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This Is Your Brain on Sex

The Science Behind the Search for Love
By Kayt Sukel

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Dirty Minds

Chapter 1


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The Neuroscience of Love: A History

(Theirs and Mine)


In 1994 a scientist named Sue Carter submitted a grant application to study a hormone called oxytocin (not to be confused with the narcotic Oxycontin, aka hillbilly heroin) in a small rodent called the prairie vole.

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A prairie vole family. Photo by Todd Ahern, University of Massachusetts.

A prairie vole (Microtus ochrogaster) looks a lot like your garden-variety mouse, but scruffier and with a shorter tail. Happily burrowing under gardens and meadows in a large stretch of central North America, these small rodents might completely escape our notice except for one special trait: they are monogamous.

Socially monogamous, that is. Unlike most other rodents—or most other mammals, for that matter—prairie voles form lifelong pair-bonds, or lasting social and sexual relationships with a single member of the opposite sex. Both males and females are also directly involved with the parenting of offspring. Because of the rarity of such habits in the animal kingdom, many animal behaviorists have become exceptionally interested in the prairie vole. One such researcher was Carter.

A professor of psychiatry at the University of Illinois at Chicago, Carter hypothesized that oxytocin, which is linked to childbirth and breastfeeding, could increase social attachment. She had already conducted research to support the idea and hoped that this grant would allow her to continue studying the hormone and its relationship to social behaviors in the prairie vole. In her application she did not mention love, marriage, or even humans. Somehow the grant review committee decided she was studying the little four-letter-word that begins with an l—love, that is—which was considered a serious no-no in the hard science climate of the day.

“I was trying to get federal grant funding to continue my work, and suddenly I was accused of studying love,” she said when I visited her lab in Chicago. Petite, white-haired, and a little bohemian in style, Carter somehow managed the feat of being both incredibly welcoming and intellectually intimidating at the same time. “Honestly, it was a shock to me. I would not have used the word love—I never used the word love. I didn’t think about the work in terms of love. I was simply talking about a preference of one animal for another—not some human construct that seemed to have little to do with what we were actually studying.”

Carter told me she was unsure of how to respond to the review. She conferred with Kerstin Uvnäs-Moberg, a fellow scientist also interested in oxytocin who was working at Stockholm’s Karolinska Institute. Could it be that their work was related to something as messy and indefinable as love? Might there be a neurobiological basis for the future study of love? Looking at newly published research by various labs concerning oxytocin, social attachment, and pair-bonding in prairie voles and other mammal species, the answer seemed to be yes. Carter and Uvnäs-Moberg thought it was time to stop ducking the topic and admit that their work did have implications for human behavior.

“It seemed like the time to really try to articulate and explain the idea that social bonds were critical to human love,” Carter said. While sex was, is, and will be of the utmost importance to propagating our species, Carter and Uvnäs-Moberg were convinced that love needed to be articulated in the context not only of genetic propagation but also of survival—specifically, the ways social bonds can help people thrive in the face of stress and other complexities of life on a daily basis. Perhaps our brains promote social relationships in order to ensure that more than one person is on tap to avoid dangers, to make sure there is enough food around to feed the family, and to help raise the young’uns. Investigating how the neuroscience underlying social bonds might promote these behaviors seemed a pertinent line of study.

Though this line of inquiry seemed very clear to Carter and Uvnäs-Moberg, it was difficult to get respect (and, perhaps more important, funding) to study such ideas experimentally. There was already ample evidence in neuroscience literature to suggest that love was a worthy topic of research. But the scientists never called it such, avoiding it like the dirty word it is. Instead they referred to the related topics of pair-bonding, monogamy, attachment, and mating behaviors. If you read between the lines, there was a lot of information out there, perhaps even enough to make the neuroscientific study of love its own field. Still most professional scientists were afraid to call love by its true name.

There was no sense in talking about the neuroscience of love without a proper working definition—a common standard that scientists across disciplines could use to test and validate hypotheses. Sadly, as fitting (and poignant) as Ted Nugent’s “tire iron” characterization might be as a song lyric, it would be limiting to use as the basis for a credible, replicable scientific study. To that end Carter and Uvnäs-Moberg invited thirty-eight prominent scientists in the field of neurobiology to a meeting at the 1996 Wenner-Gren Symposium in Stockholm titled “Is There a Neurobiology of Love?”

