The Chemistry Between Us: Love, Sex, and the Science of Attraction

by Larry Young

Men are still led by instinct before they are regulated by knowledge. —THEODORE DREISER, SISTER CARRIE

the brain is made up of a number of structures that respond to a galaxy of neurochemicals. Contrary to common wisdom, no one region of the brain is “higher” or “lower” than any other. Behavior doesn’t always develop as part of a stepwise hierarchical process; it’s more of a parallel operation.

The brain circuits of desire and love have such enormous influence, they routinely overrule our rational selves, making our behavior subject to the force of evolutionary drives.

hermaphrodites—the outcome of two different eggs being fertilized by two different sperm, and the resulting opposite-gender embryos fusing and sharing a blood supply.

The organizational hypothesis has held up remarkably well through years of refinements and additions. It holds that the default setting is on “female” during fetal development. At about the eighth week of human gestation, testicular cells will start making testosterone from cholesterol. In a typical 46,XY fetus, testes begin to form and they make more testosterone. Later, they will get a boost from the adrenal glands that also make a small amount of testosterone. This testosterone and other androgens that derive from it, like DHT, spark the construction of male genitals from the raw materials available. Testosterone, typically through conversion to other hormones like DHT (and even estrogen), also act in the brain to lay down male-typical neural circuits and permanently change the brain’s chemistry. Later in life, when androgens are secreted, they activate these male circuits giving rise to masculine behavior.

preferences are wired into our brains. Indeed, from the very first day of life, most baby girls prefer to look at human faces, whereas most boy babies prefer mechanical objects.

organizational hypothesis that gender behavior is built into our brains by the actions of hormones.

Some fish—groupers, porgies, bluehead wrasses—live as transsexuals. They change, typically from female to male. All bluehead wrasses, for example, are born female. They stay that way until a resident male disappears or dies. Then, the most dominant females immediately begin acting like males. They battle for preeminence. When one finally takes over, her ovaries degenerate, testes develop, and her brain creates new behaviors. She becomes a he.

Many mammals, including primates, engage in homosexual behavior. The argument over same-sex coupling has always been whether or not any of these animals prefer gay to straight sex. What’s true is that primates like gay sex. 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 about 95 percent of their sex lives having homosexual encounters.

what psychologist Paul Ekman called the “Duchenne smile”—a sign of unself-conscious pleasure that involves the muscles around the eyes as well as those around the lips. Susan giggles and laughs at his banter, though she’s not supposed to say anything when he speaks. She flips her hair, cocks her head, plays with her earring, and leans forward in her chair—a tactic, explains social psychology and consumer-marketing researcher Kristina Durante, who’s running this experiment, “to draw attention to her chest,” even when he’s talking and she knows he can’t see her.

The time just before, during, and after ovulation is a brief part of this cycle. But it is the time—the only time—a woman can become pregnant, and her brain knows this. So these hormonal shifts don’t just affect a woman’s physiology; they work in her brain to influence behavior toward maximizing the egg’s chances of being fertilized so it won’t go to reproductive waste.

the hard truth for men is that no matter how nice a guy you are, at their most fertile moments females of all species appreciate winner types most.

Men try to help this process along by lighting candles, opening champagne, and playing bossa nova. We might think we’re being suave, showing off our moves, but we’re really turning down the anxiety volume so a woman can hear her estrogen talking.

Ovulation lowers a woman’s resistance to risk taking, just as an increase in testosterone lowers such resistance in men.

Testosterone will drop in certain contexts besides missing testicles. When male marmosets sniff a test tube containing the scent of their own infants (but not strange infants), their testosterone drops within twenty minutes. This could be a protective mechanism, a way of tamping down a father’s sex drive and aggression when he’s around his babies. Studies in humans have shown that when men become fathers (and especially when their babies are newborns), they experience a significant drop in testosterone. Men who are very involved in parenting their babies show the biggest drops—probably because the act of caregiving itself lowers testosterone and allows the brain to focus more on nurturing and less on fighting (even if it’s only a job promotion at stake), sex seeking, or anger. Testosterone falls when we lose a sports match, and it even slumps when the team we root for loses a game. (Sorry, Cleveland.) Driving an old, clunker family car reduces a man’s testosterone levels. Long-married men have lower testosterone readings than single men do (read into that what you will). Being fired from a job isn’t good for our levels, either. Though menopause reduces sex hormones more dramatically in women, aging men still experience a significant falloff.

