Why is yawning contagious and what purpose does it serve?

The answer

Yawning serves primarily as a brain-cooling mechanism according to research by Andrew Gallup at SUNY Oneonta. A 2014 study found yawn frequency correlates with brain temperature and ambient temperature, with yawning decreasing when subjects held cold packs to their foreheads. The average yawn lasts 6 seconds, during which heart rate increases by up to 25%. Contagious yawning, which affects about 60-70% of people, is linked to empathy circuits in the brain - individuals scoring higher on empathy tests yawn more in response to others. Children develop contagious yawning around age 4-5, coinciding with the development of theory of mind. Psychopaths show significantly reduced contagious yawning. Even dogs catch yawns from their owners, with a 2013 study showing dogs were more likely to yawn contagiously from familiar people.