CHAPTER 3
Freaks, Geeks, and Prodigies
DON GIOVANNI: Zitto: mi pare sentir odor di femmina!
[Hush! I think I scent a woman!]
LEPORELLO: Cospetto! Che odorato perfetto!
[My, my! What a nose!]
DON GIOVANNI: All’aria mi par bella.
[And a pretty one at that.]
—MOZART, Don Giovanni
TAKE A FEW DOZEN PEOPLE AT RANDOM, AND YOU WILL find a range of olfactory talent that stretches from American Idol–tryout bad to unbelievably excellent. There are people who cruise untroubled past the fetid plumes of garbage cans and subway vents, and others for whom the faintest milk fart escaping from an elderly relative is a nasal crisis. Olfactory sensitivity (technically, the lowest concentration at which someone detects a smell) is just one dimension of smell talent; other factors include an awareness of smells, and the ability to identify them and discriminate among them. Extreme variability is a hallmark of odor perception, and sensory scientists have identified many factors that contribute to it. It is now possible to answer a fundamental question: Who has a good nose and who doesn’t?
THE FIRST THING to note is that people are not accurate judges of their own ability. When we asked people taking the National Geographic Smell Survey to rate their own sense of smell, we found a Lake Wobegon effect: most people were above average. The only way to assess someone’s ability impartially is with a smell test. These come in two types: identification tests and threshold detection tests. The former ask you to put names to odors, the latter ask you to detect progressively lower concentrations of a smell. Smell tests have been commercially available for years, but were formally recognized as a medical device by the FDA only in 2006; this may explain why they are an underutilized part of the physician’s diagnostic arsenal. The tests range from one-shot sniff tests, appropriate for quick screening during an office exam, to elaborate, hours-long testing with scores of odors that takes place in a research lab. Normal smelling is generally defined as a certain proportion of correctly identified odor samples, or a specific, very dilute concentration at which an odor should be smellable. An odd feature of smell tests is that the best one can score on them is “normal” there is no test that rates levels of excellence, no equivalent to a 150 IQ. In fact, there is not even an official medical term for smell genius.
Because smell tests are designed to identify people with dysfunctional noses, they are finely calibrated for degrees of underachievement. At the lowest end of the scale are people unable to smell anything at all; they suffer from anosmia, the technical term for complete smell loss. One notch up the scale are people with hyposmia, which is the olfactory equivalent of being hard of hearing; like deafness, it can be mild or severe. It has been estimated that 1 to 2 percent of the U.S. population suffers from anosmia or hyposmia. In both cases, the most common cause by far is infectious disease. Severe colds, flu, and sinus infections inflame the tissues lining the nose and kill off sensory nerve cells. In severe cases, or after a lifetime of accumulated damage, areas that were once rich with nerve cells are replaced with nonsensory mucus membrane, and the tissue takes on a moth-eaten appearance.
Head injury is the second leading cause of smell loss. A blow to the head can sever some of the olfactory nerve fibers that travel to the brain through tiny holes in the base of the skull, at a location between the ears and behind the eyes. There’s an old (and possibly true) story about a waiter carrying a tray of food at head height. As he exits the kitchen, the swinging door slams the tray into his forehead. Being a professional, he maintains his balance and proceeds into the dining room. As he serves the dishes, he realizes he can’t smell a single one. The speed with which the waiter discovers his loss might be unusual—most people don’t notice for days or weeks—but the mildness of the damaging blow is not. It takes very little force to cause smell loss. I cringe when I see kids heading the ball in a soccer game. I wouldn’t bet on them becoming chefs or perfumers.
With the exception of a stuffy nose, smell loss is a long-term condition. Smell may return after a flu or sinus infection, as the damaged sensory cells are gradually replaced by new ones, but recovery can take months and your abilities may never return to their original levels. The probability of recovery declines with age. In cases of head trauma, the prospects are bleak; the severed nerve fibers rarely reconnect. Consider the results of a typical study: a year after their initial visit to the doctor, 32 percent of postinfection patients showed improvement, compared with only 10 percent of the post-injury group. The realization that millions of Americans suffer from smell loss spurred the National Institutes of Health to underwrite basic research into odor perception. The ultimate goal of this work was to find ways to cure smell loss. Yet, despite decades of substantial funding, effective medical treatment remains elusive.
Sudden smell loss is psychologically devastating. By far the biggest impact is on eating: anosmia steals the pleasures of the table. Without its aroma, food in the mouth becomes a bland, chewy mass, and drinks become equally flavorless. Faced with dull food, some people lose appetite, eat less, and lose weight; others eat to feel full and end up gaining weight. Smell loss can alter mood—patients often show symptoms of depression, and psychological well-being, friendship, emotional stability, and leisure activities all take a hit. Some people find that their sex life suffers. In the wake of smell loss comes the anxiety of constant vigilance. Anosmics worry about gas leaks, undetected fire, spoiled food, and lapses in personal hygiene. They adopt coping strategies such as frequent bathing and laundering. Anosmics report smell-related hazardous events—burning a pot or eating spoiled food—more often than normal smellers, but there is little data to suggest a higher rate of actual injury.
