Essentials: How Smells Influence Our Hormones, Health & Behavior | Dr. Noam Sobel
In a Nutshell
Dr. Noam Sobel’s research reveals that human body odors function as chemical signals that directly influence hormones, aggression, and social bonding. Hexadecanal from children’s heads reduces male aggression while increasing female aggression, and women’s emotional tears lower testosterone in both sexes within 20 minutes. An algorithmic framework now predicts and reproduces odor similarity from molecular mixtures, enabling the first internet-transmitted scents and laying groundwork for medical diagnostics via smell.
These notes were generated by AI and may contain inaccuracies.
Welcome to Huberman's Lab Essentials, where past episodes are revisited to extract the most powerful, actionable scientific tools for improving mental and physical health and performance. Andrew Huberman, Professor of Neurology and Ophthalmology at Stanford University School of Medicine, introduces a discussion with Dr. Noam Sobel about the landscape of chemical sensation, specifically smell, and related perceptual abilities including pheromones.
The primary sense of smell is referred to as direct nasal smell through the nose. Airborne particles travel through the nose for a distance of about 6 or 7 cm in humans, interacting with a complex structure of receptors. Humans have about 7 million receptors lining the olfactory epithelium, the sensory surface of the olfactory system. There are about 350 different types of receptors in humans. A significant proportion of the genome is dedicated to subtypes of olfactory receptors in the nose.
Aromatic substances interact with receptors in the epithelium where they undergo conversion. Aromatic substances bind to a receptor and are converted into a nerve signal, specifically an action potential that travels through the olfactory nerve via the thinnest part of the skull, a perforated part with many holes. The nerve passes through those holes and attaches to its first target in the brain, the olfactory bulb.
The pattern of entanglement follows what is referred to as extreme convergence in the mammalian nervous system. All receptors of one subtype converge at one location in the bulb, referred to as the globulus or globules. Each subtype of receptors responds to a small subset of different molecular shapes, sometimes referred to as odorotopes, the molecular aspects of smell. Each odoritope activates a different subset of receptors. Due to this convergence, a map forms on the bulb that reflects the identity of the odor.
The traditional view is that when coffee activates type 1, 3, and 7 receptors, the glomeruli of those receptor types light up when smelling coffee, creating a theoretical map of the coffee. Information is transferred from the bulb to several targets in the brain, including the primary olfactory cortex (piriform cortex) and the internal nasal cortex, located on the ventral surface of the brain in the lower part of the temporal lobe. Information also goes directly to the amygdala, possibly the hypothalamus, and possibly the cerebellum.
Humans possess an incredibly amazing sense of smell. When observing human behavior through the lens of smell, dogs smell each other when interacting, but humans don't usually approach strangers and smell them carefully. However, it is not culturally taboo to smell babies or loved ones. Research is revealing mechanisms by which humans engage in chemical sensing of others.
Research began examining handshakes, which have poor understanding of their behavioral origin. Wikipedia suggests handshakes demonstrate lack of weapons, but there is no good evidence for this. Observation revealed that people shake hands and then touch their faces. A study led by Don Fromkin brought participants into a lab, seated them in an experiment room, and told them the experiment would start soon. They were filmed for 3 minutes without knowing why they came.
After 3 minutes, an experimenter entered and said a fixed text about equipment setup. In half the cases, a handshake occurred with a new person. The behavior of putting the hand on the nose after shaking hands showed a noticeable increase. Nasal airflow measurements confirmed this was olfactory behavior, with people smelling their hands. People didn't just smell the hand they shook hands with, but also smelled the other hand, suggesting something related to comparing oneself to others.
A study published by Inbal Raviv examined instant harmony friendships, where people meet and get along immediately. The theory was that similarity in body odor might contribute to this. Same-sex, non-romantic friends whose friendship began with immediate harmony were recruited. Participants used unscented shampoo and soap for 3 weeks, slept two nights in a T-shirt alone, and body odor was extracted from the shirts.
Using an electronic nose with metal oxide sensors, it was found that close friends are more similar to each other than expected by chance through random pairings. When other participants smelled close friends versus distant friends, they judged close friends as more similar to each other. To address causality, complete strangers were recruited, sniffed using an electronic nose, then engaged in the mirror game where one person moves hands and the other must mirror them without talking.
The prediction that people who smelled similar would be better at the mirror game failed. However, at the end of the experiment, each person rated the other on willingness to befriend them and other ratings like kindness and friendliness. All of this was predicted by the electronic nose. People who smell more similar believe the other person is more likely to be their friend and more likely to be a nice person.
Humans emit chemical signals from their bodies that affect other humans and influence their behavior. The topic of the smell of fear involves humans emitting certain body odor when in a state of fear that affects other humans, increasing their involuntary arousal and sympathetic state. Fear is somewhat contagious through smell. Dogs can smell fear in humans, proven about a year and a half ago in a study.
Several laboratories, starting with Dennis Chen and Haviland Jones, and then in Sobel's lab and others, collected body odor from people in a state of fear and from the same people when not in fear. Others could determine which was the state of fear and this affected their behavior.
