London :People are most likely to "catch or pass on" a yawn when interacting with family members, followed on a decreasing
scale by friends, then acquaintances and lastly strangers - the same pattern that is seen for other measures of empathy, the researchers found.

Children do not develop contagious yawning until the age of four or five - the same point at which they develop the
ability to interpret other people's emotions properly.

The findings, published in the Public Library of Science ONE journal, confirms the theory that yawn contagion is a form of empathetic bonding that happens within social groups, the scientists said.

For their study, the researchers analysed 480 bouts of yawning among 109 adults in Europe, North America, Asia and
Africa over a 12-month period.

They found that half of all yawns are contagious between family members, compared with about a quarter of those between friends, an eighth between acquaintances and fewer than one in ten between strangers.

 The results also showed that the delay in which a yawn is passed on is longer between strangers than between people who know each other well.

"We wanted to try to understand what are the most important factors affecting yawn contagion in humans so we had
the idea to collect data for one year in different countries in the world," said study co-author Prof Elisabetta Palagi.

"We found that the most important factor is not nationality, the colour of skin, different cultural habits, sex or age of the people involved, but the type of relationship that linked the two people," Prof Palagi said.

Similar behaviour in monkeys suggests that the relationship between yawn contagion and empathy may have
developed before the last common ancestor of humans, other apes and monkeys, she added.