In rare instances, people are born without a sense of smell. As it’s hard to miss what you’ve never known, people who are anosmic from birth tend to regard their condition with bemusement. A few even manage to find a silver lining. The ex-boyfriend of a young English anosmic told her, “You were the best girlfriend in the world. You let me bring curry home from the pub every night, and I could fart as much as I liked.” One newspaper reporter who is smell-blind from birth regularly covers smelly stories for a major U.S. daily. This is either a heartwarming story of a disability overcome, or journalistic malpractice of the first order. Perhaps, in a zany way, it is both.
Somewhere off the main continuum of normal to partial to complete smell loss lie the bizarre pathologies of odor perception. A person with phantosmia, for example, perceives a smell when none is there. These olfactory hallucinations can be vague (“a chemical smell”) or quite specific (one patient said, “It reminds me of a flower I smelled in Samoa”). Phantosmia is a tricky diagnosis for a doctor to make: the hallucinated smell comes and goes and may not occur in the course of an office visit. The physician must first rule out all possible organic sources for the weird smell, especially sinus or gum disease. The physical causes of phantosmia are diverse and include seizure, migraine, and brain tumor. When a real odor gives rise to a distorted perception, the condition is called parosmia. The distortions in such cases are almost always unpleasant; patients say things smell foul, rotten, or burned. Such was the case of a sixty-year old woman who awoke one morning to find that every odor smelled like burnt toast. Eleven years later, despite treatment with antibiotics, antivirals, vitamins, beta-blockers, anticonvulsants, and zinc sulfate, her condition was unchanged. Most parosmics can tell you which smells are distorted; the most common are gasoline, tobacco, coffee, perfumes, fruits (mainly citrus and melon), and chocolate. Parosmia almost always occurs after an upper-respiratory-tract infection or head trauma, where smell function is reduced but not completely gone. This leads researchers to speculate that parosmia is an “incorrect rewiring” of the connections among regenerating nerve cells following damage to the olfactory system. Among smell pathologies, the most appalling is cacosmia, in which everything smells like shit.
In Philip K. Dick’s sci-fi novel The Simulacra (1964), there is a character named Richard Kongrosian, a psionic pianist who plays the instrument telekinetically. He also has a history of mental instability. An annoying advertisement triggers in Kongrosian the delusion that he has a bad body odor. He becomes obsessed with BO and washes compulsively, but in vain; the smell lingers. His ability to play the piano from a distance notwithstanding, Kongrosian is a poster child for a real-life psychiatric disorder known as olfactory reference syndrome, which is characterized by persistent hallucinations of body malodor.
IT PROBABLY COMES as no surprise that men and women differ in smell ability. This has been confirmed many times with a variety of test methods and in cultures around the world. Women rate themselves as having a better sense of smell, and the data back them up. Women detect odors at lower concentrations and are better able to identify them by name. A German psychologist found that men and women are equally good at remembering colors and musical tones, but women are better at remembering smells. Humorist Dave Barry’s wife would not be surprised:
At least five times per week, my wife and I have the same conversation. She says: “What’s that smell?” And I say, “What smell?” And she looks at me as though I am demented and says: “You can’t SMELL that?” The truth is, there could be a stack of truck tires burning in the living room, and I wouldn’t necessarily smell it. Whereas my wife can detect a lone spoiled grape two houses away.
Sex differences are based on group averages; there is much variability within each sex, and large overlap between them. But in general, women are better. Or, as Dave Barry put it, men suffer from Male Smelling Deficiency Syndrome.
What explains the female superiority? There is little evidence of sex differences in the nose. Dave Barry’s nose probably looks and operates much like his wife’s. The brain is a different story. Recent evidence suggests that brain structures related to odor perception differ in size and cellular architecture between men and women. Whether these anatomical variations explain Barry’s quip remains to be seen. We do know that some male-female differences in perception (the fact that women often rate smells more intense and unpleasant) are mirrored by differences in the underlying brain-wave response.
Female smell superiority is partly due to women having higher verbal fluency; verbal skills boost performance on tests of odor memory and odor identification. Another factor is hormones. A woman’s smell sensitivity varies across her menstrual cycle and is greatest around the time of ovulation. Hormone effects are not simple; they interact in complex ways with cognitive factors. This interaction produces one of the most dramatic olfactory sex differences ever observed in the lab. Sensory researchers Pam Dalton and Paul Breslin tested men and women for their sensitivity to a specific odor. With repeated testing over the course of thirty days, the women became much more sensitive to the odor, while men did not. The effect was confined to the tested smell; sensitivity to a different odor did not change for men or women. The enhanced sensitivity can’t be attributed to practice; the women weren’t getting better at threshold tests in general. They became more sensitive because they paid close attention to low levels of odor while being exposed to it multiple times. Most remarkably, Dalton and Breslin didn’t find enhanced sensitivity in prepubescent girls and postmenopausal women. The phenomenon is limited to women of reproductive age. This implies that female hormones are needed to make it happen, and in fact it can be observed in postmenopausal women who take hormone replacement therapy.
