly different ways of manifesting gender identity (loc. 1528). Nevertheless, it remains the case that these cultural differences operate at a more superficial level than the foundations of gender-specific behaviour, which are themselves largely universal.

Section 2: Sex and Sexual Orientation

3. Sexual Desire and Human Estrus

Sexual desire in both males and females begins in earnest with the onset of puberty; however, there are some significant natural differences between the sexes with regards to the experience of sexual desire. When it comes to men, things are pretty straightforward. As Young explains, “for boys and men, [sexual] activation remains at a fairly constant level (assuming good health) until later in life when testosterone falls, but even then, androgens help keep sex on the male brain” (loc. 548). When it comes to women, though, things are quite a bit more complex, as “unlike boys… their hormones rise and fall dramatically as their bodies prepare for pregnancy” (loc. 551).

Now, in other mammals, a female’s sexual behavior is very clearly influenced by her ovulatory cycle. Specifically, when her eggs are ready to be fertilized (a period called estrus), she shifts from being entirely uninterested in sex, to being very receptive to sexual advances indeed (in plain terms, she gets really horny). Different mammals advertise this receptivity in different ways. So, for instance, in primates the females’ rumps turn red, like a bull’s eye (loc. 889), while female rodents will hop and dart in front of prospective mates (loc. 746).

When it comes to our own species, our sexual behavior is not nearly so limited by the females’ ovulatory cycle. Indeed, as Young notes, “unlike cats or rodents, humans will have sex any time of year and any time of day or night” (loc. 817). This is a clear indication that sex has a function other than just reproduction in our species (loc. 817) (at least part of the function of sex in our species would appear to be its role in creating bonds between people—there will be much more on this below).

Given that sexual behavior in our species is not tied to the ovulatory cycle, it had been thought, until recently, that women had evolved to do away with estrus, such that they no longer had the experience of going into heat during ovulation. However, as Young notes, “evidence is now mounting that women do have an estrus—and that men can sense it” (loc. 823).

Some of the most sensational evidence here comes out of a study that looked at human behavior in strip clubs in New Mexico. Specifically, researchers out of the University of Mexico (led by the psychologist Geoffrey Miller) tracked “stripper incomes across ovulatory cycles” (loc. 857). What they found is nothing short of shocking. As Young explains, “when the strippers Miller chronicled were in estrus, they made about $354 per five-hour shift. Anestrous women made about $264… Menstruation cut dancers’ earnings in half. The difference can’t be attributed to one woman’s attractiveness over another’s, or to fashion choices, because the research took place over two months” (loc. 864).

Even more interestingly, none of the strippers were remotely aware that they made more money while they were ovulating (loc. 869), which was corroborated by the fact that “none of them scheduled their work shifts to maximize earnings” (loc. 872) by taking advantage of their ovulatory cycle. And, of course, none of the men were aware that they were shelling out more money for women who were ovulating either (loc. 866). Somehow, all of this was going on completely unconsciously.

Now, it has been shown that men who are exposed to the scent of an ovulating woman respond by way of producing more testosterone than when they are exposed to the scent of a woman that is not ovulating (loc. 879); so this at partly explains why men end up shelling out more money to strippers that are in estrus. However, other evidence indicates that these men are probably also responding to cues that are not entirely invisible. Indeed, a bevy of evidence has now been gathered that shows that women alter their behavior when they are ovulating (especially if they are single, and under 40 [loc. 921]).

To begin with, it has been shown that women who are ovulating prefer (and wear) more revealing clothing (loc. 912); flirt more (at least with certain types of men [loc. 604-21]); enjoy porn more (loc. 646) and “fantasize more often about having sex with someone who is not their current partner” (loc. 649). And, of course, it’s not like all of this sexy behavior is leading nowhere. Women also report more sexual encounters, and instances where they initiate sex, when they are ovulating (loc. 647). In the case of the strippers, then, it seems likely that their ovulating may have found its way into just how flirtatious they were being with their customers (loc. 886), which helps explain how they were able to earn more money (loc. 886).

What is most incredible here is that all of this sexy behavior during estrus seems to be going on completely unconsciously, since, as Young notes, female subjects involved in these experiments “weren’t conscious of making different choices or aware that their behaviors were shifting” (loc. 918).

But while women may be unconscious of how their ovulatory cycle influences their behavior, men certainly seem to be picking up on it. Indeed—in addition to shelling out more money to strippers that are ovulating—it has been found that men are both more possessive and jealous, and also more loving towards their partners when they are in estrus: “boyfriends engage in more ‘mate-guarding’ behaviors when their girlfriends are ovulating… they display more jealousy. But some mate-guarding behavior is positive. Men more frequently compliment their female partners, try to spend more time with them, and express feelings of love and commitment to them when they’re ovulating” (loc. 931).

