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

by Larry Young

mating, and then living with a female, physically changed the male brain, increasing the density of vasopressin-emitting nerves and reorganizing the nucleus accumbens. Both changes strengthened the bond and stimulated paternal care for young.

In nature, about 60 percent of males will settle down with a female partner. The rest play the field.

somewhat disturbing conclusion that for men, sex, love, and aggression are inextricably mixed in the brain. It makes sense, in light of Larry’s theory that vasopressin’s role in territorial and mating behaviors has been adapted by humans so women become an extension of territory in the male brain. If Larry is correct, a man is likely to bond strongly to his mate and to be aggressive in defending her.

“Most of us think of testosterone as the hormone that is responsible for male aggressive behavior,” Heinrichs argues, “but vasopressin explains this effect much better.”

“Bonding itself produces high CRF,” Bosch says. “But this does not mean the system is also firing.” There is something fundamental about living with a mate that results in more CRF stress hormone in the brain, but that also prevents the engagement of the HPA stress axis as long as the mates stay together.

For humans, falling in love is like putting a gun to your head. You’re lured into a relationship, enjoy its pleasures, and then, over time, those pleasures fade and compulsion takes over. “It is a quite similar situation, you know, to when a person at first has the high feeling in a relationship, the CRF is silent, and the dopamine reward is taking over,” Bosch says. “You feel high. Everything is cool. Everything is nice. Then, after a while, nature makes sure you still want to stay with a partner. And this system makes you feel sick as soon as you leave the mate. This is the idea of the whole story.”

Parenting follows the same pattern, further supporting the idea that love between adults has its roots in parent-baby bonding. As we’ve explained, nurturing is rewarding. If it weren’t, we wouldn’t do it, just like we wouldn’t have sex or fall in love. And like love, it shares pathways with drug addiction, including, among others, the amygdala, the VTA, and the nucleus accumbens. Parents “fall in love” with their babies, but in time, as with a love match between adults, there’s the risk of boredom, not to mention aversion. After a number of sleepless nights, dirty-diaper changes, and general baby crankiness, the bliss of the early days can turn into drudgery. Parental loss of interest is a life-and-death matter for a baby, so nature built in a system to make sure parents feel compelled to give care whether or not they like doing it.

males separated from their male buddies didn’t grieve the separation. But females separated from other females, like cage mates and sisters they’ve lived with for a long time—any female with which they’ve shared social support—do grieve. Males put all their emotional capital in one bank, with their female partner. Females will display depression behavior if they lose their mothers, sisters, close female friends, or their bonded mates, perhaps a clue that helps explain why women suffer from depression at roughly double the rate as men.

“Drug addiction and love are absolutely parallel,”

There’s a lot of indirect evidence to support the love-as-addiction scenario. Lovers, for example, can act like painkilling drugs for each other. In one test, fifteen people who were nine months into new relationships—long enough to be in love, but not so long as to have grown sick of each other—were placed in fMRI machines and subjected to varying degrees of pain caused by heat. They looked at pictures of attractive acquaintances, their partners, and words that were previously shown to reduce pain. The words did reduce the amount of pain the people said they were feeling, but imaging revealed that the reduction was due to simple distraction: the words took their minds off it. Pictures of acquaintances had no effect on pain. But pictures of their lovers reduced pain by activating the reward system, including the accumbens, amygdala, and prefrontal cortex, just like drugs of abuse do. The addiction model also explains the allure of the long-distance relationship. It’s really just extended foreplay. When appetitive reward is won only intermittently, explains Jim Pfaus, “not only do you not get tolerant, you are actually more sensitized to it. Sex works the same way, and that’s how long-distance relationships work. It’s dramatic, right? ‘We’ll see each other every two weeks.’ And then you anticipate, and for two days before you see each other, you are full of anticipation for the reward, and then engaging in that appetitive response, as well as hot sex, well, that glues those...

The immediate stress of a breakup, and the chronic stress after, can be so great it can affect our health. When people go through a marital separation, they show significant declines in immune system strength. Newly separated people visit doctors more often, have more acute and chronic health problems than married people, die from infections at a greater rate. Contrary to the usual image of a newly separated man celebrating his “freedom” by prowling nightclubs for beddable women, men appear to suffer more severely, perhaps because men invest all their emotional bonding in their partner, whereas women often enjoy emotional and social support from other women.

