Scents + Sensibility

By Charlotte Bruce Harvey '78 / March / April 2008
March 26th, 2008

Blame it on the pill. If Rachel Herz is right, one cause of escalating divorce rates may be birth control pills, but not in the way you might think. Herz, a visiting professor of psychiatry, says the pill alters a woman's sense of smell. The danger is that if she quits taking it the man she once found irresistible may turn out to smell, well, safe. Poof! The chemistry may vanish. 

Leah Fasten
The closest thing to a universally beloved smell is vanilla, which is similar to the scent of breast milk.
Herz is an expert on how smell affects human emotions and behavior, and if that seems an odd thing to study, you may be missing out on a way to help ease your stress or find happiness in love. And you're certainly not alone. Herz believes that most people underestimate the importance of odors. We all know we depend on our noses to sniff out a leaky gas line or a toaster oven that's about to catch fire. And many of us have experienced the sudden wash of emotion that strikes when we encounter a memorable scent: an inexplicable sense of comfort, say, as we pass a perfume counter testing a beloved grandmother's favorite fragrance, or a sudden sadness as a whiff of lilacs brings back that same grandmother's burial.

In fact, Herz believes that smell affects our emotional state in ways we're only beginning to understand. Our olfactory sense heightens our emotional range, and without the ability to smell we become anxious and depressed. Studies have shown that men are attracted first by a potential partner's looks and second by her smell, while heterosexual women home in on a man's body odor. Each of us, Herz says, has a unique scent, like a fingerprint. That particular chemical mixture signals the genotype of our immune system, the fifty-plus genes that scientists call our major histocompatibility complex, or MHC. Biologists believe that to protect their potential offspring against a wide range of illnesses, women sniff out mates with an MHC that complements their own.

Then there's the pill. It's been known for decades that women's sensitivity to smell fluctuates throughout their menstrual cycles, peaking at the time of ovulation to help them identify an appropriate mate at an appropriate time. The birth-control pill, however, throws off this instinct. Herz believes that by simulating a state of pregnancy—a time when women's noses are primed to seek safety in the company of family, rather than with randy breeding partners—the pill tricks women into selecting men who smell familiar, like a relative, rather than guys with the sexy smell of complementary MHC.

Add men's colognes, and it's no wonder the divorce rate is so high. "The Axe commercials have it right," Herz says. "A man's cologne really can trick a woman into making a bad choice." After a few months or years of marriage the guy switches cologne, the woman stops taking the pill, and they wind up in a lawyer's office with her saying she can't stand even the smell of her husband. If women want to guarantee that their relationship chemistry will last, Herz says, they should get off the pill for a while before marrying and take a good hard sniff. A radical solution to an exaggerated example, perhaps, but it underscores Herz's central point: how things smell to you may influence your decisions more than you think.

Herz describes her research and the theories she's developed from them in The Scent of Desire: Discovering Our Enigmatic Sense of Smell, which was published last fall. In it she explains how our olfactory and neurological systems interact and help shape our emotions. She outlines the evolution of our sense of smell and its long role in helping us avoid danger and pursue the good life, one that includes a spouse who smells enticing.

In smell there is one absolute truth: everything is relative. Herz says no odors are inherently repugnant; we learn our emotional responses to them through either experience or culture. One man's rotten fish is another man's durian. The fruit of an East Indian tree, the durian is considered the king of fruits in Thailand, but it's frequently described as smelling like rotting fish or sewage and is banned from many Southeast Asian hotels. Efforts to breed a less pungent hybrid have met with the kind of enmity the French reserve for bureaucrats who would meddle with their beloved raw-milk cheeses (whose aroma, in turn, revolts many Asians).

Herz herself learned to like the smell of skunk from her mother. She says all this variability in people's preferences has stymied the search for a "universal stink bomb" that might be used instead of tear gas to disperse crowds. In countries where private plumbing is rare, people find the ubiquitous odor of raw sewage inoffensive. In cultures where the dead are burned ceremonially, even the smell of burning bodies has no horrific associations.

Similarly, it's hard to find scents loved everywhere. Vanilla seems to top the list; breast milk smells like it and babies turn toward it naturally. But infants whose mothers ate lots of garlic while pregnant gravitate toward that smell. Herz describes a woman who hated the smell of roses because she associates them with her mother's funeral.