This makes sense in evolutionary terms, of course, because estrus is the one and only time that a woman can get pregnant, so it is also the one and only time that a man can get his genes into the next generation. This means that there is adaptive value for men to have a vested interest in getting romantic with their partner (or any woman) around this time, and also in ensuring that their partner is not getting romantic with anyone else. And again, none of this altered behavior need be picked up on by the man doing the behaving (since it will ultimately produce its intended effect whether he is aware of it or not).

4. Sexual Orientation: The Mystery of Homosexuality and Transgendered Individuals

Now, a significant part of sexual behaviour and gender identity is sexual orientation, and indeed, most who have one set of sexual equipment gender-identify with the sex whose equipment they have, and are sexually attracted to the opposite sex. However, this is, of course, not always the case, as the existence of homosexuals, bisexuals and transgendered individuals (people who “have a strong desire to change their gender [loc. 410]) makes clear. These phenomena, then, present a bit of a mystery.

Nevertheless, the mystery is not confined to our own species, since, as Young puts it, “gender-bending is more common in nature than you might think” (loc. 412). Indeed, both homosexual behavior and gender flip-flopping show up in many other animals. With regards to the former, Young explains how “many mammals, including primates, engage in homosexual behavior… They have orgasms with same sex partners. They ejaculate and make happy noises when they do. Dominant silverback gorillas have boyfriends. Some male langurs spend 95 percent of their sex lives having homosexual encounters” (loc. 428).

Gender transitioning, too, shows up in other species: “some fish—groupers, porgies, bluehead wrasses—live as transsexuals. They change, typically from female to male… Other fish are cross-dressing transvestites. [In] Male round gobies… some males, called sneakers… are smaller, mottled brown, and have narrower heads, like females. What’s more they behave like females” (loc. 420), and use this ploy to help them get their genes into the next generation (loc. 421).

All of this goes to show, Young argues, that “brains have an intrinsic capacity to display homosexual, bisexual, and transgender behavior. Whether that capacity is expressed depends upon how a brain is shaped during development” (loc. 437). And scientists have been able to shed light on just how this happens.

In broad terms, the phenomena can be explained by the fact that the hormones that are responsible for the development of sexual organs, gender identity, and sexual orientation are all active at different times during development (loc. 463, 499); and therefore, each can become separated from the other. Indeed, to begin with, it has been found that “the hormonal actions that trigger the formation of our genitals occur early in a woman’s pregnancy. The hormonal influences that shape our brain happen later in a pregnancy. The two… can become dissociated” (loc. 528). Likewise, within the realm of behavior itself, the parts of the brain that shape gender-identity and sexual orientation also develop at different times, and can also become separated (loc. 456-463).

The fact that hormonal activity in development is responsible for the mixing and matching that we see is reflected by the fact that the brains of heterosexuals, homosexuals, and transgendered individuals are all different from one another, and in key areas of the brain that are associated with sexual excitation and sexual behavior (including the sexually dimorphic nucleus [SDN], the bed nucleus of the stria terminalis [BNST], and the interstitial nucleus of the anterior hypothalamus [INAH]) (loc. 498-520).

The following is a very good documentary about the influence of hormones on gender and sexual orientation: Documentary: Brain Sex (Part I)

In short, then, it is clear that sexual orientation is neither a result of environmental influences nor choice; rather, it is a function of nothing other than hormonal exposure during development.

Here is an excellent documentary that brings the human side to the transgender story:

5. The Mystery of Fetishes

While the broad brushstrokes of sexual orientation may be painted by biology, the finer details thereof are apparently capable of being modified by experience.  This occurs mainly through simple association. For instance, if we experience sexual arousal and consummation in the presence of a particular object or experience, we naturally come to associate the two. The more often the two are paired together the stronger the association becomes, not only psychologically, but chemically, in our brains. Soon enough, the paired object or experience itself will trigger sexual desire. As Young explains “when we experience sexual satisfaction and receive the consummatory reward, we… have primed ourselves to receive an appetitive reward from any number of cues associated with that experience. The more often we receive the consummatory reward, the stronger the associations become. What was he wearing? What did she look like? What music was playing? Where was I? These have all become antecedent conditions because the amygdala, which is wired to the accumbens, has logged the circumstances of this very pleasant feeling” (loc. 1333). The end result is a sexual preference, or even a fetish.

Interestingly, the pairing process here can be set in motion not only through direct experience, but through fantasy as well. As Young explains, “repeated fantasy and masturbation, perhaps coupled with the general arousal that can come from fear of getting caught or breaking a taboo, seals a fetish in place” (loc. 1368).

When it comes to the formation of a sexual preference or fetish, we appear to be particularly susceptible to this during our formative years, or even before. For example, many fetishists can recall specific experiences from their youth that first set their fetish into motion. For instance, “female rope fetishists describe climbing ropes and feeling the rough texture rubbing against their clitorises… Spanking fetishists recall being spanked as children—which can create general arousal—and placing that spanking into a sexual context. A male shoe fetishist may have masturbated in his mother’s closet, surrounded by her shoes, just to pick one possible childhood scenario” (loc. 1399).

