The term psychopath is sometimes thrown around to describe someone who is a bit unhinged. In reality, research suggests every one in 100 people meet the criteria to be a true psychopath - which includes antisocial behaviour, remorselessness, deception and interpersonal manipulation.
While that might not sound like a lot, it does mean that we're likely to come into contact with psychopaths regularly - but how can you spot one?
Scientists have come up with five giveaway signs to spot a psychopath, which you can use to work out if you know one.
They are less likely to yawn
You may have heard before that not yawning when you see someone else do it is a trait of a psychopath.
The theory is that contagious yawning is linked to empathy, which is a characteristic psychopaths usually lack.
A 2015 study by Baylor University in Texas looked at the personality traits of 135 students, including cold-heartedness, rebellious nonconformity and egocentricity.
The study found that those who scored higher for cold-heartedness were less likely to yawn if someone else did.
Brian Rundle, a PhD student who was 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 to show that in baboons, dogs, or chimps, the alpha male tends to yawn first. 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 simultaneously."
They have odd sleeping patterns
The checklist of psychopathic traits also includes pore positive personality aspects like charisma, charm, creativity and productivity.
This means they need to keep their brains stimulated, which can lead to an odd sleeping pattern, as they usually end up staying up late.
According to Dr Kathryn Smerling, psychopaths typically have an inability to sleep. She told Bustle: "A psychopath tends to have unusual sleeping patterns and is generally nocturnal."
A paper from Dr Peter Jonason from the University of Sydney backed up this theory.
The study found that across 263 students who were surveyed online, the people who said they tended to be a stay up late were more likely to display anti-social personality traits such as narcissism, Machiavellianism (ability to be manipulative) and psychopathic tendencies.
They use a lot of hand gestures
Using your hands when you talk doesn't necessarily mean you're definitely a psychopaths, but research suggests they use more beat hand gestures than non-psychopaths.
By beat gestures, it's meant up-and-down or back-and-forth, which emphasises certain parts of speech.
This can be a form of manipulation which can be used to emphasise a particular part of a sentence, or steer your attention away from something.
For example, in a study of videotaped interviews by David J. Cooke, Adelle E. Forth and Robert D. Hare in 1998, it was found that psychopaths used more hand gestures than non-psychopaths during narrations about their crimes and family life.
They don't move their heads much
Scientists have also found that those with psychopathic tendencies don't move their heads as much during speech as non-psychopaths.
This was found by a team at the University of New Mexico in Albuquerque. They analysed the head movements of 507 inmates during recorded conversation, finding that those with "severe and life-course-persistent antisocial behaviour" were more rigid than others.
This was an interesting find because as far back as 900 BC it has been found that people who were being deceptive would fidget, move their heads and blink a lot.
One theory is that a static head movement could be a deliberate tactic used by those with psychopathic traits, according to the study.
They talk about food, money and sex a lot
Do you know someone who loves to talk about food, money and sex? It doesn't necessarily mean they're a psychopath, but they certainly like to talk about these subjects more than others.
This was found by researchers at Cornell University, who interviewed 14 psychopathic male murderers and 38 who weren't diagnosed as psychopaths.
During talks about the crimes they committed, it was found that the psychopaths would not only show less remorse for their crimes, they were also more likely to talk about three standout topics - food, money and sex.
Meanwhile, those who weren't diagnosed as psychopathic would more likely discuss topics such as social needs, religion and family.
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