Contagious yawning is known to be more than coincidence: studies have shown that 40 to 60% of people who watch videos or hear talk of yawning end up joining in. But psychologists have long wondered what causes it. "It seems like such a hokey phenomenon," says psychologist Steven Platek.

To try to get to the bottom of it, Platek and his colleagues at the State University of New York in Albany sat subjects in front of videos of people yawning and tallied their responses to find out why we are susceptible or immune to contracting yawns.

The tests showed that those impervious to the trigger also struggle to put themselves in other people's shoes, the psychologists say. For example, they might be less likely to recognise that a social faux pas or insult could cause offence.

Identifying with another's state of mind while they yawn may cause an unconscious impersonation, the team suggests. The findings, reported in the journal Cognitive Brain Research, might also explain why schizophrenics, who have particular difficulty in doing this, rarely catch yawns.

This makes evolutionary sense, agrees Ronald Baenninger, who has studied yawning at Temple University in Philadelphia. Contagious yawning may have helped our ancestors coordinate times of activity and rest. "It's important that all group members be ready to do the same thing at the same time," Baenninger says.

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