They say a picture is worth a thousand words. But a facial expression is worth many more. Subtle changes on the face can dramatically affect the viewer’s opinion of a person—a phenomenon referred to as face-ism. And all it takes is a fleeting glance. According to research conducted by psychology professor Dr Alexander Todorov of Princeton University, a mere 40 milliseconds of looking at a face will establish an impression of someone’s personality. That’s about a tenth of the time it takes to blink an eye.

Although they are formed so quickly, these impressions can have a major impact. “Within a split-second of laying eyes on you, others will have decided whether you are competent and trustworthy,” claims science journalist David Robson on the BBC website. “Whether you are a leader or a follower. And those prejudices might shape key events in your life, determining everything from your friendships to your bank balance.”

Armed for subtlety with more than 40 muscles

The face has 43 muscles, well equipping it to transform itself. Additional muscles (in the neck, for instance) are also called into play for certain expressions. Research on muscle movements involved in different expressions has helped us define the emotions they communicate. According to psychologist Dr Paul Ekman–a leading authority on nonverbal language and the creator of the Facial Action Coding System (FACS)—there are seven basic human emotions that can be recognized via facial signals: anger, sadness, fear, surprise, disgust, contempt and happiness. (The FBI, CIA and other law enforcement agencies have used FACS to read subtle emotional cues in facial expressions, voices and body language.)

Our faces are powerful crystal balls that can predict certain types of future success with startling accuracy. Military cadets whose expressions convey dominance, for example, are much more likely to achieve higher ranks. Dr Todorov echoed this finding when he had participants look at photographs of aspiring politicians and rate them for “competence.” These judgments were made with only one second of viewing, but perceptions of competence predicted who would be elected with nearly 70% accuracy.

Face-ism can work for or against a person in a court of law as well. People who look more trustworthy are more likely to be set free than those perceived as having a “guilty” face, even when the evidence is the same.

Of course, it has long been known that people tend to respond more positively to an attractive face. But it goes beyond that. In a study reported in Psychological Sciences, a group of people who had never met before spoke in a round-robin format for three minutes. Those rated as physically attractive were evaluated more accurately, with opinions being a closer match to the person’s self-reported personality traits. This may have happened because the viewers paid closer attention overall when looking at someone they deemed attractive.

It’s disheartening to think we may be swayed by perceptions that have nothing to do with the true character of a person, but it’s a common phenomenon. “Although we like to think we make decisions in a rational way, we are often swayed by superficial cues,” notes psychologist Christopher Olivola of Carnegie Mellon University in the Huffington Post. “And appearances are a particularly superficial, yet very strong cue.”

Future research may tell us more about the power of facial expressions. For example, what would happen if a meek person started “faking” a confident expression on a regular basis? Would others start to perceive him as authoritative and fuel his rise up the career ladder?

Keep a careful eye on the candidates’ facial expressions during the presidential debates in the months to come. You may be able to predict who will move into the White House in 2017.

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