“It’s likely there has been an evolutionary arms race, with males being under selection to detect fertility as accurately as possible, and women being under selection to send these cues only to select possible mates,” Miller says.

the wish to create a reasonable facsimile of ovulatory fertility is what drives the plastic surgery and cosmetics industries.

Sometimes the estrus-associated behavior is only indirectly aimed at attracting a high-quality mate. The more direct target is often other women competing for access to Clooneys.

The more attractive a woman’s direct competitors, the more pressure a woman feels to match up. “There could be a million Clooneys in the room, but going after them, dressing for them, is only effective if you have first ensured you are more attractive than other women in the local environment.”

When men already mated to ovulating women sense estrus, they not only want to be around the “hotness” of their mate, they want to keep other men away from it. Boyfriends engage in more “mate-guarding” behaviors when their girlfriends are ovulating; Susan’s behavior in her boyfriend’s absence from the lab helps us understand why. Mate-guarding men tend to interrogate their girlfriends and wives about where they’re going and whom they’re seeing. They snoop through their lovers’ belongings; 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.

Back in 1953, Hugh Hefner was a skinny, bookish editor, formerly with Esquire, who sat down at a table in his home and pasted together what became the first issue of Playboy. His creation was not entirely original; it was a combination of two existing forms: the sophisticated, urbane text of Esquire and the lowbrow, pulchritudinous imagery of cheaply produced, under-the-counter nudie magazines. But by the 1970s, Playboy had a monthly circulation of over six million, and Hefner was flying around the world in a custom DC-9 painted with the famed bunny-head logo.

Cocaine and amphetamines can greatly enhance sexual motivation because they stimulate the release of large amounts of dopamine. Simply doing a lot of mental work can loosen the reins. Executive thinking involves comparatively massive brain real estate—the human prefrontal cortex is about ten times larger than the hypothalamus—and consumes much more energy. Loewenstein has found that doing math problems before being offered cookies can lower a dieter’s resistance to the reward of a good Toll House: all that thinking has drained our executive brain’s batteries.

Eduard von Hartmann, argued that we have to be bribed into sex, our reason shut down, because sex leads to no good: marriage, the pain of childbirth, disappearing money, disillusionment in love. Just about the worst thing that can happen, Hartmann thought, is to become conscious that “the dreamed-of bliss in the arms of the beloved one is nothing but the deceptive bait” used to get us to procreate. We have the illusion of control, but, of course, it’s our unconscious breeding instinct, working through our brains, that’s driving our behavior.

(This is where confusion arises over whether or not one can be “addicted” to sex. In contrast to sex-addiction promoters [TV-based celebrity counselors, for instance], Pfaus insists that there is no such thing, and that what looks like sex addiction is really a version of obsessive-compulsive disorder. A man who masturbates five times daily isn’t addicted to it; he’s become obsessive about achieving five orgasms a day, which is, after all, quite a feat.)

When we experience sexual satisfaction and receive the consummatory reward, we (like Everitt’s rats that turned on the light because it had become rewarding in its own right) 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. We have developed a fetish. “The mechanisms of diagnosable fetish are just extremes of the very same mechanisms that help people have normal sex,” Pfaus says. We are all, to one degree or another, fetishists. Or, put another way, we all, to one degree or another, develop strong preferences.

People taking a class of antidepressants called selective serotonin reuptake inhibitors, or SSRIs, can suffer with diminished libido. The drugs keep serotonin available to neurons, which eases the sense of despair common to depression but also quashes sex drive, just as it does in the moments after orgasm.

roughly one-third of all births in the United States occur as the result of an unplanned pregnancy—as did most of the births on the planet until the advent of modern contraception.

