Chasmology! The study of yawning! It’s a real thing! There’s even an international conference!
Even human fetuses!
Seeing, reading, or thinking about yawning triggers more yawning in humans (except those of us with schizotypal personality traits). Contagious yawning happens more often when you’re warm and when you breathe through your mouth and most probably is a primal, uncontrollable expression of human empathy.
So it’s no surprise that children don’t yawn contagiously until they’re a few years old.
If you’re out with your Romeo or Juliette and they start to yawn, don’t fret! In people, yawning is triggered by some sex-related agents like oxytocin and androgen, and many males get sexually aroused when they yawn.
And one more thing, to set the record straight: Research shows we DO yawn when we’re bored, we DO yawn because we’re sleepy, and we do NOT yawn because of high CO2 levels or low O2 levels in the blood and brain.