Along with the whereabouts of Bigfoot and the answer to whether we are alone in the universe, the mechanism behind contagious yawning remains one of life’s great mysteries. Though previous studies have suggested a link to empathy, new research suggests this is not the case, rendering it still largely unexplained.

The study, published in the journal PLOS ONE, was conducted by researchers from the Duke Center for Human Genome Variation in North Carolina. They say their findings show that contagious yawning may decrease with age, and that it is also not linked with tiredness or energy levels.

Contagious yawning occurs not only in humans, but also in chimpanzees and other animals, in response to hearing about, seeing or thinking about yawning.

In 2013, Medical News Today reported on a study in dogs, which suggested that the contagious yawns dogs get from their owners are a result of empathy rather than stress.

But Elizabeth Cirulli, author of the latest study and assistant professor at Duke University School of Medicine, says:

“The lack of association in our study between contagious yawning and empathy suggests that contagious yawning is not simply a product of one’s capacity for empathy.”

Cirulli and colleagues say their study is the most comprehensive research to date looking at factors that influence contagious yawning.

They explain that this phenomenon differs from spontaneous yawning, in that it does not occur merely when someone is tired or bored. While spontaneous yawning is observed in the womb, contagious yawning begins in early childhood.

By better understanding the biological mechanisms involved in contagious yawning, the researchers say it could provide better insight into disorders such as schizophrenia or autism.