And, of course, we are capable of forming a strong preference for things that are not only out of the ordinary, but for very ordinary things as well. As young explains, “we are all, to one degree or another, fetishists. Or, put another way, we all, to one degree or another, develop strong preferences. Reward circuitry and the neurochemicals that act upon it are responsible for why individual people prefer tall, short, blond, dark-haired, skinny, chubby, bespectacled lovers” (loc. 1335).

Interestingly, scientists have even induced fetishes in animals by way of manipulating the pairing process. For instance, the neuroscientist Jim Pfaus took young male rodents and outfitted them in custom-made leather jackets: “when a rat is trussed up in one, it looks like Marlon Brando in The Wild One” (loc. 1357) (who said scientists don’t have a sense of humor!). Pfaus then gave his leather-clad rats their first sexual experience. Not surprisingly, the leather jacket had no effect on the rat’s sexual appetite or sexual behavior one way or the other (loc. 1360). Later, though, Pfaus introduced his newly de-virginized rats into a cage with a female in heat, but without their leather jackets on. Here’s Young to explain what happened: “when the former Brandos were placed in an arena—sans jacket—with a female in heat, about 30 percent of them refused to copulate at all, and many of the ones that did appear to have sex were really just going through the motions because they didn’t intromit (which means they had trouble getting erections). The ones that did copulate took far longer than normal, and the female really had to work for it. ‘There was failure to activate arousal, simply because the jacket was missing,’ Pfaus says” (loc. 1364).

As impressive as this is, Pfaus was able to take his experiment even one step further, as he managed to engender a fetish in his rats of something that is not only neutral to them (as the jackets are), but of something that is downright objectionable to them: the smell of death. To begin with, Pfaus took female rats in heat and doused them in cadaverine (“a synthetic form of the death smell” [loc. 1347]), and then placed them in a cage with virgin males. The smell took some getting used to on the part of the males (as rats have an innate disgust of the smell of death [loc 1345]), but eventually they overcame their reservations and copulated with their gothic girlfriends on a regular basis (loc. 1350). The next part of the experiment involved looking at how the experience had affected the males. Here’s Young to explain the results: “later, Pfaus placed these males with a selection of estrous females, including one he’d perfumed with cadaverine. The males preferred the death-scented over the naturally sweet-smelling (to a rat, that is) females. He even tried scenting some females with lemon, but the males who had their first matings with the deathly females still preferred the nasty odor. Some would mate only with females that smelled like death. They had become cadaverine fetishists” (loc. 1353).

PART II: LOVE AND PARENTING

6. The Maternal Instinct

A big part of a woman’s gender identity is motherhood, and maternal behavior. However, this is not always and ever the case. Indeed, until women have children of their own, many of them are very much averse to the idea of children and mothering. However, as many a new mother has discovered, these reservations are often overcome with pregnancy and the arrival of a new baby—even when the pregnancy was not necessarily planned or wanted (loc. 1507, 1517). As Young explains, some “women feel maternal urges long before they actually have a baby, of course, but sudden converts often marvel at the way babies, once seen as drooling snot factories, become sweet human cupcakes (‘I’m gonna eat you up!’). Women who fret about their own diffidence, or outright aversion, toward babies before giving birth can be mystified by the dramatic way their baby becomes so all-consuming that they catch themselves regaling their childless friends with sophisticated analyses of their infant’s poo. They describe looking into their offspring’s eyes and experiencing a welling up of caring emotion that feels like a tsunami of mother love surging through their bodies” (loc. 1511).

And, of course, this welling up of feelings towards baby has a host of behavioral effects in the mother as well. In its most general form, this takes the shape of responding to the baby’s needs as best as one is able (even at the expense of one’s own needs [loc. 1495]), by keeping the infant fed, and warm and soothed (loc. 1662); and also by responding to the baby affectionately—for example, by reacting to its cries by fawning over and cuddling it (loc. 1502).

We are not the only species wherein the females exhibit maternal behavior, of course. Indeed, as Young points out, maternal behavior, and the mother-child bond, has a long evolutionary history: “this bond, the first love any of us experience, is the most fundamental one of all. Its evolutionary antecedents are ancient and shared, to one degree or another, among widely diverse species, even some fish. While most fish are content to lay eggs and hope for the best, Amazonian discus fish mothers stay with their offspring, feeding them with a mucus exuded by their skin… if you try to separate a mother from her fry she’ll thrash and swim in a panic” (loc. 1490). The reason behind the evolution of maternal behavior is simple: those mothers that possessed a genetic mutation that led to them to care for their offspring more had offspring that stood a better chance of surviving and reproducing and hence proliferating their maternal genes (loc. 1496).

