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

Humans can detect some odorants at extraordinarily low concentrations (e.g., mercaptans at ~0.2 parts per billion; certain odorants at 10⁻¹² molar—akin to distinguishing a single drop in two Olympic swimming pools). Experiments from Sobel’s lab show that untrained humans can track scent trails on the ground with impressive accuracy and can be trained to track as fast as they can crawl. This overturns the myth that humans are “poor smellers” compared to other mammals and suggests untapped olfactory capacity that can be enhanced with training.

Airflow naturally alternates between nostrils roughly every 2.5 hours, with one side more open and the other more constricted, a pattern that becomes even more pronounced during sleep. This “nasal cycle” is tightly linked to sympathetic–parasympathetic (autonomic) balance. Sobel’s lab built a wearable “nasal halter” that tracks airflow in each nostril and can distinguish adults with ADHD from controls and even identify whether someone is on Ritalin—purely from nasal airflow patterns. This raises the possibility of using simple airflow monitoring as a biomarker for autonomic state and various neuropsychiatric or medical conditions.

Humans constantly self‑sniff and sniff others, usually without realizing it. After handshakes, people markedly increase bringing the shaken hand to their face and sniffing, and nasal airflow recordings show they actively inhale—an unconscious chemical check of the other person. Body-odor similarity predicts “click” friendships: friends who report instantly clicking smell more similar (measured both by an electronic nose and by human judges), and strangers who smell more similar to each other are more likely to feel they could be friends after a brief nonverbal interaction. Fear sweat reliably alters others’ physiology, increasing autonomic arousal and likely spreading vigilance through a group.

Emotional tears from women are odorless but significantly lower free testosterone in men (~14% drop within ~20–30 minutes) and dampen activity in brain regions including the hypothalamus. Follow-up work shows that smelling such tears also reduces male aggression in a standard behavioral paradigm. Separately, a specific molecule, hexadecanal, emitted strongly from infant heads, decreases aggression in men but increases it in women. Functional MRI reveals that hexadecanal changes connectivity between social appraisal regions and aggression circuits in opposite directions by sex, supporting the idea that baby odors chemically promote protective maternal aggression and suppress potentially harmful paternal aggression.

In many mammals, the Bruce effect shows that a pregnant female exposed to the odor of a non-fathering male will abort the pregnancy, mediated by the vomeronasal system. Humans likely lack a functional vomeronasal organ, but they experience high rates of spontaneous miscarriage and some cases remain unexplained. Sobel’s group found that women with unexplained repeated pregnancy loss are dramatically better at identifying their partner’s body odor than controls and show altered hypothalamic responses to stranger male odors, suggesting a human analog to chemosensory pregnancy modulation. His lab is now running causal experiments where smell is blocked in couples with recurrent miscarriages to test if chemosensation is part of the mechanism.