The Sense of Smell in Humans is More Powerful Than We Think

Imagine walking into a meeting room. You shake hands with colleagues, then everyone sits down. Within seconds they all start sniffing their palms, picking up clues about you from the chemical traces left over from the handshakes.

Sniffing palms after a handshake, usually within 30 seconds of the interaction, would likely help people learn about someone’s health and genetic compatibility, according to a 2015 study by researchers in Israel. Sniffing can also offer information on people’s emotional state, such as if they are happy, sad or fearful. The smeller gleans these emotions subconsciously, of course.

For decades, scientists believed humans were not very good at detecting and identifying odors. Our animal ancestors used their noses way more than we do in modern society, says Jessica Freiherr, a neuroscientist at RWTH Aachen University, in Germany, and the author of several studies on the human sense of smell. “We are disconnected from our noses,” she says. “We need them much less in everyday life. And our vision overrides the sense of smell in a lot of situations.”

But that doesn’t mean we don’t have powerful smell potential. A 2014 study showed that we can distinguish at least 1 trillion different odors — up from previous estimates of a mere 10,000.

Awareness of our innate smelling abilities, however, is complicated because the human language doesn’t have words for a trillion smells, and much of smelling happens under the radar of our consciousness. Unlike our other senses, the olfactory nerves do not proceed directly to the brain’s thalamus, the gateway to consciousness. Instead, information feeds from the nose to cortical areas to arouse emotions and memories without our awareness. When it comes to smells, people can be influenced and not realize it.

An animal schnoz is obviously superior to our own mediocre noses, right?

Not so fast. Matthias Laska, a biologist at Linköping University in Sweden, has been comparing senses of smell across species — including humans — for more than two decades. “The more data I collected on different species over the years, the more interesting the picture became,” Laska says.

But sizing up how sensitive the snout of, say, a seal is compared with a bat or human isn’t straightforward. People can tell you when a certain scent is no longer detectable. But each animal has to learn to associate a particular odor with a reward and then do something, like press a button, to let researchers know when they smell it.

The odors compared between species also have to be the same. That sounds obvious, but while humans have sniffed around 3,300 different scents for science — out of the trillions possible — the highest number for animals is 81, by spider monkeys. Laska only found solid enough data to compare humans with 17 species, all mammals.

However, human noses held their own. Humans tested as generally more sensitive sniffers than monkeys and rats on a limited range of odors. In fact, humans detected certain scents at lower concentrations than the notoriously top-notch nostrils of mice and pigs.

Humans even beat the indomitable dog for at least a handful of scents. These include aromas produced by plants, a logical evolutionary advantage for our ancestors seeking fruits. The majority of the odors in which dogs bested us were the fatty acids, compounds associated with their own meaty prey. “Odors that are not relevant for you, you are usually not good at [smelling],” Laska says.

Bottom line: Humans, Laska says, “are not as hopeless as the classical wisdom will tell us, and dogs are not the super nose of the universe for everything.” — Ashley Braun

If you were assaulted by a stranger you didn’t get a good look at, could you identify the person by smell in a police lineup? Would the perpetrator’s body odor be enough? It very well could be, according to a 2015 study by scientists in Portugal and Sweden.

Researchers collected body odor samples from 20 male university students. Other students then watched a video of an actual assault by a man on a woman (to stir them emotionally), while sniffing a scent they were told was that of the suspect. In reality, it was the scent of one of the 20 male students. Afterward, the sniffers were given a “lineup” of five odor samples and asked to identify the person whom they had smelled — presumably not a very enjoyable task. Results were quite impressive, though. The “witnesses” were able to pinpoint the would-be suspect 75 percent of the time.

Every person has a unique scent. “It’s like a fingerprint,” says Johan Lundström, a neuroscientist at the Karolinska Institute in Sweden. “There is a large genetic component to body odor. Even trained sniffer dogs have a hard time distinguishing between identical twins, unless the twins are on different diets.”

Scientists still don’t know how human body odor can act like a scented fingerprint. It could be from the apocrine sweat glands in the armpits, which produce odorless substances made smelly by skin bacteria. In 2015, scientists from the University of Düsseldorf identified unsaturated, or hydroxylated, branched fatty acids as the “olfactorily most dominant,” or stinkiest.

Human scent affects our brain differently than other scents. When we catch a whiff, the areas of the brain responsible for social processing light up, according to a study that used positron emission tomography (PET) to measure brain function. “There is much more information in body odor than we can extract from normal odors,” says Lundström, the study’s lead author.

Another reason you might be able to identify a criminal, or at least someone feeling agitated, is that he or she may simply smell dangerous. In one of Freiherr’s experiments published in 2015 in the journal Chemical Senses, researchers obtained sweat from 16 men. The men took a timed math test and were falsely told they had performed below average. Disgruntled, they then participated in a workout where sweat was collected. As a control, the men took the math test again under no time constraint and were told they got an average score. Again, they followed up with a sweaty workout.

Volunteers sniffed the men’s sweat samples while taking a test that measures cognitive performance. When sniffing the sweat of the men told they scored below average, the volunteers were distracted and slower to respond during their own test. When sniffing the sweat from the men’s second workout, the volunteers scored in a manner indicating emotional neutrality.

A hefty pile of evidence suggests that emotions have a scent. What’s more, such smelled emotions may be contagious. Say you go out to meet a friend who had been watching funny videos on her mobile phone, making her feel happy. As you approach her, you catch a whiff of her scent and automatically smile. But had your friend just watched a scary movie, her body odor would have likely made you feel apprehensive.

