Birds of a feather yawn together, a new study has found.
Researchers discovered that yawns are more contagious between family members or friends than strangers.
Everyone knows when one person yawns it can set others off - but why the phenomenon occurs is little understood.
Now new research suggests social empathy plays an important role.
Scientists found that yawning contagion increased according to how strong the bonds between people were.
Relatives were most likely to spark off yawns in each other, followed by friends, acquaintances and lastly strangers.
The Italian researchers spent a year recording the yawns of 109 adults - 53 men and 56 women - from around the world.
Participants were divided up into "triggers" - who instigated yawning - and "observers" who responded by yawning themselves.
In total there were 480 instances where one person yawning triggered yawns in another within three minutes.
Writing in the online journal Public Library of Science ONE, authors Dr Ivan Norscia and Dr Elisabeta Palagi from the University of Pisa concluded: "Related individuals showed the greatest contagion, in terms of both occurrence of yawning and frequency of yawns."