Aromatherapy is increasingly popular because, as Herz points out in a chapter on the subject, the emotional associations triggered by certain scents in turn prompt physiological changes. Accustomed to smelling lavender in bath oils and soaps, we associate it with lounging in a tub, so when we encounter it elsewhere, we relax. Our heart rate does go up or down in response to certain smells and contexts, but the cause is psychological, not pharmacological, Herz insists. Even more remarkable, we don't have to experience these psychological associations to respond to them: you don't have to live through a house fire to experience an adrenaline surge when you smell smoke.

When Herz joined Brown's psychology department in 2000, she was delighted to follow the path of her intellectual mentor, psychology professor Trygg Engen, with whom she had coauthored several papers in the 1990s. Now retired, Engen pioneered the psychological study of smell and taught generations of undergraduates how closely memory and odor were linked. Herz has built on Engen's work, focusing on the relationship between odor, memory, and emotion. She emphasizes that what distinguishes the memories that odors evoke is not their clarity or their detail, but their emotional content. When a smell sparks a memory, it's likely to bring a rush of emotion and to make us feel as if we've been rocketed back in time—as if we were physically back in the place where we first encountered it. To Herz, this all makes perfect sense, because in the brain the olfactory cortex and the amygdala, the part of the limbic system that is thought to process memory and emotion, are interconnected.

To study the way experience influences odor preferences, Herz conducted an experiment. She asked students to complete a task using a computer while exposed to an unfamiliar odor. Some participants were given a particularly frustrating assignment on the computer and lost play money if they failed to complete it. Others were given an easy task and were rewarded with real money. Afterward, the students were asked to rate the odor. Those who had been given the hard test were more likely to dislike the scent; the second group liked it.

Leah Fasten
Psychologist Rachel Herz says smell influences our emotional state in complex ways researchers are only beginning to understand.
Not all reactions to smells depend on personal experience or culture, though. Herz says everyone responds to odors containing chemicals that irritate the nasal lining and stimulate the trigeminal nerve, the cranial nerve that processes feeling in your face. This is the nerve that makes you cry when you cut onions or sneeze when you inhale pepper. It's why peppermint scent feels cool, and why the ammonia in smelling salts rouses you from a faint.

The most basic of our five senses, smell evolved way back when our forbearers were single-celled organisms. Long before our microscopic ancestors developed the ability to see or hear or taste or touch, they needed to know whether the chemical molecules they were approaching were good (food) or bad (foe). If something "smelled" safe, they moved toward it; if it smelled nasty, they beat a hasty retreat.

As more complex beings evolved, the limbic system developed. (Herz calls it "the ancient core of the brain." It is sometimes called the rhinencephalon—"literally the nose-brain," she says.) The first part of the limbic system to develop was the olfactory cortex, which processes odors, and out of that grew the other structures of the limbic system, such as the amygdala. Thus related, olfaction and emotion serve the same function: to tell us whether to proceed or to flee.

"Brain imaging studies have shown when we perceive a scent, the amygdala becomes activated and the more emotional our reaction to the scent, the more intense the activation is," Herz writes in her book. "No other sensory system has this kind of privileged and direct access to the part of the brain that controls our emotions." Smell smoke, feel frightened. Get a whiff of your perfect mate, and you swoon.

But lose your sense of smell, and the effect can be devastating.

On August 30, 2005, just three months after graduating from Brown, one of Herz's former students, Molly Birnbaum, was struck by a car while jogging in Boston. The impact fractured her skull, and her brain bounced, severing her olfactory neurons. An avid and talented cook, Birnbaum had won a scholarship to the Culinary Institute of America and was planning a career as a chef. Although her body recovered quickly, the head trauma left her unable to smell and so unable to enjoy eating or cooking.

The ability to perceive flavor is a two-step process, Herz explains. We use the ten thousand or so taste buds in our mouths to pick up a food's sweet, sour, salty, and bitter components, as well as umami, the meaty so-called "fifth taste" that's in MSG and Vietnamese fish sauce. Supplementing these five tastes is the aroma we inhale as we bring each bite of food to our mouth and then smell again as we chew it and force the chemical molecules up toward the back of the nose, where their chemical components are broadcast to the olfactory cortex. Without smell, we can only perceive taste, Herz says, but not flavor. This is why eating is so dull when you suffer from a cold.