Sex differences are evident within days of birth: baby girls turn toward novel odors and spend more time smelling them than baby boys do. The anthropologist Lionel Tiger attributes the difference to evolution. In our long history as hunter-gatherers, he says, it was the females who gathered fruits and vegetables, and a good sense of smell was valuable in judging ripeness and safety. Tiger’s view—essentially a biologized version of “women spend more time cooking”—will not be received warmly in some quarters. Yet it’s hard to see how a cultural explanation can explain sex differences in two-week-old infants.
WITH AGE, OLFACTORY performance begins to deteriorate. The first signs of decline are detectable in the early forties—at least under laboratory conditions—and the pace accelerates in the sixties and seventies. Interestingly, the rate of decline varies with the odor. Rose and banana, for example, are easily perceived until people are in their seventies, while mercaptans (the natural-gas warning odor) show a drop among people in their fifties. Some age-related smell loss can be traced to the nose itself—the accumulated wear and tear of infections and minor blows to the head. Some of the loss is traceable to the brain. For example, odor identification ability depends on how much short-term memory the test requires. Because short-term memory declines with age, elderly people score better when the odor test is presented in a simple yes/no format than in a multiple choice format that requires more memory capacity. In any case, decline is not inevitable; a given seventy-five-year-old may outperform a given twenty-five-year-old. Perfumers, in fact, usually get better with age. Experience and skill more than compensate for any dimming of acuity that comes with age. I know of no fragrance house with a mandatory retirement age for perfumers.
TO THE AVERAGE person it seems obvious that smoking must dull the sense of smell. Surprisingly, the evidence is equivocal. Some studies find adverse effects of smoking but many, including several recent ones, do not. One, an Australian study of 942 people, found that having a smoke within fifteen minutes of smell testing put a temporary dent in performance. Other than that, “smoking did not reduce olfactory performance or self-assessment of olfactory ability in this group, contrary to previous findings.” The National Geographic Smell Survey reported mixed results. For example, smokers found the artificial musk scent of Galaxolide more intense than did nonsmokers, but the reverse was true for the musky-urinous smell of androstenone. Pleasantness ratings for the skunky-smelling mercaptan sample were higher among smokers, but so were their ratings for rose and cloves. It’s possible that smokers become sensitized to some odors and desensitized to others. In any case, minor effects of smoking observable in clinical testing may have little appreciable impact on everyday smell function. Indeed many perfumers, including the best in the business, have smoked like chimneys.
So strong is the conventional wisdom about the negative effect of smoking that researchers worry when they fail to confirm it. Take the case of a large population-based study in Skövde, Sweden. It linked decreased olfactory performance to several factors including being older, being male, and having nasal polyps. Smoking was not one of the factors. Similarly, diabetes and nasal polyps predicted complete anosmia, but sex and smoking did not. The authors didn’t find that smoking improved odor perception; they merely failed to find that smoking harmed it. One can see them bracing for a wave of politically correct indignation when they say, “The lack of a statistically significant relationship between olfactory dysfunction and smoking may be controversial.”
Blind Faith
When, at a party, I own up to being an expert on the sense of smell, I get peppered with questions. (I don’t mind this—if I’m not in the mood for Q&A, I tell people I’m “in the chemical business” and the conversation grinds to a halt.) People often ask about smell ability. Who is better: men or women? perfumers or normal people? Curiously, one comparison doesn’t come as a question but as an assertion. Wineglass in hand, someone will inform me in earnest tones that “blind people have a heightened sense of smell.” Others confidently assure me that “Helen Keller had an incredibly sensitive nose.”
Helen Keller has been dead since 1968, but remains a powerful symbol of the belief that blindness turns people into super-smellers by way of compensation. (The Marvel Comics hero Daredevil embodies the same idea.) Despite her iconic status, Helen Keller herself did not claim to have a supersensitive nose. In her famous essay “Smell, the Fallen Angel” she describes what she is able to smell. Amid lyrical, somewhat overripe prose (“Smell is a potent wizard that transports us across a thousand miles and all the years we have lived”), she gives specific examples of her olfactory ability. Let’s compare her talents to ours. Smells trigger memories—check. Approaching rainstorms have a smell—check. Can smell if a house is old-fashioned and long-lived-in—check. Can smell a person’s occupation (painter, carpenter, ironworker)—check. Close friends have distinctive odors—check. Babies smell sweet—check. Nothing extraordinary so far. Helen Keller does not sound like a nasal genius. Indeed, nowhere does she claim to have a more sensitive nose as a result of being blind, or that her sense of smell is better than that of sighted people. On the contrary, she writes, “I have not, indeed, the all-knowing scent of the hound or the wild animal.” She also says, “In my experience smell is most important.” It is not surprising that, being blind and deaf, she finds smell to be her primary way of sensing the world.
Helen Keller’s modest assessment of her own ability has done little to dampen enthusiasm for the idea of smell compensation in the blind. It seems so reasonable it must surely be correct. But is it? There is plenty of experimental evidence that addresses the question—in the last twenty years, six studies have compared smell in the blind and the sighted. Without exception, they find that the blind are no more sensitive than the sighted—both groups detect odors at about the same concentration. Nor do blind and sighted people differ in the ability to discriminate one odor from another. Even the brain waves triggered by odor stimulation are similar in blind and sighted people.