Under the thumb of the stress system, we will do things we never imagined doing, like drunk dialing the departed lover at 2:00 a.m. or listening to sad Edith Piaf songs though we haven’t any idea what the words mean. We drink, especially men, who release much more dopamine with alcohol than women do. We are out of control because we’re denied access to the one natural source of relief—the person we love.

In animals, CRF induces drug relapse, and in humans, the stress of a breakup leaves both former partners vulnerable to breakup sex. Dumpers may rationalize by thinking that perhaps they made a big mistake by ending the relationship. Dumpees aren’t thinking of self-respect because they’re rationalizing with “Aha! She wants me back!”

sex itself is not an addiction, contrary to what some pop psychologists and “sex rehab” entrepreneurs say. But sex does trigger oxytocin release, which quiets the heightened stress response to separation from the loved one. People who are newly separated from a love relationship, even if they initiated the breakup, can still experience the drive to relieve that stress, leading them into a new pairing.

Merely contemplating a breakup can create the stress and fear of the real thing. When college freshmen in love were asked to predict how upset they’d be, and for how long, if their partner ended the relationship, those who described themselves as more in love, unlikely to begin a new romance, and didn’t want the breakup significantly overestimated how bad they would feel and for how long. This is undoubtedly why some people choose to remain in a relationship after some transgression by one of the partners.

Even men and women who have been verbally or physically abused by their partners refuse to leave those relationships, just as Murray couldn’t leave his relationship with drugs. They rationalize their choice to stay, for example, by focusing on positive traits their partner might possess. Those who remain, but then do manage to leave later, often look back on their former mental state as being “brainwashed” or confused. When Murray uses phrases like “My disease told me…,” he’s reflecting the same state of mind.

The question of why this or that relationship fails is unanswerable, partly because everybody’s circumstances are so different. But there is a general truth about sexual relationships: As you’ll see below, passion fades. Before it does, however, it can fill a multitude of lurking potholes. Once it’s gone, the simple recognition that two people are ill-matched in personality or temperament probably accounts for many separations, and at least some of the roughly 43 percent of all first marriages in the United States that dissolve. Even then, though, breaking up really is hard to do, more testimony to the power of the addiction designed to keep us together.

A multinational survey of nearly 17,000 people, in fifty-three nations, from all parts of the globe, revealed that men and women, or at least the college-age men and women in the sample, were doing a lot of what social scientists and wildlife biologists call “mate poaching.” Roughly half had made at least one attempt, and many of them succeeded. In North America, 62 percent of men, and 40 percent of women, had tried to poach somebody else’s partner for a short-term fling.

Sixty percent of men who said they’d been a poaching target said they’d agreed to short-term sex with the poacher. Just under half of women said so. Similar successes were reported by poachers who said they’d made an attempt to steal a mate in order to establish a long-term relationship. Interestingly, societies in which women had more political power also had more mate poaching by both women and men. Sex, of course, leads to babies. Right now, all over the world, millions of human babies are being unwittingly raised by cuckolded men.

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.

for the sake of argument, let’s accept a rough estimate of 30 to 40 percent infidelity in marriage; 50 percent in nonmarital, but monogamous, relationships; and up to 10 percent of babies not being genetically related to the man who believes he’s the father. All over the world. Among all races, and creeds, and cultures. The conclusion has to be that infidelity is an inherent behavioral trait for at least some part of the human population.

In Chaucer’s day, female erotic desire was taken for granted. In fact, three hundred years before Chaucer, and lasting at least until the 1700s, churchmen viewed women as cesspools of wicked temptation. Female desire was so fearsome, it gave rise to the myth of the vagina dentata, the vagina with teeth. But by “the nineteenth century,” Coontz says, “the view that emerged of marriage was based on the idea that women were pure and virtuous.”

for over 1,800 years of Western civilization, much of the world has been trying to come to grips with the central paradox of love—how it can coexist with infidelity. At first, sexual pleasure was suspect, often sinful, at the very least frowned upon, even in marriage. During the past hundred years or so, sexual bliss has become one of the main objects of marriage. And yet, regardless of how society has viewed marital sexual relations, both men and women still had sex outside marriage, and outside every other type of supposedly sexually monogamous relationship. This is because infidelity isn’t caused by lax social mores or rigid social mores. A tendency toward extra-pair-bond sex can be programmed into our brains.

