For years, John McGann has been studying the science of smell by working with rats and mice at Rutgers University. But when he turned his attention to humans, he was in for a shock. The common wisdom is that our sense of smell stinks, compared to that of other mammals. McGann had always suspected that such claims were exaggerated, but even he wasn’t prepared for just how acute his volunteers’ noses were. “We started with an experiment that involved taking two odors that humans can’t tell apart—and we couldn’t find any,” he says. “We tried odors that mice can’t tell apart and humans were like: No, we’ve got this.”
In a new paper, McGann joins a growing list of scientists who argue that human olfaction is nothing to sniff at. We can follow smell trails. We discriminate between similar odors and detect a wide range of substances, sometimes more sensitively than rodents and dogs can. We live in a rich world of scents and sensibility, where odors deeply influence our emotions and behavior. “I was taught in school that human olfaction isn’t a great sense,” he says. “It’s taught in introductory psychology courses and it’s in the textbooks. But this whole thing is a crazy myth.”
For this crime against olfaction, McGann accuses Paul Broca, a 19th-century French neuroscientist. Broca was a materialist who argued that the mind arose from the brain—a position that brought vigorous opposition from the Catholic Church, which believed in a separate and disembodied soul. This intellectual battle colored Broca’s interpretation of the brain.
For example, he noted that the lobes at the front of the human brain, which had been linked to speech and thought, are relatively bigger than those of other animals. By contrast, he noticed that our olfactory bulbs—a pair of structures that govern our sense of smell—are relatively smaller, flatter, and positioned less prominently. Broca thought that those differences were meaningful and related. Our enlarged frontal lobes were the seat of our free will—the thing that made humans unique. Our olfactory bulbs, however, controlled an animalistic sense that compelled base behaviors like eating and sex. He reasoned that “for there to be free will, there must have been a reduction in smell,” says McGann.
In 1879, Broca divided mammals into smellers and non-smellers. Humans went into the latter category, along with other primates, and aquatic animals like whales. Broca wasn’t arguing that we can’t smell; he just felt that we could consciously ignore the influence of odors in a way that other mammals can’t. And his successors agreed. In 1890, riffing off Broca, William Turner classified humans and other primates as “microsmatics” for whom smell is a “relatively feeble” sense—a category and a description that persisted for centuries.
“These erroneous keystone concepts are still part of scientific canon, even amongst olfactory scientists,” says Kara Hoover from the University of Alaska Fairbanks. “McGann's paper is a welcome check on the state of our thinking, and where we have erred.”
For example, he notes that Broca made a great deal of the fact that our olfactory bulb is a relatively smaller part of our brains than the bulbs of other mammals. But so what? In mammals, the size of the bulb doesn’t change in proportion with the rest of the brain: If you’ve got a big brain, you’re not necessarily guaranteed a big bulb. That might be because, as McGann suggests, “a bigger animal isn’t smelling a smellier world.” Whatever the reason, it means that considering the size of the bulb isn’t very informative.
McGann thinks it’s better to look at the number of neurons in the bulb. And surprisingly, that number is typically around 10 million, give or take a factor of 10 in either direction. Put it this way: Your olfactory bulb has roughly the same number as a mouse’s, even though it’s physically bigger and relatively smaller compared to the rest of your brain. Or put it another way: there’s a 90,000,000-fold difference in weight between the heaviest and lightest mammals, but only a 28-fold difference between the olfactory bulbs with the most and least neurons.
What really matters isn’t how many neurons you have but what you do with them. And somewhat astonishingly, Broca never actually measured the sensory abilities of the creatures they discussed. He and others made grand inferences by looking at the shape and size of our brains, without ever testing the behaviors that those brains produce—a tendency that, as I’ve argued before, persists in modern neuroscience. (“It’s strange because … he could smell! Right?” says McGann.)
Other groups of scientists have made the same error. For example, humans have around 1,000 genes for odor receptors—the proteins that allow us to detect airborne chemicals. But apparently just 390 of these make working receptors, and the rest are broken “pseudogenes.” By comparison, mice have 1,100 working odor receptor genes and just 200 pseudogenes. Many scientists have interpreted these numbers as yet more evidence for our impoverished sense of smell.
