The phenomenon of "contagious yawning" may not be as potent in rooms full of elderly people, research suggests.

Older people are less likely to be susceptible to "catch" a yawn from someone else, a study implies.

But age only accounts for a small amount of the variation in why some are susceptible to an infectious yawn and others are not, the US researchers said. The team of experts from the Duke Centre for Human Genome Variation said their study also dispelled the theory that contagious yawning is linked to empathy.

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The study, published in the journal Public Library of Science ONE, examined 328 healthy participants who underwent a number of tests to assess their cognitive function and levels of empathy, energy and sleepiness. They then watched a three-minute video of people yawning and experts recorded how many times they yawned while watching the clip.

Some did not yawn at all while others yawned 15 times during the film - 222 yawned at least once.

The researchers did not find a strong connection between yawning and empathy or intelligence. The only factor that appeared to influence whether or not someone was susceptible to a contagious yawn was age - as age increased, participants were less likely to yawn, they said.

Study author Elizabeth Cirulli said: "Age was the most important predictor of contagious yawning, and even age was not that important."