One of the products of that meeting was a definition. Instead of going with Merriam-Webster’s basic statement about love being a case of “strong affection for another,” the group consensus was that love is “a life-long learning process that starts with the relationship of the infant to his or her mother and the gradual withdrawal from the mother with a search for emotional comfort and fulfillment.” This definition was included in the summary report written by the prominent neuroscientist Bruce McEwen.1 It offers more detail than the definition of love as strictly an emotion or a basic mammalian drive, like hunger or thirst—even if it is less romantic than “sweet surrender” or “my first, my last, and my everything.” Though a mouthful, this definition would serve as the standard to which future studies across the neurobiology field could refer.

The meeting also started a renaissance of sorts, a green light for neuroscientists, neurobiologists, and neuroendocrinologists to finally call love, well, love. This allowed them to start studying the nuances of this human phenomenon from the perspectives of brain and biology. Two years later many of the meeting’s prominent attendees published studies in a special issue of the journal Psychoneuroendocrinology on topics ranging from the evolutionary antecedents of love to the physiological consequences of withholding it. With such respected scientists backing the concept, researchers could more easily study the l-word within the space of the brain and neurobiology.

Sexy Baby Banning


Fast-forward ten years. Many great studies concerning neuroscientific aspects of that “life-long learning process” we call love were published in the late 1990s, with a great number appearing in high-profile journals like Nature and Science. Brains, it seemed, have quite a bit to do with love—certainly far more than do our proverbial hearts. While working on a story for a neuroscience website, I accidentally stumbled across McEwen’s meeting report. A simple misclick on a library database brought me to it, and even though it was completely off-topic I was compelled to read it.

Maybe I was drawn to the question raised in the title, “Is there a neurobiology of love?” It was not a subject I’d had occasion to study before. Maybe it was the fact that it was written by McEwen, an acclaimed neuroscientist from Rockefeller University. His work had impressed me since I was a graduate student. Maybe I was just procrastinating. I might well have read the phone book as an excuse to take a break that muggy afternoon. Or maybe it had something to do with my sleep deprivation. Did I mention I had recently become a mother?

If there is a stereotype of a new mom—think bedraggled, beleaguered, and baggy-eyed—I fulfilled it, and then some. From the stains on my shirt to the state of my house, there was not one part of my life left untouched by the effects of motherhood. As much as I do not subscribe to the notion of “mommy brain,” or the idea that motherhood makes you stupid, I have to admit that I sometimes wondered what was going on upstairs. But honestly, what had changed the most—somewhat inexplicably—since becoming a mother was my marriage.

The arrival of my son had completely altered my relationship with my husband. Though I certainly expected my marriage to change once we had children, I was not prepared for a complete loss of intimacy. We had been a tight-knit team, albeit a motley one, but now we were satellites in separate orbits, crossing paths only when it came to our child. My friends with kids assured me that the situation was natural and would right itself over time, after the shock of our new addition wore off. One friend, a mom of three, went beyond that: “You can’t expect to feel the same way about your husband now. Your relationship needs to change so your son can be your focus. Our brains are wired so our kids can come first. It’s an evolutionary thing.”

Her statement stuck with me. I could not understand how an “evolutionary thing,” as she had so eloquently put it, would rule out a nurturing, loving relationship between two adults or an active sex life. Now that I had checked into the breeder category, wasn’t I supposed to keep popping out kids to guarantee propagation of the ancestral line? Sex, if not a little passionate love, was required to fulfill that goal. Perhaps I had missed something.

It was a conundrum. Like most new moms, I was bone-tired. Yet I was enthralled by this small baby boy who somehow managed to brighten each moment of my life as he sucked the energy out of me. Like my friend’s puzzling evolutionary edict, it was a contradiction I could not quite figure out.

As someone who wrote about neuroscience for a living, I began to wonder what role the brain played in what was happening to me. Maybe all of it—my crazy love for my son and my imploding marriage—could be explained by changes in my brain that occurred during and after childbirth. I remained convinced that my husband’s brain may have been altered a little too. Picking up a copy of a meeting report about the neurobiology of love seemed as good a place as any to start learning more.