Human beings are born with a bias toward satisfying immediate rewards, they argued, and our ability to counteract that bias in the service of a longer-term goal—like preserving a bonded relationship—when we sense something—sexual opportunity, for example—that sparks our reward circuits, may depend on the strength of the interaction between those circuits and the PFC.

What happens when people get married?” Jim Pfaus asks. “Now that they can have sex any time they want, they stop having sex!” He’s deliberately exaggerating, but it’s true that the longer people are married, the less sex they have.

Sixteen percent of American married men age 40 to 49 said they have sex “a few times per year to monthly” in a 2010 national survey conducted by the Kinsey Institute for Research in Sex, Gender, and Reproduction. Only 20 percent of married men age 40 to 49 said they have sex “two-to-three times per week.” Thirty-seven percent of married men age 25 to 29 (and presumably married for a shorter period of time) said the same. Women showed similar trends. Many factors affect sexual frequency and motivation, including children, work, bills, and how fit and healthy you are, but there’s little doubt that the falloff is neurochemically based. Married men have significantly lower testosterone than single men, just like the marmosets. They have higher estrogen and lower stress hormones. They are bonded and settled. They give back rubs that are just back rubs. Living with somebody for a long time diminishes our interest in having sex with that person. It’s sad, but true.

There is a possible adaptive upside to a loss of sexual interest. Males that run around looking for sex aren’t very good fathers. It may be that the chemical changes that accompany bonding help us pay attention to the job at hand: supporting babies. Also, when male guppies live with one female for a long time, they focus much less on sex and spend much more energy and attention trying to find food. As a result, they grow bigger and stronger than guppies who encounter a constantly changing cast of females. Those males put more effort into copulating and less into foraging for food, proving once again that being a playboy is a costly hobby.

As the story goes, Coolidge and his wife were touring a farm, guided separately by the resident farmer. As the farmer was showing Mrs. Coolidge around the barnyard, a rooster mounted a hen. The farmer was a little flustered by the display and, in an effort to ease his embarrassment by acknowledging the obvious, Mrs. Coolidge decided to ask a technical question: “How often does the rooster mate?” “Dozens of times a day,” the farmer replied. Mrs. Coolidge smiled, and said, “Tell that to the president.” When the farmer took the president through the barnyard, and spotted the rooster, he dutifully informed him that Mrs. Coolidge had requested he pass along this fascinating bit of husbandry trivia. “Same hen every time?” Coolidge asked. “No, all different hens,” the farmer answered. “Tell that to Mrs. Coolidge,” the president quipped.

The first part of the Coolidge Effect, the slow death of passion, is experienced by many human couples. When physical passion disappears, there’s less glue to bind them over the long haul of life, less excitement, less reward, often less closeness. If there are problems that had been buried under the passion, they can rise to the surface. The second part of the Coolidge Effect, the rejuvenation of sexual appetite and performance, is a perfect example of the lure of novelty, and therefore an example of the lure of infidelity. It turns out that individual animals and people differ in just how powerful that lure can be when facing a conflict between desire and reason.

For the Coolidge Effect to take place, though, an individual has to appreciate the value of novelty and be daring enough to go after it. You have to be willing to leave the comfortable shelter of an established order—sometimes actually leave, as in leave home, and sometimes leave metaphorically, as in step outside the bond. There’s risk involved in any such adventure. You may have to poach another individual’s mate, perhaps over the objections of that individual, who may engage in some pretty serious mate guarding.

while mate guarding is ingrained in many human societies, and is a basic feature of monogamy, there’s also a sociological explanation for it. “There does seem to be less mate guarding when there is less stuff at stake, or when survival depends on sharing rather than hoarding,” she says, referring to Indians of the Amazon basin who traditionally practice a form of multiple social parentage with pregnant women having sex with several different men, all of whom are considered to have contributed something of their substance to the child, and therefore feel obligated to contribute to its support. “But once you have developed substantial differences in wealth and social status in a society, then there is a kind of narrowing of obligations.”

Property and status are handed down to one’s genetic offspring. Families weave connections based on genealogy. A bastard child is an intruder. “In my reading of history, families get really strict about female chastity when they do not want a child introduced to the family whose father or other relatives could make claims” on wealth or property, Coontz says.

Roman Catholic priests were often married men until the First Lateran Council of 1123, when the church declared: “We absolutely forbid priests, deacons, subdeacons, and monks to have concubines or to contract marriage. We decree in accordance with the definitions of the sacred canons, that marriages already contracted by such persons must be dissolved, and that the persons be condemned to do penance.” One of the reasons for this injunction—a...

