Smell is our oldest sense. One of our earliest functions as simple organisms was to detect helpful or harmful molecules in our environment and then seek them out or avoid them. The brain's olfactory bulb still sits alongside regions processing emotion . As a result—although scientists aren't sure of the exact mechanism—dysfunctions of smell are closely associated with mood disorders.
What Is the Sense of Smell?
Olfaction, the human sense of smell, can bring someone pleasure (the sweet fragrance of flowers) or generate disgust (e.g., the stink of rotting meat). It can boost mood, stir up old memories, and even drive a person to romantic distraction. People also use their sense of smell to detect hazards, such as smoke or gas, and without that ability, they can become anxious about their safety. In aromatherapy, different kinds of scents are breathed in or applied to the skin to reduce stress , promote sleep and relaxation, and ease pain.
While sight and hearing are often regarded as the top of the senses hierarchy, smell is an old and powerful sense. People often don’t realize the crucial role that smell plays in their understanding of their environment and many everyday decisions. Social odors help humans build relationships, find mates, retain key memories, avoid dangers, and prevent disease.
Humans have both a main and accessory olfactory system to detect smells and chemicals. An odor enters the nose through the nostrils or through the back of the throat while someone is swallowing or chewing. The odor molecules dissolve when they come into contact with the mucus lining in the nasal cavity. Olfactory neurons identify the odor and transmit information about it to the brain.
The notion that humans have a poor sense of smell is a widespread myth. For a long time, scientists believed that the brain could only differentiate approximately 10,000 odors. However, new research suggests that people can discriminate as many as a trillion odors (much closer to the half a million tones and millions of colors that humans are said to be able to detect).
Smell Loss and Dysfunction
Research has found that people who lose their sense of smell become more depressed, and people with depression have a worse sense of smell. Loss of smell, or anosmia , also reduces the pleasures of eating and socializing over food, reduces sex drive, and raises levels of concern about bodily hygiene. People with anosmia may shower several times a day, overload on fragrances, or avoid going out altogether.
The most common causes of smell loss have traditionally been disease, infection, and trauma , but olfaction faces a new threat from air pollution. Several studies have found that residents of smoggier cities have worse senses of smell than people living elsewhere. Compared with residents of Mexico City, for example, people in the cleaner city of Tlaxcala required weaker aromas to detect coffee and orange drink and could better distinguish between closely related smells.
Some types of smell loss respond to antihistamines or topical steroids. Researchers are also exploring gene therapies and stem-cell treatments to enhance the system's partial ability to repair itself.
A complete loss of smell is called anosmia , while a partial loss or diminished sense of smell is often referred to as hyposmia.
How Scent Affects Attraction
Smell may be the most important, albeit the most subtle, factor in physical attraction , because the nose can suss out complex mechanisms like sexual compatibility, although the conscious mind is unaware of it. The human body has more than 100 immune system genes known as the MHC, or major histocompatibility complex; these genes help our immune system to identify unwanted pathogens. The MHC determines, for example, whether an organ donor is compatible with a patient in need of a transplant.
MHC also determines histocompatibility, influencing human mating choices. In MHC experiments, women are generally more discriminate and fussy about smells and odors, possibly because they are more invested in the results of reproduction, bearing children, and caring for them.
In one famous study, researchers had male subjects wear new T-shirts for two consecutive nights. Female study participants later sniffed each shirt and deemed which ones had the most attractive scent. They preferred the shirts worn by men more immunologically dissimilar to themselves. This makes biological sense, as pairing dissimilar immune systems can mean healthier offspring. Some studies indicate that dissimilar MHC genes, which recognize foreign substances, resulted in lower rates of early miscarriages for couples.
While perfumes and colognes can initially attract someone’s attention , they pale in comparison to the major impact that human body odor has on sexual attraction. People are unconsciously attracted to pheromones that indicate sexual interest, as well as natural body odors indicating a strong and healthy immune system. Healthy foods, such as garlic , can also enhance one’s natural scent and attractiveness .
Explore More About Scent
For a comprehensive understanding of scent, read our complete guide: