Key Moments
Essentials: How Smells Influence Our Hormones, Health & Behavior | Dr. Noam Sobel
Andrew HubermanKey Moments
Your sense of smell can directly influence hormones and social behavior, from triggering attraction to detecting fear, yet much of this processing happens unconsciously.
Key Insights
Humans have approximately 7 million olfactory receptors of about 350 different subtypes, indicating a significant portion of the genome is dedicated to smell.
The handshake may be a subconscious olfactory behavior, as people tend to touch their nose or face after shaking hands, suggesting they are smelling their hand.
Individuals who experience 'instant rapport' or immediate friendship harmony are more similar in body odor than would be expected by chance, suggesting scent plays a role in forming friendships.
The scent of fear is contagious, increasing others' physiological arousal and sympathetic state, and this has been scientifically proven in both dogs and humans.
Inhaling emotional tears, which are odorless, can significantly reduce free testosterone levels by approximately 14% in both men and women within 30 minutes, a potent effect difficult to achieve pharmacologically.
Hexadecanal, a chemical signal, reduces aggression in men but increases it in women, and appears to be a chemical 'war' waged by infants to manipulate the aggression levels of their caregivers for their own benefit.
The intricate biological machinery of smell
The human sense of smell, or olfaction, begins with the direct inhalation of airborne molecules through the nose. These molecules travel about 6-7 cm to the olfactory epithelium, a complex structure lined with approximately 6-7 million olfactory receptors of about 350 different subtypes. A significant portion of our genome is dedicated to these receptor subtypes, highlighting the importance of smell. When odorants bind to these receptors, they trigger a neural signal – an action potential – that travels via the olfactory nerve through a sieve-like part of the skull to the olfactory bulb in the brain. Here, all receptors of a single subtype converge onto a specific location called a glomerulus. This convergence creates a 'map' on the olfactory bulb, where specific glomeruli light up in response to particular odor profiles. For instance, coffee might activate receptors 1, 3, and 7, causing their corresponding glomeruli to activate. This olfactory map then projects to various brain regions, including the primary olfactory cortex (piriform cortex and entorhinal cortex), the amygdala, hypothalamus, and cerebellum, indicating that olfactory information is widely distributed throughout the brain for processing.
Unconscious olfactory behaviors in social interactions
While humans may not overtly sniff each other like dogs, we engage in subtle, often unconscious olfactory behaviors. The act of shaking hands, for example, might have an underlying olfactory function. Research observed that after shaking hands, people tend to touch their noses or faces, suggesting they are subtly smelling their hands. This behavior is not random; it occurs with precise timing, indicating a deliberate, albeit subconscious, act of smelling. Further analysis revealed that individuals don't just smell the hand they shook but also the other hand, possibly indicating a form of self-sampling or comparison with the other person. This suggests that even in seemingly non-olfactory social rituals, smell plays a role in information gathering and social dynamics.
Scent as a foundation for friendship and social bonding
The phenomenon of 'instant rapport,' where individuals experience immediate harmony and connection, may be influenced by body odor. Studies involving friends who reported instant bonding revealed that their body odors were more similar than expected by chance. Initially, this similarity was measured using an 'electronic nose,' a device with sensors that respond to airborne molecules. To explore causality, researchers paired complete strangers and had them engage in a non-verbal interaction called the 'mirror game,' where they mimicked each other's movements at close proximity, allowing for scent exchange. While the prediction that similar scents would lead to better mirroring didn't hold, the individuals whose scents were more similar were later rated by each other as more likable and more likely to be friends. This suggests that scent similarity acts as a subconscious factor contributing to the initial stages of friendship formation, rather than being a consequence of it.
The contagiousness of fear and other chemical signals
Humans emit specific body odors when experiencing fear, and this scent can influence others, increasing their physiological arousal and sympathetic state, effectively making fear contagious. While this has long been anecdotally recognized, especially in dogs' ability to 'smell' human fear, it has been scientifically validated. Studies have collected body odors from fearful individuals and found that others exposed to these scents can identify the fearful state and exhibit altered behavior. Beyond fear, chemical communication between individuals, broadly termed pheromones, is a complex area of study. While the term 'pheromone' is debated, it's clear that human bodies release chemical signals that impact others' behavior. This highlights our profound reliance on olfaction for navigating social and emotional landscapes.
The paradoxical effects of hexadecanal on aggression
Hexadecanal, a chemical signal studied for its role in social calming in rodents, exhibits different effects in humans. In controlled experiments, men exposed to hexadecanal showed consistently reduced aggression. However, women exposed to the same chemical displayed a significant increase in aggression. This paradoxical effect led to a re-evaluation of the chemical's function. Further studies using fMRI revealed that hexadecanal reduces activity in areas related to arousal in men, while increasing aggression in women. The proposed explanation for this divergence lies in evolutionary biology: hexadecanal might act as a signal that reduces paternal aggression while increasing maternal aggression. This dual effect would be beneficial for infants, who are vulnerable to aggression from both sexes. The high concentration of hexadecanal emitted from infants' heads suggests they may be intentionally manipulating the emotional states of their caregivers for their own protection.