Late in pregnancy, estrogen stimulates the production of prolactin, the hormone that starts milk production in the breasts, and the receptors for it. It also sparks a striking increase in the number of oxytocin receptors in the uterus. As its derivation from the Greek implies, oxytocin (“quick birth”) stimulates the smooth-muscle cells of the uterus to rhythmically contract, forcing out the baby during childbirth. By the time a woman goes into labor, she can have three hundred times more oxytocin receptors on those cells than she did before she got pregnant. Oxytocin is also needed to eject milk from the breasts, so more receptors are made there, too.

The method of delivery also seems to be important for activating this oxytocin-reward axis. Roughly one-third of all births in the United States today are by cesarean section, a procedure that bypasses the birth canal. This bypass prevents nerve signaling from the vaginocervical area from reaching the brain, muting the oxytocin release from the PVN. In an imaging study similar to the breast-feeding test, Swain 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.

As organisms interact with their environment, genes can be turned down, or switched off, through a process called methylation. When a gene is methylated, a chemical group gloms on to parts of it in the same way a bodyguard runs paparazzi interference by sticking close to a celebrity leaving a Hollywood nightclub. This can make the gene’s on-off switch, its promoter, less accessible to the RNA polymerase enzyme that copies the gene’s instructions for making a protein. The gene is there, but it’s been turned off, or down. The study of this phenomenon is called epigenetics. In addition to a genetic inheritance from our parents, we also receive an epigenetic legacy by the way our parents treat us.

Because the maternal behavior circuits are so integral to other kinds of behaviors, this critical period in a newborn rodent’s life has major effects on an animal’s personality. Since a pup, or a human baby, gets a sense of the world through its mother, the actions of the mother go a long way toward teaching the newborn what it can expect from the world. To the pup of a low-LG mother, the world is fearsome.

Studies have proven that oxytocin, and brain sensitivity to oxytocin, enhances the ability to accurately read faces. When people view photos of eye regions on human faces that are expressing some sort of emotion, they correctly decipher the expression more often if they’ve been given a dose of oxytocin. People who are, for whatever reason, less socially literate at judging how others feel during conversations about emotional events are also better able to empathize if they’re given oxytocin beforehand. The silenced oxytocin response in those who were deprived of proper nurturing as babies may result in a blunted ability to calm down when experiencing stress, in less empathy toward others, and in an autistic-like social blindness.

greatly heightened stress responses in women who suffered abuse, neglect, or disturbed relationships early in life with their parents. When she, Larry, and other colleagues tested women with a history of childhood abuse, they found that concentrations of oxytocin in the women’s cerebral spinal fluid were lower than in women who didn’t suffer abuse. This was especially true if the abuse was emotional rather than physical. Men experience similar effects.

there’s now a growing body of evidence that the consequences of variation in early human mother-infant bonding do show up later in life, influencing the way we love others, our sexual styles, the way we raise our own children, even the way our cultures and societies develop.

early-life experiences epigenetically programming the development of the oxytocin system.”

He also wonders if family stress and the stress of the modern world in general may be re-creating in humans what lab rodents experience. “It may be a function of the increasing exposure to stress earlier in life that the population is being exposed to,” he suggests. “I guess, in humans, stress plays a similar function. If you are in an environment that is stressful and dangerous, the chance of having progeny that survive and thrive is lower, so having kids earlier could be adaptive.”

only an estimated 3 to 5 percent of all mammal species live monogamously.

In short, both oxytocin and dopamine are required for female voles to bond, and both are released when they have sex. If you recall from the last chapter, both dopamine and oxytocin are also necessary for maternal behavior.

oxytocin, dopamine, and opioids are all required to initiate the female prairie vole’s version of love. Oxytocin facilitates approach. Opioids act on their receptors to create the “wow” of sex. Dopamine helps the brain learn exactly what’s causing that wow by imprinting an association between the stimulus—a particular male—and the reward.

