Essentials: How Smells Influence Our Hormones, Health & Behavior | Dr. Noam Sobel

Episode Overview

• Dr. Andrew Huberman and Dr. Noam Sobel discuss how the human olfactory system functions, how subconscious chemical signals influence social bonding, hormone levels, and aggression, and whether smell can be digitized.
• The conversation begins by detailing the anatomy of human smell, moving from receptor transduction in the nose to convergence in the olfactory bulb and projections into the brain.
• Next, they examine behavioral studies revealing how subconscious body odor sampling drives human handshakes, instant friendships, and the transmission of fear.
• They then explore how specific chemical compounds emitted on baby scalps and in odorless emotional tears powerfully alter testosterone and sex-specific aggression.
• Finally, the episode covers breakthroughs in mapping odor perception, creating identical smells from completely different molecules, and transmitting scents over the internet.

Key Concepts

• Anatomy and Physiology of Orthonasal Olfaction: Dr. Andrew Huberman, Professor of Neurobiology and Ophthalmology at Stanford School of Medicine, and Dr. Noam Sobel, an olfactory researcher who leads a human olfaction lab in Israel, explain that primary olfaction is called orthonasal olfaction, occurring when airborne molecules travel about 6 or 7 centimeters up the nose to a convoluted sensory surface called the olfactory epithelium. This surface contains about 6 or 7 million olfactory receptors divided into about 350 different subtypes, representing a meaningful percentage of the human genome. When odorants dock at receptors, they transduce into action potentials (not gradient potentials) that propagate via the olfactory nerve through the thinnest, perforated part of the skull to synapse at the olfactory bulb. All receptors of one subtype converge to two glomeruli per subtype in the olfactory bulb, representing the most extreme case of convergence in the mammalian nervous system. Each receptor subtype responds to a small subset of molecular shapes ("odotopes"), creating a spatial map of receptor identity on the olfactory bulb. From there, information projects to primary olfactory cortex (piriform and entorhinal cortex on the ventral surface of the temporal lobe), directly to the amygdala, probably directly to the hypothalamus, and possibly directly to the cerebellum. However, the spatial map on the olfactory bulb does not exist in the rest of the brain, where scientific understanding of odor coding remains "murky."
• Handshaking as a Subconscious Sniffing Ritual: Sobel notes there is "really poor understanding" and "no good evidence" for the popular theory that handshaking evolved to show one is not holding a weapon. Instead, a Sobel Lab study led by Idan Frumin covertly video-recorded participants waiting alone in a room for 3 minutes to quantify baseline face and nose touching (which was high), and then measured nasal airflow and video after an experimenter greeted them with or without a handshake. The researchers found a remarkable increase in participants bringing their hand to their nose and sniffing it after a handshake compared to no handshake. Participants smelled both the shaking hand and their other hand; Sobel hedges that his lab thinks this is a mechanism for comparing self to other.
• Body Odor Similarity and "Click" Friendships: A Sobel Lab study led by Inbal Ravreby investigated non-romantic, same-sex "click" friends (people who bonded immediately upon meeting) recruited across Israel via social media. Participants used odorless shampoo and soap for 3 weeks, then slept alone for 2 nights in a provided T-shirt to extract body odor. Both human smellers and an electronic nose with metal-oxide-covered sensors (MOX) found that click friends had significantly more similar body odors than random dyads. To test causality, total strangers were smelled with an electronic nose and paired in a non-verbal "mirror game" (moving hands in sync while standing very close) in a round-robin design. While odor similarity did not predict mirror game performance, electronic nose measurements of body odor similarity significantly predicted how much strangers liked each other and wanted to be friends afterward.
• Chemosignals of Fear: Sobel explains that whether human chemical signals qualify as "pheromones" is purely semantics ("nerdy inside ball"), so his lab avoids the word in publications because it would only hurt them, preferring the term "chemosignals." Research by Denise Chen, Haviland-Jones, the Sobel Lab, and others demonstrates that humans can distinguish between body odor collected from people in a state of fear versus a non-fear state. Smelling fear-induced body odor increases autonomic arousal and sympathetic nervous system state in the smeller, making fear somewhat contagious. Additionally, an unnamed study from about 1.5 years prior to the recording confirmed experimentally for the first time that dogs can smell fear in humans.
• Hexadecanal (Baby Scalp Odor) and Sex-Specific Aggression: Building on research by Heinz Breer and Strotmann—who discovered that hexadecanal is a chemosignal emitted in mouse feces that promotes "social buffering" (feeling good in a group) and identified its highly conserved mammalian receptor—Sobel Lab researcher Eva Mishor studied its effect on human aggression (after initial work collecting sweat from MMA clubs "never went anywhere really"). Using the Taylor Aggression Paradigm (TAP) and replicating the experiment inside an fMRI scanner, Mishor found that hexadecanal significantly reduced aggression in men, but equally significantly increased aggression in women. Because hexadecanal is the most abundant semi-volatile molecule on human baby heads, Mishor and Sobel interpret this as an evolutionary adaptation where mammalian offspring emit hexadecanal to reduce paternal/male aggression (protecting against infanticide) while increasing maternal/female protective aggression.
• Emotional Tears as a Chemical Blanket: In Chapter 6 of The Expression of the Emotions in Man and Animals, Charles Darwin concluded that emotional tears were a functionless epiphenomenon. However, the Sobel Lab harvested emotional tears from women and exposed male participants to them. Although emotional tears are completely odorless to human perception, within 20 to 30 minutes of sniffing them, men experienced a pronounced reduction in free testosterone of about 14%. Under an arousing state in an fMRI scanner, sniffing tears caused a pronounced dampening of brain activity in the hypothalamus and the fusiform gyrus (face recognition area), though regarding the fusiform gyrus dampening, Sobel notes, "for whatever reason, I don't know... we don't know why." The testosterone reduction was independently replicated about 2 years later by Oh et al. in South Korea, and a Nature study published 2 years after Sobel's study found that mouse pup tears contain a tear pheromone that lowers aggression in adult male mice toward the pups in a smell-dependent manner.
• Digitizing Smell and Olfactory Metamers: A Sobel Lab study led by Aharon Ravia and Kobi Snitz (published in Nature) used a map of 4,000 molecules with known perceived smells to build an algorithmic framework predicting the perceptual similarity of any two molecular mixtures with very high accuracy. Using this, they created "olfactory metamers"—molecular mixtures with zero overlapping molecules in common that smell identical to humans, including a metamer for Chanel No. 5 containing no components of Chanel No. 5. Furthermore, in collaboration with Jonathan Williams at Max Planck in Munich, Williams measured odorants in Germany using GC-MS (gas chromatography–mass spectrometry) and transmitted the data over IP to Sobel's lab, which used a device mixing "odor primaries" to recreate 4 odors: rose failed (perceived by most as bubblegum), anise failed (perceived by most as cherry), gasoline succeeded slightly but significantly better than chance, and violets succeeded (15 of 16 people identified it as violets, making it the first odor transmitted over IP). Because every disease is a specific metabolic process producing metabolites with a smell, Sobel suggests high-resolution digitized olfaction will eventually enable continuous medical diagnostics (such as an electronic nose in residential toilets), though he strongly caveats the breakthrough, telling listeners "don't hold your breath for this to be on your table tomorrow," comparing current capability to "1956 black-and-white TV," and stating that diagnostic bathroom e-noses are "not anywhere in the very close future" and might only happen in his retirement.
• Physiology of Sleep Temperature and Supplementation (Sponsor Context): During sponsor segments, Huberman discusses physiological protocols and products with which he has business ties:
• Sleep Temperature (Eight Sleep): Huberman states that to fall and stay deeply asleep, core body temperature must drop by about 1 to 3 degrees, while waking up feeling refreshed requires body temperature to increase in the final hours of sleep. He recommends Eight Sleep smart mattress covers (Pod 6, eightsleep.com/huberman) for cooling, heating, and sleep tracking to improve deep and REM sleep, noting he has used it for nearly 5 years.
• Hydration and Electrolytes (LMNT): Huberman recommends drinking 1 packet of LMNT electrolyte drink mix (containing sodium, magnesium, and potassium with no sugar; drinklmnt.com/huberman) dissolved in 16 to 32 ounces of water first thing in the morning and during physical exercise.
• Foundational Nutrition (AG1): Huberman states he has taken AG1 daily for nearly 14 years and recommends AG1 Pro (containing vitamins, minerals, probiotics, adaptogens, 5 grams of creatine monohydrate per serving, calcium HMB, and zinc carnosine; drinkag1.com/huberman) for muscle strength, recovery, brain health, and gut lining support.