Blind people may have one advantage: in three of the six studies, they were better at naming odors. Even here, their success depended on cognitive factors such as memory rather than hyperacute perception. Based on her own words, and on what has been observed in experiments, Helen Keller’s ability to navigate the smellscape was not the result of a supersensitive nose. Rather, it was a triumph of the adaptable human brain making the most out of a perfectly ordinary nose.
The Nose on Freud’s Face
Sigmund Freud was not a big fan of the nose. He believed odor perception was vestigial, the sensory equivalent of the appendix. In his view, smell became obsolete when our evolutionary ancestors took on upright, bipedal posture and put distance between the nose and the ground. At the same time, Freud’s ape-man discovered shame and disgust in the exposure of his genitals. This led him to turn away from the stink of excrement and to suppress his sense of smell in general. To Freud, this was a vital precondition for the emergence of civilization—the repression of smell meant the repression of wild sexual impulses and their redirection to more refined behavior. Freud thought that children recapitulated the history of the species as they grew up, and thus that the infant’s early interest in smell fell away like embryonic gill slits. Freud’s leading American disciple, A. A. Brill, summarized the master’s view: “All children make good use of the sense of smell in early life; some of them, as we shall learn later, retain it even in adult life; most of them, however, lose it, so to speak, as they grow older.” To the orthodox analyst, psychologically mature adults move on and leave fascination with smells to perverts and neurotics.
Like many of Freud’s theories, his views on smell are difficult to summarize without making them sound simpleminded and ridiculous. The original texts consist entirely of a few sentences in a letter to his confidant Wilhelm Fliess, a German ear-nose-and-throat physician, and two footnotes in the book Civilization and Its Discontents, and are part of what historian Peter Gay called Freud’s “audacious, highly speculative venture into psychoanalytic prehistory.” Nevertheless, after becoming part of the bedrock of psychoanalytic theory, they helped devalue smell in the wider intellectual world.
It is puzzling that Freud, who found a sexual angle in every other facet of psychology, thought it had so little to do with the sense of smell. Is sexual attraction no longer an affair of the nose? Are modern women scentless and modern men oblivious, or vice versa? In a recent University of Texas study, men said T-shirts worn by women near the time of ovulation smelled more pleasant and sexy than T-shirts worn during a nonovulatory part of the cycle. Modern women, it seems, continue to produce a scent cue associated with ovulation and modern men continue to respond to it. This low-technology experiment could have been done in Vienna in 1930 or New York in 1932, had either Freud or Brill cared to test their theories.
Brill toed the party line when he wrote in 1932 that “the sense of smell unlike the sense of sight plays a very small part in the life of civilized man,” and “modern man has little need of his sense of smell.” Though surrounded by modern, civilized men, Freud and Brill never bothered to ask their opinion. The psychologist Paul Rozin and colleagues got around to it a few years ago. They asked people to rank the unacceptability of permanent loss of sense of smell, loss of hearing in one ear, and loss of the left small toe. According to about half the respondents, loss of smell was the least acceptable alternative. The average person is not as dismissive about the sense of smell as Freud thought he was. What could have motivated Freud to construct a psychoanalytic conjecture so flimsy it could be blown up with a simple opinion poll?
The experts think it was something, well, Freudian. The psychoanalyst Annick Le Guérer attributes it to Freud’s “repression” of his “transferential relationship with Fliess.” The anthropologist David Howes thinks Freud’s conflicted emotions toward Fliess led to his “denial of nasality” and a desire to “cut the nose out of psychoanalytic theory.”
I have a more straightforward hypothesis. Based on the facts of his medical history, I suspect Freud suffered from hyposmia. The repeated insults of cocaine, nose surgery, influenza, sinus infection, cigar smoking, and finally aging left him with a clinically impaired sense of smell.
Freud caught influenza in the spring of 1889, at the age of thirty-three. The infection was severe enough to leave him with a persistent cardiac arrhythmia, so it could easily have affected his nose. In his letters to Fliess from 1893 to 1900, Freud often complains of nasal congestion with discharge of pus and scabs, both symptoms of sinus and nasal passage infection. Freud suffered from migraine headaches, which he treated with nasal applications of cocaine prescribed to him by Fliess. Fliess operated on Freud’s nose twice to remove and cauterize part of the turbinate bones. On top of all this, Freud smoked heavily; his typical rate in the 1890s was twenty cigars a day.
Freud’s nose was already a medical disaster zone when he hatched his smell theory in 1897, and my hunch is that he was already smell-impaired. When he wrote Civilization and Its Discontents in 1930, he was seventy-four years old and suffering from cancer of the jaw as well. In my view, Freud’s intellectual indifference to smells was the result of sensory deprivation—the gradual onset in adulthood of severe hyposmia. His ludicrous idea that smell was active in children but ceased to matter for adults had nothing to do with his feelings about Wilhelm Fliess. It was simply an overgeneralization of his unfortunate personal experience.
Our Rank in the Animal Kingdom
No doubt there is a vast difference in power in the sense of smell in both these animals [deer and dog] and in man; nevertheless, I do not think so meanly of man’s olfactories as some physiologists appear to do.
