What makes us yawn?

Episode Summary

Executive Summary: The episode explores the mystery of yawning on Stuff You Should Know, using humor and personal anecdotes to examine its biology, triggers, and competing theories. Hosts discuss involuntary yawning, contagious yawning, links to empathy and autism, brain-cooling hypotheses, and why older oxygen/CO2 explanations seem weak. The show ends with listener mail about Rodriguez and a memorable fan encounter.

Main Topics: What yawning is physically (Priority: 5/5): The hosts define yawning as a combined yawn-and-stretch behavior (pandiculation), describing its bodily mechanics: mouth opening, inhaling air, diaphragm movement, abs flexing, and brief changes in heart rate and brain activity. Competing theories about why humans yawn (Priority: 5/5): They review and critique major explanations: oxygen/CO2 regulation, boredom/sleepiness, social signaling, evolutionary signaling, and the brain-cooling theory. Most older theories are treated as insufficient or disproven. Contagious yawning and empathy (Priority: 5/5): The episode discusses research suggesting contagious yawning may relate to empathy and mirror neurons, including references to autism studies and the idea that sensitivity to others’ yawns reflects social cognition. Yawning across development and species (Priority: 4/5): The hosts note that fetuses yawn in utero and that yawning appears across vertebrates, which suggests deep evolutionary roots and undermines simple theories like oxygen deprivation. Physiology, arousal, and alertness (Priority: 4/5): The discussion frames yawning as a shift between arousal states rather than a sign of only boredom or fatigue, with examples from nervous dogs, athletes, and sexually aroused humans. Listener mail and Rodriguez anecdote (Priority: 2/5): The back half includes a listener story about meeting Rodriguez in Detroit, reinforcing the show’s tradition of audience engagement and cultural storytelling beyond the main topic.

Key Arguments: Yawning is involuntary and appears even in fetuses, so it cannot simply be about conscious behavior or oxygen intake. The oxygen/CO2 theory does not hold up because changing oxygen and carbon dioxide levels did not reduce yawning in experiments. Contagious yawning may be connected to empathy and mirror-neuron activity, though autism-related findings are mixed and possibly confounded by attention cues. Brain cooling is currently the most plausible explanation discussed, but the hosts emphasize that yawning remains scientifically unresolved. Yawning likely reflects a change in arousal state rather than a single purpose like sleepiness, boredom, or oxygenation.

Data Points: Adults susceptible to contagious yawning: 40% to 50% - The hosts cite the share of human adults who yawn in response to seeing, hearing, or reading about yawning. Range of reported susceptibility: 41% to 55% - A later reference gives a slightly broader estimate for contagious yawning among adults. Fetal yawning observation window: 11 to 20 weeks - Yawning has been observed in fetuses during this developmental period, used to challenge oxygen-based explanations. Age threshold for contagious yawning: around age 4 - Children are said to become susceptible to contagious yawning only after this age. Heart rate increase during yawning: up to 30 beats per minute - A study referenced by the hosts found heart rate rises during inhalation in a yawn before quickly returning to normal.

Pivotal Quotes: "yawns and orgasms share a neuro-behavioral heritage" — Robert Provine: Used to illustrate a theory that yawning and sexual arousal may have evolved from related behavioral roots. "we all eat, we all pee and poop, and we all pandiculate" — Chuck Bryant: A humorous summary linking human and animal stretching/yawning as a shared bodily behavior. "that’s the hot explanation du jour" — Josh Clark: Referring to the brain-cooling theory as the current leading explanation for yawning.

Implications: Listeners come away with a clearer sense that yawning is real, involuntary, and scientifically unresolved. The episode highlights how a mundane behavior can reveal links among arousal, empathy, development, and evolution.