Why do my armpits smell?

Episode Summary

Executive Summary: The episode investigates why humans get body odor, showing that sweat itself is odorless and smell comes from skin bacteria breaking down oily apocrine sweat. It explores genetic variation, especially the ABCC11 variant that leaves many East Asians without underarm odor, and argues that body smell may be more a byproduct of being mammal than a crucial evolved signal, though odors can still convey social and health cues.

Main Topics: Origin of body odor: Explains that body odor is produced when bacteria on skin break down odorless sweat, especially the oily secretions from apocrine glands in armpits and related areas. Genetic variation and the ABCC11 gene: Discusses how a common gene variant means many people, especially those of East Asian ancestry, do not produce the usual underarm odor and therefore may not need deodorant. Historical attitudes to washing and cleanliness: Traces how bathing practices changed from Greek and Roman times through Christianity, the Black Death, and the rise of modern soap advertising. Could smell aid mate choice?: Reviews research on pheromones and the ‘sweaty T-shirt’ study, while emphasizing that evidence for human sex pheromones remains weak and attraction is multifactorial. Odor as information rather than signal: Distinguishes evolved odor signals from passive odor cues that can reveal stress, illness, or emotional state and influence others unconsciously. Smell in human evolution and perception: Notes that our sense of smell is subjective and historically variable, with Neanderthal and modern human odor perception differences illustrating how smell can differ across populations.

Key Arguments: Body odor is not the same as sweat; bacteria generate the smell by metabolizing odorless sweat. Apocrine sweat glands concentrate in armpits, groin, nipples, and ear canals, which is why odor is localized. Genetics strongly influences whether someone produces noticeable body odor, with many East Asians carrying an ABCC11 variant that reduces odorous compounds. The absence of body odor in a large share of the world suggests it may not have a vital biological function for everyone. Historical disgust toward bathing was culturally produced and changed over time, especially through religion, plague fears, and advertising. Human pheromones remain unproven, and attraction likely depends on many factors, with smell being only one possible input. Odors can still matter as cues: illness, fear, and stress can alter smell and affect others, even without deliberate signaling.

Data Points: Population with ABCC11 variant: about one third of the world - People lacking the functional gene variant do not produce typical underarm body odor. Deodorant-free body odor in East Asian ancestry: most people - Dr. Wynne explains many people with East Asian ancestry do not produce the proteins and lipids that bacteria convert into odor. Black Death mortality in Europe: a third or a half - Catherine Ashenburg says the plague killed roughly 33% to 50% of Europeans. Louis XIV baths: 2 baths - Reportedly, Louis XIV of France had only two baths in his lifetime. Sweaty T-shirt study sample: around 40 female students - Participants smelled men’s worn shirts and rated them for pleasantness and sexiness. Main sweat gland types: 2 - Eccrine glands produce watery odorless sweat; apocrine glands produce oily sweat associated with odor.

Pivotal Quotes: "Body odor is actually not from our sweat itself." — Dr. Madeline Wynne: Defines the biological source of smell as bacteria acting on sweat rather than sweat alone. "I think that body odor is not truly 100% understood yet, but I can say from anecdotal experience personally, as well as from the experience of such a large portion of the population, that no, we don't necessarily need that." — Dr. Madeline Wynne: Explains uncertainty about any universal evolutionary function for body odor. "The soap manufacturers fell into this unholy marriage, as I call it, with advertisers." — Catherine Ashenburg: Describes how modern hygiene norms were shaped by marketing and consumer culture.

Implications: The episode suggests body odor is largely a biological byproduct shaped by microbes and genes, while culture determines how offensive it seems. For listeners, it reframes hygiene, attraction, and smell as context-dependent rather than universal truths.