Contagious yawning may decrease with age and is not strongly related to variables like empathy, tiredness and energy levels, according to a recent study.

Contagious yawning is a phenomenon that occurs only in humans and chimpanzees in response to hearing, seeing or thinking about yawning. It differs from spontaneous yawning, which occurs when someone is bored or tired. Spontaneous yawning is first observed in the womb, while contagious yawning does not begin until early childhood.

"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," study author Elizabeth Cirulli said in a statement.

For the study, researchers from the Duke Center for Human Genome Variation recruited 328 healthy volunteers, who completed cognitive testing, a demographic survey, and a comprehensive questionnaire that included measures of empathy, energy levels and sleepiness.

The participants then watched a three-minute video of people yawning, and recorded the number of times they yawned while watching the video.

The researchers found that certain individuals were less susceptible to contagious yawns than others, with participants yawning between zero and 15 times during the video. Of the 328 people studied, 222 contagiously yawned at least once. When verified across multiple testing sessions, the number of yawns was consistent, demonstrating that contagious yawning is a very stable trait.

They did not find a strong connection between contagious yawning and empathy, intelligence or time of day. The only independent factor that significantly influenced contagious yawning was age.

As age increased, participants were less likely to yawn. However, age was only able to explain 8 percent of the variability in the contagious yawn response.

"Age was the most important predictor of contagious yawning, and even age was not that important. The vast majority of variation in the contagious yawning response was just not explained," Cirulli said.

Because most variability in contagious yawning remains unexplained, the researchers are now looking to see whether there are genetic influences that contribute to contagious yawning.