Using electrodes, European researchers in 2015 measured the facial movements of volunteers who sniffed sweat samples of people who had watched either pleasant or scary videos — happy-go-lucky scenes from Disney’s The Jungle Book versus hair-raising clips from Stanley Kubrick’s The Shining. After inhaling the scent of The Jungle Book watchers, participants “assumed a genuine happy facial expression,” says Jasper de Groot, a psychologist from Utrecht University in the Netherlands. “It was subtle, yet significant.”

Meanwhile, smelling the body odor of stressed-out people ups our vigilance, while the odor of people who had just watched something disgusting makes our faces twist in disgust. In fMRI scans, people sniffing the sweat from first-time parachute jumpers lit up the brain’s left amygdala, where basic emotions are processed, suggesting fear is contagious, too.

“These chemosignals ring an alarm bell in your brain to attract your attention,” Freiherr says. “Maybe you can smell a dangerous place because somebody was there five minutes ago feeling scared.”

Inhaling body odor can offer more information about people than their emotional state. The health and biological compatibility of the opposite sex might also be gleaned, all the better to help pick the perfect mate.

In an experiment published in 2014 in the journal Psychological Science, people could tell who showed signs of sickness by their body odor (the researchers injected the sweat donors with a toxin that prompted an immune reaction). From an evolutionary standpoint, smelling sickness or disease has advantages. Choosing an unhealthy partner is not the best way to pass on your genes.

Yet of maybe even greater gene-spreading significance is the ability to tell differences in MHC — the major histocompatibility complex, a gene family linked to the immune system and body scent. Scientists have long known that animals such as mice and rats can tell how genetically related they are to others of their species by smelling one another’s urine. Studies show humans are masters of this skill, too — and thankfully, no urine smelling is necessary. When scientists from the University of Chicago asked a group of women to sniff T-shirts worn for two consecutive nights by different men, the women pinpointed their closest genetic matches — even though there could be millions of unique combinations of MHC genotypes.

A study by researchers from McGill University in Canada involving neuroimaging, which creates pictures of the brain’s structure and neural activity, showed that smelling the body odor of someone closely related activates the dorsomedial prefrontal cortex, a part of the brain responsible for recognizing family.

“Biologically, it makes sense. We want to protect our own gene pool,” Lundström says. But “it’s not so much picking the best partner, it’s deselecting bad partners.” Research shows that people — and women in particular — prefer potential partners who are somewhat genetically related, but not too related. Having children with someone with an MHC genotype that is too similar, studies show, can lead to spontaneous abortion or low birth weight. Conversely, pursuing someone with a close (or semi-close) genetic makeup means preserving adaptations to an environment — think regional people having immunity to local strains of pathogens.

Meanwhile, some scents can make us appear more attractive to potential partners. Take the aroma of grapefruit. In a study that involved guessing the age of women shown in photos, participants knocked off 12 years from actual ages if they smelled, and enjoyed the smell of, grapefruit. If the participants smelled spicy and floral notes, the women appeared four pounds slimmer.

And it’s much safer to buy cologne for people within your family rather than outside it. Genetic kinship seems to influence smell preference. In one study, people with similar genotypes chose similar perfume ingredients.

If you’ve ever thought there is something special about the smell of babies, you’re right. In 2013, scientists from Germany, Canada and Sweden took fMRI scans of 30 women while they sniffed the cotton undershirts of newborns. The new moms’ thalamus lit up more than that of women without kids, suggesting the mothers’ increased attention. All the women showed activity in the brain’s neostriate areas, where the reward system lies.

The fresh scent of newborns activates the same biological mechanism in women as a baby’s “very round eyes, the round face, the cute voice,” says Lundström, who was involved in the study. It is nature’s way of bonding mother and child. Although only women were tested in that particular study, Lundström suspects that similar results would be found in men.

For now, researchers haven’t managed to pinpoint the molecules responsible for that new-baby smell. Lundström and his colleagues have some chemicals under the microscope (figuratively and literally), and are even researching whether the newborn smell could be used to treat depression. The team is also investigating whether women who suffer postpartum depression lack receptors for newborn scent molecules or don’t receive the reward signals from the baby smell.

Similar to our ability to winnow out incompatible mates by scent, new moms can distinguish their biological babies by sniffing them. In one classic study, mothers identified the smell of their child from two other newborns six hours after birth, even though mother and child were separated for most of that time. Sixty-one percent of mothers guessed right. (Chance would be 33 percent.)

This works the other way, too. Newborns know the scent of Mom by the second day of life. In a 2015 study, breast-fed babies turned their heads toward scent pads of their mothers for nearly twice as long as the pads of lactating strangers. “Mother’s body odor might be learned to some degree, as this odor is related to the chemosensory signature of the amniotic fluid, which the unborn senses,” says Katrin T. Lübke, an olfaction researcher at the University of Düsseldorf in Germany who was not part of the study.

Yet simple exposure is not enough for parents to identify the smell of their nonbiological children. In one study, mothers were able to pick the scents of their biological kids in 90 percent of cases, but with stepchildren, they were only 28 percent accurate. Among families in Wales interviewed for a government-funded study on failed adoptions, several parents mentioned that the distinctive body odor of their child had a negative impact on the relationship. One mom said her adopted daughter “didn’t smell right.”

Although our noses can sometimes lead us astray, in general they send us important messages about other people. Be careful, a dangerous person was here and may be lurking nearby. Be cautious, a person is sick and may be contagious. Be alert, your newborn needs your care. Be flirtatious, this person is a potential partner. Being more open to our sense of smell has payoffs, even in modern times.

“Listen to your inner voice, because your inner voice might be your nose telling you what to do,” Lundström says.