Now, some have argued that the maternal instinct doesn’t actually exist in our species at all. According to this argument, it may be the case that other species have the maternal instinct, but in our own species all instincts have been wiped out and replaced by culture. As Young explains, these people “insist[] the human maternal instinct does not exist—that such nurturing in humans is a choice made under pressure from cultural expectations” (loc. 1518).

However, Young argues that this is basically pure nonsense, as all of the evidence indicates that the maternal instinct is just as much a part of a woman’s biology as it is for the females of any other mammal species. It may be the case, Young contends, that “human societies place high demands on mother behavior and that a woman who contravenes those demands will almost certainly face harsh social judgement” (loc. 1522); but, the author continues, “mothers are driven to mother by their brains, and the culture of motherhood merely builds itself around nature” (loc. 1531).

Now, just as in all other animals, the maternal extinct in women is driven by hormones. Specifically, oxytocin, estrogen and prolactin. These hormones begin to be activated in the middle of pregnancy, to help prepare the woman for both the birthing process and, subsequently, childcare. To begin with, becoming pregnant triggers a woman’s body to start pumping out estrogen (loc. 1605). This estrogen surge explains why “partway through a pregnancy, a woman may begin to feel ‘maternal’ toward her child, even though it hasn’t yet been born. She could, for example, become preoccupied with the rituals of making a baby’s room, of buying baby gear, of selecting a name, or of obsessively weighing the pros and cons of organic-cotton Onesies” (loc. 1606).

Following this, as a pregnant woman approaches her due date, her body will begin releasing prolactin, which “triggers milk production” (loc. 1609). Then, during labour itself, “as the cervix ripens, a nerve signal travels up to the brain, and the oxytocin pulses begin” (loc. 1609). Now, oxytocin has been dubbed the ‘love hormone’, and for good reason. It (in conjunction with dopamine [loc. 1636]) is essentially the hormone that is responsible for maternal affection, and for the mother-child bond (loc.1599). The rush of oxytocin that is triggered by the birthing process causes new mothers to “instinctively cradle their newborns in their arms, tucked against their breasts” (loc. 1609), and ensures that they will begin to form a strong attachment with their child the moment it is born (loc. 1648).

Importantly, since the surge of oxytocin that a mother receives from the birthing process is triggered by cervical dilation, it occurs only with a vaginal birth, and not with a cesarean section (loc. 1648). As we might expect, then, the birthing method can have an effect on how a new mother regards her child immediately following birth. As Young explains, “in an imaging study… [researchers] found that the motivation and reward centers in the brains of mothers who gave birth by cesarean rather than vaginally were less responsive to their own baby’s cry. Mothers who gave birth by cesarean also tended to score higher on measures of depression” (loc. 1652).

Within 25 minutes of being born an infant will begin preparing its mother for its first feeding by way of “reach[ing] his hand up to his mother’s breast and… massaging the areola and nipple” (loc. 1612). This not only triggers the mother’s milk to start flowing, but also triggers the flow of more oxytocin in her brain (loc. 1612), thus building up the budding bond in the mother even more. As the newborn begins suckling, still more oxytocin is triggered in the mother’s brain and body, and this time, as the infant imbibes his mother’s milk, he also draws in oxytocin, thus helping form the bond from child to mother (loc. 1615).

The bond between mother and child continues to strengthen every time the mother breastfeeds her baby (largely due to the fact that the suckling causes oxytocin to be released in the mother’s brain and body, which the mother then shares with her child along with her milk [loc. 1636]). When breastfeeding does not occur, then, this can compromise the mother-child bond. Indeed, it has been shown that the bond between mother and child is strongest where breastfeeding does occur. As Young explains, “breast-feeding mothers, as a group, tended to show a greater activation response in certain brain regions, including the amygdala, when they heard their own baby cry… this more intense activation of the amygdala in breast-feeding mothers was, in turn, associated with an increase in the affectionate behavior mothers showed toward their own babies when they played together” (loc. 1645).

Now, the importance of exhibiting a high amount of attention and affection for a baby simply cannot be overstated. Indeed, people who experience neglect in infancy, or who simply have low-affection mothers, have been shown to carry a range of difficulties into later life. These difficulties include “an empathy deficit” (loc. 1783); increased anxiety and depression (loc. 1786, 1872, 1957); and difficulty forming relationships (loc. 1796). What’s worse, women who have experienced neglect or low-affection mothers in infancy tend to have much more difficulty developing an attachment to their own children, thus causing them to be low-affection mothers themselves, thereby causing the cycle to start all over again (loc. 1872, 1886-1910).

Fortunately, we are not slaves to our biology, and low-affection mothers can learn techniques to help them improve the care of their infants (loc. 1973). Over and above this, early indications are that the administration of hormones such as oxytocin may be able to help mothers that are having difficulty forming a strong attachment with their newborns (loc. 1973).

