Huberman LabHow Smells Influence Our Hormones, Health & Behavior | Dr. Noam Sobel
CHAPTERS
- 0:00 – 34:00
Foundations of Human Smell: System Overview and Myths
Huberman introduces Dr. Noam Sobel and frames the episode as an exploration of how smell and chemosensation shape human behavior and physiology. Sobel outlines the basic anatomy and function of the olfactory system, correcting myths about humans having poor smell and describing how olfactory information reaches deep brain structures involved in memory and emotion.
- •Olfaction (smell) and chemosensation (response to environmental chemicals) are central to how humans interact with the world, often unconsciously.
- •Odorants enter via orthonasal (through the nostrils) and retronasal (from the mouth up into the nasal cavity) routes, both contributing to flavor perception.
- •Humans have roughly 6–7 million olfactory receptors across ~350 receptor types, making up a surprisingly large fraction of our genome.
- •Odorants bind receptors in the olfactory epithelium, transduce to neural signals, travel through the cribriform plate to the olfactory bulb, then project widely to piriform cortex, entorhinal cortex, amygdala, hypothalamus, and other regions.
- •Olfactory pathways are relatively short and direct compared to vision or hearing, helping explain strong odor–memory links.
- 34:00 – 1:00:00
Smell Loss, Trauma, COVID, and Olfactory Training
The discussion turns to how people lose their sense of smell through head trauma or viral infection and the nervous system’s capacity for regeneration. Sobel clarifies misconceptions about recovery and describes evidence-based olfactory training, as well as the early diagnostic significance of smell loss in neurodegenerative disease.
- •Head trauma, especially to the back of the head, can shear the olfactory nerve at the cribriform plate via contrecoup injury, sometimes causing permanent anosmia.
- •Olfactory neurons regenerate, but if the nerve is fully severed, new fibers may not find their way back to the bulb; partial sparing can allow recovery.
- •For COVID-related smell loss, most meaningful recovery occurs within 12–18 months; beyond that, full recovery is unlikely.
- •Alpha-lipoic acid has limited and mixed evidence; in contrast, “smell training” with diverse household odors has solid evidence supporting improved recovery.
- •Loss of smell is a very early sign of Parkinson’s and possibly Alzheimer’s, but it is nonspecific, limiting its current clinical use as a single diagnostic marker.
- 1:00:00 – 1:12:40
Human Scent Tracking and Bilateral Smell
Huberman recalls seeing Sobel’s UC Berkeley experiment where blindfolded subjects tracked buried scent trails on hands and knees. Sobel describes how that lab bet became a Nature Neuroscience paper showing that humans can track smells like dogs and that having two nostrils provides a measurable performance benefit.
- •In controlled experiments, naïve humans deprived of other senses can follow underground odor trails with notable accuracy.
- •After just a few days of training, tracking speed becomes limited mainly by how fast participants can crawl.
- •Custom nasal prostheses showed that two separated nostrils yield better tracking than a single centralized nostril, even with equal overall airflow.
- •Each nostril feeds into largely unilateral olfactory pathways, creating mirrored maps in the two hemispheres, though this asymmetry does not appear to be an “olfaction-only” phenomenon.
- •These findings challenge the notion that only animals like dogs have functional scent tracking abilities.
- 1:12:40 – 1:33:20
The Nasal Cycle and the ‘Sniffing Brain’
Sobel introduces the nasal cycle—alternating airflow dominance between nostrils—and its relationship to autonomic balance and cognitive processing. He and Huberman discuss experiments showing that breathing phase (inhalation vs exhalation; nose vs mouth) modulates performance on non-olfactory tasks and reflects states like ADHD.
- •Most people can feel that one nostril is easier to breathe through at a given moment; this side alternates roughly every 2.5 hours, especially strongly during sleep.
- •The nasal cycle is tightly coupled to sympathetic–parasympathetic shifts, acting as a live marker of autonomic balance.
- •Sobel’s “nasal halter” device records airflow separately for each nostril over 24 hours, enabling classification of ADHD vs non-ADHD adults and identifying Ritalin use based purely on airflow patterns.
- •Pilot data suggest that acute autonomic challenges, like immersing a hand in near-freezing water, shift nasal airflow balance.
- •The “sniffing brain” hypothesis proposes that nasal inhalation acts as a timing cue for broader brain processing: people perform better on a visuospatial task during inhalation than exhalation, especially through the nose.
- 1:33:20 – 1:46:40
Olfactory Memory and Its Emotional Power
They briefly explore why smells evoke vivid memories, referencing work from Sobel’s lab showing that first exposures to odors are encoded especially strongly. This supports the widely known but often anecdotal idea that scent-linked memories of people and places are particularly robust.
- •Early olfactory experiences form “privileged” representations in the brain that are stronger than equivalent first exposures in other senses.
- •This likely underlies phenomena like the “Proust effect,” where a smell instantly brings back vivid autobiographical memories.
- •Olfactory pathways reach hippocampal and limbic structures with very few synapses, facilitating potent odor–memory associations.
- •Although olfactory memory is a large field, Sobel’s lab has contributed mainly to the notion that earliest odor associations are especially enduring.