Reading McEwen’s report, I was immediately struck by a single line, a quote McEwen added from British researcher and fellow Symposium attendee Nicolas Read: “If we realized how sexy babies are they would have been banned.”

Frankly, that kind of sentence is an anomaly in the scientific literature. You do not find lines like that in many research papers. Trust me on this one. Most of the papers I read when I’m working on a neuroscience story include lines like, “Aβ deposition stimulates a local immune response by the microglia, which become macrophagic.”2 Though fascinating (once translated into plain human English, that is), it’s not exactly the kind of stuff that makes you laugh out loud.

“It was a quip, obviously,” McEwen said when I asked him why he decided to include it. “But the mother-child bond seems to be such a strong, remarkable phenomenon.” What might researchers learn if they studied such things from a biological and mechanistic perspective? That is exactly what the attendees of the symposium wanted to start doing—as well as take a more mechanistic and biological approach to the study of monogamy, sex, and other love-related behaviors.

Even at such an early juncture in the neuroscientific study of love, McEwen had me at sexy baby banning. It has a certain perverse poetry to it. Not to mention that it resonates strongly for a new, sleep-deprived, and biologically curious mom like myself. To nonparents out there (and admittedly some of the parents too), I am sure this line sounds pretty creepy. Context is everything, after all. But to my mind, it fit. My baby was pretty sexy—much more so than I’d been prepared for. Not in a sweaty, naked-hot-guy kind of way, but in an irresistible, compelling way that had altered my body, my mind, and my life from top to bottom. I had no idea if these changes were due to evolution, neurobiology, or my particular situation, but I wanted to know more about how motherhood—and, yes, love—was facilitating them. So while I became more infatuated with my son each day, I was also just as intrigued by neuroscientific studies that gave me more insight into motherhood, monogamy, sex, and love.

Learning the In’s and Out’s of Love


I have already confessed that I know nothing about love. I’ve learned that you become much less reluctant to say so as soon as you are familiar with the inside of a divorce lawyer’s office. The disintegration of my marriage was not sudden; it took years and years to circle the drain. You’d think there would have been ample opportunity to correct course during those few years when our unhappiness became apparent. But no matter how deeply I wanted to fix things, I just never found a chance. I doubt my former husband did either. Even in hindsight I cannot tell you where my husband and I went wrong. Just when I knew it would never again be right.

Apparently I have not mastered that “life-long learning process”; despite the fact that my baby, now a charming and curious kindergartener, remains as “sexy” as ever, my search for emotional comfort and fulfillment with a partner continues. Might it have something to do with my hormones? The way my brain is wired? My choice of partner? The length of time we were together? How often we had sex? The way my body, including my brain, changed after having a child? All of the above? It was something I needed to figure out in order to truly move on after my marriage fell apart, not to mention bolster me as I reentered the dating realm.

Like most, I hoped to find some easy answers, some actionable advice that might help me understand my past relationships and, more important, avoid making the same mistakes in the future. With the dating pool beckoning, I hoped that acquiring the right knowledge would make up for developing crow’s feet, a postchildbirth figure, and postdivorce gun-shyness.

My quest to better understand the scientific nature of love started when I stumbled on a research paper and continued as I read all I could find on love, sex, and the brain. It was not enough to satiate my curiosity—I needed to talk to these scientists about their work and the different ways their findings might be interpreted. I had to visit a few labs and see some of the science in action. And surely it couldn’t hurt to participate in a few studies myself.

I learned a lot over the course of this journey. I discovered that the study of love—in any manner, neuroscientific or otherwise—is a tricky, complicated thing, more so than a natural skeptic like me had ever realized. But despite many enlightening discussions and personal adventures, nothing I experienced offered any surefire intelligence on what the majority of us are hoping to figure out: how to find love and then keep it around for a while. As it so happens, there is no magic formula, no rule book for understanding the brain’s role in love. There are no hard and fast answers. However, there are plenty of interesting surprises to be discovered about our “dirty” minds, both in animal models and in the fMRI scanner. To start I need to provide a little background, a taste of the brain areas and chemicals that fuel this most intoxicating of human emotions. This tale of brains in love begins with a bit of scientific history and the region of the brain responsible for reward processing called the basal ganglia.

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