7. Women in Love

While all female mammals (and the females of many other species) display the instinct to bond with their offspring, few also display the inclination to pair-bond with a sexual partner. However, the phenomenon is not unheard of elsewhere in the animal kingdom. Indeed, as Young points out, “an estimated 3 to 5 percent of all mammal species live monogamously” (loc. 2106). Now, it is important to point out that the monogamy being referred to here is social and not sexual monogamy. There’s a big difference. As Young points out, social monogamy refers to being “emotionally and socially bonded to one person” (loc. 2116), while sexual monogamy refers to sexual exclusivity (loc. 2118). The two can, of course, be separated. Indeed, as the author points out, “certain professional golfers, a raft of politicians, and middle-aged, Lycra thong-wearing swingers come to mind” (loc. 2116).

In any event, the point is that pair-bonding and social monogamy is not unheard of in the animal kingdom, and it is certainly an important aspect of our own species—which, when we do speak of it, we more commonly refer to as L.O.V.E. ‘love’.

Again, some have argued that romantic love is but a cultural construct, but the evidence indicates that it does indeed have a biological backing. Interestingly, this biological backing appears to have been borrowed from the very biology responsible for the mother-child bond that we have just finished discussing (in women, at least). As Young argues the point, “what we call romantic love is really the result of evolutionary adaptation—a tweaking—of the neural circuits driving maternal bonding” (loc. 2027). There are several bits of evidence that lead the author to this conclusion.

To begin with, the hormone responsible for the mother-child bond (oxytocin) is also at the heart of romantic love in women (and men, to some degree). In its role as love potion, oxytocin works mainly by way of getting both women and men to be more ‘in tune with’, and trustful of one another. Specifically, oxytocin has been shown to play a role in encouraging positive social communication (loc. 2299), accurately reading emotions off of faces (with a little positive bias [loc. 2308-13, 2324]), and cultivating trust in others (loc. 2314-17). Perhaps not surprisingly, then, it has been found that oxytocin levels can actually be used to predict relationship success. For example, in one study, scientists “measured oxytocin in new couples, then followed those couples for six months. The lovers who were still in the original relationships after six months had the highest levels at the start” (loc. 2307). What’s more, “there is mounting evidence that slight differences in the gene containing the recipe for oxytocin receptors can help explain… variations in love and bonding among individual people” (loc. 2328).

Interestingly, while it is vaginal birth and breastfeeding that is primarily responsible for triggering the oxytocin that works to bond a mother to her baby, it would appear to be sex that is primarily responsible for triggering the oxytocin that bonds a woman to a romantic partner. And while intercourse may have nothing to do with either vaginal birth or breastfeeding in terms of function, it would appear that both (somewhat creepily) trigger the flow of oxytocin according to the same basic mechanics.

To begin with, it has been shown that the same cervical nerve that is stimulated during childbirth, and which causes the woman’s brain to release oxytocin, is also stimulated during sexual intercourse resulting in the same oxytocin release (loc. 2160, 2378-84). This helps explain the results of a study conducted out of the University of the West of Scotland which found that “neither oral sex, nor masturbation, nor any other form of sexual activity gives women the feeling of overall relationship satisfaction, including ‘feeling close to one’s partner,’ that penile-vaginal sex creates” (loc. 2409).

Interestingly, this may also help explain a long-standing mystery (no pun intended) as to why the male penis is so large. As Young explains, “it may come as welcome news to many men that, relative to our body size, we are very well endowed. We have the biggest penises of any primate. The average erect gorilla penis measures only about an inch and a half… [whereas] the average non-porn-star’s erect penis is about 13 centimeters long or 5 inches” (loc. 2391). Trying to explain the relatively inflated size of the human phallus is actually quite difficult, given that, for reproductive purposes, the penis need only be long enough to “make a sperm deposit near the opening of the cervix” (loc. 2387). In us humans, this means the penis only needs to be about 2.5 inches long (loc. 2387).

Now, several theories have been advanced to help explain this mystery (loc. 2393-2400); but given what we have just learned about cervical stimulation during sex, and given that, as Young points out, “the bigger the penis, the more effective it is at triggering an oxytocin surge during intercourse” (loc. 2403), some have speculated that the human penis may have evolved to its present size for no other reason than to better  “stimulate the vagina and cervix so oxytocin is released into a woman’s brain” (loc. 2403)—since this naturally causes her to be better disposed towards him (loc. 2402). In other words, the penis in our species has evolved to be not only an instrument of sex, but of love.

