Why are yawns contagious?

Yawns are contagious because your brain is built to notice and copy people near you. When you see a wide sleepy face, movement-planning parts of your brain can start preparing the same action before you decide anything. Scientists link this to social copying, though the full reason is still being studied.

One lovely clue is that yawns pass more easily between people who know each other well, and even sometimes between dogs and their humans. Very young children usually do not catch them yet, which suggests the habit grows along with attention to other minds.