Why do humans scream and make odd faces when they're scared?

Whether it’s from a visit to a haunted house or a scary movie that makes you leave the lights on for days, fear is a thrilling yet terrifying feeling. For many, a sudden fright triggers a scream — a response so instinctive that we hardly think about its scientific basis. But what exactly triggers us to react to fear with wide eyes, a dropped jaw, and a loud scream?

The faces we make hold real power. As hosts Julie Douglas and Robert Lamb explain in the Stuff to Blow Your Mind video above, even when there's no actual danger, just pulling a scared face can heighten your alertness. Your eyes open wider, scanning quicker, granting you better vision to spot potential threats. You breathe faster, with your nostrils becoming more sensitive to smells (perfect for detecting zombies). A fearful expression can also stir emotions. In fact, many facial expressions can influence how you feel, says psychologist William James. Just as smiling can improve your mood, a terror-stricken face can trigger vigilance and a full-on fight-or-flight response throughout your body.

Despite the universal nature of the fear expression, not everyone can form the same facial expression. The risorius muscle, which controls expressions of extreme fear, is typically found near the sides of the mouth. This muscle appears in about two-thirds of people, but its size varies greatly between individuals. Some may have it on both sides of the face, while others have it only on one side. The presence or absence of this muscle impacts the intensity of the nonverbal cue known as the expression of terror.

The scream itself? The idea is that it serves as an audible defense, much like the blaring alarm of a car or the cry of a baby — impossible to overlook. Research reveals that the more erratic, wild, and primal the scream, the more difficult it is for others to ignore it. Maybe that's why the terrifying shrieks of victims in horror films are so gripping.