How Smells Influence Our Hormones, Health & Behavior | Dr. Noam Sobel
Dr. Noam Sobel, professor of neurobiology, explains how human olfaction and chemosensation profoundly influence behavior, cognition, social connections, and hormones. His lab's work reveals humans' remarkable sense of smell, its role in social sensing, and potential for disease diagnosis and digitization.
Deep Dive Analysis
18 Topic Outline
Components and Architecture of the Olfactory System
Olfactory Neuron Regeneration and Smell Loss
Smell and Memory Formation
Humans' Remarkable Olfactory Abilities and Odor Tracking
The Nasal Cycle and Autonomic Nervous System Balance
Nasal Breathing's Influence on Cognitive Processing
Olfactory Loss as an Early Sign of Neurodegenerative Disease
Congenital Anosmia and its Social/Physiological Impacts
Human Chemosensation: Handshaking and Self-Sampling
Body Odor Similarity and the Formation of Friendships
Olfaction and Romantic/Reproductive Attraction
The Bruce Effect and Unexplained Pregnancy Loss in Humans
Social Chemo-Signals: The Smell of Fear
Hexadecanal: A Baby Odor Influencing Aggression
Menstrual Cycle Synchronization and Olfaction
Emotional Tears as a Chemo-Signal Affecting Hormones
Human Olfactory Perception: Similarity vs. Subjectivity
Digitizing Smell and the Future of Olfactory Diagnostics
8 Key Concepts
Retronasal Olfaction
This is the process where odorants travel up through the back of the throat and out of the nose, contributing significantly to the perception of food flavor and taste.
Olfactory Epithelium
This is the sensory surface of the olfactory system, located deep within the nose, containing millions of receptors that interact with airborne molecules and convert them into neural signals.
Glomerulus
In the olfactory bulb, all receptors of a given subtype converge to one or two specific locations called glomeruli, creating a map that reflects receptor identity for different odorants.
Nasal Cycle
This is a constant, alternating shift in airflow between the left and right nostrils, occurring roughly every two and a half hours on average. It is linked to the balance of the sympathetic and parasympathetic components of the autonomic nervous system.
Vomeronasal Organ (Jacobson's Organ)
A small pit in the nasal passage of most terrestrial mammals, part of the secondary olfactory system. It processes specific odorants (pheromones) that influence reproductive and aggressive behaviors, though it is considered vestigial in humans.
Bruce Effect
Discovered in mice, this is a phenomenon where a pregnant mouse, in an early critical stage of pregnancy, will miscarry if exposed to the odor of a male who did not father the pregnancy. This effect is mediated by chemosensation.
Social Chemo-Signals
These are chemical signals emitted from the body of one individual that influence the physiology or behavior of another individual of the same species, often operating at a subconscious level.
Olfactory Metameras
These are molecular mixtures that are completely non-overlapping in their chemical structure but smell exactly the same. Creating them is a significant step towards digitizing smell by linking molecular structure to perceived odor.
10 Questions Answered
Airborne molecules enter the nose, interact with ~7 million receptors on the olfactory epithelium, transduce into neural signals, travel through the cribriform plate to the olfactory bulb, and then project widely to various brain targets including the piriform cortex, amygdala, and hypothalamus.
Yes, humans possess a remarkable sense of smell and can track odor trails on the ground, especially with training and when other sensory inputs are blocked. Performance is enhanced by having two separate nostrils, similar to how two eyes aid depth perception.
Airflow alternates between nostrils in a 'nasal cycle' approximately every two and a half hours, reflecting the balance between the sympathetic and parasympathetic nervous systems. This cycle can be influenced by autonomic arousal, like stress or pain.
Yes, nasal inhalation appears to time and drive aspects of neural activity and cognitive processing. Studies show people perform significantly better on visual-spatial tasks during nasal inhalation compared to exhalation, suggesting nasal breathing shapes cognition.
Yes, loss of the sense of smell is one of the earliest signs of neurodegenerative diseases like Parkinson's and Alzheimer's, often appearing years before other symptoms. However, it's not a specific diagnostic tool due to many other causes of smell loss.
Congenital anosmia is being born without a sense of smell, affecting about half a percent of the population. It's often diagnosed late (average age 14) and is associated with reduced social contacts, fewer romantic relationships, and a shorter lifespan.
Yes, humans constantly exchange chemical signals. After handshaking, people significantly increase touching and sniffing their own hands, suggesting they are subconsciously sampling chemicals from the other person. This behavior can be influenced by the pleasantness of the transferred odor.
Research suggests that people who 'click' and become friends quickly tend to have more similar body odors than random pairs. This similarity in body odor can predict initial social affinity even among strangers, indicating a causal role in friendship formation.
Emotional tears from women, though completely odorless, cause a pronounced reduction (around 14%) in free testosterone levels in men within 20-30 minutes of sniffing. This effect also leads to a significant decrease in male aggression.
Human olfactory perception is remarkably similar across individuals, contrary to common belief. While there are outliers for certain polarizing odors (e.g., cilantro), most people agree on the pleasantness and similarity of the vast majority of smells, with a high correlation in perceptual ratings.