Emotional tears as a potent hormonal regulator
Emotional tears, though odorless, possess a powerful ability to regulate hormones and influence social behavior. When humans, both men and women, inhale emotional tears from others, a significant decrease in free testosterone levels (around 14%) is observed within 30 minutes. This hormonal shift is substantial and difficult to achieve through pharmacological means. This finding was independently replicated by a South Korean research group. Further investigation using fMRI showed that tears also reduce brain activity associated with arousal, particularly in the hypothalamus and the fusiform gyrus. Intriguingly, subsequent research on rodents found that tears from infant mice reduce aggression in adult male mice, mediated by scent. This suggests tears may act as a chemical 'coat' that offers protection against aggression, underscoring the complex interplay between emotion, olfaction, and hormonal regulation.
The quest for digital olfaction: predicting and recreating smells
The ultimate goal of 'digital olfaction' is to create technologies that can generate or transmit smells computationally. While early attempts, like a Google April Fool's joke, demonstrated the concept, recent research has made significant strides. A breakthrough framework has been developed that can predict the perceptual similarity between any two molecular mixtures with high accuracy. This means that given a mixture of molecules, the algorithm can predict how similar it will smell to humans. More importantly, it enables the design of mixtures with specific, known similarity profiles. A notable achievement has been the creation of 'olfactory isomers'—mixtures with no overlapping molecular components that smell identical. This has been demonstrated by creating a scent with the same perceived smell as Chanel No. 5 without using any of its original ingredients. This technological advancement lays the groundwork for digitally encoding and transmitting smells, potentially revolutionizing fields from perfumery to medical diagnostics.
The future of smell digitization and its medical applications
The development of a computational olfactory system is progressing, with the aim of accurately predicting and recreating any perceivable smell. The current focus is on identifying 'primary olfactory ingredients' that can be combined to synthesize any smell. While early attempts to recreate scents like rose or anise via digital transmission have yielded mixed results (e.g., rose was perceived as bubblegum, anise as cherry), progress is being made. The successful digital transmission of the scent of violet, with 15 out of 16 people identifying it correctly, marks a significant proof of concept. Although a fully realized digital smell experience comparable to high-definition audio or video is likely decades away, the potential applications are vast. In medical diagnostics, for instance, as diseases have unique metabolic processes with characteristic odors, highly accurate digital olfaction could lead to 'electronic noses' in homes capable of continuous, non-invasive disease detection. This future, though distant, promises to integrate smell into our technological landscape in unprecedented ways.
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Common Questions
The olfactory system involves inhaling airborne molecules that interact with millions of receptors in the olfactory epithelium. These receptors convert the chemical signals into neural impulses, which are then transmitted via the olfactory nerve to the olfactory bulb in the brain, and further to other brain regions like the amygdala and hypothalamus.
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Mentioned in this video
Host of the Huberman Lab podcast, professor of neuroscience at Stanford University School of Medicine. He leads the discussion on the science of smell.
Guest on the podcast, an expert in the field of olfaction and chemical senses. He shares his research on how smells influence hormones, health, and behavior.
An atmospheric chemist at the Max Planck Institute in Munich who collaborates with Noam Sobel's lab on olfactory digitization, specializing in GCMS techniques.
Led a study in Noam Sobel's lab investigating the olfactory basis of handshakes and the tendency for people to touch their faces after shaking hands.
A professor at Columbia University known for his work in vision, olfaction, and thirst sensing. He discussed the pleasant aspects of breath in romantic partners and the scent associated with children.
A scientist who studied hexadecanal, a chemical signal in mice that enhances social calmness. He discovered the specific receptor for this molecule.
A former graduate student in Noam Sobel's lab who investigated aggression and later worked on the effects of hexadecanal on aggression in humans.
A key contributor to a Nature research paper on developing an algorithmic framework for predicting and designing molecular mixtures with high perceptual similarity.
Author of 'The Expression of the Emotions in Man and Animals', who studied emotional expressions. He found tears to be an evolutionary puzzle, labeling them a secondary phenomenon.
Mentioned as the 'late great Ben Paris,' whose words about continuing one's work for the benefit of others are quoted by Andrew Huberman at the end of the podcast.
Led a research paper published in Nature that developed an algorithmic framework to predict and design molecular mixtures with high perceptual similarity.
A company that produces smart mattress covers with cooling, heating, and sleep tracking capabilities. They have launched the 'Pod 6', their most advanced sleep system.
A company that has attempted to digitize the sense of smell, including a past April Fools' joke video about 'Google Smell'.
An electrolyte drink mix without sugar, containing sodium, magnesium, and potassium. Recommended for hydration and optimal brain and body function.
A nutritional supplement with vitamins, minerals, probiotics, and adaptogenic herbs. The podcast mentions its new formulation, AG1 Pro, which includes creatine, HMB, and zinc carnosine.
A famous perfume for which the researchers created a scent isomer using their algorithmic framework, without using any of the original ingredients.
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