People are less likely to yawn when others do as they get older, a study has found.
Contagious yawning is linked more closely to a person's age than their ability to empathise, as previously thought, US-based scientists said.
It also showed a stronger link to age than tiredness or energy levels.
Researchers are now looking at whether the ability to catch yawns from other people is inherited, with the hope of helping treat mental health disorders.
Autism and schizophrenia sufferers are reportedly less able to catch yawns, researchers said, so understanding the genes that might code for contagious yawning could illuminate new pathways for treatment.
In the study, published in the journal Plos One, 328 participants were shown a three-minute video showing other people yawning.
LEVELS OF TIREDNESS
Overall, 68 percent of the participants yawned. Of those, 82 percent of people aged under 25 yawned, compared with 60 percent of people aged between 25 and 49, and 41 percent of people aged over 50.
Dr Elizabeth Cirulli, assistant professor of medicine at Duke University in Durham, North Carolina, led the study.
She said: "This is the first study to look at a whole bunch of factors. It is the largest study, in terms of the number of people involved, to date."
Dr Cirulli said she did not know why contagious yawning decreased with age.
She added that although age was the most important predictor of contagious yawning, only eight percent of the variation in whether or not a participant yawned was explained by their age.
"The vast majority of variation in the contagious yawning response was just not explained," said Dr Cirulli.
The study used questionnaires to test the participants' empathy, levels of tiredness and sleep patterns.
Meanwhile, intelligence was assessed using cognitive tests.
Robert R Provine, professor of psychology at the University of Maryland in Baltimore County, said the study was "unique" as it marked the first time a link between ageing and contagious yawning had been shown.
He said the study would "help to get down to the neurological nitty-gritty of contagious behaviours" and mental health disorders such as autism and schizophrenia.
Prof Provine said the findings could also help to understand why laughing and coughing were so contagious.
He added: "Contagious acts such as yawning and laughing remind us that we are often mindless beasts of the herd, not rational beings in full conscious control of our behaviour."