Yawns are contagious. If one person in a room yawns, soon there will be lots of people in the room yawning. Even hearing a yawn seems to be enough to get people to yawn. (Apparently, writing about them is enough as well *yawn*.) I have always called this yawn tennis, though in the psychological literature they call it contagious yawning.
A paper in the September, 2009 issue of Cognitive, Affective, & Behavioral Neuroscience by Stephen Arnott, Anthony Singhal, and Melvyn Goodale suggests that yawn tennis may be caused by mirror neurons in the brain.
Mirror neurons are cells in the brain that are thought to influence imitation in humans and some other animals. Essentially, these neurons are active both when the person or animal performs an action and also when they witness someone else performing that same action. These neurons are thought to be the basis of people's ability to imitate others and also to understanding the actions of others.
But back to yawning.
These investigators put people in an Magnetic Resonance Imaging (MRI) scanner. A technique called functional MRI (fMRI) allows investigators to measure changes in blood flow to particular areas of the brain. When neurons in a particular brain area get more active, they need more energy, and so more blood flows to those areas to help supply the energy and oxygen needed for this activity. So, blood flow to a region is used as a stand-in for activity.
While people were lying in the MRI machine, they heard either yawns played to them, normal breathing, or yawns that were scrambled up so that they had all of the same auditory properties of yawns, but didn't really sound like a yawn. On each trial of this study, they rated how much they needed to yawn on a 4-point scale.
People are able to play yawn tennis in an MRI. They rated that they needed to yawn more after hearing other yawns than after hearing normal breathing or scrambled yawns. In addition, the amount that they felt they needed to yawn was related to blood flow (and therefore activity) in a particular area of the brain called the posterior inferior frontal gyrus, which is an area that is believed to contain mirror neurons. The other area of the brain whose activity was particularly related to the need to yawn was an area known to be involved in actions involving the mouth.
So, the next time you get a strong urge to yawn because you see or hear someone else yawning, just remember that your mirror neurons made you do it.