- 1:46:40 – 2:00:40
Congenital Anosmia, Development, and Reproduction Links
Sobel discusses people born without a sense of smell and how surprisingly late this is usually diagnosed. He describes links between smell and endocrine function, such as Kallmann syndrome, and emphasizes the social and lifespan costs of congenital anosmia.
- •About 0.5% of the population may have congenital anosmia, yet the average age of formal diagnosis is around 14, indicating massive under-detection.
- •Congenital anosmia is associated with shorter lifespan, reduced social and romantic relationships, and various quality-of-life deficits.
- •Many congenitally anosmic adults lack olfactory bulbs or have only small remnants, but it’s unclear whether bulbs were absent at birth or degenerated over time.
- •Kallmann syndrome (in men) involves hypogonadism and is almost always accompanied by anosmia, highlighting a tight olfaction–endocrine relationship.
- •Similar conditions in women may also couple reproductive hormone dysfunction with smell loss, though details are less clear.
- 2:00:40 – 2:18:00
Self-Sniffing, Handshakes, and Click Friendships
Turning to everyday social behavior, Sobel describes how humans constantly smell themselves and others, especially via their hands. His lab’s work shows that handshakes are followed by unconscious self-sniffing, and that body-odor similarity predicts instant “click” friendships and perceived likeability among strangers.
- •High-speed video in a hidden-camera setup revealed that people touch their faces (especially near the nose) frequently as baseline behavior.
- •After a handshake, face-touching and nasal sniffing of the shaken hand increase dramatically; nasal airflow recordings show deliberate inhalations at the moment of contact.
- •When the experimenters subtly added a pleasant or unpleasant odor to their hand via a watch-like device, they could modulate how much participants sniffed afterward, confirming olfactory involvement.
- •Body odors from “click friends” (mutual, instant friendships) are more similar to each other—both to an electronic nose and to human judges—than random pairs’ odors.
- •In stranger dyads, pre-measured odor similarity predicts who will later rate each other as nicer and more friend‑worthy after a silent mirroring task, suggesting smell helps preselect social partners.
- 2:18:00 – 2:38:00
Chemosignals, Reproduction, and the Bruce Effect Analogy
Sobel explains pheromone-like chemosignaling in animals via the vomeronasal organ and discusses the Bruce effect in mice, wherein exposure to a non-father male’s odor causes pregnancy loss. He then draws cautious parallels to humans, focusing on women with unexplained recurrent miscarriages who show distinctive olfactory abilities and brain responses.
- •Many terrestrial mammals possess a vomeronasal organ (Jacobson’s organ) and accessory olfactory system specialized for pheromonal cues governing mating and aggression.
- •In the Bruce effect, early-pregnant female mice exposed to the odor of a non-stud male abort ~80% of the time, a robust effect abolished by destruction of the vomeronasal organ.
- •Humans likely have a vestigial vomeronasal pit without a functional nerve, but some pheromone-like behaviors can be mediated by the main olfactory system.
- •Women with unexplained repeated pregnancy loss are far better at identifying their partner’s body odor than control women and show altered hypothalamic responses to stranger male odors in fMRI.
- •These findings are correlational but consistent with a human analog of chemosensory pregnancy modulation; Sobel’s lab is now running interventional studies that block smell in such couples to test causality.
- 2:38:00 – 2:59:00
Fear, Sweat, and Sex-Differential Baby Odor Effects
The conversation broadens to include fear chemosignals and the remarkable case of hexadecanal, a molecule abundant in baby head odor. Sobel explains how fear sweat alters others’ arousal and how hexadecanal pulls male and female aggression in opposite directions, likely serving offspring protection.
- •Decades of work (Denise Chen, others) show that humans emit a specific “fear sweat” that can be distinguished from neutral sweat and increases autonomic arousal in others; dogs can also smell human fear.
- •Sobel’s group initially collected human fear sweat (e.g., tandem skydiving first jumps) to search for specific molecules; they now maintain a large “fear bank” of samples.
- •They pivoted to test hexadecanal, known to mediate social buffering in mice, and discovered that in humans it decreases aggression in men but increases it in women in a standardized aggression paradigm.
- •fMRI reveals hexadecanal increases connectivity between social appraisal regions and aggression circuits in men (more top-down control) and decreases it in women (less control), mapping directly onto the behavioral effects.
- •Re-analysis of baby-head odor data from another group showed that hexadecanal is the most abundant semi-volatile emitted from infant heads, suggesting that babies chemically reduce male aggression and enhance maternal protectiveness.
- 2:59:00 – 3:25:20
Tears as Chemosignals: Testosterone and Aggression
Sobel recounts the work that made his lab widely known: demonstrating that women’s emotional tears carry an odorless chemosignal that lowers male testosterone and aggressive behavior. He details how emotional tears differ from other tear types, replication attempts, and the broader idea of tears as a protective “chemical blanket.”
- •Darwin devoted a full chapter to tears in his book on emotions but could not find a functional explanation beyond eye maintenance, calling them an enigma.
