The Power of Smell

Research shows that our olfactory sense is more influential than we realise.

A Dog noses are renowned for sensitivity to smells, which human noses are thought to be poor by comparison, yet that might be a misconception. According to recent studies, human noses are in fact surprisingly sensitive instruments that guide our everyday life to a surprising extent. Subtle smells can change your mood, behaviour and the choices you make, often without you realising it. Our own odours meanwhile indicate emotional states such as fear or sadness to those around us. The big mystery is why we aren’t more aware of our noses’ influence. Noses have certainly never been at the forefront of sensory research, and were sidelined until recently in honour of the seemingly more vital senses of sight and hearing. "There has been a lot of prejudice that people are not that influenced by olfactory stimuli, especially compared to other mammals," says Liliana Mullca-pnech, who studies the neurobiology of human stress at Stony Brook University in New York, in the United States.

B One of the first people to assert the relative unimportance of human smelling was Pierre Paul Broca, an influential 19th-century anatomist. After comparing the proportion of the brain devoted to smell in different animals, he suggested that mammals can be classed into two broad groups: macrosmatic mammals, such as dogs, have a finely tuned sense of smell which they rely on to perceive the world, while we, along with other primates, are microsmatic--having such small olfactory organs that we only rely on them to a small extent. That idea seemed to fit with more recent studies which found that the majority of mammals have genes coding for about 1,000 different types of smell receptor. Most of these genes aren’t expressed in humans, giving our noses just 400 different types of receptor.

C Yet these findings may have been misleading. Brain scans now show that more of the brain is devoted to smell processing than Broca’s anatomic studies suggested. And although we may have fewer types of receptor than other mammals, Charles Zuker at Yale University in the United States has shown that the human nose and brain are unusually well connected, with each group of receptors linking to many more neurons than is the case in other animals. That should give us a good ability to process incoming scents. Once researchers began looking, they found the nose to be far more sensitive than its reputation suggested. One study, for example, found that we can detect certain chemicals diluted in water to less than one part per billion. That means that a person can detect just a few drops of a strong smell like ethyl mercaptan in an Olympic-sized swimming pool.

D Moreover, the human brain seems to be adept at discriminating between closely related scents. Experiments conducted by neuroscientist Noam Sobel and his team at the Weizmann Institute of Science in Israel revealed that humans can differentiate between odorants that are nearly identical in chemical structure. This suggests that our olfactory system is more sophisticated than previously credited.

E Smell also wields subtle power over our decisions. In one experiment, researchers subtly introduced the scent of cleaning products into a room. Participants who smelled the clean scent later showed a greater tendency to choose tidy, organised options over messy ones in unrelated tasks--demonstrating how smells can unconsciously steer behaviour.

F Marketers have long exploited this link between smell and psychology. For instance, certain bakeries and coffee shops pump the aroma of fresh bread or roasted coffee beans into the air near their entrances to attract customers. The pleasant smell not only draws people in but can also make them more likely to make a purchase, proving scent’s role as a silent salesperson.

G Despite its influence, olfaction remains the least understood of the human senses. This may be because smell operates largely below the level of conscious awareness. As research continues to uncover its hidden impact, it becomes clear that our sense of smell is not a relic of evolution, but a vital, active guide in social interaction, memory, and decision-making.