However, the link between yawning and empathy is inconclusive. A study of the Duke Human Genome Variation Center was published in the journal PLOS ONE to identify factors that contribute to infectious yawning. In this study, 328 healthy volunteers underwent a survey that included sleepiness, energy levels, and empathy measurements. Survey participants watched a video of people yawning and calculated how many times they played while watching. Although most people yawn, not everyone does it. Of the 328 participants, 222 had yawned at least once. Repeated video tests have shown that it is a stable feature for a given person to have contagious yawning. The Duke study found no correlation between empathy, time, intelligence, and contagious yawning, but there was a statistical correlation between age and yawning. Older participants are less likely to yawn. However, because age-related yawning accounts for only 8% of the response, the researchers plan to find the genetic basis of infectious yawning. Studying the infective yawning of other animals may provide clues as to how people can capture yawns. A study conducted at the Institute of Primate Research at Kyoto University in Japan investigated the response of chimpanzees to yawning. The results of a study published in the Royal Society Biology Letters show that two of the six chimpanzees in the study suffered serious injuries in response to videos of other chimpanzees yawning. The three baby chimpanzees in the study did not catch yawns, suggesting that young chimpanzees, like human children, may lack the intellectual development needed to capture yawns. Another interesting finding of the study was that the chimpanzees only responded to the actual yawning video, not the video of the chimpanzee’s mouth.