It's not usually a good idea to quickly judge someone based on a quick glance, but Psychology Today points out that in some cases—especially when your safety is potentially threatened—it's good to go with your gut when you view someone as possibly aggressive.
Citing several different studies, Psychology Today points out that in most cases, we're very good at immediately recognizing someone as a potential threat based on their face alone. While it's not foolproof, it is often accurate, and as Psychology Today notes it's good to trust your instincts:
Don't dismiss your instincts, especially if your safety and well-being are at risk. Guesses about trustworthiness based on headshots tend to correlate with those individuals' self-reports and judgments by their acquaintances. In one experiment, people who were perceived as dishonest were likelier to mislead their peers than were those whose faces were thought to look honest... Consciously or not, you may be more cautious and attentive around potential cheaters and other offenders.
In several studies, people have been able to distinguish criminals from non-criminals based on their face alone. While facial profiling isn't the most reliable thing in a court room, it does have real world applications if you need to scan a room for threats. Psychology Today even suggests that over time people may take on slightly more aggressive facial features when they're repeatedly angry or deceptive, which might explain why Bond villains are so consistently evil looking:
But nurture (experience) may also change a face in telltale—albeit subtle—ways. Over time, people who have acted disingenuously may develop a slightly crooked mien because they make more asymmetrical facial expressions. Or, we may be picking up on slightly angry expressions; they're associated with untrustworthiness, as found in a study by Alexander Todorov at Princeton. Not coincidentally, anger increases the face's WHR (raising the upper lip and lowering the brow), and chronic anger may linger on a face. Committing repeated aggressive or violent acts might also change the perpetrator's face in telling ways. [Cornell University Psychologist Jeffrey] Valla speculates that this behavior exposes a person to chronic high levels of testosterone and the stress hormone cortisol, and that we may detect the influence of these hormones on facial skin tone and shape.
While stereotyping people at a glance is a dangerous way to view the world, if you combine it with other tricks, like spotting a liar, or watching out for negative body language, you can quickly piece together a situation and keep yourself safe.
What's In a Face? | Psychology Today
Photo by Robert Couse-Baker.