Just as interestingly, oxytocin is also at the heart of a theory that attempts to explain an even bigger mystery: the mystery of why women’s breasts are so large, and why they are so heavily sexualized in our species. As Young explains, “men are the only male mammals fascinated by breasts, and women are the only female mammals whose breasts remain enlarged, even when they’re not nursing” (loc. 2440). The reason behind this seems completely inexplicable—until, that is, you consider that nipple and breast stimulation during sex (like intercourse itself) also releases oxytocin in a woman’s brain (in exactly the same way as breast-feeding does) (loc. 2431-37). When you connect the dots, it becomes tempting to think (and some do) that “in a recapitulation of those earliest days, we use breasts to help create and maintain the romantic bond. Breasts, like penises, have evolved into tools for stimulating oxytocin release via the mother-infant bonding neurocircuit” (loc. 2431).

8. Men in Love

While oxytocin does have a role to play in explaining romantic love in men (as indicated above), there is evidence to indicate that a man’s love is actually influenced more so by a separate hormone: vasopressin (loc. 2609-12). Actually, both oxytocin and vasopressin are active in both men and women (loc. 2581); however, as Young explains, “while oxytocin sensitivity is more dependent upon estrogen, vasopressin production in neurons projecting into behavior-controlling regions of the brain depends more on testosterone” (loc. 2584). Since men produce much more testosterone than estrogen, and vice-versa when it comes to women, it seems natural that vasopressin would play a much bigger role in men’s behavior than oxtocin (and vice-versa when it comes to women) (loc. 2584).

Interestingly, while oxytocin has its ancient evolutionary roots in the mother-child bond, vasopressin has its ancient evolutionary roots in territoriality (loc. 2565). Given that this is the case, some have argued (and Young agrees) that romantic love in men has evolved out of the ancient mechanism responsible for territoriality. As Young puts it (somewhat provocatively), “Just as love has its evolutionary and neuronal roots in the mother-infant bonding circuit for female humans, for male humans… females are territory” (loc. 2581).

Now, at first blush, it may seem like a bit of a stretch to think that evolution took this path, since territoriality may not immediately strike one as a good candidate to morph into romantic love. However, when you really think about it, territoriality does involve a kind of emotional attachment. Young himself makes the connection even more explicitly and emphatically: “if you’re willing to defend [a piece of territory], risking pain, injury, maybe even death, then you must be pretty attached to it. You could even be said to be bonded to your territory. You have a home. No other place is your home, just this place. You’ve become, so to speak, territorially monogamous” (loc. 2575). Looked at in this light—and given that men did not have the same maternal instinct to draw on as women did—the mechanism responsible for territoriality may have been the very best candidate available to be evolutionarily morphed into romantic love in men.

In any event, this certainly helps explain men’s possessiveness when it comes to their romantic partners (loc. 2729, 2768, 2901). Now, it is important to point out that Young is in no way justifying such possessiveness, nor is he “suggesting that this is the only component of a man’s bond to a woman” (loc. 2874); but, he adds, it should be recognized that “the territorial urge plays an important role” (loc. 2874).

When it comes to why humans evolved the capacity to bond with a romantic partner, the answer is quite clear: child rearing. That is, the circumstances that our species encountered at some point in our evolutionary past were such that it favored men and women coming together to cooperate in satisfying their needs and raising children. Since love is what draws people together and makes them inclined to do this, love was selected for and allowed to evolve (loc. 3282).

The only thing left that was needed to close the circle was for men to evolve a capacity to love not just a romantic partner but his own children, and it would appear that this was accomplished by a simple modification of his already-evolved capacity for romantic love. In accordance with this hypothesis, the same vasopressin that is responsible for pair-bonding in men is also implicated in paternal behavior (loc. 2613).

PART III: LOVE AS AN ADDICTION, AND INFIDELITY

9. Love as an Addiction

While romantic love in men and women may be built on separate biological mechanisms, it nevertheless takes on a very similar form in both. And interestingly, the form that it takes may best be thought of as an addiction—not at all unlike a drug addiction. As Young puts it, “love could well be an appropriate and healthy evolutionary adaptation, but love has the same hold on us, using the same brain chemistry, as addictive drugs” (loc. 2982).

To begin with, addictive drugs initially give the user a high by way of activating the reward system in the brain, thereby laying the ground for addiction. George Koob, chairman of the Committee on the Neurobiology of Addictive Disorder at the Scripps Institute, explains: “drugs are addictive because early on they make people feel wonderful with ‘a massive release, or a massive activation, of reward systems” (loc. 3012).

The pleasure experienced by the drug user new to an addictive drug is echoed by a lover freshly falling in love. Indeed, (as anyone who has ever fallen in love will know), people who are falling in love experience deep pleasure in seeing and being in the presence of their love. Within the brain, the same reward system that is activated by exposure to a new drug is also activated by the presence and company of a new lover. As Young explains, the latter experience “activat[es] the reward system, including the accumbens, amygdala, and prefrontal cortex, just like drugs of abuse do” (loc. 3290).

