Turns out that yawning IS contagious.  And, it's your friends that might be the leading cause.

[C]lose friends and family are more likely than acquaintances or strangers to catch someone's yawns, a new study finds.

The researchers suggest this yawning contagion is, in part, the result of empathy, in which we can attempt to see things from another person's angle and respond to that person's emotions.

That empathy can and usually is contagious.

While yawns would not seem to have a connection with any particular emotion (unlike, say, smiles, which could indicate happiness), in some ways we are forming an emotional connection by mimicking a yawn or another expression, Campbell said. By mimicking the yawn we see, we become better able to understand how tired, perhaps, or bored the other person is.

In fact, past studies have shown stronger empathetic responses of all kinds toward kin and loved ones. Past research has also found kids with autism don't experience contagious yawning, also strengthening its ties with empathy since autism involves problems with social interaction and communication. On the other hand, canine pals can catch yawns from humans, suggested a study on various dog breeds.

The yawning research study spanned the globe, so that results aren't specific to one country or another.

The 109 adults in the study were from Europe, North America, Asia and Africa, and they were about evenly split by gender. The researchers were able to analyze 480 bouts of yawns. After considering for factors that could have affected the time between a person's yawn and an observer's imitation, they found social bond was key.

The Body Odd - Yawns more contagious among friends.