- •Sobel’s lab recruited women who could cry easily to emotionally evocative films and collected large volumes of purely emotional tears (tears of joy/laughter do not produce comparable volumes).
- •Male subjects smelling these tears (which they cannot consciously perceive as having a scent) show ~14% decreases in free testosterone within 20–30 minutes and reduced activity in hypothalamus and fusiform gyrus.
- •Independent researchers in South Korea replicated the testosterone-lowering effect, strengthening confidence in the finding.
- •New work from Sobel’s lab indicates that smelling emotional tears also lowers male aggression, consistent with the idea that tears broadcast a signal that reduces potential threat and harm.
- 3:25:20 – 3:45:20
Menstrual Synchrony, Human Pheromones, and Scientific Controversy
Huberman raises the classic question of menstrual synchrony among co-housed women. Sobel reviews the original McClintock studies, a follow-up showing sweat-based cycle modulation, and subsequent statistical critiques, explaining why he remains cautiously open-minded and how his lab might revisit the question rigorously.
- •Barbara McClintock’s 1971 Nature paper reported menstrual synchrony among college women living together, inspired by related rodent phenomena like the Whitten effect.
- •A 1998 Nature follow-up by Stern & McClintock showed that applying donor women’s sweat to recipients’ upper lips could lengthen or shorten cycle length depending on the cycle phase of the donor, implying an olfactory effect.
- •Later re-analyses questioned the statistical methods used, and at least one replication attempt reportedly failed; as a result, much of the field is now skeptical.
- •Sobel personally remains on the fence, seeing enough signal to justify a rigorous, large-scale replication but acknowledges the immense logistical complexity.
- •The broader notion of “human pheromones” remains contentious and partly semantic; Sobel prefers the term “chemosignals,” emphasizing that humans clearly emit and respond to behavior- and hormone-altering chemicals regardless of classification.
- 3:45:20 – 4:00:00
Objectivity of Smell and Cross-Individual Similarity
They tackle the belief that smell is highly subjective. Sobel describes quantitative work showing that people’s odor pleasantness ratings and perceptual spaces are much more similar than most assume, especially when controlling for language and focusing on similarity judgments.
- •Despite common anecdotes about cilantro or guava polarization, overall correlations of odor pleasantness ratings across people are around 0.8—very high.
- •Certain odorants are true outliers (e.g., cilantro) that capture attention, but the vast majority are perceived similarly across individuals.
- •Language distorts our sense of olfactory similarity: unlike color terms (“red,” “blue”) taught from childhood, odor naming is rarely trained, so people assume their internal experiences differ more than they actually do.
- •By asking participants to rate similarity across many odor pairs and comparing the resulting “similarity matrices” between people, Sobel’s group showed that olfactory perceptual spaces are at least as aligned across individuals as those for tones and more aligned than for color.
- •This shared perceptual structure underpins the feasibility of algorithmic odor prediction and digital olfaction.
- 4:00:00 – 4:27:00
Digitizing Smell: Algorithms, Metamers, and First IP Odor Transmission
In the final technical segment, Sobel outlines his lab’s work on building a quantitative mapping from molecular structure to odor perception. He explains olfactory metamers, describes recent proofs of concept transmitting smells over the internet, and speculates about future applications in communication and medicine.
- •Historically, no one could reliably predict how a new molecule would smell solely from its structure, unlike vision (wavelength→color) or audition (frequency→pitch).
- •Sobel’s lab built an algorithm that predicts perceptual similarity between any two odor mixtures, and they used it to design olfactory metamers—mixtures with no shared components that smell indistinguishable.
- •They demonstrated metamers for complex odors, including Chanel No. 5 perfume, using entirely different ingredient sets.
- •In collaboration with an atmospheric chemist, they measured airborne odorants in Germany (via gas chromatography–mass spectrometry), transmitted the data over IP, and reconstructed one odor (violet) elsewhere using a set of “primary” odorants; 15/16 participants correctly identified it as violet.
- •Current hardware is large and expensive, but longer-term, high-resolution digital olfaction could enable remote smell communication, enriched virtual experiences, and disease diagnostics via continuous odor monitoring (e.g., an “e-nose” built into toilets).
- 4:27:00
Closing Reflections: The Power and Future of Human Smell
Huberman closes by emphasizing how Sobel’s work transforms our understanding of smell from a minor sense into a powerful regulator of hormones, behavior, and health. They briefly touch on scientific culture and replication before ending with mutual appreciation and a look ahead to future olfaction breakthroughs.
- •Chemosensation shapes who we befriend, how we respond to babies, how we handle conflict, and possibly whether pregnancies succeed—all largely outside awareness.
- •Olfaction is a privileged gateway to the brain: olfactory neurons are directly exposed to the environment, making them both uniquely informative and vulnerable.
- •The COVID-19 anosmia wave has raised public and scientific awareness, triggering a renaissance in smell research and new funding and technological efforts.
- •Digital olfaction is progressing from basic algorithms to early hardware, with tech giants and academic labs racing to develop practical systems.
- •Huberman highlights Sobel’s combination of rigor and creativity as a model for impactful science and invites him back to share future work.
