Question: why is it when we yawn someone else yawns?

Alex Reid answered on 7 Nov 2018:

Yawning is a bit of a mysterious behavior but we think it may be your body helping you stay awake by injecting a quick burst of oxygen into your blood stream. The act of seeing someone else yawn and yawning yourself is really weird, but where there is weird stuff there is also often interesting science to be done. According to one study even reading the word yawn can make you yawn more! As to why this happens we are really at a bit of a loss, but we can have an educated guess. If you are with a group of people and you are all staying up late it may be for an important reason (for example our ancestors might have been out late night hunting or avoiding predation). Contagious yawning in this situation might help synchronize group alertness. As such it might be an adaptive (useful) group level behavior from our evolutionary history. This is really really speculative though (unproven) and the honest answer to your question is: we don’t really know but we are looking into it!