How Smells Influence Our Hormones, Health & Behavior
Summary
Dr. Noam Sobel, professor of neurobiology at the Weizmann Institute of Science, reveals the extraordinary sophistication of the human olfactory system and its far-reaching effects on behavior, cognition, hormones, and social bonding. His lab’s research demonstrates that humans constantly sample chemical information from themselves and others — largely without conscious awareness — and that this information drives fundamental decisions about friendship, romantic attraction, and emotional state. The nasal cycle, tear chemistry, and handshake sniffing are among the surprising mechanisms through which smell shapes human life.
Key Takeaways
- Humans are exceptional smellers: People can detect certain compounds at concentrations as low as 10⁻¹² molar — equivalent to detecting one drop in two Olympic-size swimming pools.
- You unconsciously sniff others after handshakes: After shaking someone’s hand, people reliably bring that hand to their nose and sniff within seconds — a chemosensory sampling behavior confirmed by measuring nasal airflow.
- Friend selection is predicted by smell similarity: People who “click” as friends tend to have more similar body odor profiles, and this similarity can be measured with an electronic nose before any social interaction occurs.
- The nasal cycle reflects your autonomic nervous system: Airflow alternates between nostrils roughly every 2.5 hours, mirroring the balance between sympathetic and parasympathetic activity — and can distinguish ADHD from non-ADHD adults, and detect Ritalin use.
- Nasal inhalation enhances cognition: Visual-spatial task performance is significantly better during nasal inhalation than exhalation, even for tasks entirely unrelated to smell.
- Olfactory loss is an early warning sign: Loss of smell often precedes neurodegenerative disease (e.g., Parkinson’s) by up to 10 years and is associated with shorter lifespan, reduced social connection, and worse health outcomes.
- Olfactory training works: Regularly and attentionally smelling a variety of odors after smell loss (e.g., from viral infection) has strong evidence for promoting recovery — no expensive products required.
- Tears contain chemical signals that suppress testosterone: Human emotional tears contain compounds that directly reduce testosterone levels and arousal in men who smell them, without the men being aware of it.
- You constantly self-sample your own odor: People habitually bring their hands to their nose throughout the day — a behavior the Sobel lab believes functions as a continuous comparison of self-odor versus other-odor.
Detailed Notes
The Architecture of the Olfactory System
- Orthonasal olfaction: Sniffing in through the nose — the primary pathway.
- Retronasal olfaction: Odorants traveling up through the back of the throat and out the nose in reverse — responsible for much of how food tastes.
- Olfactory epithelium: ~6–7 million receptors of approximately 350 different subtypes line this sensory surface, located ~6–7 cm inside the nose.
- A meaningful percentage of the human genome is devoted to coding olfactory receptor subtypes.
- Receptors of each subtype converge onto paired structures called glomeruli in the olfactory bulb, creating a spatial map of odor identity.
- The olfactory bulb connects directly to the amygdala, hypothalamus, piriform cortex, and entorhinal cortex — bypassing the thalamic relay used by vision and hearing.
- This short path to memory centers may explain why olfactory memory is formed quickly and is particularly robust, especially for the first exposure to a given smell.
The Trigeminal System and Smell
- The trigeminal nerve (cranial nerve V) provides a parallel chemosensory pathway with endings in the nose, throat, and eyes.
- Many common smells stimulate both the olfactory nerve and the trigeminal nerve (e.g., lemon, ammonia, menthol).
- Pure olfactants — such as coffee and vanilla — stimulate the olfactory nerve alone, with no trigeminal activation.
- Onion-induced eye tearing and throat burning are trigeminal reflexes, not olfactory ones.
- After olfactory nerve damage (e.g., from COVID-19), residual smell perception may actually be trigeminal sensation.
Olfactory Loss and Recovery
- Causes of traumatic smell loss: Counterintuitively, hits to the back of the head are more damaging than frontal impacts due to the shearing motion of the brain against the cribriform plate, severing the olfactory nerve.
- Recovery timeline: If smell does not return within ~1–1.5 years after injury, it is unlikely to recover.
- Olfactory training: Repeatedly and attentionally smelling a diverse set of odors supports neuronal survival and recovery. Strong published evidence supports this approach.
- Alpha-lipoic acid: Some published studies suggest it may accelerate recovery; evidence is present but not overwhelming.
- Anosmia (congenital): Affects ~0.5% of the population. Average age of diagnosis is 14 years. Associated with reduced lifespan, reduced social and romantic contacts, and in some cases (e.g., Kallmann syndrome) with hormonal deficits including disrupted gonadotropin signaling.
- Neurodegenerative disease marker: Olfactory loss precedes Parkinson’s disease symptoms by ~10 years. One theory proposes that Alzheimer’s pathology may enter the brain via the olfactory route.
The Nasal Cycle and Autonomic Nervous System
- Nasal cycle: Airflow alternates between nostrils approximately every 2.5 hours, driven by changes in the nasal turbinates.
- The cycle becomes dramatically pronounced during sleep (one nostril nearly fully closes while the other opens).
- The cycle reflects real-time balance in the autonomic nervous system — sympathetic versus parasympathetic dominance.
- No yoga practitioners tested (n=14) could willfully shift the nasal cycle, despite believing they could.
- The Sobel lab built a wearable “nasal halter” device to record 24-hour nasal airflow patterns.
- Can distinguish ADHD from non-ADHD adults
- Can detect whether an ADHD adult is currently on Ritalin
- Ongoing research suggests acute stress (e.g., cold water hand immersion) may shift nasal cycle balance, suggesting bidirectional causality.
Nasal Breathing and Cognition
- Nasal inhalation vs. exhalation significantly affects visual-spatial processing — performance on impossible-figure discrimination tasks is measurably better on inhalation.
- Mouth breathing shows a similar but smaller effect — inhalation still outperforms exhalation.
- The Sobel lab’s “sniffing brain” hypothesis: The mammalian brain evolved to time information processing to nasal inhalation, not just for olfactory input but for cognition broadly.
- Nasal breathing is advocated over mouth breathing for both health and cognitive reasons.
Social Chemosignaling: Handshakes and Self-Sampling
- After a handshake with a stranger, people reliably and subconsciously bring the shaken hand to their nose and sniff.
- Nasal airflow measurements confirmed active sniffing, not passive contact.
- The behavior could be amplified or suppressed by covertly introducing pleasant or unpleasant odors via a scent-emitting watch on the experimenter’s wrist.
- People also sniff the opposite (non-shaken) hand — possibly for self-vs.-other odor comparison.
- Baseline rates of hand-to-face touching are already very high; handshaking significantly elevates nose-directed touching specifically.
Body Odor, Friendship, and Romantic Attraction
- Click friendships: Same-sex, non-romantic friends who “clicked” immediately were shown to have more similar body odor profiles than random pairs, as measured by an electronic nose.
- Among strangers, odor similarity predicted how much each person rated the other as a likely friend and as a nicer/more affectionate person — before any verbal interaction.
- Romantic partner selection and MHC: In contrast to friendship, romantic attraction tends to favor individuals with