—W. H. HUDSON, On the Sense of Smell (1922)
After finishing my PhD at the University of Pennsylvania, I began working a few blocks away, at the Monell Chemical Senses Center. I received a fellowship to study there with Dr. Kunio Yamazaki. He had several lines of inbred mice used for cancer research; the lines were genetically identical except for a set of genes known as the Major Histocompatibility Complex (or MHC), which controls the body’s tissue-rejection response. They are the genes used to find whether a person is a suitable match as an organ donor. Yamazaki’s mice preferred to mate with individuals bearing a different MHC type apparently on the basis of smell. My plan was to study the behavior behind the odor-based mate choice using competitive mating experiments, where the female had access to multiple males of different MHC types.
Watching the mice choose mates, I became curious. Could humans detect the odor differences that were so apparent to the mice? Soon I was running my first experiment on human odor perception. I had blindfolded people sniff live mice in Tupperware containers with holes cut in the sides. Occasionally a mouse tail would get up someone’s nose; this seemed to bother some people more than others. The judges also sniffed tiny test tubes filled with mouse urine or dried fecal pellets. (Thankfully, no one inhaled a mouse turd.) For every odor source the results were clear: untrained humans could distinguish between the mouse strains based on smell alone. The uncanny scent powers of mice were well within human reach. I wrote up the results as a man-bites-dog story for the Journal of Comparative Psychology, and it eventually became one of the mostcited scientific papers I have ever published. By encouraging me to continue exploring human odor perception, it also led to my career in the perfume industry.
Deborah Wells and Peter Hepper discovered an even more impressive man-smells-dog story. They had dog owners sniff two identical blankets, of which one had been slept on by their pet and the other by an unfamiliar dog. The owners correctly identified their dog 89 percent of the time. The strength or pleasantness of the smell was not a factor, nor were non-doggy household odors.
Stories about the amazing ability of the canine nose highlight the dog’s talent and ignore how the feat is stage-managed by humans. (“Pay no attention to that man behind the curtain!”) Consider the recent finding that dogs can sniff out bladder cancer. The dogs in question were trained exhaustively with human urine samples. Training began with search-and-find games and progressed to more complex tests. Urine samples were carefully selected so the dogs would learn to ignore irrelevant dietary odors. The trainers also counterbalanced samples from smokers and nonsmokers, patients and healthy people. After seven months of training, the dogs were ready for the decisive test: to pick the single positive sample from a set of seven. As a group they were correct 41 percent of the time, which successfully beat the random odds (which were one in seven, or 14 percent). The resulting scientific report made headlines around the world.
So, yes, dogs can smell odors associated with bladder cancer. But this is a far cry from “What’s that, Lassie? Timmy has bladder cancer?” To make use of this canine talent, your local hospital would have to maintain a half-dozen dogs and their trainers, supply copious medically certified human urine samples, and provide ongoing statistical support and chemical analysis. At which point six out of ten bladder cancers would go undetected.
If the human nose received the same gee-whiz treatment given to animal stories, we would sound as impressive as any dog. Here’s an example: Just by smelling some ice cream that once had a wooden popsicle stick in it, regular folks can tell whether the stick came from Wisconsin, Maine, British Columbia, or China. Amazing, no? How do those monkey-people do it? In this case, wooden sticks from each locality were frozen in vanilla ice cream for six days. The samples were melted and the sticks removed. The sniffing primates—Ohio State graduate students—had to pick the same sample from a repeatedly presented pair of samples five times in a row to be declared a success. All possible pairs of wood source were tested. Two judges failed—they couldn’t tell one stick-scented ice cream from another. Eight judges succeeded—they could reliably discriminate anywhere from five to nine of the ten possible pairings. Not bad for humans. Could the judges explain how they did it? Unfortunately not, but then, neither could the cancer-sniffing dogs.
The physicist Richard Feynman had a great party trick in which he would identify by smell objects briefly handled by other guests when he wasn’t looking. He said it was easy to do because peoples’ hands have surprisingly different scents. (A 1977 study confirmed that hand odor is individually distinctive and discriminable.) There are other stupid human tricks besides Feynman’s. For example, in a lineup of dirty laundry we can pick out the T-shirt worn by our spouse or partner. A mother can identify the smell of her own baby, and a baby can pick out the scent of its mother’s breast.
How do humans measure up at the quintessential doggy task—scent tracking? Researchers at UC Berkeley had people get on their hands and knees and follow a 10-meter-long chocolate-scented trail using only their noses. The test subjects wore goggles, gloves, and kneepads, which restricted nonolfactory input. Two-thirds of the people tested successfully followed the trail under these conditions. (When subjects wore nose plugs, none of them could follow the trail.) With a few days of training, tracking speed was doubled and people strayed less from the scent trail. Dog lovers (of which I am one) may also be surprised to learn that drug dogs and humans have almost identical sensitivity to methyl benzoate, the smell used to track cocaine. Dogs have great noses, but it’s time to stop the trash talk and give ourselves more credit.
