Why do we yawn and why is it contagious?
Nobody is certain, which is unusual for something so universal. Yawning occurs in almost all vertebrates, appears in human foetuses from around 11 weeks, and has resisted a settled explanation for decades.
The oxygen theory — that yawning corrects a shortage of oxygen or excess of carbon dioxide — was the standard explanation for a long time and is now largely discounted. Experiments giving subjects extra oxygen or raised carbon dioxide did not reliably change how much they yawned.
The brain cooling hypothesis currently has the most support. A yawn draws in a large volume of air and stretches the jaw, increasing blood flow around the skull and potentially cooling the brain by a fraction of a degree. Supporting evidence includes findings that people yawn less when holding something cold to the forehead, and that yawning frequency varies with ambient temperature — dropping when the air is as warm as the body, where cooling would not work.
The arousal and state-change hypothesis notes that yawning clusters around transitions — waking, falling asleep, becoming bored, and immediately before demanding activity. Athletes and performers frequently yawn before competing, which is hard to square with drowsiness.
Contagion is a separate puzzle. Seeing, hearing or even reading about yawning triggers it in roughly 50–60% of adults. It appears to involve social and empathetic processing rather than anything physiological: contagion is stronger between people who are closer emotionally, develops in children around age four or five, and is reportedly reduced in some conditions affecting social cognition.
Contagious yawning has been observed between dogs and humans, and within several other social species.
Many readers will have yawned while reading this.