19 Actionable Insights
1. Optimize Hydration & Electrolytes
Ensure adequate hydration and electrolyte intake (sodium, magnesium, potassium) by dissolving an electrolyte mix like Element in 16-32 ounces of water first thing in the morning and during physical exercise, as proper levels are critical for optimal brain and body function and nerve cell activity.
2. Prioritize Nasal Breathing
Opt for nasal breathing over mouth breathing whenever possible, as it is beneficial for overall health and plays a significant role in shaping and modulating cognitive processing.
3. Restore Energy with NSDR/Meditation
Utilize meditation apps offering Yoga Nidra or Non-Sleep Deep Rest (NSDR) protocols, even for short 10-minute sessions, to restore cognitive and physical energy and explore different brain-body states.
4. Olfactory Training for Recovery
If experiencing smell loss, intentionally and constantly sniff a variety of common items from your home to keep olfactory neurons active, as olfaction is a ‘use it or lose it’ system and this training can aid recovery.
5. Enhance Cognition via Nasal Inhalation
During cognitive tasks, particularly those involving visual-spatial processing, prioritize nasal inhalation, as it has been shown to time and drive neural activity, leading to significantly better performance compared to exhalation.
6. Prioritize Smell in Key Choices
Recognize that in fundamental choices like food and romantic partners, smell often overrides visual appeal, acting as a primal driver of preference or aversion that strongly influences behavior.
7. Odor Similarity Predicts Friendship
Recognize that individuals who smell more similar to each other are more likely to form ‘click friendships,’ as body odor similarity can predict initial social affinity and perceptions of likability even among strangers.
8. Recognize Constant Odor Sampling
Be aware that humans constantly and often subconsciously sample their own body odor (e.g., by touching their nose after a handshake) and the odors of others, as this is a natural behavior influencing social interactions.
9. Understand Baby Odor’s Influence
Be aware that babies emit hexadecanal, a semi-volatile molecule that can chemically influence adult behavior by reducing aggression in males and increasing it in females, serving a protective function for the offspring.
10. Recognize Tears as Chemosignal
Understand that emotional tears, despite being odorless, act as a chemosignal that can significantly reduce free testosterone and lower aggression in men who sniff them, suggesting a non-verbal communication function.
11. Use Scent for Partner Connection
When a romantic partner is absent, consider smelling their articles of clothing to evoke positive connotations and comfort, as scent plays a significant role in maintaining emotional connection.
12. Leverage First Olfactory Experiences
Be mindful that the brain forms particularly robust and lasting memories from the first exposure to a smell, a phenomenon that can etch experiences into one’s being more profoundly than other sensory stimuli.
13. Regular Olfactory Health Checks
Advocate for or undergo regular olfactory testing, such as the UPSIT or Sniffing Sticks, as a non-invasive way to track changes in smell over time, which could serve as an early indicator for potential neurodegenerative diseases.
14. Observe Nasal Cycle for ANS
Be aware that your nostrils alternate airflow every 2.5 hours on average, reflecting the balance of your autonomic nervous system, which can serve as a marker for physiological states and disease.
15. Cold Exposure Shifts Nasal Balance
Be aware that exposure to cold, such as cold water hand exposure, can generate a shift in your nasal balance, indicating that autonomic arousal may drive changes in the nasal cycle.
16. Cannot Willfully Control Nasal Cycle
Do not expect to willfully shift your nasal cycle (airflow between nostrils) through thought alone, as scientific studies indicate that even experienced practitioners cannot achieve this.
17. Challenge Olfactory Subjectivity Myth
Recognize that human olfactory perception is largely shared and not highly subjective, with most people agreeing on the pleasantness of 90% of odors, and dissimilarities often stemming from a few outlier scents or a lack of common language anchors.
18. Acknowledge Shared Olfactory Preferences
When discussing smells, understand that despite a few polarizing exceptions like cilantro, humans largely agree on the pleasantness of most odors, with a high correlation across individuals.
19. Fear is Chemically Contagious
Recognize that humans emit a specific body odor when experiencing fear, which can increase autonomic arousal in others who perceive it, making fear chemically contagious.
8 Key Quotes
Humans have an utterly remarkable sense of smell.
Noam Sobel
You're walking around with a marker on balance in your autonomic nervous system and we do nothing with it.
Noam Sobel
It's the only place where your brain meets the outside world.
Noam Sobel
The Bruce effect implies that the female has to have a very clear memory of the fathering male.
Noam Sobel
Fear is contagious a bit. So the smell of fear is contagious.
Noam Sobel
Babies are conducting chemical warfare.
Noam Sobel
If something is published in nature or science that doesn't necessarily mean it's not true.
Noam Sobel
You can almost say that every disease will have an odor.
Noam Sobel
2 Protocols
Olfactory Training for Smell Loss
Noam Sobel- Select a number of different smells (e.g., from refrigerator, makeup cabinet).
- Smell these items attentionally and constantly.
- Consistent exposure helps maintain and recover olfactory neuron connections.
Body Odor Collection for Friendship Study
Noam Sobel- Participants use odorless shampoo and soap for approximately three weeks.
- Participants sleep alone for two nights in a specific t-shirt.
- Body odor is then extracted from the t-shirt for analysis.