The new study was conducted by Duke Center for Human Genome Variation and was published Friday, the Duke Center said in a release.
The researchers said contagious yawning may decrease with age and is not strongly tied to variables such as empathy, tiredness and energy levels.
Contagious yawning -- yawning in response to hearing, seeing or thinking about yawning -- is a phenomenon occurring only in humans and chimpanzees.
"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," said study author Elizabeth Cirulli, assistant professor of medicine at the Center for Human Genome Variation at Duke University School of Medicine.
Why some people are more susceptible to contagious yawning than others has not yet been determined, the release said.
"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.
"It is possible that if we find a genetic variant that makes people less likely to have contagious yawns, we might see that variant or variants of the same gene also associated with schizophrenia or autism," Cirulli said. "Even if no association with a disease is found, a better understanding of the biology behind contagious yawning can inform us about the pathways involved in these conditions."