Quotes

• At 4:46 - "Humans have an utterly remarkable sense of smell." - Sobel dispelling the common myth that human olfaction is weak compared to other animals.
• At 8:03 - "And people not only bring their hand to their nose, they sniff it." - Sobel explaining the covert video and nasal airflow findings from his lab's handshake study.
• At 13:49 - "We could actually predict friendship using the electronic nose. So this is not a result of friendship, it plays into the causal elements of building friendship." - Sobel describing how body odor similarity measured before interaction predicts whether strangers will bond.
• At 20:24 - "Babies are conducting chemical warfare, right? They're reducing aggression in their fathers or males around them and increasing aggression in their mothers or females around them, and both of those things are good for them." - Sobel explaining the evolutionary function of hexadecanal on human baby scalps.
• At 25:27 - "You can think of tears as like a chemical blanket in a way that you're covering yourself up again with, you know, to protect against aggression, right?" - Sobel summarizing the functional role of odorless emotional tears in lowering male testosterone and aggression.

Takeaways

• Recognize and trust rapid, subconscious social "clicks" with new people, as your olfactory system is actively sampling body odor similarity through proximity and handshakes to assess interpersonal compatibility.
• Monitor your own autonomic arousal when in close proximity to fearful or stressed individuals, keeping in mind that human fear is chemically contagious through body odor and can automatically elevate your sympathetic nervous system state.
• Regulate your sleeping environment so that your core body temperature can drop by 1 to 3 degrees to help you fall and stay deeply asleep, and allow it to rise in the final hours of sleep to wake up feeling refreshed.