US researchers say yawning is more complex than what we think of it. They discovered two types of it in humans, named contagious and spontaneous.
The first type occurs in humans as well as chimpanzees in response to thinking, seeing or hearing, but the second yawning type happens by tiredness or boredom.
Researchers say the contagious yawning type starts in early childhood where as the spontaneous one is first seen in the womb.
Experts at the Duke University Medical Centre for Human Genome Variation say the contagious yawning reduces with age.
The researchers studied video of more than 300 people and recorded their yawning.
They discovered some of the participants were less susceptible to contagious yawns and the only factor linked to it was the age. The research team said the volunteers with more age were less likely to yawn.
Assistant professor of the university, Dr Elizabeth Cirulli, said age played important role in contagious yawning. However, the research team is now looking ahead whether yawning is also linked to genetics.
Researchers also found people with schizophrenia or autism was less likely to yawn.