When it comes to the novice drug user, they begin to associate the objects and experiences that are present during drug use with the pleasure of the drug itself. As a result, “after just a few drug sessions, they begin to receive a burst of dopamine just by anticipating the stimulus even if the drugs are nowhere around… Imaging studies have shown that in response to cues like a crack pipe, the brain of an addict will respond in much the same way as if he or she had access to crack itself” (loc. 3021).

This association process is not unlike what happens to a new lover who comes to adore everything associated with his love: “the smell of the back of her neck, the texture of the hair on his chest, the softness of her lips, the sound of his voice… the Lautrec prints on his walls, her collection of Vogue magazines—all of these sensory cues are not only sharply vivid but also important for inexplicable reasons. The mere thought of her perfume can distract him from his work for long minutes of reverie” (loc. 3277).

Soon enough, though, this initial infatuation begins to wane some, as the lover becomes more and more sensitized to his love (loc. 3290). This does not occur nearly as quickly (or even ever), in a long-distance relationship. As Young explains, “neither of you ever has a chance to become bored. You’re both forever fascinating to each other, a relationship idling in that happy zone between first irresistible passion, and later disgust over the wearing of sweatpants to bed” (loc. 3297).

Just as the lover’s initial infatuation begins to wane, so do too does the pleasure that the drug user initially received from their drug begin to diminish. This does not mean that the drug user loses a taste for the drug, though, for while the pleasure involved in taking the drug is diminishing, addiction is setting in. The difference is that whereas once the drug user desired the drug because of the pleasure that it would bring, now the user begins to feel terrible without it, and desires the drug because it’s the only thing that will make them feel better. As Young explains, “at first, dopamine action provides motivation that kick-starts an addict’s pleasant appetite. It stamps the brain with positive associations of the contextual cues that enhance eagerness to consume. Later, though, the addict becomes subject to… ‘negative motivation.’ Rather than keen expectancy, euphoria, and impulsiveness, the drive now is full of anxiety, dysphoria, and compulsion. The addict feels compelled to act because if he doesn’t, something bad is going to happen” (loc. 3109).

In terms of the neurochemistry involved, addiction causes the brain of an addict to be flooded with a stress hormone called corticotrophin-releasing factor (CRF), which engages the body’s stress system (known as the HPA axis) (loc. 3119). The drug, and the drug alone, can prevent the engagement of CRF in the addict’s brain, so when the addict is separated from their drug they are in a permanent state of stress (loc. 3129).

Likewise, when the flames of love’s fire begin to die down, this does not meant that the lover loses all taste for his love. Rather, the fire itself may continue to burn, as the individual develops an enduring bond for his love. The lover has now entered the addiction stage of the relationship: “the nature of love changes with time. We enjoy it less as the wild urgency recedes. Yet we remain in our relationships as initial passion turns into durable social monogamy because we’ve become addicted to each other” (loc. 2989).

And indeed, the brain chemistry that accompanies this shift is the same as occurs in the drug user’s brain as they become addicted to a drug. Specifically, just as the drug addict’s brain churns out the stress hormone CRF when they are in the throes of addiction, so too does the lover’s brain churn out CRF when they have developed an enduring bond (loc. 3230). In the drug addict, only the drug can prevent the CRF from engaging in their brain, while in the lover only the presence and company of the loved can do this. As Young explains, “There is something fundamental about living with a mate that results in more CRF stress hormone in the brain, but that also prevents engagement of the HPA stress axis as long as the mates stay together” (loc. 3230). As a result, the same feeling of stress that keeps the drug addict going back for their drug, is also experienced by the lover, and keeps them drawn to and coming back to their love (loc. 3299).

And again, when we consider what love evolved for (to keep couples together for the purpose of raising children), this makes perfect sense. What better way to ensure that two people will stay together long enough to raise children than to have them become addicted to one another?

In any event, this also helps explain why break-ups are often so hard—even when you know that the person you have broken up with is not right for you (loc. 3335). And, in a similar vein, this also helps explain why many people continue on in a relationship even when it is clear that it has become destructive (loc. 3347). Still, if a relationship truly does become destructive, it is possible to fall out of love, just as it is possible to lose an addiction to a drug. Nevertheless, the hold that each has on someone may continue to persist well into the future, thus making relapses a distinct and ever-dangerous possibility.

Here is a clever little song by someone who agrees with the premise that love is similar to a drug (but not exactly like it).

10. Infidelity: Cheating and Adultery

Now, while people are who are in love with another may be addicted to one another, this doesn’t always stop them from cheating on one another. Indeed, surveys indicate that roughly 20% to 25% of women in America have cheated on their husbands, while about 30% of husbands have cheated on their wives (loc. 3408-12). Meanwhile, numbers among non-married (but supposedly monogamous) couples are even higher, with “cheating rates hav[ing] been reported at upward of half” (loc. 3415). (If anything, these numbers may be a little low, given the fact that, “as you might expect, interviewees are often reluctant to tell the truth in face-to-face interviews, and even anonymous written surveys are somewhat unreliable” [loc. 3405].)

