LONDON (Reuters) - Showing empathy can make for a lot of yawning.

A study of autistic children has bolstered evidence that people who identify better with others are more prone to contagious yawning, Japanese researchers say.

Scientists have long known that one yawn often leads others to follow suit but what triggers the phenomenon is not as clear, said Atsushi Senju, a researcher at the University of London who worked on the study.

Some believe it is simply a reflex. Others suggest the same mechanisms in the brain that make people feel empathy also cause them to yawn when they see others doing the same, he said.

In the study the team tested the reaction of autistic children and normal children when watching video clips of people yawning and then simply moving their mouths.

The researchers found the children with autism, a developmental condition that severely affects social interaction and communication including empathy, yawned less than other children during clips of people yawning.

Both groups of children yawned the same amount when watching the video of people only moving their mouths, showing that empathy was the key, Senju told Reuters.

"It supports the claim that contagious yawning is based on the capacity for empathy," the researchers wrote.

Contagious yawning is seen in only a few other primates and studies have suggested the behavior has played an evolutionary role in helping groups avoid danger by keeping animals awake and alert.

The researchers said the findings offer a jumping-off point to investigate the nature of social and communication impairment in those with autism.

"Further studies are required to investigate the relation between contagious yawning and other symptoms of (autism) such as empathy, imitation and/or face fixation," the researchers wrote.

The study is published in the Royal Society's Biology Letters on Wednesday.