How You React When Startled Is a Window Into Your Soul

The startle reflex evolved to protect the body, but it also reveals your thoughts and feelings.

A group of people in a movie theater make scared faces and clutch each other.
Image Source / Getty

To understand the expressive range of the human face, nothing beats watching a colleague scream his head off in slow motion. When my lab began to study protective reflexes in the early 2000s, the video cameras came out and the place became a scare factory. Graduate students took to lurking in hidden corners and lunging out with Velociraptor shrieks. Sundry plastic bugs and a pair of taxidermized monkey arms found their way inside the lunch refrigerator. I confess, I once took a cow eyeball from a dissection class, wrapped it in foil, and gave it to a colleague as a chocolate truffle.

By filming the reactions and reviewing the videos frame by frame, we began to realize that the startle reflex might be an evolutionary point of origin for many of our most common human emotional expressions.

This article is adapted from Graziano’s new book

When you look at still frames of a startle reaction, two features stand out: the pursing of skin around the eyes and the flashing of teeth. As the face scrunches, the upper lip pulls up, baring the upper teeth in a way that looks like a fleeting smile or a laugh. A lot of guesses have been floated about the purpose of this part of the movement. If you’re about to be attacked, maybe it’s good to appear as if you’re ready to bite. But a close look at the movement, especially if you measure muscle activity in the face, suggests a different function: eye protection. If you expose your teeth to bite a hamburger, you recruit a set of muscles that ring the mouth. In contrast, the startle reflex recruits muscles around the eyes and in the cheeks. The forehead is mobilized downward and the cheeks are mobilized upward, dragging the upper lip with them—and shielding the eyes in wrinkles.

You’ll know what I mean if you’ve ever walked from a dark indoor space into a bright, sun-saturated summer day. Your whole face contracts into a kind of sun smile, or maybe a sun grimace, exposing your upper teeth, bunching your cheeks upward, and wrinkling the skin around your eyes to protect them from the excess light.

They say that the eyes are the windows to the soul. But the eyeballs themselves aren’t really the windows. If you could look at a pair of eyeballs minus the rest of the face, you’d only learn where the eyes are looking and how dilated the pupils are. For reading another person, it’s everything surrounding the eyes that matters the most. The windows to the soul are the eyelids that can narrow skeptically or open wide, the eyebrows that move and shape expressively, the sly wrinkles at the outer corners or on the bridge of the nose, the upward bunching of the cheeks—the many tensions and relaxations rioting around the center.

Ask any professional portraitist and you will be told that although the eyes are the most important part of the portrait, the eyeballs themselves are not so interesting to paint. They’re merely bluish-white ovals with a black dot and a reflection. The emotional expression, and the challenge to the painter, lies in all the subtleties crowding around the eyeballs.

It may not be a coincidence that those true windows to the soul correspond so exactly to the epicenter of the startle reflex. The startle has a side effect that has nothing to do with self-protection: It broadcasts personal information well beyond the simple fact that you are startled. The reason is that your moods and thoughts strongly influence how your startle reflex manifests itself in any given moment. As much as the reflex relies on primitive pathways through the base of the brain that evolved before any higher thought or emotion, it is nonetheless influenced by networks all throughout the brain. Emotions, attention, and expectation sift through the cerebral circuitry, affect the startle reflex, and shape the way it looks to others.

For example, startle is affected by anxiety. One decidedly unpleasant way to make people anxious in an experiment is to periodically shock them with electricity. As they’re cringing and waiting for the next shock, if they’re blasted with an unexpected loud sound, their startle reflex to that sound will be greatly exaggerated. Even if they’re subjected to less extreme irritants, like foul odors or unpleasant pictures, their startle reaction still will grow stronger. People who suffer from anxiety disorders also have a measurably enhanced startle reaction. They really are more jumpy.

Evolutionarily, the startle reflex’s variability has a monumental consequence. It’s one of those right-angle bends in evolution that makes biologists happy. Another creature, watching your startle reaction, could in principle learn a great deal about your inner state—and use that information to its advantage. Are you confident or anxious? More likely to run away, or more likely to go on the attack? Are you dominant over the people around you, or do you feel frightened by them? Your reaction to the sound of a broken twig or the unexpected touch of a fly landing on the back of your neck—just how much you crouch, and especially how much of the startle reaction flits across your face, pops out in the wrinkles around your eyes, or shows in the rising movement of your upper lip as you flash some teeth—these reactions can convey crucial psychological information to anyone watching.

The startle reflex is not, itself, a social gesture. It’s not a smile, a laugh, or a frown. It’s simply a reflex that evolved to protect the body. But it puts you at risk, too. By leaking signs about your inner state to the rest of the world, it becomes an exploitable data breach. And it’s not subtle. You might as well put a neon billboard on your head.

To spark the evolution of our familiar, human emotional expressions, all you need is a creature with enough brain capacity to look at others, especially at their eyes, perceive the subtleties in tension and movement, and use the stolen information to decide how to act next. An especially enterprising creature might even probe your startle reaction by looming at you or barking, for the specific purpose of extracting information. Evolution can take over from there, in an arms race of exploiting the signals that you see in others, and manipulating others by sending out your own, contrived signals.

The situation reminds me of a poker game. Poker players often have tells. For example, a player might unconsciously pick at his fingernails or scratch the bridge of his nose when he’s nervous. A good player will notice the tell immediately and use it to advantage. It may not convey a lot of information, but it conveys enough to give the observer a statistical edge. On the other hand, a good player can also mimic tells, seeming to appear nervous or confident, sending interference and thereby strategically influencing the behavior of his opponents. Give it a million years of evolution, and who knows what elaborate, ritualized signals and counter-signals might emerge in Homo five-card.

This post is adapted from Graziano’s new book, The Spaces Between Us: A Story of Neuroscience, Evolution, and Human Nature.