Culture is a reflection of our brains, often a reflection of the conflicts within them. Social bonding is certainly in conflict with sexual desire. And so we have had chastity belts, the burka, and female genital mutilation. We’ve instituted marriage and the enforced consequences of wrecking a marriage. Divorce isn’t cheap; there’s often a public shaming that accompanies a discovered affair, and there may be negative career implications. In the U.S. military, you could be subject to criminal charges for violating Uniformed Code of Military Justice provisions against adultery. These steps are society’s way of trying to exploit our capacity for reason by adding costs to infidelity, and so constrain our desire for extra-pair sex.

The need for these bulwarks makes it appear as if humans are working at cross-purposes. Evolution may have built in such push-pull tension between social bonding and sexual drive. Over the millions of years of evolution, males and females may have been engaged in a kind of war of self-interest. Females may be constantly seeking the best possible genes for their offspring. To succeed, they would need to be both fertile and bold enough to exploit that fertility by seeking out an extra-pair partner. Meanwhile, males may be driven to spread all that sperm around but also be driven to prevent females from mating with any other males, especially during their fertile periods. So we jealously guard our mates and construct cultural norms of sexual monogamy to institutionalize the natural mate-guarding urge. We want monogamy for those we love but not necessarily for ourselves.

women with naturally higher levels of estrogen have reported that they’re more willing to carry on a sexual relationship with a man who’s not their primary, committed partner. They also appear to be more inclined to have serial monogamous relationships and to be on the lookout for a handsomer, richer, smarter guy.

men and women who have higher levels of testosterone are more likely to have sex with more people. This may be because a higher base level of testosterone blunts the first part of the Coolidge Effect, the drop in testosterone that correlates with a drop in mating effort, so they remain more motivated to seek sex.

variations moderate our individual capacities to fight with ourselves over our urges. Many find the idea that chemical bits of matter—our DNA, dopamine, the other neurosignaling molecules we’ve discussed—contribute so heavily to what we usually think of as morality as deeply unsettling, even offensive. But nature is neither moral nor immoral. It just is.

Evidence is mounting that nature has preserved a bias toward sexual infidelity in at least some human beings and that sexual adventuring is an inherent part of monogamous social systems....

Most of us treasure social monogamy, and most people—though not all—treasure sexual monogamy as part of that bonded relationship. Yet social monogamy dampens the erotic drive to have sex with that very social partner, while leaving us susceptible to seduction by another. Certain inborn differences in our brains can make us more welcoming to that novel stimulus—the pretty coworker, the handsome husband of a friend, the rich boss. Worse, the very sorts of people who most of us find attractive—the brave, bold, fun-loving, risk-taking sensation seekers—like the “cad” in chapter 2—the ones we usually call “a catch,” may be the same people who are most biased toward having sex with people who aren’t us.

the very fact that most cultures have invested enormous resources in enforcing monogamy demonstrates how driven at least a subset of the human population is to have sex with people outside their bond.

Acknowledging a mistake does not mean that either social or sexual monogamy will die off. Rather, Coontz speculates, more couples will adopt new relationship modes. Some will feel best embracing both social and sexual monogamy, others will shuffle the deck. Some will negotiate sexual dalliances, others may issue a blanket don’t-ask-don’t-tell policy. “I also think that we should not discount the fact that as the age of marriage rises, people may have had twenty years of premarital sex, and when they marry, will say, ‘I’m tired of this,’” and retire into happy sexual monogamy, she says.

“There is undeniable evidence out there, in both animals and humans, that social environment influences the development of social behavior in children,”

stress and anxiety, especially in early life, can alter behavior far into adult life. That behavior can be transmitted to the next generation. Studies have shown that levels of neurochemicals like oxytocin and vasopressin are shared between human parents and their children and that the behaviors of both generations are associated with those levels. Parent-child pairs with lower levels of oxytocin engage less often and less rewardingly than pairs with higher ones.

There’s a school of thought within neuroscience that suggests free will is a myth, that the preconscious brain informs the conscious mind, which then acts as if it’s making a decision—when, in fact, our course of action was already decided before we were even aware of it.

Whether free will exists at all—or it contributes 10 or 20 or 30 percent to our behaviors—is less important than the fact that we act as if we have it. In other words, we tell ourselves a story. That is what it means to be a human, especially one in love. It’s why both of us believe love’s future can be as bright as ever.