Hexadecanal is a chemical signal in mice that promotes social pacification. When mice gather together, they feel comfortable being in a group, enhanced by hexadecanal secreted in their feces. Heinz Breyer studied this molecule and discovered the specific receptor, finding the future is highly conserved across mammalian evolution, hypothesizing it might be a universal mammalian signal.
Eva Michure, a graduate student, built a model for studying human aggression using the Taylor Model of Aggression (TAP), which allows quantitative measurement of aggression on a scale. Participants played the Taylor game either under hexadecanal exposure or control conditions. Hexadecanal consistently reduced aggression in men, with very powerful effects compared to typical effects in aggression research. However, hexadecanal increased aggression in women by the same large amount.
The experiment was repeated inside an fMRI machine, tracking brain activity. Results were repeated: hexadecanal made men less aggressive and women more aggressive. The insight is that if you are a small mammal, parental aggression is often directed at you. The phenomenon of infanticide exists everywhere, and male aggression towards human children exists. Maternal aggression is often protective. Hexadecanal is the most abundant semi-volatile substance in children's heads, with enormous quantities emanating from children's heads. Children reduce aggression in fathers or males around them and increase aggression in mothers or females around them, both beneficial to them.
Emotional tears are shed in emotional situations where nonverbal communication is vital. Darwin's book The Expression of the Emotions in Man and Animals has Chapter 6 dedicated to tears. Darwin systematically examined expressions of emotions but was unable to find a suitable predecessor for tears, calling it a minor phenomenon. Research considered that tears might function as a chemical signal.
Emotional tears were collected and shown to participants. Tears are completely odorless and cannot be detected at all. However, when inhaled, a significant decrease in testosterone occurs within about 20 minutes to half an hour. This includes men and women who smell women's tears. Men can smell women's tears but don't perceive any smell. There is an approximately 14% decrease in free testosterone. People who study testosterone consider this a very strong effect that is difficult to achieve medically so quickly.
An independent group from South Korea, Oh and colleagues, replicated the effect of testosterone with extreme accuracy about two years later. MRI examination showed a clear effect on brain activity with a decrease in activity in the excited state, a decrease in activity in both the hypothalamus and fusiform gyrus.
Research on chemical signals typically examines what occurs in rodents to determine if the same phenomena exist in humans. This case proved unusual because nearly identical effects were discovered in rodents after the human work was completed. A paper published in the journal Nature two years later found that baby mouse tears reduce aggression from adult male mice toward them through a mechanism that relies on smell. The researchers identified the specific pheromone component in the tears responsible for this aggression reduction.
This discovery suggests tears function as a form of chemical coating that provides protection against aggression. Given that the sense of smell serves as a rich source of behavioral, hormonal, and other deep information, computational smell generation becomes a relevant area of inquiry.
The concept involves search engines providing not only visual information and recipes for items like berry pie, but also delivering the actual smell through computer interfaces in real time. Google created an April Fool's Day prank video about "Google Smell" that featured promotional images including bringing a phone close to a rose to emit rose scent. This represents efforts to digitize the sense of smell, with other companies also pursuing this goal.
A research paper led by Aaron Ravia with major contributions from Kobe Snitz, published in the journal Nature, developed an algorithmic framework that predicts perceptual similarity between any two molecular mixtures with very high accuracy. The system can not only predict similarity but also design mixtures with known degrees of similarity. This work produced olfactory isomers—mixtures that do not completely overlap in molecular structure but have identical smells.
Traditional perfumers would consider it impossible to produce two blends with no common molecules that smell identical, yet this was accomplished and can be reproduced by anyone. The research included creating a pseudo-Chanel No. 5 perfume without containing any of the original ingredients.
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The infrastructure for digitizing smell was created through an algorithm and framework that predicts similarity. A map containing 4000 molecules exists where each molecule has a known perceived scent. Any mixture can be projected onto this map to measure its distance from all points, determining what the mixture will smell like—whether lemon, tomato, or other scents.
The next phase involves identifying a set of ingredients called primary olfactory components that can synthesize any perceivable scent. This work is conducted in collaboration with Jonathan Williams' laboratory at the Max Planck Institute in Munich. Williams, an atmospheric chemist skilled in GCMS technology for measuring molecules, measured smells in Germany and transmitted the data via internet protocol. The information was incorporated into the algorithmic framework and recreated through a device that mixes the initial components.
Four different scents were tested in proof-of-concept experiments. Rose reproduction failed, with most people perceiving the result as chewing gum. Anise reproduction also failed, with most people identifying it as cherry. Gasoline reproduction performed slightly and remarkably better than chance. Violet reproduction succeeded, with fifteen out of sixteen people correctly identifying it as violets. Violet became the first scent transmitted via internet protocol.
The current technology represents the olfactory equivalent of black and white television from around 1956. While transmitting the difference between Cabernet and Merlot remains unlikely, generating something vaguely resembling wine would constitute significant success. With 4K resolution equivalent in smell digitization, medical diagnosis becomes possible since every disease involves specific metabolic processes that produce metabolites with distinct odors.
High-accuracy smell digitization could enable continuous diagnostic capabilities through electronic noses in bathrooms and toilets. However, this level of development is estimated to be far in the future, potentially beyond the speaker's active research career.
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