Unless you're cooking, taking out the trash, or picking flowers, it's easy to forget about your sense of smell. But we rely on it for more than just detecting gas leaks or spoiled milk.
Humans are very accurate sniffers, and we use the information gathered by our noses all the time. In one study at the Monell Center that asked people to identify the source of smells, they were right eight out of nine times. "But when we asked them how confident they were, they said, 'I'm guessing,'" notes researcher Johan Lundstrom.
Even when you don't think you smell anything fishy, your nose knows quite a lot—especially about the people around you. Here are four unexpected talents of this oft-forgotten sense.
1: Recognizing Relatives
In a blind study, subjects were given used T-shirts from family members, a friend, and a stranger. The smells of siblings, fathers, and even close friends could be picked out with greater accuracy than chance, but it was the smell of their mothers that participants could identify with almost 90 percent accuracy.
While it makes sense that we might be familiar with the smells of the people we know best, a follow-up study teased apart physical proximity and biological closeness and found that strong genetic ties confer a special olfactory bond, above and beyond mere familiarity. Mothers can easily recognize the smells of their biological children but not their step-children, for example, and full siblings—but not half siblings—can accurately identify each other's odors.
Why? A number of studies in humans and animals have found that everyone's scent carries a unique "odorprint" that contains chemical information about some of our genes. Knowing who is family—and who isn't—gives us a leg up in identifying our kin and precludes the romantic pursuit of siblings and parents.
2: Finding a Mate
"If a woman does not like how a man smells, it is a visceral barrier to being intimate," says Rachel Herz, a Brown University researcher and the author of Scent of Desire: Discovering Our Enigmatic Sense of Smell. Men pick up on women's smells, too, she notes, but they're more likely to be influenced by a woman's looks than by her scent.
Still, as Herz writes, there's no Brad Pitt of smell. A man may smell really good to some women and bad to others, and there's not much he can do about it. In fact, odor-related biochemistry may be part of sexual chemistry—one clue to the mystery of why some people just click.
The same genetic odorprint that helps us recognize our relatives communicates subtle information about the immune system and plays a role in attraction. When a woman finds a man's natural scent intoxicating, that's a hint from her brain that their immune systems are compatible—an optimal blend of overlapping and diverse immunities that will maximize their offspring's resistance to disease.
But body odor is not tied just to reproduction. Gay men, for example, prefer the smell of other gay men to the smells of women and straight men, and a parallel effect is present—though weaker—among lesbians.
3: Detecting Age and Sickness
The latest research by Lundstrom and his team at Monell demonstrates a little-known nasal skill: We can pick out the smells of old age and illness. In a recent PLOS ONE study, participants who inhaled odors from 20- to 30-year-olds, 45- to 55-year-olds, and 75- to 95-year-olds could discriminate all three groups—and the scent of the oldest group was the easiest to identify.
In another, as-yet-unpublished study, participants were injected with an antitoxin that kicks the immune system into high gear, just as a sickness might. Odor samples were taken continuously from the moment of injection until symptoms manifested. Healthy subjects were able to detect the smell of heightened immunity an hour after injection—well before the participants were outwardly symptomatic.
In both cases, Lundstrom believes, we're actually smelling a chemical associated with the activation of the immune system, which can be working in overdrive in sick people and those over 75. While the whiff of disease might help us subconsciously avoid people who are contagious, the smell is most likely not strong enough to be easily recognizable.
4: Sensing Fear and Stress
Take sweat samples from two perspiring people, one running and one giving a speech, and only the latter sample will reek of fear.
While we can't always consciously identify the odor of an anxious person, it can prime our fight-or-flight response and make us more cautious and aware, says Lundstrom. In one study, women exposed to sweat collected from students awaiting an exam evaluated neutral faces more negatively. In another, they performed more quickly and accurately on a word-association task.
Scientists don't yet know which chemical causes the fear smell, but its effect is well documented—and crucial to our survival. "It might be useful to be afraid if others are afraid," explains Herz, "because we just don't know if a tiger is around the corner and we should be getting out of there as well."