Kendrick and Keverne discovered that mimicking the natural stimulation of giving birth triggered ewes to bond with a strange lamb, whether or not they’d bonded with their own and whether or not they had actually given birth at all. Birth could now be dissociated from mother-infant bonding: it’s the stimulation of the vagina and cervix, and the release of oxytocin such stimulation causes, that touches off the bonding process.

It’s known that oxytocin is emitted into the blood when people have sex. For centuries, midwives have told women that labor can be hurried along by having intercourse. In modern times, obstetricians have used dilators and water-filled balloon devices to accomplish the same job. The reason why these devices work and why intercourse can initiate labor is that both stimulate the vagina and cervix, just as in voles having sex and ewes undergoing VCS. It’s also highly probable that oxytocin is released into the brain at the same time because, as Murphy showed in rodents, the brain’s oxytocin-producing center is directly connected to the genitals.

We have the biggest penises of any primate. The average erect gorilla penis measures only about an inch and a half.

a penis needs to be only long enough to make a sperm deposit near the opening of the cervix. The human vagina has a mean depth of about 63 millimeters (or about 2.5 inches), as measured from the cervix at the back to the introitus, the spot where a hymen would be, at the front.

Evolutionary theorists have long questioned why we have longer penises than our primate cousins. One theory has it that men use them for a kind of Anthony Weiner–esque “look-at-me!” display—sort of like a lion’s mane: a symbol to other males that we are he-men to be reckoned with. Our ancestors with longer ones chased rival males away from females. Another theory suggests that longer penises evolved in men because our sperm compete with that of other males, who may mate with the same female just after us, inside the vaginal canal. The closer to the cervix we can make a deposit, the bigger head start we can give our swimmers in the great race to the egg. A third idea suggests that women are choosy. Unlike most female primates, women can orgasm more than once. When they figure out that the male penis is a handy device for inducing said orgasms, they select men who are generously equipped.

the human penis has evolved as a tool to stimulate the vagina and cervix so oxytocin is released into a woman’s brain. The bigger the penis, the more effective it is at triggering an oxytocin surge during intercourse. Surges of oxytocin help ease any apprehension or anxiety a woman may have, making her open to her lover’s emotional and social cues. She takes in his face and eyes and powerfully registers the emotional context in her amygdala. Presumably, dopamine and opioids are released. While she stares at her lover in a way that would be disconcerting in other contexts, she receives pleasure, associating it specifically with him the way a mother keys on her baby.

A study conducted by Stuart Brody, of the University of the West of Scotland, has indicated that, as fun as they are, 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.

There’s a direct neural connection between the nipples and the oxytocinergic neurons in the brain. When Barry Komisaruk, of Rutgers University, and Stuart Brody imaged the brains of women stimulating themselves in various ways, they found that stimulating the nipples activated the brain in much the same way as stimulating the cervix. They speculated that, as it does in rodents, this kind of stimulation releases oxytocin from the PVN.

This breast-oxytocin link would explain why 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. Humans are the only animal for whom breasts have become a secondary female sex characteristic.

during human evolution the mother-infant bonding mechanism, shared across all mammalian species, has been tweaked so that women can use sex to establish a bond or to maintain one. Put another way, men are using their penises and their partners’ breasts to entice women to babysit them.

The male angler fish, a deep-sea denizen, takes monogamy very seriously. Anglers live so far under the surface that light is virtually shut out of their world. So these fish have evolved a lantern, a little bioluminescent bulb-on-a-pole they use to attract prey, and, perhaps, to find each other. Even with the aid of the lantern, though, it can be tough to find a mate that far down. So when they stumble upon a female, male anglers form an actual, not just metaphorical, bond. They bite into her, fuse blood vessels, and then dissolve away until the males exist mainly as a hypothalamus and sack of testicles attached to a female’s body.