Unfortunately, your sense of smell diminishes over time. A quarter of the people between the ages of sixty-five and eighty have no sense of smell; among those over eighty, fully half have lost their ability to detect odors. This, Herz claims, is a major reason old people develop malnutrition. It's also a cause of depression, she says. Without smell our emotional range is narrowed. The process works in the other direction as well: when we're feeling down, our sense of smell is muted. It's a feedback loop that can lead to a downward emotional spiral.

In Birnbaum's case, loss of the ability to smell led to anxiety. "I couldn't smell gas or rotting food," she says, "so I didn't want to live alone." Working in a bakery, she adds, "I got really depressed listening to customers exclaim how wonderful the place smelled. I burned cookies because I couldn't smell them."

Four months after Birnbaum's accident she underwent a battery of tests to see how well her olfactory system was working. Out of fifty or so scents, she was able to identify only three: chocolate, laundry, and rosemary. Although Birnbaum's depression was milder than many Herz describes in her book, the would-be chef was forced to change her career plans. She turned down her cooking school scholarship and sought out journalism jobs. Last fall she enrolled at Columbia's journalism school. She's working on a book about losing her sense of smell and has been interviewing patients at the University of Pennsylvania's Taste and Smell Center—people who've lost their sense of smell to a virus, head trauma, or old age. "Everyone I've talked to has been depressed," she says.

The daughter of academics, Herz says she always planned on an academic career. She began her doctoral work as a sleep researcher in Toronto, before switching to smell. (She jokes that she's "always been drawn to the under-researched 'S words.' ") After coming to Brown, she began helping train the sleep researchers who intern each summer at the E.P. Bradley Hospital Sleep Center, which is associated with Brown's Alpert Medical School.

There Herz teamed up with the center's director, Mary Carskadon, a professor of psychiatry and human behavior, to research people's ability to detect odors while sleeping. Another prominent sleep researcher had proposed that alarm clocks could use odors to awaken people, and Carskadon and Herz wanted to see if that was possible. For an earlier study, one of Carskadon's undergraduate students had built an olfactometer—a machine that delivers precise levels of various odors through tubes that empty into the nose. Carskadon and Herz exposed their waking and then sleeping research subjects to varying degrees of two scents: peppermint and pyridine.

"It stinks to high heaven," Carskadon says of the latter. "It's released in certain kinds of fires." When exposed to even moderate levels of pyridine while awake, the subjects were likely to yank the tubes from their faces, Carskadon says. But in deep and REM (rapid-eye-movement, or dreaming) sleep, few could be roused by even the strongest doses of the substance. The message was clear: Don't depend on your nose to awaken you in case of fire.

"You wake up and then you smell the coffee," Herz explains. "The coffee doesn't wake you up."

Herz has written numerous articles for scholarly journals, but The Scent of Desire is clearly aimed at a general readership. And, as a consultant, Herz has embarked on a couple of non-scholarly entrepreneurial ventures as well. One is aimed at educating chefs' palates through their noses. She's created a kit called the Educated Palate that contains essential oils of twenty-five herbs and spices commonly used in cooking. Another project is a trio of scents she helped create for a "scientific" aromatherapy line called Scentology. Each of the line's three aromas is designed to produce a specific psychological effect when you spray it on your wrist. Peppermint "Endurance Enhancer" promises to energize. Citrus-scented "Bliss Booster" lifts moods. "Crave Control" smells like soft-serve vanilla ice cream and is supposed to keep you from breaking down and ordering dessert—as long as you apply it on a full stomach.

One day this winter Herz was awaiting a call from a newspaper reporter who was writing about a new dating service called Using members' DNA samples, the company proposed to match them with partners who have compatible immune systems—creating MHC matches without using smell. Was the idea a viable one? the reporter wanted to know.

Herz said she'd had a similar idea of using MHC compatibility for a matchmaking site. Only her approach had been to let members pick a slate of potential partners and then send them T-shirts that their chosen partners had worn for a couple of days. If the customers liked each other's smell, they could meet. Unfortunately, Herz's women friends didn't like the idea of sniffing some stranger's sweaty T-shirt. Herz let the idea pass. She laughed, though, recalling her male friends' responses. "The guys didn't have any problem with the idea of smelling women's T-shirts. They said they'd smell anything a woman had worn."

Charlotte Bruce Harvey is the BAM's managing editor.

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March / April 2008