Many people take it for granted that the human nose is inferior, and scientists often make the same assumption. Charles Darwin thought our evolutionary ancestors made good use of smell, but felt it was “of extremely slight service, if any” to modern man. The sex psychologist Havelock Ellis agreed: “Among the apes it has greatly lost importance and in man it has become almost rudimentary, giving place to the supremacy of vision.” The attitude persists. As recently as 2000, some French researchers asserted “The sense of smell in primates is greatly reduced (microsmatic) with respect to other mammals such as dogs or rodents.”
Scientists are taking a fresh look at the conventional wisdom regarding the sense of smell in animals. The anatomists Timothy Smith and Kunwar Bhatnagar, for example, are questioning the textbook distinction between macrosmatic and microsmatic animals, i.e., those with good and poor olfactory abilities. The long-standing assumption is that what separates macrosmatic and microsmatic species is the amount of surface area inside the nose. This turns out to be a bad assumption; internal surface area is more about air conditioning—warming and filtering incoming air—than about odor perception. Of more relevance is the amount of sensory tissue in the nose. But Smith and Bhatnagar find that the amount of sensory tissue varies from species to species independently of total surface area. Further muddying the waters, the number of olfactory nerve cells per square inch varies among species. All in all, surface area is a dubious proxy for smell ability. Smith and Bhatnagar suggest that the traditional macrosmatic/microsmatic distinction has outlived its usefulness. Size isn’t everything.
The Yale University neurobiologist Gordon Shepherd agrees that counting nerve cells is a poor way to estimate sensory talent. In his view, the number of cells available for odor detection is less important that what the brain does with the information those cells provide. He makes the analogy to hearing: humans have about the same number of auditory nerve fibers as cats and rats, yet we have far superior speech abilities. It’s the brain areas that analyze and interpret speech sounds that provide the advantage, not the number of cells in the ear.
The German sensory physiologist Mathias Laska cuts right to the chase by measuring odor perception in different animal species. He has used reward-based conditioning techniques to find odor detection thresholds in spider monkeys, squirrel monkeys, and pigtail macaques. According to conventional wisdom, these primates are less sensitive than dogs and rabbits, but Laska finds they perform quite well—monkey thresholds are comparable to those of dogs and rabbits across a variety of odors. And contrary to Darwin’s gloomy belief, Laska finds that humans have odor sensitivity similar to that of apes and monkeys.
New evidence suggests that humans and animals may be more similar in odor perception than we thought. In 1991, Linda Buck and Richard Axel discovered a large family of mammalian olfactory receptor genes, work for which they eventually received the Nobel Prize. Each gene produces a different receptor. In general, more receptors means more detectable odors, and therefore greater smell ability. Rats have about 1,500 functional receptors, followed by dogs with about 1,000, mice with about 900, and chimpanzees with about 350. Humans have somewhere between 340 and 380. Dolphins have zero.
Does this mean rats are five times better smellers than we are? Not really. We can use DNA sequence similarity to arrange odor receptors into families and subfamilies. In theory, similar receptors detect similar odor molecules, so a receptor subfamily detects a class of related odors. When we compare odor receptor subfamilies, the human-animal gap doesn’t look too large. Humans and dogs have about 300 subfamilies, rats have 282, and the mouse 241. The overlap between species is substantial. About 87 percent of human receptor subfamilies have counterparts in the mouse genome, while 65 percent of mouse subfamilies are shared by humans. This suggests to Linda Buck and her colleagues that “the majority of odorant features [i.e., smells] detectable by one species may also be recognized by the other.” Perhaps a mouse can smell more of our world than we can smell of his. (Unlike us, he may have a whole subfamily of receptors devoted to cat urine.) For man and mouse the differences are not as big as the similarities. For man and chimp this is even more the case—there is a corresponding human gene for 85 percent of chimp odor receptor genes. Chimp, dog, man, or mouse, we perceive the general features of the smellscape in much the same way.
Physical equipment—size of brain areas, number of nerve cells or receptor types—may be less important than what the brain does with the information once it arrives. For many animals, a smell is a call to action, a trigger for a biologically hard-wired survival response: “scent of lion means flee.” In contrast, human cognitive abilities turn smells into symbols and let us make flexible use of their signal value. When it comes to comparative smell ability, it’s the brain, stupid.
Better Than the Rest
One morning when I walked on my monk’s alms-rounds to collect food, my nose became like that of the most sensitive dog. As I walked down the street of a small village, every two feet there was a different smell: something being washed, fertilizer in the garden, new paint on a building, the lighting of a charcoal fire in a Chinese store, the cooking in the next window. It was an extraordinary experience of moving through the world attuned to all the possibilities of smell.
—JACK KORNFIELD, A Path with Heart
My friend Larry Clark is an ornithologist. I have hiked trails with him as he identified bird after bird by song alone. His skill leaves me awestruck. It’s the same feeling I get when a perfumer talks about a fragrance—he seems to be smelling more than I do, finding notes that my blundering nose doesn’t detect until he points them out. How do olfactory experts accomplish these feats? Are their noses that much better than yours or mine? What does it take to become an expert smeller?
