Want to see if someone is a psychopath or not? Try yawning at them. If they yawn back, you're okay and proceed to befriending them if you wish. If they don't, perhaps steer clear - they could be distinctly anti-social.

Scientists have found that the more psychopath characteristics people have, the less chance they will be affected by 'contagious yawning' which normal mammals are said to be unable to resist.

he researchers from Baylor University in Texas published their findings in the journal Personality and Individual Differences. A total of 135 students were tested for traits such as  Machiavellian egocentricity (a tendency to only consider personal needs), cold-heartedness and rebellious nonconformity.

As is widely known, most people will yawn if someone around them yawns and shows signs of fatigue. The students were shown video clips of people using different facial expressions, including yawning. The ones who scored highly for cold-heartedness were the ones less likely to yawn. 

Brian Rundle, a PhD student involved in the study, told The Times: 'One of the biggest lines of evidence is that (contagious yawning is) very much related to empathy. 'While this is a really interesting finding, it doesn't mean that if you're not affected by a contagious yawn there's something wrong with you.'

'There's some evidence that to show that in baboons or dogs or chimps the alpha male tends to yawn first,' he added. 'If you're sitting around the campfire it cues everyone else to yawn, and instead of going to bed at separate times they all do it at same times.'