Yawns are more contagious between family members and friends than strangers, a new study found.

Researchers from the University of Pisa in Italy observed 109 men and women of various nationalities for up to two hours at a time in their natural settings. Each time a subject yawned, the yawns triggered in those around them were recorded.

"Our results demonstrate that yawn contagion is primarily driven by the emotional closeness between individuals and not by other variables, such as gender and nationality," Ivan Norscia and Elisabetta Palagi reported today in the journal PLoS One.

Yawns were most contagious between kin and life partners, followed by friends and then acquaintances, according to the study.

"Strangers and acquaintances showed a longer delay in the yawn response compared to friends and kin," the authors wrote.

Little is known about the phenomenon known as yawn contagion. A yawn can signal fatigue, stress or boredom. And Norscia and Palagi suspect that, like a smile, a yawn is a form of empathy.

"Yawn contagion is affected by the empathic bond that links two people," they said in a statement.

Similarly, smiles are stronger and more sustained when inspired by loved ones, according to a 2009 study of mothers and their infants.

Seeing someone yawn activates a complex network of brain regions involved in movement, sensation and emotion, according to Norscia and Palagi.

"Thus, the neural regions linked to the emotional sphere of positive affect may be over-stimulated in subjects viewing the yawn of someone they care about," they wrote. "Such over-stimulation may ultimately lead to a potentiated yawning response."