Pure nose-sensitivity is not the answer. The average person probably detects odors at about the same concentration as the professional wine taster. What the expert has are cognitive skills that make better use of the same sensory information. The practiced wine expert can name varietals and tell one vintage from another, just as the trained perfumer classifies a new cologne with ease and zeroes in on its unique notes. The expert’s advantage, in other words, is brain power rather than nose power, and in the regular exercise of these specialized mental skills. Wine experts, for example, routinely make notes as they taste. Experts outperform novices in matching their own descriptions to wines on subsequent tastings. Their mental discipline helps experts avoid a trap called the “verbal overshadowing effect” that can snare novices when the effort to generate a verbal label interferes with the perception of the aroma itself.
The perfumers Robert Calkin and Stephan Jellinek believe their job can be done with only an adequate nose. What makes for professional success is specific mental skills and thought processes. My own research confirms that fragrance professionals think differently. Perfumers, fragrance evaluators, chemists, and sales executives have better olfactory imagery ability than nonexperts from outside the industry. The ability to bring to mind the scent of specific perfumes, and to imagine how ingredients will smell when blended, is central to the job description.
Constant honing of perceptual skills may actually change how an expert’s brain responds to scent. The brain-wave patterns of professional perfume researchers have been compared to those of less specialized workers. When smelling an odor, the pros show distinctive frontal lobe activity in an area known as the orbitofrontal cortex—one that is involved in cognitive judgments. This pattern of brain response in the pros may reflect their more analytic way of perceiving odor. Another study examined brain activity in wine sommeliers and nonexperts, as each group sipped and savored wine samples. The sommeliers had activity in areas associated with cognitive processing (the orbitofrontal cortex again) and in an area where taste and smell information are integrated. In contrast, the nonexperts showed activity in the primary sensory areas and zones associated with emotional response. Practice in making deliberate judgments about what one smells leads to changes in brain function and makes a person into a better smeller.
Superpowers
Is there such a thing as an olfactory prodigy? What talents would a Mozart of the nose possess? He would ace tests of odor identification, notice smells at trace concentrations, and quickly zero in on the difference between highly similar scents. He would effortlessly arrange samples according to concentration, name odors without hesitation, and pick the individual components out of a complex mixture. He would have an enormous store of remembered odors, and the ability to memorize new ones in a single sniff. He couldn’t be fooled into false recognition with distracters and decoys. Finally, he would have a profound ability to imagine odors and to anticipate how they would smell when mixed together.
If such a person exists, science hasn’t found him. This doesn’t stop novelists from imagining characters endowed with superhuman ability. Take Grenouille, the hero of Patrick Süskind’s novel Perfume: The Story of a Murderer. Many people have recommended this book to me, thinking I would enjoy the depiction of Grenouille’s incredible olfactory powers, but I am not impressed. Where the Laing Limit keeps normal people from smelling more than four odors in a complex mixture, Grenouille is born with the ability to recognize dozens of them. Even if we buy this fantasy, how does it instantly make Grenouille the best perfumer in Paris? Analyzing a perfume isn’t the same thing as creating one. (I can hear every note in a Mozart symphony, but that doesn’t make me a composer.) We know that perfumers work from the top down; they first recognize a perfume’s type, and then the nuances that make it unique. Grenouille starts by cracking a perfume into its raw materials, the very opposite of how real perfumers work. As a fan of slasher films, I don’t mind that Grenouille is a repellent freak with no body odor of his own who murders female virgins to extract their body scent. Neither do fans of Perfume—they are so enthralled by the romance of essential oils and blending that they ignore Grenouille’s nasal necrophilia and the novel’s soul-deadening grimness. Perfume is about perfume-making the way The Texas Chainsaw Massacre is about sausage-making.
The novelist Salman Rushdie created a hero with supernormal olfactory power named Saleem Sinai, who is born with an enormous nose. The smelly passages in Midnight’s Children are fun to read even as they verge on the phantasmagorical. Here Rushdie conjures the smellscapes of Karachi, Pakistan:
…the fragrances poured into me: the mournful decaying fumes of animal faeces in the gardens of the Frere Road museum, the pustular body odours of young men in loose pajamas holding hands in Sadar evenings, the knife-sharpness of expectorated betel and opium: “rocket paans” were sniffed out in the hawker-crowded alleys between Elphinstone Street and Victoria Road. Camel-smells, car-smells, the gnat-like irritation of motor-rickshaw fumes, the aroma of contraband cigarettes and “blackmoney,” the competitive effluvia of the city’s bus-drivers and the simple sweat of their sardine-crowded passengers.
Like Grenouille, Saleem Sinai comes from the land of make-believe. His olfactory ability goes far beyond normal experience: he uses it to detect emotions in other people, read their character, and peer directly into their souls. Similarly bizarre characters appear in Chitra Banerjee Divakaruni’s The Mistress of Spices and Tom Robbins’s goofy burlesque Jitterbug Perfume. Why are the authors of magic realist fiction so fond of supersmellers? Transforming a “primitive” animal sense into an all-knowing form of perception is apparently an irresistible literary conceit. Whatever their entertainment value, fictional supersmellers don’t shed much light on real people.
