Blink and you’ll miss it.

If you suspect someone is a psychopath, just look into their eyes, experts advise.

In a 2018 studyresearchers at Cardiff and Swansea Universities investigated the effect of “nasty” images, such as mutilated bodies and threatening dogs, on offenders who are psychopathic and those who aren’t.

They found that when looking at the pictures, the eyes of the psychopaths had an “unusual reaction” — they did not experience pupil enlargement.

Meanwhile, the pupils of non-psychopathic offenders did widen and dilate when looking at harrowing images, which is a natural response.

“Our findings provide physical evidence of an emotional deficit common to psychopathic offenders,” lead author Dr. Dan Burley, from Cardiff University’s School of Psychology, said at the time.

“The pupil has long been known to be an indicator of a person’s arousal,” Burley explained. “The pupil usually dilates when an image shocks or scares us. The fact that this normal physiological response to threat is reduced in psychopathic offenders provides us with an obvious physical marker for this condition.”

This look in the eyes is often referred to as a “psychopathic stare” or “psychopathic eyes,” reflecting an unfeeling and withdrawn response when looking at something that most people would find alarming.

However, researchers were dumbfounded to discover that psychopaths’ pupils actually dilated when looking at positive and optimistic images, showing a “normal” response similar to those of non-psychopaths.

These findings suggest that psychopathy isn’t directly to a difficulty responding to emotion, but rather a specific reactivity to threatening information.

“Many psychopathic offenders appear to be bold, confident, and can act in a cold-blooded manner. It’s much easier to act bold if you have no feelings of fear, and to be cold-blooded if there is no emotion to get in the way of the act,” Professor Robert Snowden from Cardiff University, who supervised the research, explained.

Professor Nicola Gray, a clinical and forensic psychologist from Swansea University, who provided clinical supervision for the project, added, “This is one of the first times we have objective, physiological, evidence of an emotional deficit underpinning the offending behavior of psychopathic offenders that does not depend on invasive methods or expensive equipment.”

“We hope to be able to develop this methodology to assist with clinical assessment and intervention in offender populations.”

Some of the infamous people considered to be psychopaths include serial killers Ted Bundy, Fred West and Richard Ramirez.

One cannot be diagnosed as a psychopath, however, because it’s not a diagnosis. “Psychopathy can only be evaluated and assessed using the Psychopathy Checklist-Revised (PCL-R),” Dr. David Tzall, a licensed psychologist, told PsychCentralexplaining that many confuse psychopathy and antisocial personality disorder (ASPD).

“They are similar in scope but different in terms of details,” he clarified. “ASPD is a diagnosable personality disorder characterized by a pervasive pattern of disregard for the rights of others, the safety of themselves and others, impulsivity, and socially accepted norms and rules.”

Meanwhile, “psychopathy is a personality construct that describes a set of interpersonal, affective, and behavioral traits that are similar to those observed in ASPD.”