These are some pretty substantial numbers, especially when you consider that infidelity is very much frowned upon in our society (as it is, and has been, in the vast majority of societies throughout history [loc. 3385, 3433]). What’s more, even in those societies where infidelity has not just been frowned upon, but met with the harshest of punishments (which it has in most, until very recently), this hasn’t stopped it either (loc. 3446). Given that this is the case, it is difficult to avoid the conclusion (that Young himself draws) that “infidelity is an inherent behavioral trait for at least some part of the population” (loc. 3428).

The question is why. Not why people actually cheat, that’s easy; but why the pull towards it evolved in the first place, and/or why it has not died off in a socially monogamous species such as ours. When it comes to men, the answer seems fairly straightforward. Getting a little action on the side can help him get his genes into the next generation. Plus, if he plays his cards right, he may be able to get away without contributing anything to the raising of the child, which means he can focus all of his resources on the children he has with his partner, which will serve to give them as big a leg up as possible in the great biological game (so even if his bastard children don’t make it, at least his own children will get the greatest opportunity to do so) (loc. 2653). (All of this may sound more than a little immoral, but again, natural selection is not necessarily concerned about morals.)

When it comes to females, on the other hand, there are a different set of considerations. To begin with, no matter what children a woman spawns, she will be stuck with them (and have to feed them) for at least 9 months, and probably a lot longer (loc. 3656). For this reason alone, a woman’s biological legacy depends on her being a lot choosier with who she has sex with, and under what circumstances.  Still, if a woman already has a good man that is committed to her and her children, and she just happens to come across another man who is not necessarily interested in committing to her, but whose genes may confer a greater advantage on her children than her partner’s, well then, game on. (As Young notes, estimates are that “between 3 and 10 percent of children around the world are being raised by fathers who don’t know the children are not genetically related to them” [loc. 3427].)

Now, while the previous analyses paint all men and all women with the same brush (in that it understands each of them as performing the same mental calculus when considering whether or not to cheat on a partner), it turns out that people vary widely in just how susceptible they are to caving in to infidelity, and it appears that this has a lot to do with genetics. For example, those who are more bold and inclined to take risks (which is a personality trait that is largely influenced by genetics [loc. 3686]) are much more likely to cheat than those who are less so (loc. 3692). For instance, people who have a particular gene that has been found to contribute to boldness (the 7R+ allele), were found to have “had 50 percent more instances of sexual infidelity than those who were noncarriers… Half of the repeat allele carriers said that they’d cheated on a partner they’d been monogamously committed to, but only 22 percent of noncarriers said that they’d cheated” (loc. 3692). And other genes have also been implicated here (loc. 3706-10).

Given that this is the case, some have hypothesized that cheating may be less a species-wide issue, as an evolutionary strategy that appeals to some more so than others, and which is more or less adaptive depending on the circumstances. As Young explains, “when times are tough and scary, and the future uncertain, the population needs the bold and adventurous. In calm, plentiful times, there’s less need. ‘That is, in environments where “cad” behavior is adaptive, selective pressure for 7R+ would be positive; but in environments where “dad” behavior is adaptive, selective pressure for 7R+ would be negative’” (loc. 3698).

Over and above this, it may also be the case that cheaters help spread adaptive genes through the population quicker, which ends up being a boon to the species as a whole (loc. 3695). Young concludes the topic thus: “the true answer to the question of whether humans are designed to be sexually monogamous seems to be, It depends. Some are. Others maybe not so much” (loc. 3807).

The following is a very good documentary on the science of love, sex and attraction that touches on many of the issues explored in the book (as well as a few that aren’t):

11. Conclusion

Our genes and hormones may strongly influence our decisions and actions, but they do not determine them. Therefore, understanding how our behaviour is influenced by our biology need not make us resigned to this influence. Rather, it simply allows us to understand ourselves better, and should help us make better decisions and actions in our lives. This applies as much to love, and sex and relationships as anything else.

*To purchase the book at Amazon.com, please click here: The Chemistry Between Us: Love, Sex, and the Science of Attraction

*Thank you for taking the time to read this article. If you have enjoyed this summary of Larry Young and Brian Alexander’s The Chemistry Between Us: Love, Sex, and the Science of Attraction or just have a thought, please feel free to leave a comment below. Also, if you feel others may benefit from this article, please feel free to click on the g+1 symbol below, or share it on one of the umpteen social networking sites hidden beneath the ‘share’ button. Finally, if you feel inclined to tip me for my efforts, then I second that emotion! I have given you the opportunity to do so below.

Cheers,

Aaron,

The Book Reporter