“We’ve all been guilty of saying it,” says Leslie Vosshall from Rockefeller University, “but objectively, this is ridiculous.” For a start, later studies have shown that several of these supposed “pseudogenes” are actually activated and used. But more importantly, odor receptors work in combination, so having 390 doesn’t mean that you can only distinguish 390 smells. Indeed, when Mathias Laska actually tested monkeys, he showed that you can’t really predict an animal’s ability to tell smells apart by counting its odor receptor genes. And when other scientists tested humans, they’ve found that we can reasonably detect the majority of airborne chemicals, with surprisingly few olfactory blind spots. (In a recent study, Vosshall’s team controversially calculated that humans can discriminate between at least one trillion different chemicals—although that estimate has been criticized.)
Every species has its own strengths and weaknesses. Humans outclass other animals at detecting certain odors, including chemicals in our own blood and (more oddly) bananas, but we’re much less at sensitive to other smells. “That makes a lot of sense,” says McGann. “It’s not like our 400 olfactory genes are a subset of a rat’s. Every animal has a different complement.”
Asifa Majid from Radboud University notes that there’s also a lot of variation between different groups of people. “The typical study focuses on Western participants, who live in a culture where olfaction is not particularly elaborated,” she says. “But people in other parts of the world are better at odor detection, discrimination and naming.” As I’ve written before, she has shown that the Jahai people of Malaysia and the Maniq of Thailand have more dedicated smell words than Westerners, and can name smells as consistently, easily, and clearly as English speakers can name colors.
An impoverished language for smells may have influenced the Western disdain for this sense since long before Broca looked at brains. “Both Plato and Aristotle thought olfaction was too ill-formed to be put into words, and so vague as to only to give rise to emotional impressions,” Majid says, and their views have had a long legacy in Western culture. “Our myths of olfaction might have been overridden much earlier if [scientists had looked at] Jahai instead of Brits and Americans.”
“Methinks they doth protest too much,” says Alexandra Horowitz from Barnard College.* Our sense of smell might be better than people had assumed, but it still pales in comparison to that of dogs—animals that Horowitz has studied a lot. “I’ve watched dogs newly exposed to the odor of marijuana go into a new room, cruise around for 20 seconds, and find a sealed bag with a tiny amount of pot inside a closed drawer. I’ve watched a dog not yet trained for search-and-rescue find a person, whose scent they do not know, hiding in an acres-long rubble heap within a minute. I've watched my dog recognize when a dog friend has recently passed down the street, and predict my son's arrival home. Dogs simply smell what we do not.”
“I’m not crazy: there are things that dogs can do that humans can’t,” McGann admits. “But it’s also hard to generalize because we rarely test ourselves.” For example, neuroscientist Noam Sobel once took blindfolded undergraduates to a park, and tasked them with following a 10-meter trail of chocolate oil that had been drizzled on the grass. The students got on their hands and knees, snuffled about like dogs, and succeeded. “I’ve done it myself,” says McGann. “I can put my nose on the ground and follow things.”
Even with practice, we might never approach the incredible sensitivity of a dog. But the point is that we don’t really know, because so little research has been done. McGann hopes that his review will encourage scientists to take smell more seriously—and compel doctors to care about its loss. The sense fades with age, and the National Institutes of Health say that among people aged 60 to 69, one in four men and one in nine women have problems with smell. “But if you say you’ve lost your sense of smell, doctors shrug,” says McGann. “It’s not treated like the big deal it is. We need to put more resources into understanding the problems and developing therapies.”
“The whole field [of olfactory science] has a keen insecurity about the smell capacity of our own species,” says Vosshall. “McGann’s paper is a refreshing rebuke to this antique thinking, and should prompt all of us to go outside right now and enjoy the olfactory world around us.
In other words, go out, find some roses, and … well, you know.
* This article originally misidentified Alexandra Horowitz's employer as Columbia University; Barnard College is an affiliate college of Columbia University. We regret the error.