Busted
I suggest that if the police really wish to know where stills and “speakeasies” are located, they take me with them. It would not be a bad idea for the United States Government to establish a bureau of aromatic specialists.
—HELEN KELLER
In April 2005, an Indiana man arrived at the Decatur County jail to bail out his brother-in-law. As he handed over $400 in cash, the dispatch clerk noticed that the bills reeked of marijuana. Police officers got the man’s permission to search his car. They found a pipe and some pot and charged him with possession. The episode has a certain Cheech-and-Chong quality to it, but the use of odor as evidence raises serious questions about the Fourth Amendment’s guarantee against unreasonable search and seizure. In February 1999, an Ohio State Highway Patrol officer stopped a motorist for running a red light. When the driver rolled down his window, the officer smelled marijuana smoke. A search turned up rolling papers and joints in the driver’s pocket, and a stubbed-out doobie in the ashtray. The driver was arrested. At trial, he succeeded in having the charges dismissed on the grounds that a search based only on odor—without other visible, tangible evidence—was improper. The case made its way to the Ohio Supreme Court, which ruled that the “plain smell” of burning pot was, by itself, sufficient probable cause for a warrantless search. The supreme courts of Michigan, Colorado, Wisconsin, Arkansas, and at least fifteen other states have reached similar conclusions.
How good a nose does a government agent need to claim probable cause for a drug search? The Ohio court relied on the fact that the arresting officer was trained and experienced in identifying the smell of marijuana smoke. Other jurisdictions aren’t so fussy. The degree of nasal prowess claimed by police officers can, at times, beggar belief. In New Jersey, for example, a driver was pulled over for a traffic infraction. The police officer claimed to smell fresh, unburned marijuana through the open driver’s window. A search revealed a brick of Mexican pot wrapped in a plastic garbage bag in the trunk, where it had been placed after a drug buy twenty minutes earlier. In California, police searched a house where they suspected pot was being grown. They didn’t obtain a warrant because they claimed they could smell marijuana plants—from several hundred yards away in the hot air and diesel exhaust venting from the chimney.
These feats of nasal detection are all the more remarkable given the level of training most police officers receive. According to Jim Woodford, who serves as an expert witness in criminal trials, officers often learn drug smells by sniffing the real thing in the evidence room. Formal training is rudimentary. “Somebody comes in with a suitcase of stuff, everybody goes by and takes a sniff. That’s the training,” he says. The problem with this informal approach is that marijuana aroma is extremely variable, something potheads are well aware of. (Just ask the reviewer of the Beck concert in Costa Mesa…)
Of course, police officers become familiar with drug smells while busting dealers and users. They cite this on-the-job experience when defending their skills in the courtroom. They testify that “I’ve been on so many busts, and I recognize it. Over the years I’ve learned it.” Woodford says, “That’s sufficient to be deemed an expert by the court.” He says it is rare for the defendant in a drug case to challenge the officer’s smell ability via a smell test or medical exam.
Just how detectable is the smell of pot under circumstances such as these? Richard Doty and colleagues conducted some forensic sniff tests to find out, using experimental conditions modeled on the New Jersey and California cases. They found that untrained people can easily distinguish a Hefty bag containing 2.5 kilograms of Mexican pot from one holding an equal weight of shredded newspaper. But when the samples were placed in a car trunk, the panelists could not detect the smell from the driver’s window. Likewise, panelists could reliably identify mature female cannabis plants at close range by scent alone, and could distinguish immature pot plants from tomato plants by smell. But when the smell of marijuana plants was mixed with exhaust from a diesel generator, the panelists couldn’t detect it.
When it comes to detecting drunk drivers, sniff-based forensics are on even shakier scientific ground. A study by the National Highway Transportation Safety Administration found large variability in the ability of police officers to smell alcohol on a person’s breath. As a group, cops picked up the scent consistently only when the drinker had a very high blood alcohol level (the detection rate was 61 percent for BACs between 0.10 and 0.15 percent). In the most rigorous study on the topic, all variables except odor were eliminated: test subjects were hidden behind a screen and breathed at the officers through a tube. The police participants were all highly experienced and trained as Drug Recognition Experts. Even so, test performance was highly variable across officers. As a group, they detected breath alcohol 85 percent of the time at BACs of 0.08 percent or more, but caught it only two-thirds of the time at lower levels. An officer’s ability to estimate the intensity of breath alcohol odor was no better than chance.
THAT POLICE OFFICERS, like everyone else, show a wide range of olfactory ability comes as no surprise to smell scientists. That their abilities should be granted special consideration by judges and juries is another matter. Doty and his colleagues argue that skepticism is in order when marijuana is said to be “in plain smell.” Sensory claims by police are least substantiated when it comes to fresh, unburned marijuana. Yet this is just the circumstance where courts have given greatest credence to a police officer’s nose—no corroborating evidence is needed. Doty’s study has already been cited by the defense in a drug case in federal court. (A police officer with no training in pot aroma claimed to smell immature marijuana plants in an unvented grow house from a long distance away.) Can trained police officers outsniff civilians? Probably. But according to Doty, this has yet to be scientifically documented. Helen Keller would expect better from the Federal Bureau of Aromatic Specialists.